Ferrous Pipeline Corrosion Processes (1 of 4) 01-88[1]

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office ofPipe1ineSafetv

Techni cal Report No. OPS- TR- 71- 001 Oct ober 1971

w s f f PG a ~ r o s ” 6 / ~ ~

FERROUS PI PELI NE CORROSI ON PROCESSES,DETkCTI ON, AND MI TI GATI ON

Pr epared f orOf f i ce of Pi pel i ne Saf et y

Depar t ment of Transpor t at i onUnder Cont r act DOT- Ok- A9- 108 by

Mechani cs Research, I nc.9841 Ai rpor t Boul evard

Los A r r g e l e s , ‘ Cal i f orni a 90045

Cont ent s of t hi s r epor t r ef l ect t he vi ewsof t he cont r act or , who i s r esponsi bl e fort he accuracy of t he data pr esent ed, anddo not necessar i l y r ef l ect t he of f i ci alvi ews or pol i cy of t he Depart ment of

Transpor t at i on. Thi s report does notconst i t ut e a st andar d, speci f i cat i on, orr egul at i on.



CONTENTS

CONTENTS

LI ST OF I LLUSTRATI ONS

LI ST OF TABLES

LI ST OF SYMBOLS

I . I NTRODUCTI ON

1. ProgramObj ect i ves

2. Li t erat ure Survey

3. Quest i onnai re Methods and Responses

4. Per sonal Cont acts

5. Out l i ne of Repor t

11. SUPWRY

1. Factors I nf l uenci ng Cor rosi on

2. Cor rosi on Cont rol

3 . Const r uct i on, I nspect i on, and Mai nt enance

111. FORMS OF CORROSI ON

1. Basi c Corrosi on Mechani sms

2. I nt ernal and Ext ernal Pi pel i ne Cor rosi on

3. Uni f ormCor rosi on

4. Pi t t i ng

5. Crevi ce Cor rosi on

6. Gal vani c Cor rosi on

7. Corrosi on by Stray Cur rent s and Hi gh Vol t ageDi rect Cur rent

8. Mechani cal St ress Ef f ects

Page

1

V

vi

i x

1

1

1

3

7

7

9

9

13

17

18

21

25

29

30

4 2

43

45

49





-

2 . Mai ntenance

3 . I nspect i on

VI . ECONOMI CS

VI I . OTHER PROBLEMS ASSOCI ATED WI TH CORROSI O \ I OF PI PELI NES

1. Trai ni ng of Cor rosi on Engi neers and Techni ci ans

2. I nt er f erence wi t h Smal l Pi pi ng Systems

VI I I .

3 . Groundi ng of El ect r i cal Ci r cui t s on Wat er Pi pe

4. Mechani cal Damage to Pi pi ng

5. I mproper U s e of Cat hodi c Prot ect i on

GAPS I N THE TECHNOLOGY AND CONCLUSI ONS

Page

140

144

154

158

158

160

160

161

161

162

APPENDI X

I . Exampl e Abst ract Ret r i eval Run

11. Quest i onnai re

111. Al phabet i cal Li st i ng of El ectrol ysi s orCor rosi on I nt er f erence Commi t t ees

I V. Al phabet i cal Li st i ng of Organi zat i ons andSoci et i es wi t h I nt erests i n Cor rosi onand Cor rosi on Cont rol

173

176

200

205

216REFERENCES

i i i



LIS T OF ILLUSTRATIONS

Figure

1

5

6

7

Mixed Potential Diagram for Corrosion of Iron(Schematic)

Cathodic Polarization Curve

Anodic Polarization Curve

Schematic Representation of the UndergroundCorrosion Process

Mixed Potential Curve for Galvanic Corrosion of

More Noble Metal M Having Higher Rate of CathodicReaction

Effectiveness of Cathodic Protection

Nomogram of Some Oxidation Potentials at 25OC

Page

23

25

26

34

4 4

1 1 2

120

V



LI ST OF TABLES

Tabl e Page

1 Key Word Code f or Comput er Ret r i eval of Corrosi onLi t erat ure 4

Operat i ng Responses 6

Most Preval ent Causes f or Corrosi on Leaks 18

Measurements That Have Been Used f or I dent i f i cati onof Causes of Cor rosi on Leaks 19,

5 Most Preval ent Ci r cumst ances Under Whi ch Corrosi onLeaks of Coated Pi pe Have Been Found Dur i ng theLast 5 Years 28

6 Number of Corrosi on Leaks Occurr i ng i n 1969 at J oi nt s and Wel ds 29

7 Some Oxi des, Sul f i des, and Sal t s of I r on Appear i ngi n Cor rosi on Product s 35

8 Number of Corrosi on Leaks That Have Been Causedby Hydrogen Bl i st ers and Cracks 51

9 Mater i al s Transpor t ed Whi ch Cause I nt ernal Corrosi oni n t he Presence of Free Moi st ure 66

10

11

12

Composi t i on of Seawater 68

Compani es Exper i enci ng Bact er i al Corrosi on 71

Maj or Areas of Corrosi on Research Engaged i n orSponsored by the Operat i ng Compani es 75

13 Metal Treatments I mprovi ng Resi st ance to GeneralCorrosi on 78

14 Met al Treat ment s I mprovi ng Resi st ance t o Pi t t i ngand Crevi ce Cor rosi on 79

15 Met al Treatment s I mprovi ng Resi st ance to St ressCor rosi on Cracki ng 80

16 Metal Treat ment s I mprovi ng Resi st ance t o HydrogenEf f ects 80

17 Metal Treat ment s I mprovi ng Resi st ance to Sul f i deSt ress Cracki ng 81

vi



Page

Metal Treatments Improving Res is tanc e t o I n t er -granu la r Cor ros ion 8 1

Tab l e

18

1 9

20

2 1

2 2

23

2 4

25

26

27

28

29

30

31

32

33

34

3536

Values of Material Parameters i n Equation 11 from NBS Underground T e s t Data 8 1

Major Types of Protective Coatings Used by CompaniesDuring the L a s t 5 Years by Order of U s e 87

Fac to rs Conside red i n Se l ec t ing Coa ting Mate r i a l sand Wrappings fo r Sp e c i f i c A p p l i c a ti o n 88

Mate r ia l s Cur ren t ly Used t o Coa t F i e l d Jo in t sand App urte nanc es 89

N u m b e r of Companies Favoring Di ff er en t Galva nicAnodes 1 0 2

Performance of Galvanic Anodes 1 03

Types of R e c t i f i er s Used 1 04

Number of Genera tors i n U s e p e r Company 1 0 4

Impressed Cu rr en t Anodes According t o Performance 1 0 7

Major Sources of Information About the Presenceof In t e r f e r e n c e C u r r e n t s 1 0 8

C r i t e r i a Used t o Determine When In te rf er en ce H a sBeen Mitiga ted 1 0 8

Number of Corrosion Leaks p e r L inea r M i l e Occurring

i n 1 9 6 9 f o r Stee l Pipe 113

Maximum Pipe Potential or Instantaneous OpenC i r c u i t P o t e n t i a l a t t h e R e c t i f i e r L o c a t i o n

Hydrogen and Oxygen Electrode Potentials

115

1 1 7

Vo lta ge Values a t Which t h e Companies Consid erProtec t ion Has Been Achieved 1 2 7

Other Cathodic Protec t ion Cri te r ia Used by t h e

Companies 128

Normal Placement of Reference Ele ctr ode 128Conditions Under Which Pl an t Applied Coat ingsA r e Preferred 137



PageTable

37

38

39

40

41

42

43

44

45

46

47

48

49

50

Conditions Under Which Over -the-Ditch AppliedCoat ings A r e P r e f e r r e d

S tandards and Spec i f i ca t ions fo r Coa t ings

F i e l d Pract ices Used t o Ensure Good CoatingsMethods Cu rr en tl y Used f o r Att ach ing Conductort o P ip e

Methods Used t o Rep air Cor rosi on Leaks

Fac tor s Taken In t o Cons ide ra t ion i n t h e Replace-ment o r Abandonment of Corroded Pipe

C r i t e r i a f o r Replacement o f 12- Inch and S m a l l e rMains

Frequenc ies of S ur ve i l la nc e Methods

Supplementa l Observat ions on the Inspect ion ofCorrosion Leaks

Underground Pipe Inspection Report

Leak Repair Report

Pu bl ic at io ns and Inf orm atio n So urces Which HaveBeen Found t o Co ntr ibu te Most t o Corro sionControl Programs

El ec t ro ly s i s o r Cor ros ion In t e r f e r en ce Committees

O r ga n iz a ti o ns a nd S o c i e t i e s w i t h I n t e r e s t s i nCorros ion and Corro s ion Cont rol

138

139

138

140

141

1 4 2

143

145

150

152

153

159

2 0 1

20 7

vi.i i



LIST OF SYMBOLS

C a p i t a l Let te r s

A = Amperes

A242 = ASTM s t a nd a r d f o r hi g h s t r e n g t h , l o w a l l o y

s t r u c t u r a l s t e e lA285 = ASTM s t a n d a r d f o r l o w t o i n te r m ed i a te s t r e n g t h ,

low al loy s teel f o r p r e ss u r e v e s s e l s

A517 = ASTM s t andard fo r h igh s t r eng t h , low a l l a ysteels f o r p re s s u r e v e s s e l s

A = Exposed are a i n square f ee t

AC = A l t e r n a t i n g c u r r e n t

Ag = S i l v e r

AGA = American Gas Association

AIME = American In s t i t u t e of Mining, Meta l lur gic a l , andPetroleum Engineers

A I S 1 = American Iron and Stee l I n s t i t u t e

A 1 = Aluminum

A p I = American Pet roleum Ins t i tu te

A s = Arsenic

A s 2 0 3 = A r s e n i c t r i o x i d e

ASTM = American Socie ty for Tes t ing Materials

AWWA = American Water Works Association

B = Boron

BaS04 = Barium Sulphate

Br - = Bromide ion

C = Carbon

OC = Degrees cen t r ig rade

Ca = Aggressor concent ra t ion



Ca++ = C a l c i u m i o n

C a C 0 3 = Calcium carbonate

C a ( HC 0 3 ) = C a l c i u m b i c a rbona te

C a ( N 0 3 ) = C a l c i u m n i t r a t e

CaS04 = C a l c i u m s u l pha t e

Cd = Cadm i um

C 1 = Chlor ine

C1 - = Chlor ide ion

C 0 2 = Carbon dioxide

CO, = Carbonate ion-

C = P a s s i va t o r c onc e n t r a t i onC r = Chromium

2-C r 0 4 = Chromate ion

Cu = Copper

P

CuS04 = Copper sulphate

DC = Direct c u r r e n t

DOT = Department of Transpor t a t ion

E = P o t e n t i a l ( v o l t s )

E o = Standard ha l f - c e l l p o t e n t i a l for u n i t a c t i v i t i e s ofr e a c t a n t s and p roducts : f un c t i o n of t empera tu re

= C or ro si on p o t e n t i a l ( v o l t s )E c o r r

EMF = Elec t romot ive fo rce

F = Faraday ' s cons t an t (96 ,500 coulombs/g. e qu i v . r e a c t i on )

O F = Degrees Fa hren hei t

F = F l uo r i de i on.

Fe = I r o n

Fe ,Fe2+ = Fer rous ion++

c



3Fe = F e r r i c i o n

F e C 1 2 = Fer rous ch lor ide (Lawrenci te )

Fe(OH)3 = Ferr ic hydroxide

Fe203 = I r on ox i de (hematite)

Fe304 = I r on oxide (magnet i te)

Fe P = I ron phosphide2

H = Hydrogen

H+ = Hydrogen ion

H C 1 = Hydrogen chl or id e (hydr ochlo r ic ac id )

HCO; = Bicarbonate ion

-H C r 0 4 = Bichromate i o n

Hg=

MercuryHN0 3 = N i t r i c a c i d

HS- = B i s u l f i d e i o n

HVDC = H i gh vo l t a ge d i r e c t c u r r e n t

H2 = Hydrogen mole cule (g as )

H 2C 0 3 = Carbonic ac id

H 0 = Water

H S = Hydrogen sulf ide

2

2

H 2 S 0 4 = S u l phu ri c a c i d

H3B03 = Boric a c i d

I = I od i de i on-

I- E Curves := C ur r e n t- po t e n t i a l plots

13 = Indium

= Current t i m e s resis’zance

J-55 = A l l oy t ub i ng s pe c i f i c a t i ons

K+ = Potassium ion

x a



Xa = Constant dependent on m e t a l t y p e

Kn = Constant dependent on aera t ion of s o i l

L i C l = Li th ium ch lor ide

M = Metal

Mn = Metal i o n w i t h a p o s i t i v e charge of n.

Mg = Magnesium

Mg2+ = Magnesium ion

MgC12 = Magnesium chloride

Mg (OH) = Magnesium hydroxide ( B r u e i t e )

Mn = Manganese

Mo = Molybdenum

N = Normality of a SOlUtiOn

N = N itrogen

N-80 = A l l oy t ub i ng s pe c i f i c a t i ons

+ N a = Sodium ion

NaBr = Sodium bromide

NACE = Na t iona l A s soc ia t ion of Corros ion Engineers

N a C l = Sodium ch lo ri d e (common s a l t )

NaF = Sodium fluoride

NaI = SQdium i o d i d e

NAPCA = Na t iona l Associa t ion of Pipe Coat ing A p p l i c a t o r 7 -

NBS = Na t iona l Bureau of Standards

NH3 = Ammonia

NH4 = Ammonium ion

NH4N 0 3 = Ammonium n i t r a t e

+

NH 4 0 H = Ammonium hydroxide

N i = N i c k e l





Vm = Molar volume of the metal

Zn = Zinc

Zn++ = Zinc i o n

Small Letters

a = A c t i v i t y , e q u a l s c o n c e n t ra t i o n fo r i d e a l mi x tu r e

a = Equat ion cons tan t dependent on a l l o y

a 4 - = Fourth power of t h e a c t i v i t y of th e hydroxide ionOH

a = Oxygen ac t iv i t i e sO 2aH = Hydrogen io n ac t iv i t y

b = Constant

cm2 = Square cent imeters

cu = Cubic

-e = Elec t ron

exp = e (base of n a t u r a l l o g a r i t h m s ) r a i s e d t o the poweri n t h e p a r en t h es e s

f t = Length i n f e e t

g = G r a m s

i = Current

i = Corros ion current , amperesc o r r

k = Equat ion cons tant

lb/A yea r = Pounds per ampere year

log = Common logarithm (base 1 0 )

2m = Square m e t e r

mA or m a = Milliamperes

m i l s = Thousandths of an inch

mm = M i l l i m e t e r s

t

mpy = Thousandths of an inch per y ea r



mV = M i l l i v o l t s

n = Equat ion con s t an t dependent on a e ra t io n of s o i l

n = Number of e l ec t ro ns exchanged i n t h e anodic cor ros ionr e a c t i o n

ohm cm = R e s i s t i v i t y - e lec t r ica l r e s i s t a n c e of a 1 cm-cube

of material p = P i t depth

pH = Negative logar i thm ( b a s e 1 0 ) of t h e hydrogen io n

pOH = Negative logari thm (base 1 0 ) of hydroxide ionconcen t ra t ion

ppm = P a r t s p e r m i l l i o n

p s i = Pounds per square inch

Greek L e t t e r s

a = Value of rl a t c u r r e n t i = 1

u = Propor t iona l t o

3 = "Beta c ons t a n t , " T a f e l c ons t a n t , o r T a f e l s l o p e

y- Fe 0 OH 0 = Gamma f e r r i c oxide monohydrate2 3 2

rl = O ve r po t e n t i a l ( de v i a t i on of p o t e n t i a l from t h er e v e r s i b l e v a l u e )

8 = Time i n y e ar s

p = S o i l r e s i s t i v i t y i n ohm-cm



SECTION I

INTRODUCTION

1. ProgramObj ect i ves

The purpose of t hi s programwas to determi ne the current

stat e of t he ar t of cor rosi on of f er rous pi pel i nes. Thi s i n-

cl uded corrosi on mechani sms, f requency and causes of corrosi on

f ai l ure, cor rosi on mi t i gat i on, cor rosi on det ect i on, and i nspect i on

and st andards. The programgoal was achi eved by three separate

but i nt eract i ng paths: (1) a l i t erat ure survey, ( 2 ) a quest i on-

nai r e, and ( 3 ) personal contacts.

2. Li t erat ure Survev

Emphasi s was pl aced on corrosi on l i t erature publ i shed between

1945 and J ul y 1970. Only except i onal publ i cat i ons out si de of t hi s

per i od were ut i l i zed. The maj or sources of the cor rosi on l i t era-

ture were Bi bl i ographi c Surveys of Cor rosi on, Cor rosi on Abst racts,

and Suppl ement s to Corrosi on Abst racts, al l publ i shed by NACE

( Nat i , mal Associ at i on of Cor rosi on Engi neer s, Houst on) . Ot her

sources were several hundred books, t rade and professi onal

j ournal s, symposi a and research publ i cat i ons, and Corrosi on Cont rol

Abst racts ( publ i shed by Sci ent i f i c I nf ormat i on Consul t ant s Li mi t ed,

London) .

Cor rosi on Abst ract s comprehensi vel y cover 88 dom?st i c,

f orei gn, and goverr . ment sources. T le Cor rosi on Cont rol Abst racts

are a t ransl at i on of the Russi an abst ract j ournal Ref erat i vnyy

Zhurnal wl i ch covers about 800 wor l dwi de publ i cat i ons.

Approxi matel y 90 percent of the wor l d' s cor rosi on l i t erat ure

F r o m 1945 t hrough 1970 was covered i n t hese seri es. Thi s amou. nted



t o s o r t i n g th ro ug h o v er 8 0 , 0 0 0 ge ne r a l c o s m s i o n a b s t r a c t s and

p a p e r s . Approximately 32,000 of these w e r e f rom fore ign sources .

Each a b s t r a c t t h a t w a s p e r t i n e n t t o t h i s program w a s mounted on

se pa ra te sh ee ts of pap er , ed i t ed , and key word coded fo r computer

r e t r i e v a l . T h i s came t o a t o t a l o f 1826 ab s t r a c t s . Approx imate ly

35 p e r c e n t of t h e s e appeared s u f f i c i e n t l y i n t e r e s t i n g t o warran t

d e t a i l e d s t u d y of t he o r i g i n a l works. I n t hos e cases t h e a b s t r a c t s

w e r e f u r t h e r e d it e d o r e n t i r e l y r e w r i t t e n , and subjective comments

w e r e made i n t he ab s t r a c t and /or as p a r t o f t h e d i s c u s s io n i n t h e

body of t h i s r e p o r t .

The 1 8 2 6 ab s t ra c ts ment ioned above were p laced on magnetic

t ape . Th i r ty-one percent of t hese w e r e f rom fore ign journa l s ,

whi l e t h e remainder w e r e from domes t ic sources . L i s t i n gs o f t h e

a b s tr a c ts grouped by key words and i n numeri cal o rder are on f i l e

a t OPS-DOT (Of f i ce of Pipe l ine Safe ty-Department of T r a ns po r t a t i on ) .

Table 1 shows th e key words (o r key ph ras es ) used! t o code o r

c l as s i fy t he abs t r ac t s . The key word code, fo llowed by th e au thor s '

names, t he l i t e r a t u r e r e f e r e nc e and t h e a b s t r a c t i t s e l f , w a s t he nkeypunched on Ho l l er i t h car ds . A l l of t h i s inform at ion may be

r e c a l l e d on command. Thus one may, f o r example, reca l l a l l ab-

s t rac ts de a l i ng w it h a na e r ob i c- b ac t e r i a l- cor ros ion of ca rbon- s t ee l s

underground by asking t h e computer for those key phrases . The

s e a r c h f o r t he s e t h r e e key ph r as e s i s found i n Appendix I. Any

s i n g l e key word o r combinatio ns of keg words may be searched. I n

a d d i t i o n , a l l t h e paper s wr i t t en by an au thor o r g roups o f a u tho r s

can be r e c a l l e d ; f o r exam ple , a l l t h e a b s t r a c t s o r p apers w r i t t e n

by Peabody can be r e c a l l e d and p r i n t e d o u t . Other key words

2



d e a li n g w i th p i p e l i n e c or ro s io n b u t n o t l i s t e d i n Table 1 may

a l so be used t o s e a r c h f o r a b s t r a c t s .

T he a bs t r a c t s w e r e numbered sequent ia l ly as received. The

r e fe r e nc e s i n t h i s r e p o r t u s e t h e same abs t rac t numbers as t h e

l i t e r a t u r e s ea rc h, e . g . , ( 5 5 2 ) i s Reference 552 i n t h e l i s t of

r e f e r e nc e s .

A wide range of l i t e r a t u r e w a s uncovered. The l e ve l of p re-

s e n t a t i on va r i e d t h e f u l l r a nge f rom ve r y e le m en ta r y t o very ad -

vanced. Many, i f n o t m o s t , of t he pa pe rs w e r e merely r es ta te-

ments of what w a s al re ad y known. Numerous pa pe rs w e r e c l e a r l y

sa les promotions f o r one company's p roduc t s o r se rv i ces . Never -

theless, much new information has been uncovered which does not

y e t appear i n any book. This rej?ort emphasizes the new informa-

%ion t h a t w a s considered impor tant .

