BLASTING AND - Shot Peener

be t r u e . F o r example, one c o m m e r c i a l l y 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 h a s 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 k e y t o s u c c e s s 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 consistency. 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 i s 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 project. T e s t i n g pH of each b a t c h 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 very simple, inexpensive, five-minute procedure. p e r f o r m e d 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 . It can e a s i l y be Testing f o r water soluble c o n t e n t i s more c o m p l i c a t e d 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 i s 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 less. One s h o u l d be c a u t i o u s of t h e c o n f u s i o n t h a t e x i s t s r e g a r d i n g d r y When q u e s t i o n e d by phone, a s a n d s u p p l i e r r e p o r t e d a b r a s i v e and m o i s t u r e . 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 . content." 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 "moisture 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 i n d e p e n d e n t 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 m e a s u r i n g and r e p o r t i n g "water absorption." 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 "water 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 a b r a s i v e . 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 t o 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 a b r a s i v e cleaned s u r f a c e . Nevertheless, some c o n s i d e r a t i o n should a l s o be g i v e n t o t h e r e l a t i v e m e r i t s of c o a t i n g over f l a s h r u s t v e r s u s c o a t i n g o v e r i n h i b i t e d s u r f a c e s . Numerous r e f e r e n c e s i n d i c a t e i t i s f a r more d e t r i m e n t a l t o apply a c o a t i n g over a c h l o r i d e o r s u l p h a t e contaminated s u b s t r a t e t h a n over a s u b s t r a t e which i s contaminated w i t h c h e m i c a l l y 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 c o a t i n g s [ 4 , 5, 71. Also, many p a i n t companies have developed p r i m e r s 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 salt f r e e rust. Wet j e t a b r a s i v e b l a s t i n g of t h e type d e s c r i b e d above very e f f e c t i v e l y removes s o l u b l e s a l t s such a s c h l o r i d e s 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 i n p o i n t r u s t i n g which forms a f t e r wet a b r a s i v e b l a s t i n g w i t h no i n h i b i t i o n i s a c h e m i c a l l y p u r e i r o n oxide r e l a t i v e l y f r e e of c h l o r i d e o r s u l p h a t e contamination. B l e i l e , e t al., found t h a t c o n t a m i n a n t s l e f t on t h e s u r f a c e from d r y a b r a s i v e b l a s t i n g w i t h m i n e r a 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 contributed t o premature coating 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 u s e t h e s e two a b r a s i v e s i n a wet a b r a s i v e b l a s t system w i t h no i n h i b i t i o n . It i s p o s s i b l e t h a t t h e wet j e t a b r a s i v e would remove t h e c o n t a m i n a n t s t o a l e v e l such t h a t t h e b l i s t e r i n g of t h e p a i n t f i l m would n o t o c c u r , because t h e s u r f a c e provided by wet j e t a b r a s i v e b l a s t i n g is c h e m i c a l l y clean. N e v e r t h e l e s s , a non-flash-rusted a p p l i c a t i o n of p a i n t f i l m s . surface i s preferred f o r the The most commonly used c h e m i c a l i n h i b i t o r s f o r b l a s t i n g w a t e r a r e a l k o n o l a m i n e s , sodium n i t r i t e a n d / o r n i t r a t e s , ammonium p h o s p h a t e s , s u l f o n a t e s , 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 s c u s s i o n , 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 t h e

