Alloy performance in corrosive environments Alloys for use

© 2017 Swagelok Company 2

• Alloy performance in corrosive environments

• Alloys for use in specific corrosive media

• Selection of special alloy fluid system components

Presented by Dr. Robert Bianco, Swagelok Senior Materials Scientist


Introduction to Materials Science

Ceramics Polymers Composites

Semiconductors Carbon Metals


Metals: elements and alloys

• Iron → Fe + C = carbon steel

• Chromium + Nickel → Fe + Cr + Ni = stainless steel

• Nickel → Ni + Cu = Monel

• Copper → Cu + Zn = brass

Alloy: metal that contains two or more elements


• Phase: volume of material with uniform chemical

composition and physical properties

– Major phases: ferrite, austenite (stainless steel)

– Minor phases: carbides, oxides, sulfides, intermetallics

Phases in metals

single major phase:

ferritic steels (ferrite)

austenitic steels (austenite)

two major phases:

duplex stainless steels

(ferrite and austenite)


Alloy Performance in Corrosive Environments


Marvao, Portugal


Forms of wet corrosion


General (or uniform) corrosion

• Occurs uniformly

• Literature states recession rates (mm/year or mils/year)

• Sometimes literature shows weight loss (mg/cm2/day)


Stainless steel

• Addition of least 10% Cr to Fe → stainless steel

Philip Monnartz, theinventor of stainlesssteel


Surface of stainless steel

• A chromium-rich oxide layer protects stainless steel from corrosion

• This passive layer can be damaged, resulting in an active surface

– Active surfaces are more susceptible to corrosion

• Localized corrosion

• Galvanic corrosion

Cr-rich

Cr-rich

Cr-rich Cr-richCr-rich


Pitting corrosion

• Breakdown of passive oxide layer

• Formation of localized shallow pits

• Pits grow deeper in time

• Can penetrate tubing


Crevice corrosion

• Breakdown of passive oxide in oxygen-deprived crevice

• Localized corrosion causes Fe++ to build up in crevice

• pH drops and Cl--concentration increase → more corrosion

• Can occur at lower temperature than pitting corrosion


Corrosion resistance

•Measured by ASTM G48 in 10% ferric chloride

•Ref.: Practical Guidelines for the Fabrication of Duplex Stainless Steels, Int. Molybdenum Assoc., 2001

CPT

CCT

-20

-10

0

10

20

30

40

50

60

70

80

90

1 2 3 4 5 6 7 8 9 10 11304L 316L 904L 6Mo 2304 2205 255 2507317L 317LMN

Te

mp

era

ture

(ºC

)

IN 625

IN 625

2-3 % Mo

3-4 % Mo

6-6.5 % Mo

4-5 % MoDuplex SS

Austenitic SS


Galvanic corrosion

• Occurs when two different metals are in intimate contact

and an electrolyte (electrically conductive fluid) is present


Electrochemical series

Passive surfaces are

more noble than active

surfaces

Less noble materials

experience galvanic

corrosion

Rule of thumb:

galvanic corrosion if

potential difference

exceeds 0.2 Volts


Intergranular corrosion (IGC)

• Microstructure of metals exists of grains

– Within grains, atoms are periodically arranged

• Grains come together at grain boundaries


Intergranular corrosion (IGC), cont.

• Compounds can precipitate on grain boundaries

– Carbides (e.g., chromium carbide), nitride, sigma phase

– Welding, high temperature exposure, improper heat treatment

• Precipitation “robs” adjacent grains of important elements

• IGC: fluid attacks boundaries between grains & precipitates

“acid”

crack


IGC fracture surface


Stress corrosion cracking (SCC)

• Crack propagation: microstructure + fluid + tensile stress

– Microstructure of metal susceptible to SCC

– Conducive environmental conditions (fluid, temperature)

– Tensile stress (applied plus residual) above a critical level

• Example: Chloride-induced SCC of austenitic stainless

steels


• H2 causes change in mechanical properties of material

– Strength: yield strength and ultimate tensile strength

– Ductility: elongation and reduction of area

– Fatigue behavior: cycle life

– Stress corrosion: crack propagation under static

tensile load

Hydrogen embrittlement


Interaction of H2 with microstructure

• Dislocations

– Preferred diffusion path

• Grain boundaries

– Preferred location for accumulation

• Other phases: ferrite, martensite

– Preferred diffusion path

– Embrittlement

Interfaces with particles and other phases

– Preferred location for accumulation

– Weakening of interface strength


H2 reduces ductility, (RAair – RAH2)/RAair

A. W. Thompson, “Hydrogen Embrittlement of Stainless Steels and Carbon Steels” (1978) (modified)


Ductility depends on nickel content

G.R. Caskey, “Hydrogen Effects in Stainless Steel” (1983)