3 . Questionnaire Methods and Responses

The development and response of t he c o r r o s i on que s t i onna i r e

w a s a s ig n i f i ca n t coopera t ive ach ievement of t h e o p e r a t i n g i n-

dus t ry and t h e O f f i c e of P i pe l i ne S a f e t y .

I n t h e developm ent p ha se , t h e e n t i r e f i e l d of underground

and underwater fe r r ou s pi pe l i ne c orr as i on knowledge w a s o u t l i n e d .

From t h i s w a s drawn a series of que s t i ons on i n s p e c t i o n , c a us e s ,

c o a t i n g s , c a t h o d i c p r o t e c t i o n , c a t h o d i c p r o t e c t i o n c r i t e r i a ,

s u r v e i l l a n c e , c on t r o l , ma in te na nc e, i n t e r f e r e nc e , and r e s e a r c h .

The q u e s t i o n s were chosen t o supplement and. expand on the know-

,edge a va i l a b l e f r om t h e l i t e r a t u r e s ea rc h. The que s t i onna i r e

was then reviewed and expanded by a nvinber of i ndu s t r y a nd govern-

T e n t a l c o r r o s i on committees and s p e c i z l i s t s . The reviews o f t h e



TABLE 1

KEY WORD* CODE FOR COMPUTER RETRIEVAL OF CORROSION LITERATURE

-CORROSION MECHANISMS HEMICAL ENVIRONMENT ROTECTION YPE OF ARTICLE

Corrosion Mechan i sms(gen)Un if or m Co r r os io n o i l Res i s t i v i t yGalvan ic Cor rosion

Crevice CorrosionP i t t i n g C o r r o s i o nI n t e r g r an u la r Co r r o s io nSelec t ive Leach ingStress Co r r o s io n c id s - Low p H < 7 )Corrosion Fa t igue l k a l i e s - High pHErosion Corrosion rga n i cs , Pe t ro leumHydrogen EffectsBio lo g ica l A t t a c k (gen)Anaerobic Ba ct er ia l nhvdrous Ammonia

dA

'7 1

r o t e c t i o n Methods ( g e n ) Experimenta t h o d i c P r o t e c t i o n S e n e r a l R e v i e w o r Booka t e r i a l s S e l e c t i o n S p e c i f i c R e v i e w

o a t i n g s ( g e n ) Theorys p h a l t C o a t in g s Acciden tso a l T a r Co a t in g s F i e ld E x pe r i en ceax Coat ings R u l e s , L a w s , Stan d a r d sl a s t i c a nd O th e r N ew s A r t i c l e s

Organic Coat ingsL h i n F i l m Coat ings LOY BEING STUDIED

a r b o n S t e e l s

a s t I r o n si l i c o n I r o n su r e I r o n st a i n l e s s S t e e l s

CorrosionAer o b ic Bac te r i a l

Cor rosionFungal ani Other

Mic r o b io lo gica l l o g i ca l Ag en t sCorrosi o n

x t e r n a l C u r ren t

t r a y C u r r e n t snodi c P r o t e c t i o n

T es t in g

DataBas ic E lec t r o ch em ica l

f f e c t of Velo c i tyf f e c t of T i m eo i l S t r e ss

*Key words o th er than th ose l i s t e d may be U se d t o d c f l 2 - ? ::er e t r i e v a l s e a r c h area.



que s t i onna i r e were c on s t r uc t i ve , and t he c oope r at i on w a s e x c e l l e n t .

More t ha n 9 2 i n d i v i d u a l s c o n t r i b u t e d t o t h e q u e s t i o n n a i re . A copy

of t he 21- page q u e s t i o n n a i r e i s conta ined i n Appendix 11.

The r e s pons e o f t he op e r a t i ng i n dus t r yt o

t h e v o l un t a ryque s t i onna i r e w a s unprecedented fo r a nat ionwide survey. S i x

hundred ques t ionna i res w e r e s e n t t o t h e op era t in g indu s t ry . From

t h i s 6 2 . 1 p e r c e n t , or 373 op er at in g companies, respond ed. Ten

and seven- t e n th s pe r c e n t o f t he c o r r o s i o n r e s e a r c h o r ga n i z a t i ons

and 9 . 6 p e r c e n t of t he c o r r o s i on c ons u l t a n t s r e s ponde d t o t h e

q u e s t i o n n a i r e . A t o t a l of 4 0 4 que s t i onna i r e r e s pons e s were

r ece ived . E igh ty- eight perc en t of th e pet roleum companies, 59

p e r c e n t of t h e g a s o p e r a t in g companies, and 53 p e r c e n t o f t h e

water companies responded. The ma jo ri t y of t h e companies t h a t

d i d not respond w e r e s m a l l companies w i t ? - a l i m i t e d t e c h n i c a l

s t a f f who f e l t t h a t t h ey c ou ld n ot j u s t i f y t h e t i m e and expense

t o r es pond o r t h a t t he y d i d rlot have th e necessa ry co r ros ion

knowledge.

The 373 op er at in g respons es w e r e d i v i de d as shown Ln Table 2 .

The qu es t i on na ir e had two typ es of enswers, numerical and

ess ay . The numerica l answers were coded and p l aced cn Hol l e r i t h

c a r d s f o r computer ized an al ys i s . The computer ta bu la te d t h e

r esponses and pr in t ed a copy of t he ques t ionna i re wi th answers .

A s e p a r a t e r e p o r t Wac made for:

e The 373 Operating Companies

e C ons u l t a n t s

e Research Organizat ions

e O t h e r I n t e r e s t e d GroEps

C



TABLE 2

OPERATING RESPONSES

Operating Companies

G a s Gathering

G a s Storage A r e a Gathering

G a s Transmission

G a s D i s t r i b u t i o n

O i l Gathering

O i l Transmission

Petroleum Product Transmission

Pe t ro leum Produc t Dis t r ibu t ion

Water

T o t a l

Number of Companies

32

18

6 3

159 .

1 3

28

31

1

28

373

0 G a s Gathering

0 Gas Storage A r e a Gathering

0

G a s TransmissionG a s D i s t r i b u t i o n

0 O i l Gathering

0 O i l Transmission

0 Petroleum Product Traxsmiss ion

0 Petroleum Product D i s t r i b u t i Q n

0 Water

These repor t s are on f i l e a t OPS-DOT. Excep t f o r t he expec ted

d i f f e r e n c e s related. t o “,he materials t r a n s p o r t e d , t h e r e w e r e no

s i g n i f i c a n t v a r i a t io n s i n th e numerica l responses of t h e s e p a r a t e

r e p o r t s l i s t e d above. T h er e fo r e, t h e n um er ic al r e s u l t s of t h e

6



q u e s t i on n a i r e c o nt a i ne d i n t h i s r e p o r t are t h e combined r e s u l t s

o f t h e 373 o p e r a t i n g c om pa nie s. The o c c a s i o n a l d i f f e r e n c e s i n

r es po ns e o f g a s , o i l , water, g a t h e r i n g , t r a n s m i s s i o n , o r d i s t r i -

b u t i o n companies are n o t e d .

All o f t h e r e s po n s es t o e ac h p a r t i c u l a r e s s a y q u e s t io n w e r e

p l a c e d t o g e t h e r . T h i s p e r m i t t e d a r a p i d o v e r v i e w of t h e r e s po n s e s

t o e a c h q u e s t i o n . Where a p p l i c a b l e , t h e r e s u l t s o f t h e e s s a y

q u e s t i o n s were p l a c e d i n t a b l e s f o r e a s y a n a l y s i s . Many o f t h e s e

t a b l e s a pp e ar i n t h i s r e p o r t . O t he r i n t e r e s t i n g comments w e r e

a l s o i n c l u d e d a nd are r e f e r e n c e d b y q u e s t i o n n a i r e n u m b e r , e . g . ,

Q l O O ) . The c om bin ed e s s a y q u e s t i o n s a nd t h e q u e s t i o n n a i r e

r e s p o n s e s are on f i l e a t OP S-DOT.

The 4 0 4 r e s p o n s e s r e p r e s e n t t h e e x p e r i e n c e a nd k now ledg e o f

c o r r o s i o n c o n t r o l a s s o c i a t e d w i t h 1 , 36 9, 94 7 m i l e s o f p i p e .

4 . P e r s o n a l C o n t a c t s

More than 96 c o r r o s io n s c i e n t i s t s and e n g i ne e r s f rom al l

o v e r t h e U n it ed S t a t e s w e r e i n te r v ie w e d f o r b o t h f a c t and o p i n io n .

L o c a l an d n a t i o n a l m e e t in g s of WACE, E l e c t r o c h e m i c a l S o c i e t y , a n d

t h e M e t a l l u r g i c a l S o c i e t y 05 AIME w e r e a t t e n d e d .

5. O u t l i n e o f R e p o r t

The r e s u l t s o f t h e l i t e r a t u r e s ’ i rvey , t h e q u e s t i o n n a i r e , an d

- - . h e p e r s o n a l i n t e r v i e w s w e r e c om bi ne d i n s o f a r as p o s s i b l e . The

s t a t ? of t h e a r t of c o r r o s io n of f e r r o u s p i p i p . g i s summarized anti

c o n c l u s i o n s are g i v en i n S e c t i o n I T. H en ce, a n o v e r vi e w o f t h i s

s t u d y c a n be ob ta ined . by r e a d i n g S e c t i o n I a n d S e c t i o n 11.

C u r r e n t kn ow le dg e o f t h e b a s i c f o rm s o f c o r r o s i o n i s o u t l i n e d

i n S e c t i o n I I I . T hi s s t a r t s w i th t h e b a s i c Isubrne-ged c o r r o s i o n









A major t h r e a t m a y be t h e u s e of h i g h v o l t a g e , d i r e c t c u r r e n t

power t r a n s m i s s i o n . It i s l i k e l y t h a t v e r y l a r g e c u r r en t s w i l l

a t leas t o c c a s io n a l l y be r e l e a s e d i n t o t h e s o i l . Research i s

under way t o de t e r m i ne t h e m a gn i tude of t h e h a za rd t o p i p e l i n e s

and measures which can be taken t o r e d uc e c o r r o s i o n f ro m t h i s

s ou r c e . The e f f e c t s c a n be a pp r e c i a b l e a nd may, i n some cases,

r e q u i r e u s e o f a u t om a t ic c a t h o d i c p r o t e c t i o n s y s te m s.

When h igh s t r eng th steels are s u b j e c t e d s i m u lt a ne o u sl y t o

m e c ha n i c a l stress a n d t o a c o r r o s i v e e n v ir o n m en t , c r a c k s c an

d e v el o p v e r y r a p i d l y . The s u r v e y d i d n o t show t h a t t h i s i s

am a jo r c a u s e o f c o r r o s i o n , b u t

it i ss i g n i f i c a n t i n t h a t f a i l u r e

i s a p t t o b e c a t a s t r o p h i c . The v a r i o u s stress c o r r o s i o n c r a c k in g

mechanisms and p h e m m e n a a r e n o t w e l l under s t cod and a re under

a c t i v e i n v e s t ig a t i o n . A t t h i s t i m e t h e o n l y r e as o n a b l y s u r e

m etho d f o r p r e v e n t i n g s t ress c o r r o s i o n c r a c k in g i s t o a v o i d

u s e 05 h i g h s t r e n g t h s tee ls . Hard welds should a .so b e a vo ided

for t h i s r e a s o n .

Twenty-two leaks w e r e a t t r i b u k e d t o h ydrogen b l i s t e r i n g by

t h e s u r v e y e d conpan ies . Hydroqen i s g e n e r a t e d bct h by c o r r o s i o n

i n a n a c i d envi ronment .and by c a t h o d i c p r o t e c t i Q 9 , e s p e c i a l l y a t

h i g h F o e e n t i a l s . M e t al s w i t h v o i d s a nd i n c l u s i o n s are p a r t i c u -

l a r l y s u s c e p t i b le . I n - z e r y r a n u l a r c o r r o s i o n cf mar.y s t a i n l e s s

s t e e l s c a n be r a p i d when t h e t h e r m a l c y c l e c f w e l d i ng s e n e r a t e s

c e r t a i n t y p e s of i n h om o g e ne i ti e s.

T em p er at ur e i n f l u e n c e s c o r r o s i o n i r s e v e r a l ways; t h e n e t

e c f e c t i s g e n e r a l l y f o r corrosion -to a c c e L e r a t 3 a.s t e m p e r a t r r e

i n c r e a s e s . A l s o , s t ress c o r r o s i o n c r a c k i n y h a s bee? fourld to be



more common i n p ip e l e ad in g f rom compressor s t a t i o n s , p resumably

beca use o f t h e e l e va t e d ga s t e m pe r a t u r e s ge ne r a t e d by c om pr e s si on .

C or r o s i on of s teel b u r i e d i n p e r m a f r o s t soils, on t h e other hand,

has been found t o be n e g l i g i b l e .

U n i f o r m c o r r o s i on t e nds t o d e c r e a s e w i t h t i m e b ec ause o f , t h e

i

p r e s e n c e of c o r r o s i o n p r o d u c t s b l o c k in g access of c o r r o d e n t s .

P i t t i n g and stress c r a c k i ng , how ever , i n c r e a s e w i t h t i m e . The

c o r r o s i o n f a i l u r e r a t e o f u n p ro t e c te d p i p e l i n e s t e n d s t o i n-

crease e x p o n e n t i a l l y w i t h t i m e .

I n g e n e ra l , t h e most c o r r o s i v e soi ls c o n t a i n l a r g e c on ce n-

t r a t i o n s of s o l u b l e sa l t s a nd , c o n s e q u en t l y , h av e r e l a t i v e l y l o w

r e s i s t i v i t i e s . On t h e o t h e r h an d, t h e l eas t c o r r o s i v e s o i l s

u s ua l ly have hi gh r e s i s t i v i t i e s , i n d i c a t in g l o w c o n c e n t r a t i o n s

of soluble sa l t s . Soiis w i t h r e s i s t i v i t i e s g r e a t e r t h an ab ou t

5 ,000 ohm-cn c an g e n e r a l l y b e e x p e c t e d t o be m i l d l y t o non-

c o r r o s i v e . I t s hou l d be r e a l i z e d , how ever , t h a t be c a us e many

o th e r f a c t o r s may af fec t t h e c o r r o s i o n r a t e of m e ta l , c e r t a i n

h ig h r e s i s t i v i t y s o i l s may be m o r e c o r r o s i v e t h a n s o i l s of

lower r e s i s t i v i t y .The p i t t i n g r a t s cf n e t a l b u r i e d i n soil > a s beer, found t o

increase e x p o n e n t i a l l y as a f u n c t i o n o f s o i l ; . e r a t i o n . The

Lower t h e amount of a e r a t i o n , t h e f a s t e r ,he p i t t i n g r a t e i n-

c r e a s e d w i t h t i m e .

C h l o r i d e i o n i s known t o a c c e l e r a t e c o z r c s i o n . Eydrogen

s u l f i d e a c c e l e r a t e s c o r ro s i o n, and cag c au s e s u l f i d e stress

c r a c k i n g . C arb on d i o x i d e p r e s s u r e s over 30 psi i n w e t o i l and

gas c a u s e c o r r o s i o n . T h e s p l a s h a n d t i d a l zones i n E e a w a t e r ere

1 2



t h e m o s t c o r r o s i v e m a r i ne e n vi r on m e nt . F u r t h e r m o r e , c a t h o d i c

p r o t e c t i o n i s i n e f f e c t i v e t he r e .

I t i s known t h a t p r o d u c t s of m i c r o b i o l o g i c a l m e t a b o l i s m

c an a c c e l e r a t e c o r r o si o n . S u l f a t e r e du c in g b a c t e r i a c a u se I J2S

f o r m a t i o n u nd er a n a e r o b i c c o n d i t i o n s a nd a c c e l e r a t e c o r r o s i o n .

The e x t e n t t o which micro-o r g a n i s m s c o n t r i b u t e t o p i p e l i n e

c o r r o s i o n i s u n c e r t a i n . V e r y l i t t l e good f i e l d d a t a a r e

a v a i l a b l e . There t e n d s t o be a s i g n i f i c a n t p o l a r i z a t i o n o f

o p i ni o n on t h i s s u b j e c t .

I n t h e a t m o s p h e r e , c o r r o s i o n i s a c c e l e r a t e d by m o i s t u r e,

wind - blown sea s a l t , s u l f u r d i o x i d e , a nd t o some e x t e n t , o z o n e .

The combined ef fec t o f d i l u t e s u l f u r i c a c i d and ozone i s worse

t h a n t h e sum of t h e i r i n d i v i d u a l ef fec t s .

2 . C o r r o s i o n Cont rol

I t i s r a r e l y f e a s i b l e t o c om pl et el y e l i m i n a t e a l l c o r r o s i o n

i n p r a c t i c a l s i t u a t i o n s .

t o a n a cc e pt a bl e l e v e l f o r t h e lowest c o s t .

The g o a l i s r a t h e r t o b r i n g c o r r o s i o n

Near ly 3. 11 of t h e

companies su rveyed had a co r ro s i on co n t ro l prog ram.

Al though no f e r rous metal i s un touched by co r ros ion , t h e

e x t e n t of c o r r o s i o n i s s i g n i f i c a c k l y in f luenced by t h e e x a c t

compos i t ion o f the m e t a l and. by the rma l and mechan ica l t rez-L-

ments . Welds a r e n o t ed t o be p a r t i c u l a r l y s u s c e p t i b l e to

c o r r o s i o n . L n f Q r t u n a t e l y , m o s t of t h e a d d i t i v e s which markedly

improve corrcsFon r e s i s t a n c e a r e probabl-y too e x p e n s i v e f o r use

i n l a r c e F l p e l i n e s y s t e m s .

Al*:’qougb most c c r r o s i o n o c c u r s e x t e r n a L l y , i n t e r n a l c o r r o s i o n

i s z l s o a p r 0 5 l e ~ nwken m.oisture and c h l o r i d e i o n , h yd ro ge n s u l -

13



f i d e , or c a r b o n d i o x i d e are p r e s e n t . T h i s h a s been c o n t r o l l e d

by removing m o i s t u r e ( d r y i n g t h e g a s and o i l streams) , a dd i ng

i n h i b i t o r s , o r c o a t i n g . P ro pe r i n h i b i t o r s are s e l e c t e d e m p i r i-

c a l l y . I t i s i m po r ta n t t o n o t e t h a t many i n h i b i t o r s accelerate

c o r r o s i o n if t h e i r c o n c e n t ra t io n i s too l o w .

A widely used method of c o r r o s i o n c o n t r o l i s t h e a p p l i c a t i o n

o f c oa t i n gs . A lt hough m e t a l c o a t i n g s are e f f e c t i v e un der some

c o n d i t i o n s , t h i c k o r g a n ic c o a t i n g s are g e n e r a l l y f a vo r ed f o r

underground use w h e r e p r o t e c t i o n i s r e q u i r e d f o r l o ng p e r i o d s .

.

T he s e c oa t i ngs are i d e a l l y w a t e r i mp er vi ou s, e l e c t r i c a l l y i n-

s u l a t i n g , and t ou gh . G e n e r al l y , t i g h t a d h es i on t o t h e p i p e i s

a l s o r e q ui r e d, a l tho l iyh good r e s u l t s h a ve b e en r e p o r t e d w i t h

l o o s e f i t t i n g p l a s t i c sh e a th s . The r e qu i re m e et f o r w a t e r

i m p e r m e a b i l i t y i s p robab ly even more s e v e r e when t h e c o a t i n g

i s n o t b onded t o t h e p i p e . 1.: s h o ul d b e n ot e d t h a t no p e r f e c t

c o a t i n g m a t e r i a l ex i s t s .

The most common c i r cum stan ce f o r le ak s i n coated. p i pe was

p h y s i c a l damage cf t h e c oa t i ng . The secord. most common ci r -

cumstance w a s c o r r o s i o ? . a t i m pr ope r l y a pp l i e d cast ings. M i c r s -

organisms attac 'c mosC, , i f n o t a l l , o r g a n i c c o a t i n g s . T h e r e f o r e ,

one cannot assume f , F ~ , tonce 7 p i p e i s c o a t e d it yemains effec- .

t i v e l y c o a t e d f o r e v e r . I n s p c t i o n a n d r e p a i r i s n e c e s s a r y LO:

o n ly when l a y i n g p i p , b u t a l s o p e r i o d i c a l l y t h e r e a f t e r .

ma in t enance r e q u i r e y e n t s are p r oba b ly p a r t i c u l a r l y s e v e r e when

c a t ho d i c p r o t e c t i o n i s not employed s imul t aneous ly .

The

A lth ou gh s el do m u se d f o r p i p e s c a r r y i n g g a s a nd o i l , c o n c r e t e

coa t i -ngs c a n p r ov i de e f f e c t i i - e c o r r o s i o n c o n t r o l . Cmc r e t e ' s

14



e f f e c t i v e n e s s i s l a r g e l y d ue t o i t s h i g h pH. Aga in , p rope r

a p p l i c a t i o n i s r e q u i r e c ? , i n c l u ? . i n g c h oo s i ng t h e p r o p e r c o n c r e t e

mix.