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 d a t a , as y e t unpublished, s u g g e s t s t h a t most i n h i b i t o r s must be used a t c o n c e n t r a t i o n s of 1%t o 32 t o prevent f l a s h rusting. Humidity c a b i n e t and adhesion t e s t i n g of p a n e l s b l a s t e d i n t h i s program showed t h a t t h e h i g h e r c o n c e n t r a t i o n s c o n t r i b u t e d t o l o w e r adhesion and/or i n c r e a s e d b l i s t e r s i z e and frequency. The e x c e p t i o n was an i n h i b i t o r based on a l k o n o l a m i n e c h e m i s t r y 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 n o t c a u s e b l i s t e r i n g o r a d e c r e a s e i n adhesion a t a c o n c e n t r a t i o n of 5.0% T h i s h e l p s e x p l a i n why p a i n t manufacturers seem t o be more c o m f o r t a b l e w i t h t h e alkonolamine-type i n h i b i t o r s . This c h e m i s t r y i s c o n s i d e r e d t o be non- t o x i c and biodegradable i n t h e s e c o n c e n t r a t i o n s , according t o one European manufacturer. U n f o r t u n a t e l y , t h e p o t e n t i a l e f f e c t of pH and w a t e r s o l u b l e content of t h e b l a s t i n g a b r a s i v e used i n t h e s e t e s t s was n o t measured. It is therefore u n c e r t a i n how much of t h e decreased adhesion and b l i s t e r i n g may have been caused by t h e c o n t a m i n a n t s i n t h e a b r a s i v e . Another concern i s t h e use of i n h i b i t o r s i n w e t a b r a s i v e b l a s t i n g around f o o d p r o d u c t s , such a s p o t a b l e w a t e r t r e a t m e n t and s t o r a g e facilities. N a t u r a l l y , e v e r y p o s s i b l e p r e c a u t i o n must be t a k e n t o prevent t h e b l a s t w a t e r from c o n t a c t i n g t h e food product because t h e w a t e r c o n t a i n s p a i n t , rust, and s p e n t a b r a s i v e , any one of which may be f a r more dangerous than the inhibitor. A t l e a s t contaminants suspended i n b l a s t w a t e r a r e e a s i e r t o c o n t a i n , c o l l e c t , and remove t h a n 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 w a t e r i n d u s t r y s u g g e s t 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 g r a d e sodium n i t r i t e based. There i s a s u r p r i s i n g l a c k of i n f o r m a t i o n i n t h e 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 a b r a s i v e b l a s t i n g . 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 t y p e s of p a i n t systems from t h e world's l e a d i n g p a i n t m a n u f a c t u r e r s o v e r f i v e t y p e s of s u r f a c e p r e p a r a t i o n : b l a s t e d p a n e l s and one d r y b l a s t e d p a n e l a s a c o n t r o l . f o u r wet j e t a b r a s i v e These r e s u l t s w i l l p r o v i d e t h e b a s i s f o r a paper i n 1988. INSPECTION OF WET ABRASIVE BLASTED SUBSTRATES The d r y 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 d a r k e r i n c o l o r (even when w h i t e b l a s t i n g sand i s used) and l o w e r i n r e f l e c t i v i t y t h a n a w e t a b r a s i v e blasted surface. T h i s i s 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 l d p a i n t , powdered c h l o r i d e and s u l p h a t e contamination, and powdered a n y t h i n g e l s e t h a t was on t h e s u r f a c e p r i o r t o b l a s t i n g . powder i s d e p o s i t e d i n a uniform l a y e r over t h e b l a s t e d s u r f a c e . u n i f o r m l a y e r of c o n t a m i n a n t s f i r m l y a d h e r e s t o t h e s u b s t r a t e . This This Attempts t o wash i t away w i t h a 2500 PSI water j e t s t i l l would n o t produce t h e same s h e e n and r e f l e c t i v i t y a s t h a t developed w i t h wet a b r a s i v e b l a s t i n g . 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 r e s e n t on t h e s u r f a c e t h a t was n o t p r e s e n t when t h e s u r f a c e was cleaned w i t h t h e wet a b r a s i v e device. These r e s u l t s would a l s o l e a d one t o q u e s t i o n t h e v a l i d i t y of d r y b l a s t i n g f o l l o w e d by r i n s i n g o r w a t e r j e t t i n g a s a method of p r e p a r i n g s i m u l a t e d wet a b r a s i v e b l a s t e d t e s t panels. For example, t h e a u t h o r has observed t h a t a p a r t i c u l a r c o n c e n t r a t i o n of sodium n i t r i t e i n h i b i t o r i n t h e b l a s t i n g w a t e r w i l l v e r y e f f e c t i v e l y p r e v e n t f l a s h r u s t i n g , w h i l e t h e same c o n c e n t r a t i o n used i n w a t e 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 n o t prevent f l a s h rusting. The p r e s e n c e o r absence of t h e powdered l a y e r of c o n t a m i n a t i o n on t h e