Microbiologically influenced corrosion

• Caused by activity of microorganisms

– Bacterial colonies adhere to the metal surface

– Form crevice → localized corrosion underneath deposit

– Acidic waste can also lead to corrosion


Selection of Special Alloy Fluid System

Components


Metals and alloys used by Swagelok

• Stainless steels– Austenitic stainless steels 316, 304, 321

– Superaustenitic stainless steels 254 SMO, AL6XN

– Duplex stainless steel 2205, 2507

• Nickel alloys– Alloy C20

– Incoloy 825

– Inconel 600 & 625

– Hastelloy C-276, C-22, B2

– Monel (Alloy 400)

• Other materials– Carbon steels 1018, 1035, 1137, 11L37, 12L14

– Brass (Cu-Zn alloys C360, C377)

– Aluminum, titanium, zirconium, tantalum, Tungum, 904L, 28

• Materials for components– 17-4PH springs, X-750 and Elgiloy diaphragms, Ni gaskets


300-Series stainless steels

30418%Cr, 8%Ni

304L

316 316L 316LN316H

321

347

303

C20, 825

310

“6-moly” alloys

(AL-6XN,

254 SMO)

316Ti

+2% Ni

+2% Mo

+S - C

+Ti

+Nb

+Cr+Ni

+Si

- C + N+C

+Ti+Cr +N

+10% Ni

+4% Mo

+Cr +30% Ni

+Cu

348

+Ta

superaustenitic Ni-base alloy


Super Austenitic stainless steels

Superaustenitic stainless steels

“6-moly” alloys

Standard

Austenitics

825

+Cr

+8% Ni

+4% Mo

+N

+6% Ni

+Cr

+25% Ni

+Cu

+5% Ni

+Ti

+Al

C20

+Cu

+Nb

+Ta

AL-6XN254 SMO

+Cu

+20% Ni, +Mo

High nickel alloys

Nickel alloys

316: 17% Cr, 10% Ni, 2% Mo


6-Moly alloys ( 6% Mo)

• Higher levels of Cr, Ni, Mo, N, Cu than in 300-series steel

• Good mechanical properties

– Stronger than 300-series stainless steels

– High ductility, good weldability

• Very good corrosion resistance

– Good in both reducing and oxidizing conditions

– Very good resistance to localized corrosion

– Better resistance to chloride-ion induced SCC than 300-series SS

– Less expensive than nickel alloys

• UNS S31254: 254; developed as 254 SMO by Avesta

• UNS N08367: “6HN”; developed as AL-6XN by Allegheny Ludlum

• UNS N08926: developed as 1925hMo by ThyssenKrupp


6-moly alloys allowed by NORSOK

• NORSOK M-630 Piping Data standard lists 6-moly alloys

• 254 SMO: older 6-moly alloy with lower nickel content

• Newer 6-moly alloys with higher nickel content

• Similar pitting resistance equivalent number (PREN)

Alloy UNS Ni Cr Mo Cu N PREN

254 S31254 17.5-18.519.5-

20.56.0-6.5

0.50-

1.00

0.18-

0.22

42.2-

45.5

AL-6XN,

25-6HN,

367

N08367 23.5-25.520.0-

22.06.0-7.0

0.75

max.

0.18-

0.25

42.7-

49.1

1925 hMo,

25-6Mo,

926

N08926 24.0-26.019.0-

21.06.0-7.0

0.50-

1.50

0.15–

0.25

41.2-

48.1


Min. required solution annealing temperature

Alloy UNS Number

ASTM A484

(bar)

ºC (ºF)

ASTM A182

(forging)

ºC (ºF)

ASTM A480

(plate)

ºC (ºF)

254 S31254 1149 (2100)

6HN N08367 1107 (2025)

1925 hMo N08926 n/a n/a 1099 (2010)

• Lower annealing temperature for alloy 6HN

− Less grain growth → better mechanical properties

− Less oxidation → less scale, less pickling


Microstructure of 6HN vs. 254 forging

254 6HN


Mechanical properties of 6-moly forgings


Corrosion resistance

•Measured by ASTM G48 in 10% ferric chloride

•Ref.: Practical Guidelines for the Fabrication of Duplex Stainless Steels, Int. Molybdenum Assoc., 2001

CPT

CCT

-20

-10

0

10

20

30

40

50

60

70

80

90

1 2 3 4 5 6 7 8 9 10 11304L 316L 904L 6Mo 2304 2205 255 2507317L 317LMN

Te

mp

era

ture

(ºC

)

IN 625

IN 625

2-3 % Mo

3-4 % Mo

6-6.5 % Mo

4-5 % MoDuplex SS

Austenitic SS


CPT and CCT of 6-moly alloys

Critical crevice corrosion

temperature in 3% NaCl;

3 tests

Critical pitting temperature

of welds in 3% NaCl;