I n c a t h o di c p r o t e c t i o n , c o r r o s i o n i s redu ce2 by making t h e

B i p e n e ga t iv e w i t h r e s p e c t t o t h e a d j a c e n t s o i l . I t i s n o t a

panacea f o r a l l c o r r o s i o n p ro b le m s. N e i t h e r i s t h e p r op e r

a p p l i c a t i o n of c a t h o di c p r o t e c t i o n a t r i v i a l m a t t e r . I n c r e a s e d

c o r r o s i o n due t o c a t h o d i c p r o t e c t i o n h a s a c t u a l l y b een o b s er ve d

when s t e e l w a s c o n ti n uo u sl y o r i n t e r m i t t e n t l y h e a t e d and i n t e r -

m i t t e n t l y w e t t e d .

j u s t downstream from com pressor s t a t i o n s i n a r e a s where t h e

s o i l i s i n t e r m i t t e n t l y d r y a nd w e t . C a t h o d i c p r o t e c t i o n c a n

i n c r e a s e t h e l e a k r a t e a t f i r s t , when a p p l i e d t o o l d p i p e , by

l o os e ni n g t h e a d h e r en t r u s t s c a l e which p r ev ious ly cove red

s 3 a l l h o l e s .

e x t e r n z l l y a p p l i e d DC power o r by a t t a c h m e n t of s a c r i f i c i a l

a n o d e s , s u c h a s Yg Zn, o r A l , which a r e s lowly consumed.

E i t ? e r rnethod i s e f f e c t i v e when p r o p e r l y i n s t a l l e d and main-

t a i n e d .

Such con d i t i on s would be expe c ted on p i pe s

C a t h o d i c p r o t e c t i o n may b e a c c o m pl i s he d e i t h e r by

I t may b e p o s s i b l e t o m a i nt a in c o a t i n g s t o t h e e x t e n t t h a t

a d e q u a t e p r o t e c t i o n i s a c hi e ve d w i t h o u t o t h e r measures .

t h e Z i f f i c u l t y i n a ch ie vi ng t h i s l e v e l of main tenance has caused

many conpanies t o s up p le m en t c o a t i n g s w i t h c a t h o d i c p r o t e c t i o n .

O n t h e o t h e r h a n d , c a t h o z i c p r o t e c t i o n used a l o n e can g e n e r a l l y

p r o v i d e a d e q u a t e p r o t e c t i o n , e x c e p t t h a t l a r g e c u r r e n t s are

r e q u i r e d and i n t e r f e r e n c e w i th o t h e r s t r u c t u r e s t h a t may b e

nearby i s l i k e l y . Khen u se d w i t h c o a t i n g s , o n l y enough c u r r e n t

Ilowever,

1 5





t h e companies surveyed used a p i p e p o t e n t i a l of 0 . 8 5 volt rela-

t i v e t o t h e co pp er s u l f a t e e l e c t r o d e a s t h e i r c r i t e r i o n f o r

c a t h o d i c p r o t e c t i o n .

a 300-mV d i f f e r e n c e b et we en t h e e n e r g iz e d and t h e o r i g i n a l

open c i r c u i t p o t e n t i a l s .

more t h a n o ne c a t h o d i c p r o t e c t i o n c r i t e r i o n .

The second m o s t common c r i t e r i o n used w a s

Most of t h e companies surveyed used

P i p e l i n e s p a s s i n g u n de r r oa dw ay s are f r e q u e n t l y e n ca se d i n

o u t s i d e m e t a l c a s i n g s . T h e r e i s e v i de nc e t o t h e e f f e c t t h a t

s u c h c a s i n g s are u nn ec es sa ry f o r s t r u c t u r a l s a f e t y . F u rt h er -

more, when t he m e t a l c a s i n g becomes s h or t ed t o t h e p i p e , c a t h o d i c

p r o t e c t i o nis

i n e f f e c t i v e . The c om pa ni es s u rv e y e d h ad a t o t a lof

2 2 , 6 6 9 c a s i n g s s h o r t e d t o t h e carrier p i p e , Over 300 leaks w e r e

r e p o r t e d i n s i d e c a s i n gs f o r 1 9 6 9 .

3. C o n s t r u c t i o n , I n s p e c t i o n , a nd M ai nt en an ce

To e n s u r e good c o r r o s i o n c o n t r o l , o n e m us t n o t o n l y d e s i g n

t h e c o n t r o l s y st em s p r o p e r l y , b u t a l s o draw up detai led s p e c i-

f i c a t i o n s , i n s p e c t t h e w or k, a nd p e r i o d i c a l l y m a i n t a i n t h e

sys tem.

cla mp s t o r e p a i r a t leas t some of t h e i r l e a k s p w h i l e 5 4 p e r c e n t

r e p l a c e d p o r t i o n s c t h e p i p e .

p i p e w a s t h e l e a k h i s t o r y . The n o s t common annua l su rve i l l ance

Irethod w a s a p i pe- t o- s o i l p o t c 5 n t i a l s u r v e y , w h i l e t h e second

rest cc.mmon was l i n e c u r r m t measurement . The 373 o p e r a t i n g

companies r epor t ed 2 5 2 , 5 5 2 c o r r o s i o n .',eaks 6ur inc j 1 9 6 9 f o r

1 , 3 6 9 , 9 4 7 m i l e s of p i p e .

S i x t y- o n e p e r c e n t o f t h e surveyed companies used

The R ai n c r i t e r i o r l f o r r e p l a c i n g



SECTION I11

FORMS OF CORROSION

1st 2nd 3rd 4th 5 t h 6 t h

Two hundred and f i f t y - t h r e e o f t h e s u r v e y ed o p e r a t i n g com-

p a n i e s i n d i c a t e d i n t h e q u e s t i o n n a i r e t h a t t h e y d e t e r m i n e t h e

p r o b a b l e t y p e of c o r r o s i o n w i t h r eg a r d t o c o r r o s i o n l e a k s .

3 summarizes o p i n i o n s as t o t h e m o s t p r e v a l e n t c a u s e s i n e a c h

company.

Table

*

i7 t h 8 t h

MOST PREVALENT

0 :

( A ) Galvan ic C e l l

( B ) S t r a y C u r r e n t

i n t e r f e r e n c e )( i n c l u d i n g c a t h o d i c

('C) Stress C or r os i onCrack ing

(D) C or r os i on Fatig1J.e

(E) Hydrogen Embri t t l e-ment

(F: C a u s t i c E m b r i t t l e-w n t

G ) M i c r o b i o l o g i c a l

C or r o s i on

HI Other*Unlessh o t h e r w i s e s p e c i f i e

2 8 4

19

3

4

2

0

7

117

I

1 8

lS0

17

1

-.. 3

0

3

3 2

num-

unbe

?

4 b

21

17

3

3

33

1 2z r s-

of Conpani esI I

0

4

I

l5 I

4

3

1 _-

l oc

c.

onca

0

2

1

@

2

4

0

00n e d i n t

o i

0

0

0

4

1

2

0

0

0

1

71

2

0

0

t a b i e s

are t h e number of o pe ra t i n g companies 3 u t o f t h e t o t a l 373 com- p a n i e s t h a t responc?.e$ t o t b a t pax-titular cyues t ion i n t h e q u e s t i o r l -n a i r e .

18

..



G a l v a n i c c e l l c o r r o s i o n w a s t h e m o s t p r e v a l e n t c a u s e accord -

i n g t o 284 compan ies. S t r ay cu r re n t w a s t h e s e c o n d m o s t p r e v a l-

e n t c au se f o r 150 companies . For 1 9 co mp an ie s s t r a y c u r r e n t w a s

t h e major problem. M i c r o b i o l o g i c a l metabolism w a s a p p a r e n t l y

a s i g n i f i c a n t c au se f o r co r ro s i o n f o r 30 p e r c e n t o f t h e companies

p o l l e d . Many companies a p p a r e n t l y f e l t t h a t stress c o r r o s i o n

c r a c k i n g and c o r r o s i o n f a t i g u e w e r e a p r e v a l e n t c a u s e , b u t o n l y a

f ew l e a k s d u e t o stress c o r r o s i o n c r a c k i n g and c o r r o s i o n f a t i g u e

h a v e b e e n r e p o r t e d .

Table 4 g i v e s t h e s u p p l e m e n t a l m ea su re me nt s t h a t c o m pa ni es

heve used t o i d e n t i f y c a u s e s of c o r r o s i o n l e a k s .

TABLE 4

MEASUREMENTS THAT qAVE BEEN USED FOR I D E N T I F I C A T I O NOF CAUSES O F CORROSION LEAKS

--

Number o fComp a n ies

(A) Coa t ing Th ickness 137

( E ) C h e m i c a l A n a l ys i s o f S o i l 47

( C ) P i pe P o t e n t i a l 275

(D ) Maximum P i t Depths at A d j z c e n t Corroded A r e a sW i t b i n t h e Excava,",ion 187

( E ) M e t a l l - u r g i c a l A n a . l y r i s 46

(F) R e d o x Po t e n t i a l 9

(G ) So i l pF; 88

( H ) S o i l R e s i s t i v i t y 2 3 0

:I) Q u a l i t a t i v e F i e l d T e s . 1 f o r S u l f i d e I o n 5 c

;J> P o t e n t i a l or C u r r e n t w i t h T e s p e e t t o F o r e i g nS t r u c t u r e 219

19



There are two c o m p e ll i n g r e a s o n s f o r s t u d y i n g t h e u n de r -

g ro un d c o r r o s i o n p ro c e s s e s . These are: (1) To d e f i n e t h e con-

d i t i o n s and c o n s t i t u e n t s i n t h e soil w h i c h c o n t r i b u t e t o t h e so i l ’ s

c o r r o s i v e n e s s , a n d ( 2 ) T o d e f i n e t h e r e a c t i o n s a t and nea r t h e

m e t a l s u r f a c e so as t o p r o v i de a t h e o r e t i c a l b a s i s f o r cathodic p r o t e c t i o n c r i t e r i a t o s u p p l a n t t h e e m p i r i c a l m etho ds i n p r e s e n t

use .

D e f i n i t i o n of t h e s o i l c o n d i t i o n s w hich c o n t r i b u t e t o c o r r o-

s iveness would make t h e s o i l c o r r o s i v i t y su rv ey more p o s i t i v e i n

r e s u l t s a nd of g re a t e c o n o m i c v a l u e t o in du s t ry . However, on l y

a f e w of t h e o p e r a t i n g c om pa ni es i n t h e U nited S t a t e s s t r i v e for

t h i s o b j ec t i ve as shown by T a b le 4 .

Many operators re ly upon soil r e s i s t i v i t y as a measure of t h e

s o i l ’ s c o rr o si v en e ss . T h i s i n d i c a t o r i s i n a d e q u a t e t o c o m p l e t el y

d e f i n e c o r r o s i v i t y . A n e u t r a l c a r b on a t e s o i l would be r e l a t i v e l y

lnnocuous compared t o one cof i . ta in ing s o l u b l e c h l o r i d e s and s u l -

f a t e s a t t h e same r e s i s t i v i t y . Many s i m i l a r comparisons can be

made where r e s i s t i v i t y a l on e is s e e n i;o be i n s u f f i c i e n t informa-

t i o n f o r p r e d i c t i n g c or r o s i v i t y .

The p i p e p o t e n t i a l ( P a r t C of T a b l e 4 ) , i n c lu d i n g t h a t w i t h

r e s p e c t t o n e i g h b o r i n g p i p e l i n e s ( P a r t J of Table 4 ) , i s used ex-

t e n s i v e ly t o f i n d t h e c a u s e o f c o r r o s i o n leaks. The n a g n i t u d e of

a s t e a d y p o t e n t i a l c a n o f t e n detect lack of c a t h o d i c p r o t e c t i o n ,*

s v e r p r o t e c t i o n , g a l v a n i z ac t iv i ”: y and c a t h o d i c i n t e r f e r e n c e , .

F l u c t u a t i n g c u r r e n t s are h e l p f u l i n d i sc o v er i n g t h e s o u r c e of

i n t e r f e r e n c e c u r r e n t .



1. B a s i c Corrosion Mechanisms

C or r o s i on i s a n e l e c t r o c h e m i c a l p r o c e s s w i t h a n a no d i c re-

a c t i o n as

Fe Fe2+ f 2 e-

and s u c h c a t h o d i c r e a c t i o n s as2H’ + 2e- H2, o r Eq. 2)

(Eq. 3 )b

O2 + 2 H 2 0 + 4e- + 40H-

Once formed, the Fe2+ i s n o t n or ma ll y s t a b l e , b u t reacts wi th oxy-

gen and water t o fo rm magne t i t e , Fe304 , o r h e m a t i t e , Fe203*3H2.

T h i s l a t t e r compoundl b o t h de h yd r a t e d and v a r i o u s l y h y d r a t e d , i s

c a l l e d r u s t . The e q u i l i b r i u m e x i s t i n g be tw een i r o n and i t s c o r r o-

s i o n pro du cts may be found by thermodynamic c a lc u l a t i o n s . The

r e s u l t s are p r e s e n t e d i n t h e form of phase d i a g r a m s , o f t e n r e f e r -

r e d t o i n c o r r o s i o n t h e o r y as P o l x b a i x p l o t s .

I t i s g e n e r a l l y t h o u g h t t h a t l e r t a i n r e g i o n s o n a g i v e n p i e c e

of s tee l are a no de s w h il e o t h e r p a r t s are c a t hode s . T he se r e g i ons

pay be m i c r o s c o p i c a l l y n e a r one a n o t h e r o r very f a r a p a r t . Micro-

e l e c t r od e e xpe r i m e n t s ha ve c on fi r me d t h a t some r e g i ons are p r imar -

i l y a nod i c and. o t h e r s a r e c a t h o d i c ( 7 7 9 ) .

I n r e c e n t y e a r s , mclch a t t e n t i o n h a s be en d e v ot e d t o uncover-

i n . 5 t h e react ior . n e c h a n i s n s a t t h e e l e c t r o d e s , s i n c e t h e a bove re-

act ic r l s a r e m e re ly what i s e x t e r n a l l y o b s e r v ed and t e l l n o t h i n g

z b o u t t h e v a r io u s s t e p s i w o l v e c ( 7 5 8 , 760 761 , 766-768) . Even

WithoLt t h i s d e t a i l e d cnowledgc, howeiwr, o n e c a n p r o f i t a b l y make

u s e f t h e c o n c e p t s o f e l e c t r c i e k i n e t i c s . I n corrosio: , t h i s

takc.c; t he fo rm of m i x e d p o t e n t i a l t h 2 o r y ( 7 6 3 ) . C o n si d er t h e re-

ac’: -on of Equa t ion 1 t a k i n c p l a c e 3 . t a n e l e c t r o d e . E l e c t r o a s a r e

21



b e i n g l i be r a t ed a t the m e ta l s u r f a c e w hi le p o s i t i v e i o n i c c u r r e n t

flows i n t o t h e e l e c t r o l y t e . As t h e e l e c t r o d e i s made m o r e p o s i t i v e ,

t h e rc :ac t ion r a t e increases as shown i n F i g u r e 1. Soon, however,

i r o n p a s s i v a t e s and the r e a c t i o n r a te d e c r e a s e s .

t e n t i a l s a g a in ca us e t h e c u r r e n t t o i n c re a se . P a s s i v a ti o n r e s u l t s

from t h e fo rm a t i o n of a s o l i d f i l m a t t h e metal s u r f a c e w h i c h

i n t e r f e r e s w i t h i o n and e l e c t r o n f l o w .

Very high po-

On t h e o t h e r h a n d , t h e i r o n c an a l s o serve as an electrode

f o r a c a t h o d i c r e a c t i o n , s uc h as Equa t ion 3 . For t h i s r e a c t i o n ,

t h e ra te i n c r e a s es a s t h e s u r f a c e i s made more n e g a t i v e . I f , a s

i s u s u a l , t h e metal s u r f a c e h a s b o t h a no d i c an d c a t h o d i c r e g i o n s ,

t h e n bo th r e a c t i o n s p r oc e ed s i m u l t a n e o u s l y . I f n o e x t e r n a l s o u r c e

o r s i n k o f e l e c t r o n s

must be e qu al a t t h e

The l i n e a r c o r r o s i o n

i Vcor r m

nFA=

i s a v a i l a b l e , t h en t h e r a t e of b o th r e a c t i o n s

p o i n t where t h e c u r v e s i n F i g u r e 1 c r o s s .

r i t e , R , is r e l a t e d t o t h i s i n t e r s e c t i o n by

where icorri s t h e c o r r o s i o n c u r r e n t , (gLvep by t h e i n t e r s e c t i o n

o f c u rv e s i n F i g u r e 1), Vm is t he molar volume of t h e m e t a l , n i s

t h e number of e l e c t r o n s zxchanged i n t h o a n o d i c c o r r o s i o n r e a c t i o n

( t w o i n Equa t ion 11, F is F a r a d a y ' s c o n s t a n t a nd A i s t h e s u r f a c e

area of t h e m e t a l . I n t h e case o f s e v e r a l s im u l t a ne o u s a n o d ic

r e a c - c i o n s , t h e I-E c u r v e s f o r e a c h are added and t h e sys tem a g a i n

3 p e r a t e s a t t h e i n t e r s e c t : - o n of t h e c u r v e s .

The o r i g i n s of t h e a n o d i c a d c a t i i o d i c c u r v e s ?.re t h e p o i n t s

at which f q r w a r d and reverse r e a c t i o n s are t a k in g p l a ce a t e q u al

v e l o c i t i e s -.e., e q u i l i b r i u m for t h a t p a r t i c u l a r r e a c t i o n . The

22



II

i corr

LOG CURRENT

F i g u r e 1. Mixed F o t e n t i - a 1 Diagram for C o r ro s i o n of I ron(Schemzti . , - )

p o t e n t i a l s a t t h o s e p o i n t s are g i ve n by t h e h a l f - c e l l p o t e n t i a l ,

which, u s i n g Equa t ion 3 as an example, may be g iven by the famil-

i a r N e r n s t Equ at ion o f c k e r n i c z l thermodym.mics ,



where Eo i s t h e s t a n d a r d half - cel l p o t e n t i a l f o r u n i t a c t i v i t i e s

o f r e a c t a n t s an d p r o d u c t s , R i s t h e g a s c o n s t a n t , T i s a b s o l u t e

t emyje ra tu re , n i s t h e number o f e l e c t r o n s i n t h e r e a c t i o n ( f o u r

f o r t h i s e xa m pl e ) , F i s F a r a d a y ' s c o n s t a n t , and t h e a ' s a r e a c t i v -

i t i e s , which e q u a l c o n c e n t r a t i o n s f o r i d e a l m i x t ur e s . Note t h e

c o n c e n t r a t i o n of r e a c t a n t s i n f lu e n c es t h e p o t e n t i a l a t t h e o r i g i n s .

The m i x e d p o t e n t i a l t h e o r y i s n o t o n l y u s e f u l c o n c e p t u a l l y ,

b u t a l s o p r o v i d e s t h e bas is f o r i n s t a n t a n e o u s l y m ea s ur i ng c o r r o - *

s i o n r a t e s a nd s t udy i ng electrode r e a c t i o n mechanisms. An e x te r -

n a l p o t e n t i a l i s a p p l i e d a nd t h e c u r r e n t m ea su re d. The c u r r e n t

obse rved w i l l a c t u a l l y be t h e d i f f e r e n c e b e t w e e n t h e t w o c u r v e s

o f F i g u r e 1 a t some s p e c i f i e d p o t e n t i a l . Th us, p o t e n t i a l and

cur ren t measurement s of t h e sys tem shown i n F i g u r e 1 would y i e l d

t h e c a t ho d i c p o l a r i z a t i o n cur-Je of F i g u r e 2 and t h e a n o d i c p o l a r -

i z a t i o n c ur ve of F i g u r e 3 .

K i n e t i c s t u d i e s ha.ve shown tha t seve ra . 1 s t e p s are i n v o l v e d i n

n e t c a t h o d ic an2 n e t a n od i c r e a c t i o n s , T h e o r e t i c a l l y , when o ne

step c o n t r o l s t h e ra te o f a n e l e c t r o d e p r o c e s s , t he k i n e t i c s

follow t h e T a f e l equat j -on ( 7 6 6 , 7 6 5 ) ,

T = a -+ @ I n i Eq* 6)

where q i s t h e over:potectia?- Ideviatio,? of p o t e n t i a l f r o m t h e

reversible v a l u e ) , i s <;he v a l u e of q a t c u r r e n t i = 1, and f i s

t h e " beta c o n s t a n t , "Tzfe.?..constan", I o r "Ta f e l s l o p e . T h i s

e q u a t i m i m p l i e s a s ' x a i g h + l i n p v i s F n i l oga r i t hmi c p l o t s as i r

p o r t i o n s o f t h e c u r v e s of F i g u r e 1.

effec ts of v a r y i n g s o l u t e c o n c e n t r a t i o n s p e r m i t d e d u c ti o n s a b o u t

Measurements of 6 and t h e

mechanisms of r e a c t i o n s a t electrodes.

24



.IaI-Z EW co r rI-O ’a

-

I

icorr

LOG CURRENT

Figure 2 . Cathodic Polar iza t ion Curve

Corros ion engineer ing and sc ience now u t i l i z e a wide va r i e ty

of e x p e r i n e n t a l t e c hn i ques i n a dd i t i on t o p o l a r i z a t i o n s t u d i e s .

These range from s imple p i t depth , l oss i n we i gh t , and t h i c kne s s

measurements t o very so ph is t ic a t ed methods such as e l l i p s om e t r y ,

e l e c t r o n d i f f r ac t i c . n , r a d i o t r a c e r s , and t h e u se of t h e e l e c t r o n

microscope ( 6 8 8 ) .

