WET ABRASIVE BLASTING - PROBLEMS AND SOLUTIONS

by

by J e r r y P. Woodson

Surface Dynamics, Inc. Houston, Texas

INTRODUCTION

The d r i v e t o p r o t e c t workers and our environment from i n j u r y by

hazardous m a t e r i a l s is changing how s t e e l i s p ro t ec t ed from corrosion.

Indus t ry is s t r u g g l i n g , perhaps even scrambling, t o main ta in the q u a l i t y of

co r ros ion p r o t e c t i o n a s t r i e d and proven methods and m a t e r i a l s a r e

e l i m i n a t e d by government r egu la t ion , l i t i g a t i o n , and o t h e r pressures .

Dry ab ras ive b l a s t i n g ( e s p e c i a l l y w i th sand), ac id p i ck l ing , and

o rgan ic so lven t based p a i n t s a r e endangered spec ies . (Sand i s not the only

hazardous abras ive ; a r s e n i c and o t h e r contaminants have been found i n

s i l i c a - f r e e ab ra s ives [ 3 1 .) Coating o r p l a t i n g w i t h heavy me ta l s (zinc,

cadmium, chromium, etc.) is not f a r behind. Asbestos, one of t h e bes t

f i l l e r s f o r organic coa t ings , is h i s to ry .

Innovat ion i s proceeding a t a r a p i d pace t o r ep l ace e x i s t i n g processes

w i t h new ones t h a t a r e s a f e r and more c o s t e f f e c t i v e . For example, one

i n d u s t r i a l u s e r has found t h a t a r e l i a b l e wet ab ra s ive b l a s t i n g machine

e l i m i n a t e d an a i r p o l l u t i o n source, reduced s i l i c o s i s exposure, and, when

used f o r pre-cleaning, reduced a hazardous l i q u i d waste d i s p o s a l problem.

An a p p l i c a t o r of high performance c o a t i n g s has i n s t a l l e d weC a b r a s i v e pre-

b l a s t i n g t o c u t t he t ime requi red f o r d r y b l a s t i n g (and thus t h e amount of

dus t genera ted ) i n h a l f .

A s new regu la t ions go i n t o e f f e c t , and insurance r a t e s cont inue t o

skyrocket f o r s u p p l i e r s , c o n t r a c t o r s , and owners involved i n dry ab ra s ive

b l a s t i n g , ou r i n d u s t r y must f i n d ways t o reduce o r e l i m i n a t e dus t generated

dur ing s u r f a c e preparat ion. "Wet ab ra s ive b l a s t i n g o f f e r s t h e p o t e n t i a l t o

reduce o r e l i m i n a t e many of t he problems a s s o c i a t e d w i t h d ry b las t ing" [I].

"Current ly, t h e most p r a c t i c a l and wide ly used a l t e r n a t i v e s t o dry

b l a s t i n g a r e wet methods of b l a s t cleaning. The use of wa te r i n combination

w i t h a b r a s i v e s s i g n i f i c a n t l y reduces the amount of d u s t produced and the

range over which i t i s d i s t r i b u t e d " [8] .

The c l ean ing r a t e s of t en w e t b l a s t sys tems eva lua ted i n 1984-85 were

30% t o 90% a s f a s t a s dry b l a s t i n g [ 8 ] . However, a t l e a s t one wet ab ra s ive

b l a s t i n g system now a v a i l a b l e c leans a s f a s t as, and sometimes f a s t e r than,

d ry b l a s t i n g . With t h i s technology, compressed air a t 40 t o 120 P S I

t r a n s p o r t s a continuous s t ream of ab ra s ive p a r t i c l e s i n t o a s m a l l propulsion

chamber. The ab ras ive p a r t i c l e s a r e then a c c e l e r a t e d through the th roa t of

a v e n t u r i nozz le by a medium (1500-2500 PSI) pres su re water j e t and,

thoroughly wet ted , impact t h e s u b s t r a t e a t a very high ve loc i ty . This "wet

j e t ab ra s ive" system e f f e c t i v e l y removes w a t e r s o l u b l e s a l t contaminants

[21. (The au tho r notes t h a t t h i s technology does no t p r e s e n t l y f i t w i th in

t h e equipment nomenclature of e i t h e r NACE o r SSPC. The phrase "wet j e t

abras ive" seems t o convey the p r i n c i p a l o p e r a t i n g concepts.)