4 tests

Ref.: Drugli et al., Corrosion 93


Incoloy 825 (Ni-Fe-Cr-Mo-Cu)

• High resistance to reducing environments

– Sulfuric acid below 70%

• High resistance to oxidizing media

– Nitric acid, nitrates

• Resistant to chloride-ion stress cracking

• Stabilized with titanium → resistant to IGC

• Good performance in sour gas (NACE MR0175)


High nickel alloys in sulfuric acid


High nickel alloys

Nickel

(200, 201)+30% Cu

Monel 400

Monel

K-500

+Al

+Ti

600

+Cr

+Fe

690625 X-750

+Cr

+Mo

+Nb

+Ti

+Al

+Nb+Cr

Hastelloys

B-2, C-22,

C-276

+Mo

+W


High nickel alloys

• Ni content > 50%

• Excellent corrosion resistance in all environments

• Essentially immune to SCC

• Moderate strength and hardness, good weldability

– Can be strengthened through alloying, heat treatment

• Some Ni-alloys: very good high temperature properties

• Very expensive but offer superior properties


High nickel alloys

• Monel 400 (Ni-Cu alloy)

– Excellent resistance to corrosion in reducing environments

• Hydrochloric acid

• Outstanding in hydrofluoric acid

− Best in reducing conditions, less so in aerated solutions

http://www.nsme.cn/WebEditor/UploadFile/200652895351744.jpg

Monel 400 Condenser


High nickel alloys

• Inconel 625 (Ni-Cr-Mo-Fe-Nb-Ti-Al)

– Very corrosion resistant in oxidizing and reducing media

– High strength, good ductility

– Nb reduces risk of IGC in high temperature use

– Mo adds resistance to chloride pitting/crevice corrosion

– Very good performance in sour gas (NACE MR0175)

http://www.issartel.com/sites/issartel/local/cache-vignettes/L400xH278/Double_span_body-_Forged_Inconel_625-40422.jpg


High nickel alloys

• Hastelloys (Ni-Cr-Mo-W)

– Excellent corrosion resistance in oxidizing and reducing

media

– Excellent ductility/toughness/strength at high temperature

– Excellent resistance to pitting/crevice corrosion, immunity to

SCC

• C-276: widely used for excellent corrosion resistance

• C-22: even better corrosion resistance than C-276

• B-2: outstanding corrosion resistance in reducing environments


Duplex stainless steels

Ferritic

Grades

Austenitic

Grades

2205

2507

2707

Duplex

Superduplex

Hyperduplex

Higher

Alloy Content:

+Cr, Ni, Mo, N

2304

2003

Ferralium

255

Lean

Duplex


Duplex stainless steels

• Duplex steel: mixture of austenite and ferrite

– Typically 50% : 50%, min. 35% ferrite required

• Good resistance to all types of corrosion

• Lean duplex alloys: AL2003, LDX 2101, SAF 2304

– Higher strength than 316

– Corrosion resistance similar to 304 and 316

• Duplex alloy, widely used: 2205

• Superduplex alloy: 2507, offered by Swagelok

• Hyperduplex alloys: 2707, 3207

– Best corrosion resistance of duplex alloys

– Highest strength of duplex alloys


2507 super duplex stainless steel

• Characteristics of austenitic and ferritic stainless steels

• Stronger than annealed 316 SS

− Lower tubing wall thickness for same pressure rating

• Maximum use temperature 250ºC (482ºF)

− Unstable microstructure

• Minimum use temperature -46ºC (-50ºF)

− Ductile-to-brittle transition


Alloys for use in specific corrosive media


Selection of alloys for corrosive media

• 316 stainless steel is most economical choice for many

moderately corrosive fluids

– Acids

• Low concentration inorganic acids

• Some inorganic strong acids

• Organic acids

– Aqueous alkaline solutions

– Consult handbooks for media, concentrations,

temperatures


Acids and bases, higher temperatures

• Specially developed alloys

– C20 and 825 for sulfuric acid service

• High nickel alloys

– Monel (hydrofluoric acid)

– C276 (concentrated oxidizing and reducing acids)

– C22 (concentrated oxidizing and reducing acids)

– B2 (concentrated reducing acids at higher temperatures)

• Special materials

– Titanium (wet HCl gas)

– Zirconium (concentrated reducing acids at high temp.)

– Tantalum (most corrosive acids and acid mixtures)


• Pure acid versus mixed acids

• Presence of contaminants

– Halide ions, metal ions

• Aerated versus non-aerated acids

• Acid concentration

• Temperature

Parameters affecting corrosion resistance


For questions or additional information, contact one of our offices below.

email: [email protected] - web: http://nctn.swagelok.com

Charlotte, NC: 704.289.7400Raleigh, NC: 919.878.8085Knoxville, TN: 865.673.6610

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Alloy performance in corrosive environments Alloys for use