2 . I n t e r n a l a nd E x t e r na l P i p e l i n e C orr o si on

The b z - i c mechanisms f o r c o r r o s i on are t h e same f o r i n t e r n a l

and e x t e s , 11 cor;ITasion, a l though t i l e _ environments are q u i t e

d i f f e r e n t . I n n o s t i n s t a n c e s , t h e e x t e r i o r en vi ro nm en t i s much

more c o r r o s i v e Clan t h e i n t e r i o r , w h i c h i s fo r tuna te because

c o r r o s i o n m i t i g a t i o n i s much s im pler on t he o ut s i de .

Roughly 75 p e r c e n t of t h e g a s t r an s m i ss i o n l i n e s i n t h e U nite d

Sta tes w e r e surveyed dur ing the pe r iod f rom 1 J u l y 1 9 6 7 t o 30 June

2 5



conLOG CURRENT

Figure 3 . Anodic Polar ization Curve.

1 9 6 8 50 d et er m in e c a u se s o f p i p e l i n e f a i l u r e s ; t h e s u r ve y c ov er ed

3 0 t h p r e s e r v i c e t es t s and i n s e r v i c e o p e r at i o n a l f a i l u r e s ( 5 8 6 ) .

The primary cause of t e s t f a i l u r e s w a s t h e l o n g i t u d i n a l w el d,

:..-hereas e x t e r n a l dar.?.;e ( b u l l d o z e r , d i t c h e r , e tc .) w a s t h e p r i m e

cau.se i n o p e ra t io n al f a i l u r e s , w i t h cor ros ion and t h e l o n g i t u d i n a l

weld as t h e next mos t f requent causes . A number o f f a i lu r e s i n

o l d e r , u n c o a t e d l i n e s or poo r l y coa ted l i n e s w e r e found t o have

occn r r ed as a r e s u l t of e x t e r n a l c o rr o si o n . I n t h e s e f a i l u r e s ,

c

rr >





TABLE 5

MOST PREVALENT CIRCUMSTANCES UNDER WHICH CORROSION LEAKS OF COATEDPIPE HAVE BEEN FOUND DYJKING THE LAST YEARS

_ -

Order of Frequency1_1_

C a t h o d i c a l l y P r o t e c t e d1st 2nd 3rd 4 th 5 t h 6 t

Number of Companies

(A) Co r r o s i o n a t Improper ly Appl ied Coat ing 55 56 2 0 4 4

C ons t ruc t i on or Subseq uently Abraded by Oth ers 81 35 15 5 5

N Root Growth 4 22 23 1 5 1 3

(D ) Corr osio n Beneath Unbonded Coat ing 13 22 28 1 6 6

(E ) F a i l u r e of t h e Co a t i n g Material 1 2 1 3 1 3 11 1 2

(B) Corrosion Where Coating H a s C l e a r l y Been Damaged During

( C ) Corrosion Where Coating Is Ruptured by Soil Stress or

0

N o t C a t h o d i c a l l y P r o t e c t e d1st 2nd 3 r d 4 t h 5 t h 6 t

Number of C o mD a n i e s

(A) Co r r o s i o n a t Improp erly Applie d Coa ting 43 65 28 6 2(B) Co r r o s i o n Where Coating H a s C l e a r l y Been Damaged During

Co n s t r u c t i o n o r Subseque nt ly Abraded by Othe rs 1 0 8 4 2 1 7 2 0

(c j c o r r o s i o n Where Coat i r ig 1s Ruptured by S o i l S t r e s s o rRoot Growth 8 1 9 1 8 2 1 15

(Dj Corrosion Beneath Unbonded Coating 5 -10 36 23 11

(E) F a i l u r e of t h e Co a t i n g Material 11 18 15 1 2 11



TABLE 6

NUMBER OF CORROSION LEAKS OCCURRING I N 1969 AT JOI NTS AND WELDS

J O I N T S

Number o f Leaks1- 2 3-4 5-6 7-15 ove r 15

(A ) Compression Couplings 8 6 1 2 11 20

(B) Threaded Couplings & F l a n g e d J o i n t s 1 9 11 3 26 33

( C ) O t h e r 5 4 2 3 1 5

WELDS N u m b e r of Leaks

(A) Long i tud ina l Fac to ry Welds 8 4 4 2 1 2

( B ) S p i r a l F a c t o r y W e l d s 0 0 0 0 2

( C ) F i e l d Welds 6 9 3 0 2 7

few e x i s t i n q p i p i n g sy s t e m s , t h e l e a k s at. j o i n t s and w e l d s are

i n s i g n i f i c a n t when compared t o $he t o t a l number of l e a k s .

3. Uniform Cor rosi on

Uniform removal of m e t a l i s t h e l ea s t t r ou b l es o m e o f t h e

v a r i o u s forms of c o r r o s i o n . However, it i s t h e eas i e s t form of

c o r r o s i o n t o s t u d y , a n d as a r e s u l t m u c h of o u r fundamental know-

l e d g e of c o r r o s i o n comes f rom r e F e a r c h o n u n i f o r m c o r r o s i o n

p r o c e s s e s . Fo r example , t h e u n i f o r m c o r r o s i o n r a t e i s what one

d e te r m in e s d i r e c t l y from p c l a r i m t : - o n s t u d i e s ( 7 , 537) Errors

can arise , however, i f a n o t h e r e.-odic r e a c t i m i s o f c o n p a r a b l e

m ag ni tu de t o t h e m e t a l rernovE1 r e a c t l o r . . T h i s h a s a r i s e n , fo r

example, Zrom t h e o x i d a t i o n c.5 hydrogen unde r p r e s su re (hyd rogen

exchange! ? w i n g c o r r o s i o n i n a s e a l e d a c t o c l a v e ( 5 4 7 ) . C o r r o s i o n

c u r r e n t \ 73s 9.8 t i m e s t h e a c t u a l z o r r o s i o n r a t e . Cathod ic p o l a r -

2 3



i z a t i o n s t u d i e s o f i r o n i n on e n or ma l h a lo g e n a c i d showed t h e

hyd roge n c ove r a ge t o be n e a r one monolayer a t t h e c o r r o s i o n p o t e n-

t i a l ( 1 5 3 ) . T h i s s e e m s t o i n d i c a t e t h a t n e a r l y t h e e n t i r e i r o n

s u r f a c e f u n c t i o n s as a c a t h o d e d u r i n g c o r r o s i o n a n d t h a t o n l y a

s m a l l p a r t o f t h e s u r f a c e u nd er go es d i s s o l u t i o n a t a ny g i ve n t i m e .

R a d io t ra c er s t u d i e s o f i r o n i n water r e v e a le d t h a t no c o r r o s i o n

t a k e s p l a c e i f oxygen i s e x cl ud e d a nd t h a t n e g l i g i b l e e xc ha ng e

of i r o n atoms w i t h t h e s o l u t i o n t a k e s p l a c e ( 2 6 5 ) .

A f t e r a f i l m w a s fo rm ed on i r o n i n w a t e r c o n t a i n i n g o xy ge n,

t h e T a f e l p o r t i o n o f t h e p o l a r i z a t i o n c u rv e w a s found t o change

i n s l o p e 6 d e c r e a s e d ) as t h e f i l m t h i c k n e s s i n c r e a s e d ( 2 3 5 ) .

For t h i n f i l m s it w a s conc luded t h a t t h e c h a r g e p a s s e d w a s used

FYI t h e f o r m a t i on o f i r on - i o n v a c a n c y- e l e c t r o n p a i r s a n d f o r

t r a n s f e r of o xy ge n frorn t h e s o l u t i o n t o t h e o u t e r l a y e r of t h e

ox i de . The f i e l d d r i v e s i o n s th r ou g h t h e l a y e r ( 4 7 8 ) . I n t h e

t r a n s i e n t s t a t e t h e r e i s a n on ze ro c ha rg e d i s t r i b u t i o n i n t h e

l a y e r which s lo wly decays when s t ea dy s t a t e c u r r e n t i s restored.

( T h i s may be r e l a t e d t o t h e e f f e c t o f AC o n c o r r o s i o n . )

been s u q g e s t e d t h a t t h e f i l m c o n t i n u o u s l y d i s s o l v e s and so must be

r e g e n e r a t e d t o m a i nt a in p a s s i v i t y ( 6 9 2 ) . The e f f e c t s o f c om p le xi ng

a g e n t s , p o t e n t i a l s , i n h i b i t o r s , and d ou bl e l a y e r on c o rr o s i on w e r e

I t has

e x p l a in e d by t h e i r i n f l u e n c e on d i s s o l u t i o n o f t h e f i l m .

4 . P i t t i n g

S t e e l p i p e g e n e r a l l y d o es n o t f a i l by un i form c o r r o s i o n . I t

f a i l s p r ed o mi na te ly by l o c a l i z e d z t t a c k i n t h e form of p i t s .

P i t s a ppe a r t o be i n i t i a t e d wheP l oca l p e n e t r a t i o n o f t h e s u r f a c e

f i l m occu.rs on s tee l g e n e r a l l y p r D t e c t e d by t h e f i l m . That i s ,



it i s a form of a t tack i n t e r m e d i a t e b et we en g e n e r a l a t t a ck and

c o m pl e te p a s s i v a t i o n . When t h e f l u i d i n a p i p e i s u n d e r p r e s s u r e ,

a p i p e may p o s s i b l y f a i l by f r a c t u r e before t h e p i t c om p le t e ly

p e n e t r a t e s t h e w a l l ( 5 9 8 ) .

Most p i t t i n g a p p e a r s t o be a s s o c i a t e d w i t h c h l o r i d e i o n s .

B r o m i d e a nd h y p o c h l o r i te s c au s e p i t t i n g , w h il e f l u o r i d e s and

iod ides have l i t t l e e f f e c t . C a th o di c r e a c t i o n s o c c u r on t h e s u r -

face of t h e s tee l , w h i l e a n od ic d i s s o l u t i o n takes p l a c e i n t h e

p i t . Thus, an e x c e s s of p o s i t i v e c ha rge i s produced i n t h e p i t ,

which a t t r a c t s t h e n e g a t i v e c h l o r i d e i o n s ( 7 6 9 ) . I t has been

c la im ed t h a t p i t t i n g i s more s e v e r e on t h e t o p s i d e of p i p e t h a n

on the bottom because t h e s o l u t i o n i n t h e p i t s i s h e a v i e r thar

t h e su r rcmnding electrolyte 7 6 9 ) ; however, a n a l y s e s of e x t e n s i v e

f i e l d e xpe r i e nc e on bu r i e d p i p e l F n e s h av e shown t h a t p i t s o c c u r

more o f t e n o n t h e bottom of p i p e s . T h i s As a t t r i b u t a b l e t o

s e v e r a l fac tors -- a h i g h e r s a l t c o n c e n t r e t i o n a n d m o i s t u r e con-

t e n t a t t he bo t t om be c a us e of d r a i n a g e and- because t h e s o i l i s

packed more t i g h t l y there, and a lower oxygen co nc en t r a t io n be-

c a us e of t h e g r e a t e r d i s t a n c e f r o m t h e s c i l s u r f a c e . T h i s o f f e r s

o ne e x p l a n a t i o n :or p i t i n i t i a t i o n on p o l y c r y s t a l l i n e s t ee l s u r -

faces . Commonly, p i t s appear t o be p r e f e r e n t i a l i n l o c a t i o n and

d i r e c t i o n , p r o b a b l y f o l l o w i n g v e i c ? of h i g h im p u r it y c o n t e n t i n t r o -

d x e e d u ri n g c a s t i n g , r o l l i n g , e tc . S k c e p i t t i n g i s a s t a t i s t i -

cal phenomenon, %he maximum pi’, Cepth i r c r p a s e s as t h e s u r f a c e

ares of .?he sample increases , o f t e n b e l - 9 D z o p o r t i o n a l t o t h e 0 . 1 5

p w e r ~f t h e area ( 6 5 7 ) .



I t h a s been fo un d i n p o l a r i z a t i o n s t u d i e s t h a t when p o t e n t i a l s

on p a s s iv e s u r f a c e s are l owered , a p o i n t i s reached where t h e

c u r r e n t r a p i d l y i n c r e a s e s . T h i s has been i n t e r p r e t e d as due t o

i n i t i a t i o n and a u t o c a t a l y t i c growth of p i t s . The breakdown po-

t e n t i a l h a s been ca l l ed t h e "c r i t i c a l " o r " p i t t i n g " p o t e n t i a l ,

w i t h t h e s u g g e s t i o n t h a t p i t t i n g d oe s n o t o c c u r a t more a c t i v e

( n e g a t i v e ) p o t e n t i a l s . T hu s, m ea su re me nt s of p i t t i n g p o t e n t i a l

have been employed t o co mpare p i t t i n g t e n d e n c i e s of metals and

e l e c t r o l y t e s ( e . g . , 1 5 4 , 7 7 2 , 2 8 5 ) . However, t h i s p r o c e d u r e , and

i n f ac t t h e whol e c onc e p t , has r e c e n t l y been s e v e r e ly c r i t i c i z e d .

I n o n e p a p e r it w a s conc luded t h a t p i t t i n g p o t e n t i a l i s r e l a t e d

t o p i t t i n g , b u t q u e s t i o n e d t h a t it i s a b a s i c charac te r i s t i c of

t3e a l l o y ( 7 7 0 ) . I t w a s f o u n d t h a t t h e r a t e of po t en t iodynam ic

scanning and t i m e of h c l d i n g Z u r in g p o t e c t i o s t a t i c p o l a r i z a t i o n

have a marked e f f ec t on t h e measured va lue of c r i t i ca l p o t e n t i a l .

I t has been found t h a k t h e rates a nd t ype s of c o r r o s i o n of s t a i n -

less s tee l i n ch lor ide media were q u i t e d i f f e r e n t f o r c o n t r o l l e d

p o t e n t i a l c o r r o s i o n tests t h a n w i t h c o n v e n t i o n a l c o r r o s i o n testsu nd er i d e n t i c a l c o n d i t i o n s of c o n c e n t r a t i o n , t e m p e r a t u r e , a n d

p o t e n t i a ? . ( 7 7 1 ) . The electrochemical tests more o f t e n p r o d u c e d

p i t t i n g . These r e s u l t s w e r e a t t r i b u t e d t o t h e m i g r a t i o n an d

s u r Z a c e accumula t ion of C1- d u r i n g p o t e n t i a l m e a s u r e m e n t s .

C ons i de r a b l e e v i de nc e has been accum ulated t o show t h a t

c h l a r i d e i on causes b reakdown of t h e p a s s i v e f i l m . I n p o l a r i z a -

t i m s t u d i e s w i t h C1- added t o a s o l u t i o n c o n t a c t i n g p a s s iv e i r o n ,

it w z s f o u n 6 t h a t a n : x d u c t i o n p e r i o d i s n e c e s s a r y before t h e

a n o d t c c u r r e n t i n c r e a s e s ( 7 7 3 ) . " h e i n 2 u c t i o n p e r i o d w a s i n -

.

.

32



v e r s e l y p r o p o r t i o n a l t o t h e C1- c o n c e n t r a t i o n , w i t h a minimum

c o n c e n t r a t i o n r e q u i r e d f o r a t t a c k ( 3 2 8 ) . A f r e s h l y p a s s iv a t ed

s u r f a c e w a s less r e s i s t a n t t o C1- t ha n one a t s t e a d y s t a t e .

T he se r e s u l t s h av e be en e x p l a i n e d by m i g r a t i o n o f C 1 - t h r ough t h e

f i l m . A s e co n d i n d u c t i o n p e r i o d h a s a l s o b ee n o b s e r v e d , beyond

which porous y-Fe203*H20 grows on t h e s u r f a c e , a l l o w i n g e a s y l oca l

d i s s o l u t i o n l e a d i n g t o p i t t i n g ( 7 7 3 ) .

O n c e i n i t i a t e d a p i t r a p i d l y d e ve lo ps i n t o t h e c o n f i g u r a t i o n

diagrammed i n F i g u r e 4 . A l l o f t h e f a c t o r s i n f l u en c i n g t h e pr o-

gress and deve lopment of a p i t d u r i n g underground c o r r o s i o n c o u ld

n o t c o n v e n i e n t l y be i n c lu d e d i n a s i n g l e d ia g ra m . H o w e v e r , t h i s

diagram, supplemented by Table 7 , s u f f i c e s f o r d e s c r i p t i o n o f t h e

more e v i de n t c h a r a c t e r i s t i c s .

Thu.s r e f e r r i n g t o F i g u r e 4 :

(1) The l iquid - ga s i n t e r f a c e s how n a t t h e t o p r a n g e s fro m t h e

l i q u i d b ou nd ar y o f a p a r t i a l l y o r f u l l y submerged p i p e l i n e t o t h e

t h i n f i l m of m o i s t u r e e i t h e r on t h e p i p e s u r f a c e o r b r i d g i n g a

s o i l p a r t i c l e and t h e p i p e . The a v a i l a b i l i t y o f g a s e s a t t h e p i p e

may Se s e v e re l y l i m i t e d i n t h e f i r s t c a s e , w hereas i n an 3pen

t e x t w e e . s o i l t h e g a s may be a bunda n t l y a va i l a b l e t hr ough. a t h i n

m o i s t x r e f i l m a n d i s q u i ck l y re p la c ed i f consamed.

(2) Four gases are shown i n e q u i l i b r i u m w i t h t h e l i q u i d

phase . Oxygen and hydrogen s u l f i d e a r e s o l u b l e i n water , w h i l e

c a r bon d i ox i de a nd ammoni?. react w i t h water t o f or m new i o n i c

q e c i e s :

( a ) Oxygen., comprising o n e - f i f t h of t h e e a r t h ’ s a t mo sp he re ,

i s wel.’- d i s p e r s e d i n o p e n t e x t u r e d soil; it is a l s o p r oduc e d by



fV

: O i f H.=

IN H , O H Z N H i t OH-

.. W J 1

HCO; C O , , ~ S ~ S ~ ~ L

AS cot OR ncs OR PRE-

ClPrATEO AS IflON

CARBONAT€S +SULFIDES

--

Figure 4 . Schematic Representation of the Underground Corrosion'Process



TABLE 7

SOME OXIDES, SULFIDES, AND SALTS OF IRON APPEARING

Su b s t a n c e

Fe r ro u s h y d ro x i d e

F e r r i c h yd ro x id e

F e r r o s o f e r r i c o x i d e

F er ro us s u l f i d e

F e r r i c s u l f i d e

F e r r o us c h l o r i d e

F er ro us s u l f a t e

( a ) Not no rma l ly WI

I N CORROSION PRODUCTS

Formula

( a )Fe(0H) 2 ; F e O - H 2 0

Fe2O3*3H20;Fe(OH) (b

Fe 0 ; F e 2 0 3 * F e 0 C )

3 4

FeS

Fe2S3F e C l 2 - 4 H 2 0 ( d )

F e S 0 4 - 7 H 2 0(d)

t t e n as the hydra tec

S o l u b i l i t y-

C o l o r , Form

White ,c r y s t a l l i n e

Red -y el lo w t o brown

Black ,g r a n u l a r

B la ck t o brown

Yellow-green

Blue - green

Blue -green

o x i d e s i n c e F e O i s b lack( b ) R a re l y w r i t g e n as t h e h y dr o x id e(c) The oxide i s m a g n e t i c a n d h a s a n a f f i n i t y f o r w a t e r , i . e . ,

Fe3C4 'H20

(d) These s a l t s p r e c i p i t a t e w i t h f i x e 2 p r o p o r t i o n s o f w s . t e r

(1) H i g h l y i n s o l u b l e(2) S o lu b le i n a c i d( 3 ) S o l u b l e i n . a c i d i n t h e p r e s e n c e o f oxygen( 4 ) H i g h l y s o l u b l e

(5) S ol ub le i n a l k a l i

p l a n t l i f e wnich may c a u s e t h e c o n c e n t r a t i o n i x s o i l m o i s t u r e t o

read - v a l u e s w e l l above the normal 9 t o 1 0 ; a r t s p e r m i l l i o n . The

o x y g e n c c n c e n t r a t i o n i s p ro b a b l y n e a r t h e e q u i l i b r i u m v a l u e a t a l -

m o s t e ve ry p o i n t i n b a c k f i l l e d s o i l .

Ib ) Hydrogen su l f ide i s an en d p r o d u c t o f t h e r e d u c t i o n

of s u l f a t e i on by a n ae r ob i c b a c t e r i a 2nd i s , u n l i k e o xy g en , p ro-

ducec i n m i l a n d o f t e n a t t h e ~ i p es u r f a c e ,



( c ) Carbon d i o x id e , i m po r ta n t i n t h e l i f e c y c l e , reacts

w i t h water t o form t h e d i p r o t i c a c i d which d i s s o c i a t e s as shown.

I t i s of e i t h e r a tm o sp he ri c or b i o l o g i c a l o r i g i n .

i n i t i a t i n g f a c t o r .

It i s an

( d ) Ammonia, f r e q ue n t l y i n t r oduc e d i n t o t h e s o i l as a

f e r t i l i z e r , reacts w i t h water to form ammonium and hydroxide ions.

( 3 ) The p r o d u c t o f t h e c o n c e n t r a t i o n s of the components of

d i s s o c i a t e d water an d t h e c o n s eq u e nt r e l a t i o n b et we en t h e pH and

pOH are a p p r o p r i a t e l y shown i n t h e c e n t r a l p o r t i o n o f t h e d ia gr am .