Our i n d u s t r y seems t o have had r e l a t i v e l y l i t t l e experience wi th w e t

a b r a s i v e b l a s t i n g except on s m a l l jobs. As r e g u l a t i o n s and o the r pressures

f o r c e t h e u s e of w e t ab ra s ive b l a s t i n g on l a r g e r p r o j e c t s , problems unique

t o this method of s u r f a c e p repa ra t ion must be addressed and solved. This

paper i d e n t i f i e s t h r e e of the most impor tan t of t hose problems and reviews

p r o g r e s s t o d a t e toward s o l v i n g them: b l a s t i n g media q u a l i t y c o n t r o l , t h e

u s e of r u s t i n h i b i t o r s , and i n s p e c t i o n of wet a b r a s i v e b l a s t e d s u r f a c e s .

ABRASIVE BLASTING MEDIA QUALITY CONTROL

The pH and w a t e r s o l u b l e c o n t e n t a r e e x t r e m e l y i m p o r t a n t i n d e t e r m i n i n g

i f a n a b r a s i v e b l a s t i n g media is s u i t a b l e f o r wet a b r a s i v e b l a s t i n g .

B l e i l e , e t a l , r e p o r t e d t h a t t h e pH of t e n d i f f e r e n t a b r a s i v e s l a g m a t e r i a l s

ranged f rom 5.7 t o 10.1 (31. Samples of a b r a s i v e b l a s t i n g sand commerc ia l ly

a v a i l a b l e i n t h e Houston, Texas, a r e a have pH v a l u e s r a n g i n g f r o m 4.5 t o

6.8.

F u r t h e r s t u d i e s are needed t o d e t e r m i n e whe ther t h e pH of a b l a s t i n g

media f rom a p a r t i c u l a r s u p p l i e r is c o n s i s t e n t o r v a r i e s f rom ba tch t o ba tch

o r f rom p a r t i c l e s i z e d i s t r i b u t i o n t o p a r t i c l e s i z e d i s t r i b u t i o n .

S i m i l a r l y , a d d i t i o n a l r e s e a r c h needs t o be under taken t o d e t e r m i n e what

c h e m i c a l s i n t h e b l a s t i n g media c a u s e t h e pH t o be e i t h e r a c i d i c o r bas ic .

It i s known is t h a t t h e pH of t h e a b r a s i v e b l a s t i n g media (ABM)

s i g n i f i c a n t l y a f f e c t s t h e tendency of a w e t a b r a s i v e b l a s t e d s u r f a c e t o

e i t h e r f o r m a g e n e r a l f l a s h r u s t o r t o deve lop s m a l l , randomly l o c a t e d ,

s p o t s of r u s t somet imes r e f e r r e d t o as "p inpo in t rus t ing" . Using a b l a s t i n g

s a n d t h a t d rops t h e pH of t h e b l a s t i n g w a t e r t o 4.5, and a p r o p r i e t a r y wet

b l a s t i n h i b i t o r t h a t h a s no a f f e c t on pH, i t was n o t p o s s i b l e t o p reven t

g e n e r a l f l a s h r u s t i n g even a t i n h i b i t o r c o n c e n t r a t i o n s a s h igh as 2%. Using

t h e same sand , and a n a lkonolamine i n h i b i t o r which t e n d s t o r a i s e t h e pH, i t

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

i n the b l a s t i n g w a t e r o f 0.5%. However, some randomly d i s p e r s e d p i n p o i n t s

of rust appeared a f t e r a few hours. It is b e l i e v e d t h a t go ing t o a

. concen t r a t ion of 1.0% with t h i s p a r t i c u l a r i n h i b i t o r would e l i m i n a t e the

p inpo in t s of r u s t , but t h i s has not y e t been t r i e d .

B l a s t i n g sand wi th a pH of 6.5 t o 6.8 i s commercial ly a v a i l a b l e i n t h e

same geographic a r e a a t t he same p r i ce . Both f l a s h r u s t i n g and p inpoin t

r u s t i n g can be e l imina ted w i t h an alkonolamine-type i n h i b i t o r a t

concen t r a t ions of only 0.2% us ing sand wi th a r e l a t i v e l y n e u t r a l pH.