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 i n d u s t r y : i n s p e c t i o n and a c c e p t a n c e o r r e j e c t i o n of a s u r f a c e a s ready f o r p a i n t i n g . What is s e e n by an i n s p e c t o r i s t h e l i g h t r e f l e c t e d from t h e s u r f a c e . What i s g e n e r a l l y a c c e p t e d a s "white metal* is a g r a y c o l o r of uniform r e f l e c t i v i t y w i t h no d a r k e r a r e a s o r s p o t s . The c o l o r of t h e d u s t g e n e r a t e d by breakdown of t h e a b r a s i v e i s n o r m a l l y uniform. I f a l a y e r of powder of u n i f o r m c o l o r is on t h e s u r f a c e , t h e c o l o r of l i g h t r e f l e c t e d from t h a t s u r f a c e 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 a c c e p t a n c e g r e a t e r . If t h a t l a y e r of powder does n o t 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 e d , 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 t h e l i g h t t h a t s h i n e s on i t ; t h e r e f o r e , minor i m p e r f e c t i o n s a r e f a r more a p p a r e n t and the p r o b a b i l i t y of a c c e p t a n c e i s lower. There has been c o n s i d e r a b l e d i s c u s s i o n of t h e d i f f i c u l t y i n developing v i s u a l a n d / o r w r i t t e n s t a n d a r d s f o r i n s p e c t i o n of a b r a s i v e b l a s t e d s u r f a c e s . Perhaps t h e b a s 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 i s being used t o judge s u r f a c e s covered w i t h a l a y e r of w h i t e powder (sand), a l a y e r of b l a c k powder ( s l a g ) , and now, w i t h w e t a b r a s i v e b l a s t i n g , no powder. These t h r e e t y p e s of s u r f a c e s cannot look t h e 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 a b r a s i v e s . As wet a b r a s i v e 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 a g a i n have t o r e l e a r n t h e i r trade. Development and a c c e p t a n c e of a q u a n t i t a t i v e chemical t e s t f o r d e t e r m i n i n g s u r f a c e c l e a n l i n e s s w i l l h e l p 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. I n t h e 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 a c c e p t t h a t c l e a n s t e e l s i m p l y does not

look t h e same w i t h o u t a l l t h a t d u s t on i t . One s i m p l e t e c h n i q u e which has been u s e f u l i n d e m o n s t r a t i n g t h e a c t u a l c l e a n l i n e s s v e r s u s t h e apparent c l e a n l i n e s s of s u b s t r a t e s i n v o l v e s t h e u s e of s t r o n g , w h i t e f i l a m e n t tape. A two-inch l e n g t h of t h e t a p e i s p r e s s e d on t h e s u r f a c e and i n t o t h e anchor p r o f i l e , t h e n removed w i t h a s h a r p j e r k . The powdered d u s t , r u s t , and o l d p a i n t a d h e r e t o t h e t a p e and are much e a s i e r t o s e e a g a i n s t t h e w h i t e background. Spots o r p i n p o i n t s of r u s t which were hidden by t h e l a y e r of powder a r e a l s o p u l l e d o f f by t h e t a p e and a r e e a s y t o s e e a g a i n s t t h e w h i t e background. While not v e r y s c i e n t i f i c , t h i s method h e l p s e d u c a t e i n s p e c t o r s who a r e n o t accustomed t o l o o k i n g a t s u b s t r a t e s cleaned w i t h w a t e r w e t t e d a b r a s i v e . CONCLUSIONS A new e r a of a b r a s i v e b l a s t s u r f a c e p r e p a r a t i o n i s dawning - one i n which t h e u s e of w a t e r w e t t e d a b r a s i v e w i l l become t h e norm r a t h e r t h a n t h e exception. The change t o w a t e r w e t t e d 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 i s being d r i v e n by p o w e r f u l s a f e t y , e n v i r o n m e n t a l , and economic f o r c e s Water w e t t e d a b r a s i v e is e f f e c t i v e a t removing w a t e r s o l u b l e s a l t s and producing a h i g h e r l e v e l of c h e m i c a l c l e a n l i n e s s t h a n d r y a b r a s i v e blasting . Chemical c l e a n l i n e s s of t h e b l a s t e d s u r f a c e i s c r u c i a l t o good p a i n t a d h e s i o n and long-term p a i n t performance. C o n t r o l of t h e pH and t h e water s o l u b l e c o n t e n t of t h e a b r a s i v e i s e s s e n t i a l t o successful surface preparation w i t h water wetted abrasive. Technology is now a v a i l a b l e t h a t s h o u l d make b l a s t i n g w i t h w a t e r w e t t e d