The hydra t ion of h yd ro ge n i o n s a nd t h e e q u i l i b r i u m c o n s t a n t are

a l so shown there .

-

(The unhydra t ed p ro ton , H', i s e v i d e n t l y non-

e x i s t e n t . However, it i s used i n t h e f i g u r e f o r s i m p l i c i t y . I t

is t o be r e a d as the hydronium ion, H30 . )+

(4) A p i t is shown i n t he p ipe w a l l a t t h e bot tom o f t h e

page a l o n g w i t h t h e uncor roded area t o t h e l e f t , i n o r d er t o

i l l u s t r a t e t h e w e l l e s t a b l i s h e d e l e c t r o c h e m i c a l t h e o r y o f c o r r o-

s i o n. I n t h e i d e a l case a l a r g e c a t h o d i c area i s c o n c e n t r i c

w i t h t h e a n od ic p i t where i r o n g oe s i n t o s o l u t i o n a nd g i v e s

e l e c t r o n s t o t h e m e t a l . T h e s e e l e c t r o n s move t o t h e c a t h o d i c

areas where t he y p a r t i c i p a t e i n a r e d u c t i o n p r o c e s s s u c h as

i l l u s t r a t e d by Eq u at io n s 2 and 3 .

s o l u t i o n c a r r y i n g two p o s i t i v e c h a r g e s w i t h o u t n e g a t i v e l y charged

i o n s b e in g a v a i l a b l e to b a l a n c e t h e c ha rg e . E i t h e r one doubly

c ha r ge d a n i on s uc h as s u l f a t e o r a p a i r of s i n g l e ch a rg e d i o n s

such as OH- , Cl-, Q r HS- must be p rov ided . I f t h e e l e c t r o l y t e

a t t h e p o i n t of c o r r o s i o n i s a l k a l i n e , f e r r o u s hy dr ox id e p r e -

The i ron atom c a n n o t go i n t o

c i p i t a t e F ,

36



A s c o r r o s i o n p r o c e e d s , t h e a n i o n s move t h r o u g h t h e

e l e c t r o l y t e ( s o i l , water) a l ong a n arc t ow a rd t he a node , w h i l e

t h e c a t i o n s f o l l o w s i m i l a r p a t h s t o t h e c at ho d e. The l a r g e r arc

shows t h e d i r e c t i o n of f l o w of t h e i o n s u s u a l l y fou nd i n s o i l .

The s o l u t i on t h r ough w hi ch s o l ub l e s a l t s a r e t r a n s f e r r e d from o ne

area t o a n o t h e r un d er t h e e lec t r i c g r a d i e n t e s t a b l i s h e d b etw een

anode and cathode i s t h e e l e c t r o l y t e a nd , l o g i c a l l y , . t h e e l e c t r o -

l y t e a t t he a node i s t h e a n o l y t e and t h a t a t t h e c a t h o d e , t h e

c a t h o l y t e .

I f t h e t r a n s p o r t arcs are imagined t o be s h o r t e n e d , t o

t h e p o i n t t h a t t h e a d j a c en t r e a c t i n g s u r f a c e s are of atomic dimen-

s i o n s , t h e n t h e p r o c e ss i s r e f e r r ed t o as c he m i c a l c o r r o s i on a nd

a more g e n e r a l a t t a c k of t h e c o rr o di n g s u r fa c e s r e s u l t s .

A c co rd i ng t o classical t h e o ry , t h e r e a c t i o n a t t he anode

i s c o nf i ne d t o t h e f o rm a ti o n O X f e r r o u s i o n . B u t t h e r e i s e v i-

d en ce t h a t o t h e r r e a c t i o n s c an s o m e t i m e s o c c ur t h e r e . T h i s w i l l

be d i s c u s s e d l a t e r .

Shown toward t h e l e f t i n t h e f i g u r e are f o u r r e d u c t i o n

r e a c t i o n s . Two r e a c t i o n s a cc ou nt f o r t h e d i s p o s i t i o n of t h e same

number of e l e c t r o n s as r e l e a s e d a t t h e anode . Two show d i s p os i-

t i o n of monatomic (n as ce nt ) hydrogen ( a ) by e v o l u t i o n of hydrogen

.. gas, and ( b ) by r e a c t i o n w i t h oxygen t o f or m w a t e r . I n e lec t ro-

l y t e s low i n pH t h e f o r r n a t i m of molecular hydrogen i s f a v o r e d .

On t h e o t h e r hand, abundant oxyqen i s known t o d e p o l a r i z e

cathodes in e l e c t r o l y t e s 'n.j-gh i n pH. A f i f t h react ion i n t h e

f i g u r e e m p ha si ze s t h e t e nd e n cy of t h e c s t h o l y t e t o i n c r e a s e i n

pH a s c o r r o s i o n p r oc e ed s .

37





There are two i n t e r e s t i n g c h a r a c t e r i s t i c s of t h e c o rr o-

s i o n p ro du ct m a nt le . F i r s t , c o n s id e r a bl e q u a n t i t i e s of w a t e r

are consumed i n t h e p r e c i p i t a t i o n o f t h e o x id e s. Second, t h e

release o f h ydr ogen i o n t e n d s t o c a r r y water away from t h e co rr o-

d i n g area due t o h y d r a t i o n .T e s t s

on s am pl es o f r u s t t a k e n i nt h e f i e l d show t h a t t h e pH of Zone I i s o f t h e o r d e r o f 3 .5 ,

w h e re a s t h e pH o f t h e a n o ly t e i s o n l y s l i g h t l y a c i d i c , a s i n d i-

c a te d i n t h e f i g u r e .

The o r d e r l y f l o w o f a n i o n s i s i n t e r r u p t e d i n Zone I by

r e a c t i o n o f i nc om in g s u l f i d e s a nd c a r b o n a t e s w i t h hy dr og en i o n s

a n d c o n s eq u e n t e v o l u t i o n o f h yd ro ge n s u l f i d e an d c a rb o n d i o x i d e

and n e u t r a l i z a t i o n o f hy dr ox i de i o n s . I r o n s u l f i d e s h ave n o t

been found i n Zone 111. N i t r a t e i o n s , g e n e r a l l y r a r e i n s o i l ,

presumably o x i d i z e t h e i r o n o x i d e s t o t h e f e r r i c s t a t e . These

r e a c t i o n s t h u s a l lo w o n ly t h e c h l o r i d e and s u l f a t e i o n s , and

p o s s i b l y b i s u l f i d e , t o r e a c h t h e a n o l y t e . On t h e o t h e r hand,

t h e o r d e r l y f lo w o f c a t i o n s i s u n i n te r r u pt e d t o t h e m e t a l s u r -

face, w he re m agnesium a nd c a l c i u m i o n s p r e c i p i t a t e as hydrox ides

and ca r bon a te s . There hydrogen io ns , i f any , are a l s o n e u t r a l i ze d .

Sgdium and po tas s ium ions accumula te a t t h e s u r f a c e , and t h e pH

t e nd s t o i n c r e a s e .

Two f a c t s o f i n t e r e s t i n t h e movement o f t h e i o n s a re :

( a ) t h a t w hi le t h e c u r r e n t d e n s i t y i n t h e c a t h o d ic area i s a

maximum ne a r th e p i t , i t a t t e n u a t e s r a p i d l y w i th i n c r e a se i n t h e

r a d i u s o f t h e t r a n s p o r t a r c ; and (b ) t h a t d i f f u s i o n , p re do mi na te s

o v er t h e m i g r a t io n of t h e i o n s due t o t h e i n f l u e nc e of t h e elec-

t r i c f i e l d e s ta b l i sh e d by t h e c o r r o s i o n p r o c e s s e s.

3 9



( 6 ) The c o r r o s i o n p r o d u c t m a n t l e i s ne ve r as i d e a l l y con-

s t i t u t e d as d e p i c t e d i n t h e f i g u r e . The m a g ne t ic o x i d e f i l l s and

o v e r l a y s t h e p i t i n t h e absence of f e r r o u s hydrox ide . I t i s

porous and e s s e n t i a l l y f r e e of condensed m o i s t u r e . T h e p a r t i c i -

p a t i o n of water i n t h e f o r m a t i on of t h e s e v e r a l o x i d e s h a s b e e n

mentioned. Added t o t h i s d r y i n g ef fec t i s t h e r e p u l s i o n of hydro-

nium ions by t h e e lec t r i c f i e l d . However, t h e vo i d s are s a t u r a t e d

w i t h w a t e r va po r .

t h e a n o l y t e i s b a r e l y a c i d and t h e b l a c k o x i d e may e x h i b i t a

g r e e n i s h cast i n d i c a t i n g i t s s a t u r a t i o n w i t h f e r r o u s c h l o r i de .

Pale g re en c r y s t a l s of t h i s s a l t have been p i cked f rom th e b l ack

g r a n u l a r mass. They develop a t h i n b ro w ni sh coat on exposure t o

a i r . Water of c r y s t a l l i z a t i o n f u r t h e r d ep l e t e s t h e a n o l y t e of

f r e e water.

-I n accord w i t h t he r m odyna m i c c ons i de r a t i ons ,

Chemica l a n a l y s i s of material i n p i t s has been more e x t e n-

s ive i n p i p e c o at e d w i t h cement mortar t h a n w i t h p i p e e i ther bare

or c o a t e d w i t h o r g a n i c materials . I n t h e s e a n al y se s t h e r a t i o of

i r o n t o c h l o r i n e i s always i n good agreement w i t h f e r r o u s chlor -

i d e s t o i c h i o m e tr y . S u l f a t e a nd o th e r i o n s a pp ea r i n t h e a n o l y t e

i n traces only, The mortar reacts w it h s u l f a t e i on and i n h i b i t s

i t s t r a n s p o r t t o t h e p i t .

( 7 ) The mechanism f o r t h e f o r m a t i on of m a g n e t i t e i s o b s c u r e .

The p r o b l e m a t i c a l e q u a t i o n s i n t h e m a nt l e are i n t e n d e d p r i m a r i l y

t o s h o w t h e e nd p r oduc t s a nd t o g i v e a r e a s o n a b l e a c c o u n t of t h e i r

f o rm a ti o n i n a c co r d w i t h o b s e r v a t i o n . A nurqber of o t h e r e q u a t i o n s

c o u l d be w r i t t e n b u t t h e s e w o u l d be e q ua l l y s pe c u l a t i ve . However,

there i s e v i d e n c e t h a t m a g n e t i t e forms s pon t a ne ous l y f r o m f e r r o u s

.

40



h y d r o xi d e w i t h e v o l u t i o n o f h yd ro ge n .

e x p l a i n t h e f o rm a t io n of m ag ne ti te i n b o i l e r t u b es .

i s t h e r mo d y n a mi c a l l y p o s s i b l e .

formed t h e f e r r o s o f e r r i c o x i de s ee d s i t s c o n t i n u e d p r o d u c t i o n .

O n t h e o th e r ha nd , moun ting ev idence shows t h a t unde r h igh ae r a-

t i o n ferr ic i o n forms d i r e c t l y f r o m t he metal.

of t h e F e ,F e 3 + c ou pl e would f a v o r t h i s r e a c t i o n i n a s o l u t i o n

s a t u r a t e d w i t h f e r r o u s c h l o r i d e c o m b i n e d w i t h a h i g h l y aerated

c a t h o d e p r o v i d e d , of c ou rs e , t h a t t h e a c t i v i t y of t h e f e r r i c

i o n i s m a i n t a i n e d e s s e n t i a l l y a t z e r o by p r e c i p i t a t i o n of t h e

T h i s r e a c t i o n i s used t o

The r e a c t i o n

I t has been s u g g es t ed t h a t once

Thus t h e p o t e n t i a l

-

m a g n e t i c o x i d e .

(8) The o v e r a l l p o t e n t i a l of t h e go ing c e l l may be a p p r o x i-

mated by the rmodynamic ca l cu la t i on s .

e q u i l i b r i u m w i t h i t s sa l t s F e C 1 2 - 4 H 2 0 and F e S 0 4 - 7 H20 a s r e q u i r e d

by t h e schematic, are r ead i ly compu ted (Table 7 ) . T h e p o t e n t i a l s

of t h e h y d r o g e n a n d o x y g e n e l e c t r o d e s p r e s e n t problems r e l a t i n g

t o t h e a b s o r p t i o n of hydrogen by i r o n and la ck of i n f o r m a t i o n

on t h e r e a c t i o n s a t t h e e l e c t r o d e s u rf a c e s . F u r t h e r c om p li ca ti ng

t h e a p p ro x i m a ti o n s , t h e s u b s t a n t i a l r ed o x p o t e n t i a l b et we en Z ones

I and I must be t a k e n i n t o a c co u nt . I t would appea r ea sy t o

measure t h e a c t u a l p o t e n t i a l i n t h e a n o l y t e and t h e r edox po ten-

t i a l i n Zone I and t h e c at ho de p o t e n t i a l i n a case approach ing

t h e i d e a l . Some such measurements have beer! made i n t h e f i e l d .

The mixed p o t e n t i a l t a k e n r e m o t e l y , as on t h e s u r f a c e of the

earth o v e r t h e p i p e , i s p o o r l y i n f o r m a t i v e e x c e p t w i t h r e s p e c t

t o t h e order of t he r e a c t i o n on t h e m e t a l s u r f a c e . T h i s i s

g e n e r a l l y found t o be b i f a r a d a i c ( t w o e l - e c t r o x s e x ch a ng e d) , i n

The p o t e n t i a l of i r o n i n

,.

41



a c c o r d w i t h c lass ica l t h e o r y . As a p r a c t i c a l matter, o p p o r t u n i-

t i e s f o r t e s t are few. Excavat ions are g e n e r a l l y made f o r r e p a i r

o f p i p e , a nd i n su c h cases t h e c o r r o s i o n ce l l s are d e s t r o y e d by

e s c a p i n g o i l , gas, or water.

As a matter o f i n t e r e s t , it is n o t e d a t t h i s p o i n t t h a t oxy-

gen may p l ay th ree roles i n t h e c o r r o s i o n p r o c e s s , n am ely, (a )

removal of adsorbed and absorbed hydrogen, (b) i t s d i r e c t r e du c-

t i o n a t t h e c a th o d e , a nd (c) i t s r e d u c t i o n w i t h i n t h e m an t le t o -

form f e r r i c o x id e . P er ha ps t h e r e a c t i o n

4Fe2+ + O2 + 2H20 = 4Fe3+ + 4 0 H - (Eq. 7)

s h ou l d b e i n cl u d e d i n d i s c u s s i o n s of basic mechanisms.

The i n f l u e n c e o f b a c t e r i a an d s u l f i d e s on p i t t i n g w i l l be

d i s c u s s e d la ter .

5. Crevice C or r os i on

I n t e n s e l o c a l i z e d c o r r o s i o n f r e q u e n t l y o c c ur s w i t h i n s h i e l d e d

areas exposed t o c e r t a i n e n v ir o n me n ts . S h i e l d e d areas are ex-

p e c t e d when po rous g a s k e t s are employed, when la r g e den se roc ks

c o n t a c t b a r e p i p e s , u nd er f r a c t u r e d o r d i s b o n d e d c o a t i n g s , e tc .

The mechanism of attack i s t h o u g h t t o be s imi la r t o t h a t f o r p i t -

t i n g , e x c e pt t h a t t h e r e i s no n e c e s s i ty f o r i n i t i a l l y f orming a

s h i e l d e d area.

i o n hy d ro l ys i s , d i f f e r e n t i a l a e r a t io n c o n t r i b u t es t o c r e v i c e corro-

s i o n ( a n d a lso t o p i t t i n g ) , e s p e c i a l l y i n t h e e a r l y s t a g e s . S i n c e

oxygen has d i f f i c u l t y moving i n t o a c r e v i c e , t h e c o n c e n t r a t i o n

of o xy gen w i t h i n t h e c r e v i c e s o on f a l l s below t h a t a t t h e e x-

t e r n a l s u r f a c e . E qu a ti on 5 i n d i c a t e s d i f f e r e n t p o t e n t i a l s

I n a d d i t i o n t o c h l o r i d e i o n m i g r a t i o n a n d f e r r o u s

.

.

4 2



between t h e e x t e r i o r and crevice , making them ca th od ic and anod ic ,

r e s p e c t i v e l y .

Crevice c o r r o s i o n of m i l d s t ee l i n NH NO s o l u t i o n s w a s found4 3

t o be maximum under c o n d i t i o n s o f weak p a s s i v a t i o n ( 4 8 5 ) . I n

crevice c o r r o s i o n of s t a i n l e s s steel i n seawater, it w a s found

t h a t t h e e n t r ap p e d c o r r o d e n t had a pH below t w o a n d c o n t a i n e d

f e r r o u s b u t n o t f e r r i c i o n s ( 5 84 ) . E y d r o l y s i s o f c hr om i c i o n s

w a s t h ou g ht t o oc cu r w i t h s t a i n l e s s s teel i n a d di t io n t o h y d ro l-

y s i s o f f e r r o u s i o n s t o l ow er p H .

C r e v i c e c o r r o s i o n of s t a i n l e s s s teel on ly occur red be low a

C hl or id e i o ns f a c i l i t a t e dc r i t i c a l p o t e n t i a l i n d i l u t e H2S04.

s p r ea d i n g o f a t t a c k (487).

6 . Galvan ic Cor ros ion

It i s w e l l known that, when a more noble m e t a l i s i n c o n t a c t

w i t h i r o n t h e c o r r os i o n r a t e of t h e i r o n i s a c c e l e r a t e d b ec au se

i t h a s a l o w er p o t e n t i a l . C om pa ri so n of g a l v a n i c c o r r o s i o n re-

s u l t s w i th s t a n d a rd EMF v a l ue s s hows t h a t t h i s i s n o t p r e c i s e l y

t r u e . G a l v an i c c o r r o s i o n c a n o n l y b e w e l l unders t ood by s tudy-

i n g mixed p o t e n t i a l b e h a vi o r , as i l l u s t r a t e d by F i g u r e 5 . The

main r e a s o n t h e c o r r o s i o n r a te i s a c c e l e r a t e d i s t h a t t h e c a t ho d ic

r e a c t i o n i s f a s t e r , so t h a t t h e n e t a n od ic d i s s o l u t i o n r e a c t i o n i s

f a s t e r . I f t h e o t h e r meta l ' s c a t h o d i c r e a c t i o n i s s u f f i c i e n t l y

f a s t and i t s a n o d i c r e a c t i o n i s s l o w , t h e n it c a n a c c e l e r a t e t h e

c o r r o s i o n of i r o n e v en t h o u gh it h a s a lower s t an d ar d p o t e n t i a l .

The m ag n it ud e o f t h e e f f e c t de pe nd s o n t h e r e l a t i v e s i z e o f t h e

m e t a l s . The i r o n n e z x e s t t h e f o r e i g n metal i s a f f e c t e d m o s t

s e v e r e l y .



icorr

LOO CURRENT

F i g u r e 5. Mixed P o t e n t i a l C ur ve f o r G a l va n i c C o r r p s i o n of More Noble Metal M Having Higher Rate of C a t hod i c R e a c t i on

E ve n t w o d i f f e r e n t steel a l l o y s i n c o n t a ct create a g a l v a n i cc o u p l e a nd a c c e l e r a t e c o r r o s i o n o f o ne of them. Thus, s t a i n l e s s

$

d ou ble d t h e c o r r o s i o n r a t e of carbon steel i n a e r a t e d 3 . 5- p e r ce n t

N a C l when the areas w e r e e q u a l a nd i n c r e a s e d t h e r a t e by e i g h t t o

n i n e t i m e s when t h e s t a i n l e s s had an area e i g h t t i m e s as l a r g e

7 8 4 ) . C e r t a i n c o r r o s i o n p r o d u c t s c a n b e c a t h o d i c w i t h r e s p e c t

.

t o s tee l . Thus , f o r example, new p i pe i n a r u s t e d l i n e w i l l be *

a nod i c a nd c o r r ode p r e f e r e n t i a l i y . A r e c e n t s u r v e y of p i p e l i n e

c o r r o s i o n a t 23 U . S . Navy l o c a t i o n s r e v e a l e d t h a t m a n y f a i l u r e s

w e r e caused by use of d i s s i m i l a r n e t a l a .

44



Galvan ic ce l l s w e r e named as t h e m o s t p r e v a l e n t c a u s e for

c o r r o s i o n b y m o r e companies ( 7 6 p e r c e n t ) su rv eyed t h a n any o t h e r

c a u s e .

7 . Corros ion by S t r ay Cur ren t s and High Vol tage D i r e c t C u r r e n t

Equa t ion 1 shows t h a t a n o d i c d i s s o l u t i o n t a k e s p l a c e w her e

( p o s i t i v e ) c u r r e n t l e a v e s t h e p i pe . I f e lec t r ic f i e l d s are

p r e s e n t i n t h e s o i l , c u r r e n t w i l l f low i n th e p i pe . Where th e

c u r r e n t e x i s t s , c o rr o s i o n w i l l t a k e p l a c e a c c o r d i n g t o Equa t ion 4 .

This amounts t o 0 . 55 m i l s p e r y e a r f o r a c u r r e n t d e n s i t y o f 1

mA/square f t . I f a c o a t i n g o f l o w e l e c t r i c a l c o n d u c t i v i t y i s

a p p l ie d t o t h e p i p e , t h e c o r r o s i o n w i l l be even more r a p i d a t

t h e h o l i d a y s b ec au se t h e c u r r e n t i s t h e n c o n c e n t r a t e d i n s m a l l

s p o t s .