Spec i fy ing and purchasing ABM w i t h a non-acidic pH is p r e f e r a b l e t o

i n c r e a s i n g i n h i b i t o r concent ra t ion f o r t e c h n i c a l a s w e l l a s economic

reasons: t h e lower the i n h i b i t o r concen t r a t ion i n t he b l a s t i n g water , the

lower t h e p r o b a b i l i t y t h a t i n h i b i t o r r e s i d u e can c o n t r i b u t e t o p a i n t

problems.

The amount of water s o l u b l e m a t e r i a l ( o f t e n r e f e r r e d t o a s "water

s o l u b l e content") i n a b l a s t ab ra s ive can cause i t t o be u n s u i t a b l e f o r use

w i t h wet b l a s t i n g equipment. Abrasive b l a s t i n g media w i t h a wa te r s o l u b l e

content of 0.5% o r l e s s , by weight, t y p i c a l l y does no t pack up on v e r t i c a l

and h o r i z o n t a l s u r f a c e s , d r i e s out f a i r l y quick ly , and is easy t o wash of f .

One b l a s t i n g sand i n the Houston a r e a has been determined t o have water

s o l u b l e con ten t s a s high a s 4.0%. When used f o r wet ab ra s ive b l a s t i n g , t h i s

sand tended t o b u i l d t o a th ickness of 1-1/2" t o 2" even on v e r t i c a l

sur faces . The sand s tayed wet f o r hours , and was very d i f f i c u l t t o wash

down, e s p e c i a l l y ou t of corners. The longe r t h e sand keeps the water

a g a i n s t t h e f r e s h l y b las ted s t e e l , t he more e f f e c t i v e t h e i n h i b i t o r must be.

Sometimes the i n h i b i r o r concent ra t ion must be inc reased t o compensate f o r

t he l o n g e r wet t i m e .

I n i t i a l t e s t i n g suggested a c o r r e l a t i o n between t h e high water s o l u b l e

content and low pH. However, a d d i t i o n a l i n v e s t i g a t i o n has shown t h a t not t o

be t r u e . For example, one commerc ia l ly a v a i l a b l e b l a s t i n g sand i n t h e

Houston a r e a has a w a t e r s o l u b l e c o n t e n t of 0.2% and a pH of 4.5.

One key t o success on any s u r f a c e p r e p a r a t i o n and p a i n t i n g p r o j e c t i s

c o n s i s t e n c y . Without c o n t r o l o v e r pH and w a t e r s o l u b l e c o n t e n t , i t is

v i r t u a l l y i m p o s s i b l e t o o b t a i n c o n s i s t e n c y on a w e t a b r a s i v e b l a s t i n g

p r o j e c t . T e s t i n g pH of each ba tch of a b r a s i v e a s i t i s r e c e i v e d a t t h e j o b

s i t e is a v e r y s i m p l e , inexpens ive , f i v e - m i n u t e p rocedure . It can e a s i l y be

performed by t h e c o n t r a c t o r o r t h e i n s p e c t o r . T e s t i n g f o r w a t e r s o l u b l e

c o n t e n t is more compl ica ted and w i l l r e q u i r e t h e development of a k i t t o be

used a t t h e j o b site. I n t h e meantime, i t is recommended t h a t t h o s e who

w i l l u s e w e t a b r a s i v e b l a s t i n g t e c h n i q u e s i d e n t i f y and p a t r o n i z e s u p p l i e r s

of a b r a s i v e s w i t h a pH of 6.5 o r h i g h e r and a w a t e r s o l u b l e c o n t e n t of 0.5%

o r l e s s .