a b r a s i v e (and t h u s r e d u c t i o n 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 e c o n o m i c a l l y f e a s i b l e on most s u r f a c e p r e p a r a t i o n p r o j e c t s . Q u a n t i t a t i v e t e c h n i q u e s p r e s e n t l y under development t o i n s p e c t a b r a s i v e l y c l e a n e d s u r f a c e s w i l l help t o r e s o l v e i n s p e c t i o n c o n f l i c t s caused by t h e d i f f e r e n c e s i n r e f l e c t i v i t y of dry b l a s t ( d u s t covered) v e r s u s wet j e t abrasive blasted (dust free) surfaces. Chemicals a r e a v a i l a b l e f o r t h e b l a s t w a t e r t o e f f e c t i v e l y prevent f l a s h rusting. However, c o n s i d e r a b l e t e s t i n g and i n v e s t i g a t i o n i s needed t o d e t e r m i n e t h e i r a f f e c t on p a i n t performance. The f o l l o w i n g a r e i n t e r r e l a t e d and i n t e r d e p e n d e n t and must be considered t o g e t h e r a s a whole: - - Water w e t t e d a b r a s i v e 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 concentration, Abrasive pH. Abrasive w a t e r s o l u b l e c o n t e n t . Abrasive contaminants. S k i l l l e v e l of t h e equipment u s e r . S k i l l l e v e l of t h e i n s p e c t o r . REFERENCES Appleman, B.R. and Bruno, J.A., " E v a l u a t i o n of Wet Blast Cleaning Units", Proceedings of t h e SSPC 1985 Annual Symposium, SSPC 85-06. NACE Task Group TGG-22 Surf a c e s " . - "Surface P r e p a r a t i o n of Contaminated S t e e l B l e i l e , H.R., R o d g e r 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., " S p e c i f i c a t i o n f o r Abrasive B l a s t i n g Media", Proceedings of t h e SSPC 1985 Annual Symposium, SSPC 85-06.

4. West, J., "The R e l a t i o n s h i p 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. "The I n f l u e n c e of S a l t s i n Rusts on t h e Corrosion of Chandler, K.A., t h e U n d e r l y i n g S t e e l . " B r i t . C o r r o s . &, 1 9 6 6 , Vol. 1, J u l y - 6. D a s g u p t a , D., a n d R o s s , T.K., " C l e a n i n g o f R u s t y 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,, a n d C a i n , T.A., " S a l t s : T h e i r D e t e c t i o n a n d T h e i r I n f l u e n c e on Coating Performance", Proceedings of t h e SSPC 1985 Annual Symposium, SSPC 85-06. 8. "Evaluation of t h e E f f e c t i v e n e s s of Wet Blast Cleaning Methods of S u r f a c e P r e p a r a t i o n " , June 1985, US. Department of T r a n s p o r t a t i o n M a r i t i m e A d m i n i s t r a t i o n i n Cooperation w i t h Avondale Shipyards; New O r l e a n s , L o u i s i a n a , SSPC 86-02.

blasting to cut the time required for dry blasting (and thus the amount of . One key to success on any surface preparation and painting project is consistency. Without control over pH . consistency on a wet abrasive blasting project. Testing pH of each batch of abrasive as it is received at the job site is a very simple, inexpensive, five .