S t r a y c u r r e n t s w e r e second o n l y t o g a l v a n i c c e l l s i n c a us i ng

c o r r o s i o n leaks a c c o rd i n g t o t h e c o mp an ie s s u rv e y e d (T a b l e 3 ) .

A m aj or s o u r ce of s t r a y c u r r e n t i s e lec t r ic r a i l w a y s ( 3 3 3 ) .

This can be minimized by u si ng a w e l l i n s u l a t e d roadbed and 100-

p e r c e n t r a i l r e t u r n f o r t h e DC.

C a th o di c p r o t e c t i o n of o n e u n de rg rou n d s t ru c t u r e c an i n d u c e

s t r a y c u r r e n t s a nd c o r r o s i o n i n n e a r b y s t r u c t u r e s a n d i s known

as " i n t e r f e r e n c e . " This may be a l l e v i a t e d by j o i n i n g n ea rb y

s t r u c t u r e s w i t h e lec t r i ca l c a b l e and p r o v id i n g p r o t e c t i o n j o i n t l y .

A t o t a l of 1 2 , 4 0 8 dr a in ag e bonds between th e companies' p i p i ng and

o t h e r s t r u c t u r e s w a s re po r t ed by t h e survey ed companies . Many of

t h e bon ds c o n t a in e d r e s i s t o r s and d i o d e s t o c o n t r o l t h e m ag ni tu de

and t h e d i r e c t i o n o f t h e c u r r e n t flow between s t r u c t u r e s . The

l a r g e s t c u r re n t w a s 1 5 0 A w i t h . t h e z v e r a g e f o r many companies

45



around 7 A p e r major bond. T h e m a j o r i t y of t h e bonds c a r r i e d

0 . 5 t o 5 A. T h i s i m p l i e s a minimum of 1 2 , 0 0 0 amperes being ex-

changed by t h e companies p a r t i c i p a t i n g i n t h e q u e s t i o n n a i r e s u rv e y.

The e f f e c t of s t r a y c u r r e n t s o n p i p e s c an b e i so l a t e d by

i n t e r r u p t i n g t h e e l e c t r i c a l c o n t in u i t y o f t h e p i p e a t i n t e r v a l s

w i t h i n s u l a t i n g f l a n g e s . I n d i s c r i m i n a te u s e of i n s u l a t i n g 4

f l an g e s may ca us e problems due t o d i f f e r en c e s o f p o t e n t i a l be-

tween t h e t w o s e c t i o n s of p i p e . C a thod ic p r o t e c t i o n i s v e r y h e l p-

-

f u l b u t m ust be c o o r d i n a t e d w i t h o t h e r n e ar by c a t h o d i c p r o t e c t i o n

v e n t u r e s , as d i s c u s s e d l a t e r . A s o u r c e of s t r a y c u r r e n t may be

de te rmined by us ing two or more c op pe r s u l f a t e e l e c t r o d e s t o

f i n d t h e p o t e n t i a l g r a d i e n t . The vector sum of two such measure-

m en ts i n p e r p e nd i c u la r d i r e c t i o n s p o i n t s toward t h e source of

s t r a y c u r r e n t ( 56 8 ). I t h a s been p o i n t e d o u t t h a t v o l t a g e s are

in du ce d i n p i p e l i n e s p a r a l l e l t o o ve rh ea d h ig h v o l t a g e AC power

l i n e s ( 7 0 1 ) . When c a t h od i c p r o t e c t i o n w i t h rec t i f ie rs i s used

i n a d d i t i o n t o t h e induced AC, a n e t DC c u r r e n t r e s u l t s . Corro-

s i o n c a n be a vo i de d by a r r a ng i ng t h e rec t i f i e rs so t h a t i nd uc ed

c u r r e n t i s d r a i n e d o f f i n t o t h e g ro un d b ed . AC i t s e l f has been

found t o accelerate c o rr o si o n s l i g h t l y -- less t h a n 1 p e r c e n t of

t h a t f r o m e q u i v a l e n t DC ( 354 , 683 ) . O t h e r s have quo ted a f i g u r e

of 0 . 1 p e r c e n t i n de p e nd e n t of c u r r e n t d e n s i t y ( 4 0 5 ) .

H i gh vo l t a ge DC (HVDC) power t r a n s m i s s i o n l i n e s are p l a nn e d

f o r t h e n e a r f u t u r e . Power companies have claimed t h a t e a r t h .

c u r r e n t s w i l l n e i t h e r be l a r g e no r f r e q u e n t , and t h a t i n any

e v e n t , t h e c u r r e n t s w i l l l a r g e l y follow deep s t r a t a i n t h e ear th.

The p i p e l i n e i n d u s t r y and some c o r r o s i o n c o n s u l t a n t s d i s a g r e e

46



( 25 3, 3 3 6 ) . I f t h e p i p e l i n e c om pa nie s f e a r s are r e a l i ze d , l i t i g a -

t i o n f o r damage i s l i k e l y . N o t o n l y c o ul d l a r g e c u r r e n t s of even

a f e w m i n u t e s d u ra t i o n cause s i g n i f i c a n t c o r r o s i o n where t h e

c u r r e n t l e a v e s t h e p i p e s , b u t c o a t i n g s c a n be damaged where

c u r r e n t e n t e r s t h e p i p e s . S i n c e t h e power co mp an ie s o n l y e x p e c t

t o i nd uc e s t r a y c u r r e n t s s e v e r a l ho u rs p e r y e a r , a p o s s i b l e s o l u-

t i o n w i th r e s p e c t t o c o r r o s i o n i s f o r t h e HVDC sys tem t o s h u t

down, r a t h e r t h a n d i v e r t c u r r e n t i n t o t h e e a r t h and create a

d i f f i c u l t , random s t r a y c u r r e n t s i t u a t i o n .

S e v e r a l t es t s have been conducted t o d e t e rm i n e HVDC e f f e c t s .

Cu r re n t s o f 300 A n e a r L o s Angeles had l i t t l e e f f e c t on p i p es i n

Los A n g e le s , p r e s um a bl y b e c au s e o f t h e h i g h d e n s i t y o f p i p e l i n e s .

F i f t y m i l e s from Los Angeles 50 mV changes w e r e observed. How-

*

e v e r , o n s o m e p i p e l i n e s c u r r e n t w a s p i c ke d up i n damaging amounts

( 2 6 0 , 523, 252, 7 2 7 ) . O n e s u rp r i s i n g r e s u l t o f t h e L o s Angeles

measurements i s t h e random t e l l u r i c c u r r e n t s t h a t w e r e found.

These are t h o u g h t t o be d ue t o s u c h s o u r c e s as s m a l l s h i f t s i n

t h e e a r t h ' s m ag ne ti c f i e l d and f l u c t u a t i o n s i n cosmic rays . A

s t u d y o n t e l l u r i c e f f e c t s co nc lu de d t h a t c o r r o s io n i s i n s i g n i f i -

c a n t f o r c oa te d , p r o t e c t e d l i n e s and ba r e l i n e s ( 7 9 4 ) .

U s e of HVDC deep w e l l e l e c t r o d e s d o es n o t n e c e s s a r i l y l i m i t

t h e r i s k s of s t r a y c u r r e n t ( 2 5 2) . The l o c a t i o n o f t h e e l e c t r o d e s

i n r e l a t i o n t o p i p e l i n e s w a s found t o be c r i t i c a l . L o c a t i o n o f

e l e c t ro d e s i n t h e sea w a s found t o be b e s t . More t es t s are needed

and are under way ( 7 2 7 ) .

*September 1 9 6 9 meet ing of L o s A n g e l e s C a t h o d i c P ro t e c t i o nCommittee.

4 7



A computer study showed t h a t c u r r e n t s d o n o t p l u n g e t o t h e

core of t h e ear th . Depth o f b u r i a l , s o i l r e s i s t i v i t y , and p i pe

were found t o have l i t t l e e f f e c t on i n t e r f e r e n c e e f f e c t s . L ar ge r

d i a m e t e r p i p e showed l a r g e r ef fec ts . The d i s t a n c e of t h e p i p e l i n e

from t h e HVDC e l e c t r o d e w a s found t o be t h e m ajo r f a c t o r i n deter -

mining t h e e f f e c t s on a p i p e l i n e ( 4 8 8 ) .

A lt ho ug h c a t h o d i c p r o t e c t i o n i s u s e f u l a g a i n s t no rm al i n t e r -

f e r e n c e , t h e HVDC c u r r e n t s are e x p e c t e d t o change so r a p i d l y t h a t

o rd i n a ry m a nu a ll y a d j u s t e d s y s t e m s c a n n o t c om p en sa te f o r them.

L a r g e c u r r e n t s o u r c e s w i t h a u t o m a t i c c o n t r o l w i l l be r e q u i r e d --

and w i l l be e x p e n s i v e ( 3 3 5 ) . Automat ic c o n t r o l may be r e q u i r e d

o n l y f o r up t o 1 5 0 m i l e s from t h e HVDC e l e c t r o d e ( 7 9 5 ) .

A s t u d y w a s made of a h y p o t h e t i c a l p i p e l i n e 36 i n ch e s i n

diameter and 250 m i l e s l o n g , w i t h 5 0 , 0 0 0 ohms-squ are f o o t c o a t i n g

r e s i s t a n c e ( 7 9 6 ) . T h e p i p e l i n e w a s assumed t o be i n t h e e lec t r i c -

a l f i e l d e s t a b l i s h e d b y HVDC ear th c u r r e n t s a nd s e p a r a t e d 2 0 m i l e s

f r o m t h e e l e c t ro d e . U n d e r these c i r c u m s t a n c e s , a problem of c o r r o-

s i o n a n d disbanding w o u l d e x i s t . T h e s t u d y c on cl ud ed t h a t : 1)

HVDC e f f e c t s c an a t best o n l y be m i t i g a t e d a t c o n s i d e r a b l e e x p e n s e :

( 2 ) because o f t h e w i d e f l u c t u a t i o n i n t h e amperage of HVDC c u r -

r e n t , a u to m at i c r e g u l a t i o n o f p r o t e c t i v e c u r r e n t s would be n e c e s-

s a r y o v er t h e l e n g t h of t h e b u r i e d s t r u c t u r e s ; ( 3 ) f o r t h e hypo-

t h e t i c a l p i p e l i n e s t u d ie d , n i n e c o n t r o l l ed rec t i f ie r i n s t a l l a -

t i o n s w o u l d be r e q u i r e d a t a n est imated c a p i t a l cost of a b o u t

$200,000.

I n a n o t h e r s t u d y ( 7 9 7 ) , the l i t e r a t u r e a nd " o f f - t h e - s h e l f "

equipment w e r e reviewed t o d e t e r m i n e t h e best a r rangement fo r

4 8



c o r r e l a t i n g HVDC c u r r e n t s w it h e a r t h c u r r e n t s a nd c a t h o d i c p r o-

t e c t i o n c u r r e n t s . F or a b ou t $ 5 , 0 0 0 p e r a s sem b ly , equipment can

be p u t t o g e t h e r from a v a i l a b l e commercial components . Required

would be a h i g h i mpedance d i g i t a l v o l t m e t e r , a p a p e r t a p e punch,

a nd a p p r o p r i a t e s w i t c h i n g a nd m u l t i p l e x i n g e qu i pm e nt t o m ea su re

several p o t e n t i a l s a t one t i m e . The package c ou l d op e r a t e un-

a t t e n d e d f o r a n e x te n de d p e r i o d o f t i m e on AC power ( 7 9 7 ) .

8. M ec ha ni ca l S t r e s s E f f e c t s

Steel can be made t o f a i l b o t h by c o r r o s i o n and by a p p l i c a t i o n

o f s u f f i c i e n t me ch an ic al stress. When both stress and a c o r r o s i v e

environment are a p p l i e d s i m u l t an e o u s ly , f a i l u r e o c c u r s a t l o w e r

l eve l s of stress a n d /o r s o o n e r t h a n when e i t h e r a c t a l o n e . S e v e r e

c r a c k i n g s o m e t i m e s r e s u l t s from t h e a c t i o n o f c e r t a i n c or r od e nt s

on c e r t a i n metals when a t e n s i l e stress i s a p p l i e d .

Table 3 s h o w s t h a t a s m a l l b u t s i g n i f i c a n t f r a c t i o n of t h e

c om pa ni es s ur v ey e d c o n s i d e r m e c h a n i c al l y a i d e d c o r r o s i o n t o be a

p r e v a l e n t cause of l e a k s .

Some c o r r o s i o n e n g i n e e r s d i s t i n g u i s h b etw ee n t w o t y p e s of

c r a c k i n g -- stress c o r r o s i o n c r a c k i n g a n d h y d r o g e n e m b r i t t l e m e n t

o r hydrogen stress c r a c k i n g -- w h il e o t h e r s do n o t . I t i s

thoug h t by some t h a t stress c o r r o s i o n c r a c k i n g i s due t o s t r e s s -

accelerated d i s s o l u t i o n a t t h e t i p of t h e c r a ck and t h a t c a t h od i c

p o l a r i z a t i o n can e l i m i n a t e t h e phenomenon. On t h e o t h e r hand ,

hydrogen stress c r a c k i n g i s a t t r i b u t e d by some t o hydrogen a toms

d i f f u s i n g i n t o t h e i r o n and e m b r i t t l i n g it so t h a t it c r a c k s

e a s i l y . C a t h o d ic p o l a r i z a t i o n w ou ld w or se n t h i s phenom enon,

w h i l e an o d ic p o l a r i z a t i o n would r e l i e v e it. C a u s t i c e m b r i t t l e m e n t

4 9



i s t houg h t t o be hydr ogen e m br i t t l e m e n t e nge nde r e d by a h i g h p H

e nv i r onm e n t .

Related t o o t h e r h ydrog en e f f e c t s i s h y d r o g e n b l i s t e r i n g i n

which b l i s t e r s are form ed i n t h e m e t a l . Bl is te r s g e n e r a l l y o c c u r

a t e l e v a t e d t e m p e r a t u r e s . W e t h yd ro ge n s u l f i d e c a n c a u s e stress

c r a c k i n g c a l l e d s u l f i d e stress c o r r o s i o n c r a c k i n g ( 33 7, 3 3 8 ) .6

One i m p o r t a n t d i f f e r e n c e b et we en t h i s a nd o t h e r f or ms of s t ress

c r a c k i n g i s t h a t t h e e nv iro nm en t i n s u l f i d e p r o c es s es i s u s u a l l y

i n s i d e t h e p i p e r a t h e r t h a n e x t e r n a l , as w i t h m os t o t h e r c o r r o-

d e n t s . Hydrogen s u l f i d e i s f ou nd i n s o u r gas, c r ude o i l , swamps,

marshes , and open sewage d i sposa l areas.

C or ro si on f a t i g u e r e f e r s t o a c c e l e r a t e d f a i l u r e c a us ed by

s t ress c y c l i n g d u e t o t h e p r es e n ce o f a c o r r o s i ve e nv i r onm e n t .

The ex ac t mechani sms res po ns ib l e f o r these phenomena are u n c e r t a i n

and may no t a lways be t h e same. These phenomena are unde r ve r y

a c t i v e i n v e s t i g a t i o n , p a r t l y b ec au se so many unanswered qu es t i on s

r em ai n and p a r t l y be c au s e f a i l u r e c an b e c a t a s t r o p h i c .

I n p i p e l i n e s stresses a r i s e f r o m i n t e r n a l h y d r o s ta t i c p r e s-

s u r e , b en din g i n i r r e g u l a r t r e n c h es , b a c k f i l l , e t c . R e s i dua l

stresses are a l so p r e s e n t from p l a s t i c d e f o r m a t i on i n m a nufa c tu re

and p i p e l a y i n g . Even t h e t he r m a l stresses of we ld ing produce

r e s i d u a l stresses. I n 1965 a 24- inch n a t u r a l g a s p i p e l i n e f a i l e d

f r o m stress c o r r o s i o n c r a c k i n g an d 1 7 p e o p l e w e r e k i l l e d ( 6 9 4 ) .

A r e c e n t s u r v e y showed t h a t stress c o r r o s i o n c r a c k in g w a s obs e r ve d

i n a v a r i e t y of p i p e l i n e s ( 5 8 6 ) . F a i l u r e s a n d n e a rb y s e c on d a ry

c r a c k s w e r e a x i a l a nd n o t a s s o c i a t e d w i t h w e l d s , p i t t i n g , g e n e r a l

c o r r o s i o n , o r a ny o t h e r defect. C r a c k s f o l l o w e d g r a i n b o u n d a r i e s

50





such as hydrogen s t re s s c r a c k i n g or stress c o r r o s i o n c r a c k i n g ,

e t c .

The f o l lo w i n g q u o t a t i o n s are some of t h e more i n t e r e s t i n g

e x p l a n a t i o n s :

(1) "The two hyd ro gen b l i s t e r l e a k s o c c u rr e d i n areas of h i g h

C 0 2 and H2S c o n t e n t c o- e x i s t i n g w i t h free water i n t h e gas

stream u p s t r e a m o f a p r o c e s s p l a n t . I t w a s dete rmi ned by an

i n d e pe n d e n t l a b o r a t o r y t h a t hy dr og en a to ms p e n e t r a t e d v a s t

l a m i na t i o ns i n t h e p i p e a nd r e s u l t e d i n p r e s s u r e b ui ld - u p

w i t h i n t h e l a m i n a t i o n s . I n t h i s r e s p e c t , c o r r o s i o n w a s a

c o n t r i b u t i n g factor t o t h e l e a k s . I t w a s e s t i m a t e d t h a t up-

wards of 2 5 p e rc e n t of t h e p i p e area w a s l a mi n at e d i n t h e

a f f e c t e d area. The e n t i r e s y s t e m s i n v o l v e d w e r e t a k e n o u t of

service, h y d r o s t a t i c a l l y tes ted, a n d u l t i m a t e l y t h e e n t i r e

sys tems w e r e r e tu r ne d t o service. Dehydra t ion equipment w a s

t he n i n s t a l l e d a t we l l- h ea ds t o d r y t h e g a s streams. In-

h i b i t o r s were u s e d d u r i n g t h e i n t e r i m p e r i o d a nd c o n t i n u e t o

be used as a s a f e t y f a c t o r . " (Q533)

( 2 ) " I n 1951 and 195 2, we had 1 7 f a i l u r e s i n t h e g a t h e r i n g

*

sys tem due t o hydrogen b l is ters o r i g i n a t i n g from g e n e r a t i on

of hydrogen f r o m t h e c o r r o s i o n processes of the a c i d g a s

( C 0 2 and H 2 S c o n t e n t ) b e i n g g a t h e r e d . " ( 4 6 3 6 )

( 3 ) "One leak i n c i d e n t has been a t t r i b u t e d t o stress c o r r o-

s i o n c r a c k i n g based on t h e c h a r a c t e r i s t i c s of t h e o b s e rv e d

secondary cracks, f r a c t u r e i n t h e f i e l d and i n t h e l a b o r a t o r y .

The in fo rma t ion on t h i s i n v e s t i g a t i o n has been i n c l u d e d i n

* Numbers i n p a r e n t h e s e s p receded by I 'Q" refer t o i n f o r m a t i o n f r o ms p e c i f i c q u e s t i o nn a i re s .

5 2





( 7 ) "On 1 0 0 cast i r o n l e a k s , t h e p ip e was c ra c k e d . W e d o n o t

know w h et h er t h e c o r ro s i o n c a u s ed t h e c r a c k s o r t h e c r a c k s

c a us ed t h e c o r r o s i o n . I (Ql44)

( 8 ) "One c r a c k d u e t o selective c o r ro s i on i n a l o n g i t u d i n a l

seam of elect r ic r e s i s t a n c e wel de d p i p e ." (4580)

Some r e c e n t r e s e ar c h r e s u l t s on s t r e ss- co rr os io n phenomena

are r ev ie we d b r i e f l y i n t h e f o l l ow i n g p a r a gr a p h s.

P i p e l i n e s teel was h e a t t r e a t e d t o o b t a i n y i e l d s t r e n g t h s

rang ing f rom 1 1 0 , 0 0 0 t o 1 5 0, 00 0 p s i . C ra c ki n g o c c u r r e d a t stress

and c a t h o d i c p r o t e c t i o n c o n d i t i o ns l i k e t h o s e t h a t mi gh t be used

w i t h h i g h s t r e n g t h steels . S u s c e p t i b i l i t y t o hydrogen stress

c ra ck i ng depended on th e r a t e a t which hydrogen a toms entered the

s t ee l , which i n t u r n w a s g e n er a l ly l a r g e r a t h i g h pH ( 3 5 6 ) .

D u c t i l i t y and t e n s i l e s t r e n g t h o f s t a i n l e s s s t ee l f o i l s w e r e re-

duced by atomic h yd ro ge n a b s o r p t i o n d u r i n g c a t h o d i c c h a r g i n g ( 3 3 2 ) .