One shou ld be c a u t i o u s of t h e confus ion t h a t e x i s t s r e g a r d i n g d r y

a b r a s i v e and mois tu re . When ques t ioned by phone, a sand s u p p l i e r r e p o r t e d

t h e " w a t e r s o l u b l e c o n t e n t " t o be 0.25% maximum on one s i z e sand and 0.50%

maximum on a n o t h e r s i z e . L a t e r he c o r r e c t e d t h e s t a t e m e n t t o "mois ture

content." S t i l l l a t e r , i t was d i s c o v e r e d t h a t t h e independent l a b which was

p e r f o r m i n g t h e t e s t s was a c t u a l l y measur ing and r e p o r t i n g "wate r

a b s o r p t i o n . "

Both pH and w a t e r s o l u b l e c o n t e n t a r e measured by s t a n d a r d ASTM t e s t

p r o c e d u r e s n o r m a l l y used t o e v a l u a t e s o i l s . I t is t h e " w a t e r s o l u b l e

c o n t e n t " t h a t can r e s u l t i n a g l u e - l i k e b i n d e r and c a u s e d i f f i c u l t i e s i n

c l e a n i n g up t h e w e t abras ive .

WET ABRASIVE BLAST INHIBITORS

A v a r i e t y of c h e m i c a l s can be added to t h e b l a s t i n g w a t e r t o r e t a r d t h e

. development of r u s t i n g on a wet ab ra s ive cleaned sur face . Never the less ,

some cons ide ra t ion should a l s o be given t o t he r e l a t i v e m e r i t s of coa t ing

over f l a s h r u s t ve r sus coa t ing over i n h i b i t e d sur faces .

Numerous r e fe rences i n d i c a t e i t is f a r more d e t r i m e n t a l t o apply a

coa t ing over a c h l o r i d e o r su lpha te contaminated s u b s t r a t e than over a

s u b s t r a t e which is contaminated w i t h chemical ly pure i r o n oxide, s i m i l a r t o

t h a t found as a pigment i n many coa t ings [4 , 5, 71 . Also, many pa in t

companies have developed pr imers which a r e s p e c i f i c a l l y more t o l e r a n t t o

sal t f r e e ru s t . Wet j e t ab ra s ive b l a s t i n g of the type descr ibed above very

e f f e c t i v e l y removes so lub le s a l t s such a s ch lo r ides and s u l p h a t e s from t h e

s u r f a c e [ l , 2, 61. The f l a s h r u s t i n g o r p inpoin t r u s t i n g which forms a f t e r

wet ab ra s ive b l a s t i n g wi th no i n h i b i t i o n is a chemica l ly pure i r o n oxide

r e l a t i v e l y f r e e of ch lo r ide o r su lpha te contamination.

B l e i l e , e t al., found t h a t contaminants l e f t on t h e s u r f a c e from dry

a b r a s i v e b l a s t i n g w i t h minera l s l a g from two d i f f e r e n t s u p p l i e r s a c t u a l l y

con t r ibu ted t o premature coa t ing f a i l u r e ( b l i s t e r s ) [ 3 ] . It would be very

i n t e r e s t i n g t o use these two a b r a s i v e s i n a wet ab ra s ive b l a s t system w i t h

no i n h i b i t i o n . It is poss ib l e t h a t t h e wet j e t a b r a s i v e would remove the

contaminants t o a l e v e l such t h a t t he b l i s t e r i n g of t h e p a i n t f i l m would not

occur , because t h e s u r f a c e provided by wet j e t ab ra s ive b l a s t i n g is

chemica l ly clean.

Nevertheless , a non-flash-rusted s u r f a c e i s p re fe r r ed f o r t h e

a p p l i c a t i o n of p a i n t f i l m s . The most commonly used chemical i n h i b i t o r s f o r

b l a s t i n g water a r e alkonolamines, sodium n i t r i t e and/or n i t r a t e s , ammonium

phosphates , s u l f ona tes , benzoates, and numerous v a r i a t i o n s of these. There

has been a l o t of d i scuss ion , but s u r p r i s i n g l y l i t t l e hard d a t a , on the

a f f e c t of t h e above types of i n h i b i t o r s on p a i n t performance.

One s e t of t e s t da t a , as y e t unpublished, sugges ts t h a t most i n h i b i t o r s

must be used a t concent ra t ions of 1% t o 32 t o prevent f l a s h rus t ing .