I n s i n g l e c r y s t a l i r o n t h e h yd rogen a tom p e n e t r a t i o n ra te de-

c r e as e d a s t h e p ac ki ng d e n s i t y o f t h e p l a n e p a r a l l e l t o t h e s u r -

face i n c r e a s e d ( 1 9 9 ) . I t was f ou nd t h a t hy dr og en s o l u b i l i t y i n

i r o n a n d s tee l w a s i n c re a se d by t e n s i l e stress and decreased by

compres s ive stress ( 3 8 7 ) . D i f f u s i v i t y was unaffec ted . Beyond a

c r i t i c a l co nc en t r a t io n , which depended on t em pera tu re , hydrogen

produced s t r u c t u r a l damage and e m b r i t t l e d t h e metal. I r o n s u l-

f i d e f i l m s r e t a r d e d hydro gen p e n e t r a t i o n ( 3 8 5 ) .

The s u s c e p t i b i l i t y t o f a i l u r e of 9-4-4 5 h i g h s t r e n g t h s tee l

w a s obse rved t o i n c r e a s e w i t h b o t h a n od i c and c a t h o d i c p o l a r i z a-

t i o n ( 4 0 7 ) . I t w a s s u g g e s t e d t h a t h yd ro g en e m b r i t t l e m e n t may be

54



i nv o lv e d i n a l l stress c o r r o s i o n c r a c k i n g . The pI-1 a t t h e t i p o f

a c r a c k as a f u n c t io n o f p o t e n t i a l i n s a l t w a t e r w a s measured, and

it a p p e a r e d t h a t c o n d i t i o n s w e r e a lw a ys f a v o r a b l e f o r hyd roge n

r e d u c t i o n ( 4 0 0 ) . E l e c t r o n m ic ro sc op y r e v e a l e d no d i f f e r e n c e i n

f r a c t u r e f e a t u r e s a t d i f f e r e n t p o t e n t i a l s . A model has been pro-

posed which r e q u i r e s H 2 l i b e r a t i o n a t t h e c r a c k t i p ( 5 4 8 ) .

It h a s be en s u g g e s te d t h a t p l a s t i c d ef o rm a t io n a t t h e t i p o f

t h e cr a ck i n c r e a s e s t h e ra te o f d i s s o l u t i o n a t t h e t i p . However,

co ld work h as been found t o h av e no e f f e c t on t h e c o r r o s i o n r a t e

of p u r e i r o n ( 75 6 , 7 5 9 ) . C arb on a d d i t i o n s c a u se d t h e r a t e t o i n-

crease w i t h c o l d w or ki ng , e s p e c i a l l y a t nor m al l y pa s s i ve po t e n-

t i a l s ( 756 , 316 ) . High t e m pe r a t u r e a nne a l i ng r e duc e d t h e r a t e ,

s u gg e s t in g t h a t l a t t i c e i m p e r f e c t i o ns by t he m s e l ve s do no t c a us e

i n c r e a s e d c o r r o s i o n b u t t h a t c o m p o s i ti o n a l i nh o m o ge n e it i es d o.

S t u d i e s of s t r u c t u r a l c h an ge s i n A I S 4 3 4 0 h i g h s t r e n g t h

carbon s t e e l s u g ge s te d t h a t t h e e f f e c t o f hy dro ge n i s t o r ed uc e

t h e a u s t e n i t i c g r a i n b ou nd ary e n e r gy ( 8 0 0 ) . Hydrogen i n h i gh

p u r i t y i r o n c a us e s s u r f a c e b l i s t e r i n g and s o m e i n t e r n a l f i s s u r e s

a t g r a i n b o u nd a ri e s ( 6 8 1 ). The y i e l d stress w a s l owered , even

a f t e r h yd ro ge n r em ov al . T h i s s u g g e s t s t h a t e m b r i t t l e m e n t i s due

t o d i s l o c a t i o n c r e a t i o n and n o t t o d i s l o c a t i o n l o c k i n g .

Measurements of loca l e l e c t r i c a l p o t e n t i a l s showed t h a t

stresses do no t lower t h e p o t e n t i a l a t t h e p o i n t s of stress con-

c e n t r a t i o n i n c o r r o s i on f a t i g u e e xp e ri me n ts ( 6 2 9 ) . When a crack

forms, t he n t h e p o t e n t i a l d e c re a se s . C at ho di c p o l a r i z a t i o n w a s

d ed uc ed t o p r o t e c t n o t by s u p p r e s s i o n o f loca l c u r r e n t s b u t b y

f o r m a t i on o f a n a l k a l i n e e nv ir onme n t c a us i n g pa s s i va t i o n . On t h e

o t h e r hand t e n s i l e stress e . t ( i f t e d t h e c a t h od i c p o l a r i z a t i o n

55





g r a i n b o un d a ri e s w i t h d e p l e t i o n of t h e n e i g h b o r i n g s tee l i n c h r o -

miurn ( 7 6 9 ) . The chromium depleted areas are t h e n a n o d i c wi t h

r e s p e c t t o t h e r em a in d er o f t h e s t ee l and d i s s o l v e r a p i d l y .

e v e r y o n e a g r e e s w i t h t h i s m e ch an is m.

Not

I n t e r g r a n u l a r c o r r o s i o n was

f ou nd t o o c c u r o n l y when t h e c a r b i d e s f or me d a c o n t i n u o u s p a t h a n d

n o t when t he y formed as i s o l a t e d c r y s t a l s ( 5 3 4 ) . I t a l s o o c cu r re d

when h i g h t e m p e r at u r e t r e a t m e n t r emoved p r e c i p i t a t e s b u t l e f t h i g h

s o l u t e c o n c e n t r a t i o n s a l o n g t h e g r a i n b o u nd a ri e s. Othe r worke r s

o b se rv e d i n t e r g r a n u l a r c o r r o s i o n o n l y when p r e c i p i t a t e s were

p r e s e n t ( 5 3 5 ) .

1 0 . I n f l u e n c e o f T e m p e ra t ur e , V e l o c i t y , a n d T i me

T em pe ra tu re i n f l u e n c e s e l e c t r o c h e m i c a l r e a c t i o n s i n s e v e r a l

w ay s, a l t h o u g h t h e n e t e f f e c t i s g e n e r a l l y f o r c o r r o s i o n t o a c c e l-

e r a t e as t e m p e r a t u r e i n c r e a s e s . Thus, i n H 2 S , l o w a l l o y s tee l s

c o r r o d e a c c o r d i n g t o an A r r h e n i u s r e l a t i o n s h i p ; i . e . , t h e c o r r o-

s i o n r a t e w a s p r o p o r t i o n a l t o exp ( - 1 0 , 7 2 O / T ) where T i s i n d e gr e es

Rankine. A maximum r a t e i s o f t e n o b s e r v e d i n n e u t r a l e l e c t r o l y t e

s o l u t i o n s a t about 70°C ( 7 7 6 ) . The t en d en c y o f a u s t e n i t i c s t a i n -

less t o stress c r a c k i n c r e a s e s w i t h t em p e r a t ur e ( 6 9 1 ) .

The e q u i l i b r i u m p o t e n t i a l f o r a n e l e c t r c c i , z ; n i c a l r e a c t i o n

c h a ng e s w i t h t e m p e r a t u r e , d e p e n d e nt o n t h e e n t r o p y of t h e r e a c t i o n .

I n E q u a t i o n 5 , E i s a f u n c t i o n of t em per a tu re . The dependence of

E on c o n c e n t r a ti o n i n c r e a s e s a s te m p er a tu r e i n c r e a s e s by v i r t u e o f

t h e l a s t t e r m i n Equa t ion 5 .

0

A no th er e f f e c t o f t e m p e r at u r e i s t o

acceler. ; . te a l l c n e m i c a l r e a c t i o n s .

d i f f u s i o n r a t e s , b o t h i n t h e s o l u t i o n n e x t t o t h e metal and

th rough any f i l m c o v e r i n g t h e sur face .

A t h i r d e f f e c t i s t o i n c r e as e

The s o l u b i l i t y o f o xy ge n

5 7



d e c r e a s e s a s t e m pe r at u re i n c r e a s e s , w hich t e n d s t o r e d u c e t h e

c o r r o s i o n r a t e .

Stress c o r r o s i o n c r a c k i n g h a s b een r e p o r t e d t o be a problem

i n p i p e l e a d i n g f ro m c om p re ss or s t a t i o n s , p re su m ab ly b e c a u s e o f

t h e e l e v a t e d g a s t e m p e r a t u r e s g e n e r a t e d b y co m p r es s io n .

Underground cor ros ion of s tee l h a s be en f ou nd t o b e n e g l i g i b l e a

i n p e r m a f r o s t s o i l s ( 5 5 0 ) . I n s o i l s s u b j e c te d t o h i g h te mp er a-

t u r e s , c o r r o s i o n i s i n cr e as e d u n l e s s r a i n f a l l i s so s p ar s e t h a t

t h e wa te r c o n te n t o f t h e s o i l i s l ow be c a us e o f e va po r a t i on . A

p i p e l i n e c a r r y i n g h o t s p r i n g w a t e r e x h i b i t e d u n u s u a l l y r a p i d

c o r r o s i o n ( 1 4 9 ) .

I n c r e a s e s i n s o l u t i o n v e l o c i t y n e a r t h e m e ta l l o we r s t h e

r e s i s t a n c e f o r m a s s t r a n s f e r , b ot h f o r t r a n s p o r t o f r e a c t a n t s

t o t h e m e t a l s u r f a c e a nd f o r re mo va l o f c o r r o s i o n p r o d u c t s 1).

Whether t h i s i n c r e a s e s o r d e c r e as e s t h e c o r r o s i o n r a t e d ep en ds

on what i s c o n t r o l l i n g t h e r a t e . I f t h e s tee l i s n e a r t h e p a s s i v e

s t a t e , t h e n i n c r e a s e d v e l o c i t y c o u l d i n c r e a s e oxygen c o n c e n t r a t i o n

a t t h e s u r f a c e , c a u se p a s s i v a t i o n , an d r ed u ce c o r r o s i o n . Flow o f t h ,

s o l u t i o n w i l l a l s o t e n d t o remove t h e c o n c e n t ra t e d s o l u t i o n s n e c e s s a

f o r p i t t i n g and c r e v i c e c o r r os i o n . I n t h e l a b o r a t o r y , f lo wi ng se a-

water w a s found t o be more c o r r o s i v e t h an q u i e s c e n t s e aw a t er f o r

3 0 4 s t a i n l e s s ( 5 8 4 ) . On t he o t he r ha nd , s t a i n l e s s c o r r o de s more

s l ow l y n e a r t h e s u s f z c e o f t h e o ce an t h a n i n d ee p q u i e s c e n t s e a -

w a t e r , which i s much l ow er i n oxyge n c on t e n t . S t i r r i n g r e duc e d

d e p o s i t i o n o f f e r r o u s h y dr ox id e f i l m i n 0 . 1 N borax ( 3 2 7 ) . I f

p a r t i c l e s are p r e s e n t i n t h e f l u i d , p a s s i v e f i l m s can be damaged

and c o r r o s i o n g r e a t l y a c c e l e r a t e d as v e l o c i t y i n c r e a s e s . T h i s

5 8



i s known as " e r o s i o n c o r r o s i o n . " W i t h h i g h p u r i t y water f l o w i n g

i n m il d s t ee l t u b e s , c o r r o s i o n i n c r e a s e d w i t h v e l o c i t y up t o a

c e r t a i n v a l u e a nd t h en d e c r e as e d ( 3 0 9 ) . T h i s i n d ic a t e s d i f f e r e n t

r a t e c o n t r o l l in g s t e p s i n t h e t w o regimes .

U nif orm c o r r o s i o n t e n d s t o d e c r e a s e w i th t i m e b e ca u se of t h e

p r e s e n c e of c o r r o s i o n p r o d u c t s b l o c k i n g a c c e s s of c o r r o d e n t s .

On t h e o t h e r h an d, p i t t i n g and stress c r a c k i n g i n c r e a s e w it h

t i m e . Chlor ide- i n d u c ed b re ak do wn o f p a s s i v e f i l m s r e q u i r e s an

i n d u c t i o n t i m e , w hich d e c r e a s e s a s t e m p e ra t u r e i n c r e a s e s ( 7 7 3 ) .

From a n e m p i r i c a l s t a n d p o i n t t h e c o r ro s i on f a i l u r e r a t e of unpro-

t e c t e d p i p e l i n e s t en ds t o i n c r e a s e i n a n ex p on e nt i al manner w i t h

t i m e .

11. E f f e c t of S o l u t e s

The r o l e of v a r i o u s i o n s i n c o r r o s i o n of i r o n a n d s t ee l was

r e c e n t l y r ev ie we d i n R e fe r en c e 777. T h e s i t u a t i o n i s complex

because so many f a c t o r s a r e i n v o l v e d i n d e t e rm i n i n g t h e c o r r o s i o n

r a t e . T h e r e are a l s o a n i n f i n i t e v a r i e t y o f c om b in at io ns of

d i f f e r e n t s o l u t e s and c o n d it i o n s. N e v e rt h e le s s , it i s p o s s i b l e

t o s t a t e a few g e n e r a l p r i n c i p l e s and s um ma ri ze t h e o b s e rv a t i o n s .

One can see f rom the ca thoc l ic r e a c t i o n s , E q u a t i o n s 2- 3, t h a t

c o r ro s i o n s h o u l d b e a c c e l e r a t e d by l ow pH and by higher oxygen

c o n c e n t r a t i o n s a t t h e s u r f a c e . A lt ho ug h i n c r e a s i n g H c o n c e n t r a-+

t i o n i n c r e a s e s c o rr o s io n , t h e a n od i c p o l a r i z a t i o n c u r v e of F i g u r e

2 i s a c t u a l l y s h i f t e d t o t h e l e f t , w i t h c u r r e n t d e n s i t y c hange2 +H €1

p r o p o r t i c n a l t o l / a +, where a + i s t h e a c t i v i t y o f H ( 3 8 9 ) . I t

h a s be en s u g g e s t e d b o t h t h a t OH- c a t a l y z e s t h e a n od ic r e a c t i o n

(Equa t ion 1) a n d t h a t H i n h i b i t s t n e a no di c r e a c t l o n . The+

5 9





- 2+Fe3+ + e = Fe (Eq. 8 )

T h i s r e a c t i o n acts a s an a d d i t i o n a l c a t h o d i c r e a c t i o n . Fe 2+ i s

3+r e o x i d i z e d t o Fe by oxygen. Ca t i on s of nob le m e t a l s s u c h a s

Cu are d i s pl a c e me n t p l a t e d o n t o s tee l and acce le ra te c o r r o s i o n b y

g a l v a n i c a c t i o n . The p r e s e n c e o f o t h e r m e t a l s i n t h e g e n e r a l

n e ig h b orh o od o f t h e p i p e c an i n f l u e n c e c o r ro s i o n by c h an g i n g t h e

c o m p o s it i o n o f t h e s o i l (786).

R e s u l t s are d i f f i c u l t t o p r e d i c t f o r mixed s o l u t e s . N e u t r a l

+s a l t s w e r e found t o i n c r e a s e H a c t i v i t y and so t o r ed uc e t h e

a n o d i c r e a c t i o n r a te (389). I n s u l f u r i c a c i d , NaI , N a B r , and

N a C l i n h i b i t e d , w h i l e NaF a c c e l e r a t e d c o r r o s i o n . Ten p e r c e n t H C 1 ,

N a C 1 , and L i C l a c c e l e r a t e d c o r r o s i o n , w h il e F e C 1 2 i n h i b i t e d .

C h l o r i d e i o n s a l t e r e d t h e s t r u c t u r e of f i l m s form ed i n NO;. Break -

down of p rev ious ly fo rmed pas s iv i ty occur red on ly above a " b reak -

th rough" p o t e n t i a l i n b o ra x- c h l o ri d e s o l u t i o n s , w hic h became more

n e g at i v e wi t h i n c r e a s e s i n C 1 - and more p o s i t i v e w i t h i n c r e a s e s i n

borax c o n c e n t r a t i o n (329). B e l o w t h e " p i t t i n g " p o t e n t i a l , C1 -

produced no changes i n p a s s i v e f i l m s ( 4 0 1 ) .

i n h i b i t e d c o r r o s i o n , t h e amount r e q u i r e d i n c r e a s e d as C 1 - and SO;

c o n c e n t r a t i o n s i n c r e a s e d (775, 776). To p roduce a g i v en c o r r o s i o n

r a t e , p a s s i v a t o r c o n c e n t r a t i o n C w a s found t o be r e l a t e d t o ag-P

g r e s s o r c o n c e n t r a t i o n C by

P

While NO; and C r O i

a

l o g c = a + b l o g Ca ( E q . 9 )

T h i s i n d i c a t e s a c o m p e t i t i v e a d s o rp t i o n m ec han ism . C o r ro s i o n r a t eremained l o w as pH w a s d e c r e a s e d t o 2.8 i f H C r O I was employed t o

l o w e r pH, w h i l e it inc reased be low pH 5 i f H2S04 was used t o re-

duce pY (776) On t h e o t h e r h a n d, h a l i d e s e nh an ce d o r g a n i c

61



a m i n e s ' i n h i b i t o r e f f i c i e n c y , most no t a bl y w it h I- (343). Both

H2S and C1- a c c e l e r a t e d c o r r o s i o n a t 15OoC, b u t t h e i r e f f e c t s w e r e

n o t a d d i t i v e (706).

S o l u t e s ca n a l s o p l a y a s i g n i f i c a n t r o l e i n h ydrogen a bs or p-

t i o n phenomena such as e m b r i t t l e m e n t , stress c ra c k i n g , a n d b l i s -

t e r i n g . For example, o x id ize d a r se n i c compounds inc r ea se d Lydro-

g e n a b s o r p t i o n (399). T h i s w a s t h o u g h t t o arise f rom h indrance o f

t h e c o m b i na t i o n o f l i b e r a t e d hy drog en t o form molecular hydrogen,

which i s c o m p e t i t i v e w i t h a b s o r p t i o n .

12. Underaround Corrosion

Many s t u d i e s have been made on c o r r o s i o n o f bare u n p r o t e c t e d

f e r r o u s m e ta l s b u r ie d i n v a r i o u s t y p e s o f s o i l f o r p e r i o d s o f u p

t o 45 y e a r s (282, 429, 525, 539, 550, 551, 552, 639, 678, 687,

778, 788-790). F i e l d t e s t s w e r e shown t o be neces sa ry f rom t h e

o b s e r v a t i o n t h a t p i t t i n g w a s many t i m e s g r e a t e r i n t h e f i e l d t ha n

f o r t h e same s o i l i n t h e l ab o r a t o r y . V ar io us s o i l p a ra m et e rs ,

such as pH, e l e c t r o l y t e c o n c e n tr a t i o n , m o is tu r e c o n t en t , resis-

t i v i t y , e tc . , w e r e de te rmined and a t t empts made t o correlate

c o r r o s i o n r a t e wi th such pa ra met e rs . Mois tu re , which i s g e n e r a l l y

m o r e a b un d an t i n p o o r l y aerated s o i l s , p ro vi de s t h e e l e c t r o l y t e

f o r t h e c o r r o s i o n p r o ce s s . The e l e c t r o l y t e i s composed of hydro-

gen and hydroxy l ions f r o m t h e w a t e r i t s e l f , and a v a r i e t y of

c a t i o n s a nd a n i o n s t h a t d ep en d upon t h e s o l u b l e s a l t s d i s s o l v e d

i n t h e s o i l . T he se i o n s d et e r m i ne t h e e lec t r ica l c o n d u c t i v i t y ,

o r r e s i s t i v i t y , o f t h e s o i l and t h e c hem ica l p r o p e r t i e s such a s

a c i d i t y and a l k a l i n i t y .

62



I

I

The ki nd and concent r at i on of sol ubl e sal t s i n soi l s seem t o

af f ect t he i ni t i al r at e of cor r osi on and t he pr ogr ess of cor r osi on

wi t h t i me. I n gener al , t he most cor r osi ve soi l s cont ai n l arge

concent r at i ons of sol ubl e sal t s, especi al l y i n t he f or mof sul -

f ates, chl or i des and bi carbonates, and may have very aci d or

hi ghl y al kal i ne p H val ues.

conduct i vi t i es, or l ow resi st i vi t i es. The l east corr osi ve soi l s,

as a gr oup, have hi gh resi st i vi t y val ues, whi ch i ndi cat e l ow con-

cent r at i ons of sol ubl e sal t s. Si nce r esi st i vi t y measur ement s

pr ovi de as much i nf or mat i on as any ot her si ngl e soi l proper t y

r egar di ng t he cor r osi ve char acter i st i cs of a soi l , and si nce i t

can be r eadi l y measured, i t i s of t en used to pr ovi de approxi ma-

t i ons of t he cor rosi vi t y of soi l s. Resi st i vi t y measur ement s have

st i l l gr eat er val ue when suppl ement ed by observat i ons of soi l

drai nage and/ or measur ement s of pH. I n gener al , so i l s wi t h re-

si st i vi t i es gr eat er t han 5, 000 ohm- cmar e f ound t o be mi l dl y

cor r osi ve t o noncor r osi ve. I t shoul d be r eal i zed, however , t hat

because many other f act or s may af f ect t he corr osi on r at e of metal ,

cer t ai n hi gh r esi st i vi t y soi l s nay be mor e cor r osi ve t han soi l s of

l ower resi st i vi t y ( 5 5 2 ) . I nst ances have occur r ed wher e change i n

soi l r esi st i vi t y, r at her t han t he absol ut e val ue, has been f ound

t o be t he maj or cause of cor r osi on.