Humidity cab ine t and adhesion t e s t i n g of pane ls b l a s t ed i n t h i s program

showed t h a t t h e h igher concent ra t ions con t r ibu ted t o lower adhesion and/or

i nc reased b l i s t e r s i z e and frequency. The except ion was an i n h i b i t o r based

on alkonolamine chemistry t h a t provided e f f e c t i v e i n h i b i t i o n a t 0.5Z and d i d

not cause b l i s t e r i n g o r a decrease i n adhesion a t a concent ra t ion of 5.0%

This h e l p s e x p l a i n why pa in t manufacturers seem t o be more comfortable w i t h

t h e alkonolamine-type inh ib i to r s . This chemis t ry is considered t o be non-

t o x i c and biodegradable i n these concent ra t ions , according t o one European

manufacturer .

Unfortunately, t he p o t e n t i a l e f f e c t of pH and water so lub le content of

t h e b l a s t i n g ab ras ive used i n t hese t e s t s was not measured. It is therefore

u n c e r t a i n how much of t he decreased adhesion and b l i s t e r i n g may have been

caused by t h e contaminants i n the abrasive.

Another concern is the use of i n h i b i t o r s i n w e t ab ra s ive b l a s t i n g

around food products , such a s potab le wa te r t r ea tmen t and s t o r a g e

f a c i l i t i e s . Natura l ly , every poss ib l e precaut ion must be taken t o prevent

t h e b l a s t w a t e r from con tac t ing the food product because the water contains

p a i n t , rust, and spen t abras ive , any one of which may be f a r more dangerous

than t h e i n h i b i t o r . A t l e a s t contaminants suspended i n b l a s t water a r e

e a s i e r t o con ta in , c o l l e c t , and remove than a d u s t cloud. Discussions t o

. d a t e w i t h t h e p o t a b l e water i ndus t ry suggest t h a t t h e i n h i b i t o r of choice

f o r t h i s a p p l i c a t i o n w i l l l i k e l y be food grade sodium n i t r i t e based.

There is a s u r p r i s i n g l ack of i n fo rma t ion i n t he l i t e r a t u r e on

i n h i b i t o r s f o r wet ab ra s ive b l a s t ing . A major p r o j e c t has been i n i t i a t e d t o

t e s t s e v e r a l types of pa in t systems from t h e world's l ead ing p a i n t

manufacturers over f i v e types of s u r f a c e prepara t ion : f o u r wet j e t a b r a s i v e

b l a s t e d panels and one dry b las ted panel a s a con t ro l . These r e s u l t s w i l l

provide the b a s i s f o r a paper i n 1988.

INSPECTION OF WET ABRASIVE BLASTED SUBSTRATES

The dry a b r a s i v e b l a s t e d s u r f a c e i s always da rke r i n co lo r (even when

wh i t e b l a s t i n g sand i s used) and lower i n r e f l e c t i v i t y than a w e t ab ra s ive

b l a s t e d sur face . This is because t h e r e is a uniform l a y e r of powdered

a b r a s i v e , powdered o ld pa in t , powdered ch lo r ide and s u l p h a t e contamination,

and powdered anyth ing e l s e t h a t was on the su r f ace p r i o r t o b l a s t ing . This

powder i s depos i ted i n a uniform l a y e r over the b l a s t e d sur face . This

uniform l a y e r of contaminants f i r m l y adheres t o t h e s u b s t r a t e . Attempts t o

wash i t away w i t h a 2500 PSI water j e t s t i l l would not produce t h e same

sheen and r e f l e c t i v i t y a s t h a t developed wi th wet a b r a s i v e b l a s t ing . This

would i n d i c a t e t h e r e i s s t i l l some contaminant p re sen t on t h e su r f ace t h a t

was not p r e s e n t when t h e su r f ace was cleaned w i t h t h e wet ab ra s ive device.

These r e s u l t s would a l s o l ead one t o ques t ion t h e v a l i d i t y of dry

b l a s t i n g fo l lowed by r i n s i n g o r wa te r j e t t i n g a s a method of preparing

s imu la t ed wet a b r a s i v e b l a s t ed t e s t panels. For example, t h e au thor has

observed t h a t a p a r t i c u l a r concent ra t ion of sodium n i t r i t e i n h i b i t o r i n t he

b l a s t i n g wa te r w i l l very e f f e c t i v e l y prevent f l a s h r u s t i n g , whi le t he same

concen t r a t ion used i n wa te r t o wash o f f a dry b l a s t e d s u r f a c e w i l l no t

prevent f l a s h rus t ing .