Such soi l s al so have rel at i vel y hi gh

I t has been f ound t nat t he r esi st i vi t i es of most soi l s exhi bi t

a l ogar i t hmi c nor mal di st r i but i on, i . e. , t hat t he cumul at i ve di s-

t r i but i on of t he l ogar i t hms of many resi st i vi t y measur ement s al ong

a pi pel i ne r i ght - of - way f al l s 07 a st r ai ght l i ne on probabi l i t y

paper. Such a soi l i s def i ned 2 s "stat i sti cal l y uni f orm. "

63



I n one s tu d y , p i t s were found o n ly a t p o i n t s where r e s i s t i v i t y

w a s below a b o u t 1 2 0 0 ohm-cm, w i t h i n c r e a s i ng f r e q ue nc y be low

t n a t ( 7 6 3 ) .

Ra tes appeared t o i n c r e a s e as pH i n c r e a s e d , a l t h o u g h pH a l s o

t e nde d t o i n c r e a s e w i t h d e c r e as i n g r e s i s t i v i t y . A no th er s t ud y

r e v e a le d n o c o r r e l a t i o n w i t h pH, water h o l d i n g c a p a c i t y , or s a l t

c o n c e n t r a t i o n s f o r v a r i o u s i r o n s a nd s t ee l s ( 6 3 9 ) . Tests o f n i c k -

e l cas t i r o n s a mp le s r e v e a l e d no c o r r e l a t i o n w i t h m o i s tu r e , re-

s i s t i v i t y , pH, o r C1- c o n t e n t ( 5 2 5 ) .

Oxygen a v a i l a b i l i t y h a s a l a r g e i n f lu e n c e on c o r r o s io n r a t e .

I t h a s b een f ou nd t h a t un d i s t ur b e d s o i l s are so d e f i c i e n t i n oxy-

gen a f ew f e e t be low t he g r ound l i n e o r be low t he w a t e r t a b l e

z o n e , t h a t steel p i l i n g s were n o t a p p r e c i a b l y a f f e c t e d by c o r r o-

s i o n , r e g a r d l e s s o f t h e soil t y p e s o r p r o p e r t i e s ( 5 3 9 ) . U nf or tu-

n a t e l y , p i p e l i n e s a r e a lm os t i n v a r i a b l y found i n t r e n c h e s , where

t h e s o i l h as be en d i s t u r b e d .

Temperature can have a s i g n i f i c a n t e f f e c t . Thus, s tee l p i l e s

i n Alaska showed no co r r os io n when bur i ed i n pe rm af r os t (below

5 f ee t ) f o r 6 t o 11 y e a r s ( 5 5 0 ) . A few p i t s were found i n t h e

a c t i v e t ha w r e g i o n ( a b o u t 5 f e e t ) an d r a n ge d up t o 0 . 7 mm i n

d e p t h . ( I t s hou l d no t be c o n c l u 6 d from t h i s t h a t o i l p i p e l i n e s

i n p e r ma f ro s t iy.ii-1 n o t c o r r o d e p s i n c e t h e y w i l l be w a r m e d t o p r e - L

v e n t s o l i d i f i c a t i o n of t h e o i l ) . C onve r s e ly , s e r i ou s p r ob le m s

-2ere e n co u nt er ed w it h h e a t d i s t r i b u t i o n l i n e s b elow t h e w a t e r

- ,able b u t n o t w i t h t h o s e above t h e w a t e r t a b l e ( 4 7 9 ) .

Iri a n e x t e n s i v e NBS program, it w a s found t h a t t h e i n i t i a l

p i t t i n g r a t e d e c r e a s e d i n w e l l d r a i n e d , h j g h r e s i s t i v i t y s o i l s ,

6 4



b u t i s m a in ta in e d i n p o o rl y d r a i n e d , low r e s i s t i v i t y s o i l s ( 5 5 2 ) .

T h e p i t t i n g r a t e R changed wi th t i m e 0 a c c o r d i n g t oP

l og R = log k + n log 0 (Eq. 10)P

where n = 0 . 1 9 f o r w e l l a e r a t e d a n d n = 0 . 6 8 f o r p o or ly a e r a t e d

s o i l s .

The f o l l ow i n g e qu a t i o n w a s d e r i v e d f o r maximum p i t d e p t h i n

m i l s ( 7 5 4 ) :

p = K K 10 - ~ H Pe / p I n (Eq. 11)n a

where p H i s t he s o i l pH, p i s t h e s o i l r e s i s t i v i t y i n ohm-cm, 0

i s t i m e i n y e a r s , a n d A i s t he e xpos e d area i n s q ua re f e e t . I .

and "a1' are d e p e nd e n t o n t h e a l l o y u s e d , a s shown i n Ta b l e 1 9 .

T h e o r e t i c a l v a lu e s f o r n w e r e d e r i v e d by c o n s i d e r i n g t h e p r o b a b l e

a

r a t e l i m i t i n g s t e p . They co mpare w e l l w i t h t h e NBS e m p i r i c a l

v a l u e s i n E q u at io n 1 0 . Thus , i n w e l l a e r a t e d s o i l s n = 1 / 6

f o r Taco3 f i l m f o r m a t i o n o n c a t h o d i c areas b e i n g r a t e l i m i t i n g .

Fo r f a i r l y a e r a te d s o i l s n = 1 / 3 f o r f o rm a ti on o f f e r r i c c o r ro-

s i o n p r o du c ts o v e r t h e p i t . F or p o or ly a e r a t e d s o i l s n = 1 / 2 ,

which i s a n a r e a e f f e c t w i th co n s t a n t c o r r o s i o n c u r r e n t . I n

v er y p oo r ly a e r a t e d s o i l s t h e c o r r o s i o n p r od u c ts a r e s o l u b l e

and lower t h e r e s i s t i v i t y of t h e s o i l e l e c t r o l y t e , l e a di ng t o

n = 2/3. Compar ison wi th NBS f i e l d e x p er i en c e g av e v a l u e s o f

K n= 1 7 0 , 2 2 2 , and 355 f o r good, f a i r , and po or s o i l a er a t io n , .

A S o v i e t s t ud y i n d i c a t e d t h a t t h e -:umber and d e p th o f p i t t i n g

of g a s p i p e l i n e s i n c r e a s e d w i th :-.,zcreas'.?g f r equency of changes

i n s o i l p r o p e r t i e s w i th d i s t a n c e 7 3 C ; . Changes i n e l e v a t i o n ,

s o i l t y p e , and r e s i y t i v i t y - w e r e c o n si d er e d . G r e a t e s t p i t d e p th s

were caused by a?. :ernat ing from c l a y s t o s a nd y s o i _ ' s .

65



13 . G a s a nd P e tr o l e um C or r o s i on E f f e c t s

S e ve n t y- t h r ee o f t h e o i l a nd ga s op e r a t i n g c ompan ie s r e s pond -

i n g t o t h e q u e s t i o nn a i re t r a n s p o r t materials w hi ch , i n t h e p r e-

s e nc e of f r e e m o i s t u r e , may c a u s e i n t e r n a l c o r r o s i o n . T ab le 9

l i s t s t h e mater ia ls t r a n s po r t e d by t h e s e c om pan ie s .

C o r r o s i o n i n o i l a nd ga s w e l l equipment w a s r e v i e w e d i n

Refe rence 553. Cor ros io n w a s p r i m a r i l y i n t e r n a l . Sour o i l and

g a s c o n t a i n H2S, w hi l e sweet o i l c o n t a i n s o n l y C 0 2 and f a t t y a c i d s .

S w e e t c o r r o s i on i s a s s o c i a t e d w it h h i g h saltwater p r o d u c t i o n ,

TABLE 9

MATERIALS TRANSPORTED WHICH CAUSE INTERNAL CORROSION

I N THE PRESENCE OF FREE MOISTURE

Mater ia1 Number o f Companies

7 7Sour Crude ( H 2 S ) -~ _.

Sweet Crude ( C 0 2 ) 1 4

Crude o i l 7

ivater 6

S a l t w a t e r 5

S u l phu r

LPG

R e f i ne d P e t r o l e um P r oduc t s

C a s i ng he a d , g a s , & f r a c k i n g s o l u t i o n

C a u s t i c s

c12

s04

G d s o l i n e s

6 6



i . e . , 4 0 t o 50 p e r c e n t .

c a u se d c o r r o s i o n , w h i l e p r e s s u r e s b elo w 7 p s i w e r e n o n c o r r o s i v e .

A C 0 2 p r e s s u r e o f o v e r 30 p s i u s u a l l y

The g e n e r a l c o r r o s i o n r a t e o f l o w a l l o y steels i n aqueous

H S s o l u t i o n s i n c r e a s e s as t h e H 2 S c o n c e n t r a t i o n i n c r e a s e s .

Hydrogen s u l f i d e i n s o u r w e l l s c a us e d c o r r o s i o n by p r oduc ing

c a t h o d i c i r o n s u l f i d e and by hy dro gen c r a c k s a nd b l i s t e r s ( 5 5 3 ) .

2

I t h a s bee n n ot e d p r e v i o u s l y t h a t H 2 S c a n i nduc e s t ress c o r r o s i o n

c r a c k i n g . On t h e o t h e r h an d, i r o n s u l f i d e f i l m s p ro du ce d on i r on

by H2S-C02-H20 c o r r o s i o n were p r o t e c t i v e ( 3 7 4 ) . S t u d i e s o f pH

e f f e c t showed tha t f i l m was l e a s t p r o t e c t i v e i n pH r a n g e 6 .5 t o

8 . 8 and e s s e n t i a l l y c o n ta i n ed k a n s i t e ( F e S ) . The n i o s t p r o t e c -

t i v e f i l m a l s o c o n ta in e d p y r i t e (FeS ) and t r o i l i t e ( F e S ) .3 4

2

The c o r r o s i v i t y o f p e tr o le u m i n c r e a s e s as i t s water c o n t e n t

i n c r e as e s . I n f a c t , t h e r e i s v e r y l i t t l e o r n o c o r r o s i o n u n l e s s

m o i s t u r e i s p r e s e n t ( 7 0 6 ) . A 1 9 5 3 surv ey showed re qu i r ed main-

t en anc e of 348 sweet w e l l s i n C a l i f o r n i a in c r ea s ed i r r e g u l a r l y as

t h e p e r c e n t a g e of produced water i n c r e a s e d ( 6 1 8 3 ) . Water i n l o w

s p o t s i n p i p e s i s e s p e c i a l l y d e l e t e r i o u s p a r t i c u l a r l y w i t h so ur

o i l a nd g a s . Such water i s e l i m i n a t e d or r e duc e d by p r ope r de s i gn

and by p e r i o d i c a l l y p i g g i n g t h e l i n e . The d e t e r m i n a t i o n of s a f e

l i m i t s f o r H2S, C 0 2 , and H 0 c o n t e n t s i n r e l a t i o n t o p ip e g r ad e ,2

o p e r a t i n g p r e s s u r e , stress l e v e l , e t c . , are needed (4126).

1 4 . C or r os i on i n N a t u r a l Waters--

O f f s ho r e o i l p r o d u c t i o n h a s i n c r e a s e d d r a m a t i c a l l y i n r e c e n t

y e a r s . Accompanyi??g t h i s h a s been a n i nc r ea s ed p u b l i c s e n s i t i v i t y

t o m ari ne o i l l e ak a g es . P i p e l i n e s a l s o cross r i v e r s , h a r b o r s ,

b a y s , e t c . Some of t h e s e e n v ir o n m en t s are v e r y c o r r o s i v e .

6 7







o f a e ro b i c- a n a e ro b i c z on es are fo un d i n s e d i m en t s c o n t a i n i n g,

d i f f e r e n t am ounts o f o r g a n i c mater ia l s . T h i s i s v e r y c o r r o s i v e .

S u l f . i r i c a c i d i s g e n e ra t e d from s u l f u r and s u l f i d e s ( 7 8 0 ) . pH

l eve l s of 5 and 6 have been produced by gen er a t io n of humic a c i ds

i n t h e s o i l . C 0 2 i s a u n i v e r s a l m e t a b o l i c p r o d u c t o f a l l s a p ro-

p h y t i c microbes. Ammonia can be p roduced by b a c t e r i a l h y d r o l y s i s

o f u r e a .

The m o s t s t u d i e d m i c r o-organisms from a c o r r o s i o n s t a n d p o i n t

are t h e s u l f a t e r e d u ci n g b a c t e r i a ( 78 0, 53 8, 549, 5 24 , 3 7 5 ) .-

T he se a n a e r o b i c b a c t e r i a r e d uc e SO, t o H 2 S , which i s i n i t s e l f

c o r r o s i v e . I r o n s u l f i d e- i r o n ce l l s are a l s o formed. However,

t h e main i n f l u e n c e seems t o be d i r e c t d e po l ar i za t io n o r accelera-

t i o n o f t h e c a t h od i c r e a c t i o n .

The sche mat ic of F i g u r e 4 o b v i o u sl y d o e s n o t a p p l y t o c o r r o-

s i o n which r e s u l t s f rom b a c t e r i a l r e d u c t i o n of s u l f a t e i on s .

L i t t l e h a s b ee n p u b l i s h e d on t h e c o m p o s it i o n o f t h e c o n s e q u en t

c o r r o s i o n p ro d u c t . However, s i n c e t h e r e d u c t i o n o c c u r s u nd e r

a n a e r o b i c c o n d i t i o n s , f e r r o u s s u l f i d e would b e e x p e ct e d as t h e

p r e v a l e n t form. But b o t h it and t h e l i g h t c o l or ed f e r r i c s u l-

f i d e , b o t h c om pa ra bl e i n f o r m ul a t o t h e c o rr e sp o n di n g o x i d e s , are

h i g h l y i n s o l u b l e . The m an tl e o f t e n t a k e s t h e f or m o f a s l a b

d e f i n i n g , no d ou bt , t h e e x t e n t o f t h e b a c t e r i a l p r o l i f e r a t i o n .

T h e s l a b i s used by some as an i t e m Df i d e n t i f i c a t i o n of hydrogen

s u l f i d e c o r r o s i o n .

S u l f a t e r e d u c e r s ( d e s u l f o v i b r i o ) pr od uc ed F e 2 P and a v o l a t i l e

phosphorous compound when growing i n c o n t a c t w i t h s tee l and phos-

p h a t e s (5 38 , 5 4 9 ) . S o i l b a c t e r i a i.: s..Jntact- w i t h b a r e s t e e l

7 0



g e ne r at e d p o t e n t i a l d i f f e r e n c e s i n t h e l a b ( 5 0 2 5 , 5 0 2 3 ) . L i g h t

i n c r e a s e d t h e c o r r o s i o n r a t e . Methane- genera t ing bacter ia a l s o

i n c r e a s e t h e c o r r o s i o n r a t e ( 4 ) .

T h e impor tance and p reva lence of t h i s t yp e of c o r r o s i o n h a v e

One ex tens ive random inspec t ion of c o a t-l o ng b een i n q u e s t i o n .ings on a p i p e l i n e from t h e s o u t h w e s t t o t h e eas te rn seaboa rd

f a i l e d t o d i s co v e r s u l f i d e s i n t h e s o i l . Table 3 shows tha t 2 4

p e r c e n t of t h e c om p an ie s s u rv e ye d c o n s i d e r m i c ro b i o l o g i c a l c o r r o-

s i on t o be a s i g n i f i c a n t c a us e o f l e ak s . Table 11 e x e m p l i f i e s t h e

u n c e r t a i n ty i n e x t e n t of b a c t e r i a i c o r r o s i o n .

The u n c e r t a i n t y c a n a l s o be shown by q u o t i n g from r e s p o n d e n t s

t o t h e q u e s t i o n n a i r e :

1) "One of o u r c o n s u l t a n t s (c la ims) t h i s a s a major c a u s e

of c o r r o s i o n . O t h e r c o n s u l t a n t s h av e s i n c e e x p r e ss e d d o u b ts

a s t o t h e v a l i d i t y o f h i s a s s u m p t i o n . " ( Q 6 2 5 )

( 2 ) " We f i n d . . . t h a t a n ae r ob i c s u l f a t e r ed u ci n g b a c t e r i a

are t h e major cause of c o r r o s i o n ." (Q357)

( 3 ) " We b e l i e v e t h a t m a j o r i t y o f a l l c o r r o s i o n leaks on s tee l

p i p e s ( i s ) c a us e d by s u l f a t e r e d u c i n g bac ter ia ." ( 4 5 0 0 )

TABLE 11

COMPANIES EXPERIENCING BACTERIAL CORROSION

D o N o tY e s No Know R a re l y

( A ) Anaerobic 37 120 158 33

1 73 16( B ) Aerobic 9 1 3 4



( 4 ) " We d o n o t c o n s i d er t h a t it e v e r h a s b e en d e m o n s t r a t e d

t h a t b a c t e r i a c a us e c o r r o s i o n on p i p e l i n e s . " ( Q 5 8 5 )

Aany of t h o s e who a t t r i b u t e c o r r o s i o n t o b a c t e r i a l a c t i o n base

t h e i r c o nc l us i on on t h e p r es e nc e o f s u l f i d e i o n i n t h e s o i l , n o t

on a d e t a i l e d ex am i na ti on of t h e c o r r o s i o n p r o d u c t m a n tl e . T h i s

s i m p le an d p r o f i t a b l e way t o d e t er m i n e t h e loca l c a u s e o f c o r r o -

s i o n h a s b ee n n e g l e c t e d .

1 6 . Atmospher ic Cor ros ion

P o r t i o n s o f e v e r y p i p i n g s y s te m are above ground a t pumping

s t a t i o n s , end p o i n t s , e t c . A l t h o u g h c o r ro s i o n i s n o t as g r e a t a

problem aboveground a s underground, i t i s p r e s e n t n o n e t h e l e s s .

A t low p r e s s u r e s o f d r y ox yg en , a n i r o n o xi d e f i l m f i r s t

forms a t a l i n e a r r a te and then approaches a l i m i t i n g t h i c k n e s s

of 26 Angstroms. Tra ces of wat er vapor lower t h e l i m i t i n g t h i c k -

n e s s t o 18 Angstroms ( 1 0 2 ) . Under real a t m o s p h e r i c c o n d i t i o n s

c o r r o s i o n i s s imi la r t o t h a t f ou nd u n de rg rou n d. Water i s , o f

c o u r s e , n e c e ss a r y f o r t h e s e c o r r o s i o n p r o c e s s e s . R a i n f a l l s u p p l i e s

o c c a s i o n a l m o i s t u r e . A t a t r o p i c a l m a r i n e s i t e c o r r o s i o n w a s more

r a p i d i n p e r i o d s of h i g h r a i n f a l l an d h i g h h u m id i ty ( 2 3 4 ) . Once

c o r r o s i o n p r o d u c t s ha ve f o rm ed , t h e y c a n s t o r e w a t e r by s h i e l d i n g

it f ro m e v a p o ra t i o n , by forming s o l u t i o n s w i t h lower v a p o r p r e s-

s u r e s , e t c . I t w a s f ou nd t h a t c o r r o s i o n was n e g l i g i b l e when t h e

r e l a t i v e h u m i d i t y w a s h e l d below 4 0 p e r c e n t w i t h t h e s tee l b e in g

c ov er ed by s y n t h e t i c r u s t s c o n t a i n i n g s u l p h a t e s and c h l o r i d e s

( 7 8 1 ) . Hygroscopic s a l t s and ac ids f rom the a tmosphere can s e t t l e

o n t h e s tee l s u r f a c e a n d k e e p it mo . d , s t . I t w a s s u g g e s t e d t h a t

f e r r o u s s u l f a t e h y d r at e cap. s u p p l y water by d e h y ? . r a t i o n ( 7 8 2 ) .

7 2



Many f i e l d t es t s of a tmo sphe r i c co r r os io n have been made

( e . g . , Refs . 189 , 4 3 , 1 6 9 , 2 3 4 ) . C o r r o s i v i t y var i e s w i t h t h e

m e t a l b e i n g t e s t e d ( 1 8 9 ) . Sea s a l t and SO2 a c c e l e r a t e d c o r ro -

s i o n ( 4 3 ) . C o r r o s i o n o f a v a r i e t y of steels w a s f ou n d t o b e i n -

f lu en ce d t o t h e grea te s t e x t e n t b y SO2 c o n t e n t ( 1 6 9 ) . Specimens

e xp o se d i n t h e w i n t e r h av e a much higher S c o n t e n t a t t h e metal

s u r f a c e t h an t h o s e e x po se d i n t h e summer ( 1 7 3 ) . S 0 2 - f o r m s hygro-

s c o p i c an d c o r r o s i v e H2S04 i n t h e a tm o sp he re a nd a l s o on t h e i r o n

s u r f a c e , w hich may, i n f a c t , accelera te t h e r e a c t io n .

The i n f l u e n c e o f a i r b o r n e s a l t w a s i l l u s t r a t e d by t h e o b s e r -

v a t io n t h a t t h e c o r r os i o n r a t e o f i r o n w a s a n o r d e r o f m a gn i tude

g r e a t e r a t 80 f e e t ' f r o m t h e o ce an t h a n a t 800 f e e t away ( 7 84 ) .

Ozolle, a component o f smog, caus ed p i t t i n g of s t e e l i r d e i o n i z e d

water b u t reduced p i t t i n g i n sea w a t e r ( 7 8 5 ) . Ozone a l so accel-

e r a t e d c o r r o s i o n of s tee l i n d i l u t e H 2 S 0 4 .

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