The presence o r absence of t h e powdered l a y e r of contaminat ion on the

s u r f a c e f u r t h e r f u e l s a major c o n f l i c t i n t h i s indus t ry : i n s p e c t i o n and

acceptance o r r e j e c t i o n of a su r f ace a s ready f o r pa in t ing .

What is seen by an in spec to r i s the l i g h t r e f l e c t e d from the surface.

What is g e n e r a l l y accepted a s "white metal* is a gray co lo r of uniform

r e f l e c t i v i t y w i t h no darker a reas o r spots . The co lo r of the dus t generated

by breakdown of t h e ab ra s ive is normally uniform. I f a l a y e r of powder of

uniform co lo r is on t h e sur face , t h e co lo r of l i g h t r e f l e c t e d from t h a t

su r f ace w i l l n a t u r a l l y be uniform and t h e p r o b a b i l i t y of acceptance grea te r .

If t h a t l a y e r of powder does not e x i s t because t h e s t e e l w a s wet

a b r a s i v e b l a s t ed , t h e appearance of s t e e l i s b r i g h t s i l v e r a s opposed t o a

n e u t r a l gray. The b r i g h t s i l v e r s u r f a c e r e f l e c t s more of the l i g h t t h a t

sh ines on i t; the re fo re , minor imper fec t ions a r e f a r more apparent and the

p r o b a b i l i t y of acceptance is lower.

There has been cons iderable d i scuss ion of t h e d i f f i c u l t y i n developing

v i s u a l and/or w r i t t e n s tandards f o r i n s p e c t i o n of ab ra s ive b l a s t ed surfaces.

Perhaps t h e bas i s of t h e d i f f i c u l t y is t h a t a s i n g l e v i s u a l c r i t e r i o n is

being used t o judge su r f aces covered w i t h a l a y e r of wh i t e powder (sand), a

l a y e r of b lack powder ( s lag) , and now, w i t h w e t ab ra s ive b l a s t ing , no

powder. These t h r e e types of su r f aces cannot look the same.

I n s p e c t o r s have had t o r e l e a r n t h e i r t r a d e a s a t r a n s i t i o n i s being

made from s i l i c a sand t o slag-type abras ives . As wet ab ra s ive b l a s t i n g

r e p l a c e s d r y b l a s t i n g i n many a p p l i c a t i o n s , i n s p e c t o r s w i l l aga in have t o

r e l e a r n t h e i r t rade . Development and acceptance of a q u a n t i t a t i v e chemical

t e s t f o r de te rmining s u r f a c e c l e a n l i n e s s w i l l he lp e l i m i n a t e much of t h e

s u b j e c t i v i t y of t h e i n s p e c t i o n method. In t he meantime, owners,

c o n t r a c t o r s , and i n s p e c t o r s must accept t h a t c lean s t e e l s imply does not

look the same wi thout a l l t h a t d u s t on it.

One s imple technique which has been u s e f u l i n demonst ra t ing t h e a c t u a l

c l e a n l i n e s s versus the apparent c l e a n l i n e s s of s u b s t r a t e s i nvo lves t he use

of s t rong , whi te f i l a m e n t tape. A two-inch l eng th of t he t ape i s pressed on

t h e s u r f a c e and i n t o the anchor p r o f i l e , then removed w i t h a sha rp je rk .

The powdered dus t , r u s t , and old p a i n t adhere t o t h e t ape and are much

e a s i e r t o s e e a g a i n s t t he white background. Spots o r p inpo in t s of r u s t

which were hidden by the l a y e r of powder a r e a l s o pul led o f f by the t a p e and

a r e easy t o s e e a g a i n s t t he whi te background. While not very s c i e n t i f i c ,

t h i s method he lps educate i n s p e c t o r s who a r e not accustomed t o looking a t

s u b s t r a t e s cleaned w i t h water wet ted abrasive.

CONCLUSIONS

A new e r a of ab ra s ive b l a s t s u r f a c e p repa ra t ion is dawning - one i n which

t h e use of wa te r wet ted ab ras ive w i l l become t h e norm r a t h e r than t h e

except ion.

The change t o wa te r wet ted a b r a s i v e t o reduce o r e l i m i n a t e d u s t is being

d r i v e n by powerfu l s a f e t y , environmental , and economic fo rces -

Water wet ted a b r a s i v e is e f f e c t i v e a t removing wa te r s o l u b l e salts and

producing a h ighe r l e v e l of chemical c l e a n l i n e s s than dry a b r a s i v e

b l a s t i n g . Chemical c l e a n l i n e s s of t he b l a s t e d s u r f a c e is c r u c i a l t o good p a i n t

adhes ion and long-term p a i n t performance.

Cont ro l of t h e pH and t h e water s o l u b l e content of t he a b r a s i v e is

e s s e n t i a l t o succes s fu l s u r f a c e p r e p a r a t i o n w i t h water wet ted abras ive .

Technology is now a v a i l a b l e t h a t should make b l a s t i n g w i t h wa te r we t t ed

a b r a s i v e (and thus reduct ion o r e l i m i n a t i o n of d u s t ) t e c h n i c a l l y and

economical ly f e a s i b l e on most su r f ace p repa ra t ion p ro j ec t s .

Q u a n t i t a t i v e techniques p re sen t ly under development t o i n s p e c t ab ra s ive ly

cleaned s u r f a c e s w i l l help t o r e so lve in spec t ion c o n f l i c t s caused by the

d i f f e r ences i n r e f l e c t i v i t y of dry b l a s t (dust covered) ve r sus wet j e t

a b r a s i v e b l a s t e d (dus t f r e e ) sur faces .

Chemicals a r e a v a i l a b l e f o r t h e b l a s t water t o e f f e c t i v e l y prevent f l a s h

rus t ing . However, cons iderable t e s t i n g and i n v e s t i g a t i o n is needed t o

determine t h e i r a f f e c t on pa in t performance.

The fo l lowing a r e i n t e r r e l a t e d and interdependent and must be considered

toge the r a s a whole:

- Water wet ted ab ras ive b l a s t i n g equipment.

- I n h i b i t o r chemistry.

- I n h i b i t o r concent ra t ion ,

- Abrasive pH.

- Abrasive water so lub le content .

- Abrasive contaminants.

- S k i l l l e v e l of t he equipment u se r .

- S k i l l l e v e l of t he in spec to r .

REFERENCES

Appleman, B.R. and Bruno, J.A., "Evaluat ion of Wet Blast Cleaning Units", Proceedings of t he SSPC 1985 Annual Symposium, SSPC 85-06.

NACE Task Group TGG-22 - "Surface P repa ra t ion of Contaminated S t e e l Surf aces " . B l e i l e , H.R., Rodger s , S.D., P o r t e r , F., S m i t h , A.E., G r i f f i n , J., "Spec i f i ca t ion f o r Abrasive B las t ing Media", Proceedings of the SSPC 1985 Annual Symposium, SSPC 85-06.

4. West, J., "The Re la t ionsh ip Between Coating Thickness and S a l t Contamination on B l i s t e r i n g of Coatings." UK Corrosion '85 a t Harrogate.

5. Chandler, K.A., "The In f luence of S a l t s i n Rusts on t h e Corrosion of t h e U n d e r l y i n g S tee l . " B r i t . Corros . &, 1966, Vol. 1, J u l y -

6. Dasgup ta , D., and Ross, T.K., "C lean ing of Rus ty S t e e l f o r P a i n t i n g " . B r i t , Corros . J., 1971, Vol. 6 , November.

7. Weldon, DOG,, and Cain , T.A., " S a l t s : T h e i r D e t e c t i o n and T h e i r Inf luence on Coating Performance", Proceedings of t he SSPC 1985 Annual Symposium, SSPC 85-06.

8. "Evaluation of t h e Ef fec t iveness of Wet Blast Cleaning Methods of Surface Preparat ion", June 1985, US. Department of T ranspor t a t ion Mari t ime Adminis t ra t ion i n Cooperation w i t h Avondale Shipyards; New Orleans, Louisiana, SSPC 86-02.