SALINE WATER CONVERSION

Corrosion of Metals in Sea Water FREDERICK W. FINK

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 5, 2015 | http://pubs.acs.org Publication Date: January 1, 1960 | doi: 10.1021/ba-1960-0027.ch005

Battelle Memorial Institute, 505 King Ave., Columbus 1, Ohio

The Office of Saline Water is directing a large number of investigations into the feasibility of new processes for producing fresh water starting with sea or brackish water as a source. It is desired that these plants last for 20 years or more. This paper points up ways in which the economic waste resulting from corrosion in saline water plants can be avoided. The article is based on a review of the corrosion literature and on consultations with marine experts in the field. Of the many materials for distillation plants, steel is the most important. It can be used to handle sea water below 250° F., if proper steps are taken such as the removal of all air (oxygen) from solution. For severe service and better performance metals like titanium, Hastelloy C, Monel, cupro-nickels, aluminum, aluminum brass, or Admiralty brass are used.

T h e ocean, w h i c h covers more t h a n 7 0 % of the earth's surface, is considered b y m a n y t o b e a c h a l l e n g i n g f r o n t i e r . S u c h g o v e r n m e n t agencies as t h e N a v y , t h e D e p a r t m e n t of t h e I n t e r i o r , t h e B u r e a u o f M i n e s , t h e M a r i t i m e C o m m i s s i o n , t h e D e p a r t m e n t o f A g r i c u l t u r e , etc., h a v e r e c o g n i z e d t h e i m p o r t a n c e of t h e sea f r o m t h e i r o w n s p e c i a l v i e w points for m a n y years. G o v e r n m e n t activities, such as the demonstration plants of the Office o f S a l i n e W a t e r t o c o n v e r t sea w a t e r t o f r e s h w a t e r , t h e 7 m i l e d e e p - s e a - d i v i n g " B a t h y s c a p h e " under N a v y sponsorship, a n d plans f o r studying hydrofoil ships, p r o v i d e e x a m p l e s o f n e w e r a c t i v i t i e s c o n n e c t e d w i t h t h e sea. T h e l a t e s t n u c l e a r - p o w e r e d s u b m a r i n e s c a n v i s i t m o s t p a r t s o f t h e oceans, e v e n those u n d e r t h e ice. P r i v a t e i n t e r e s t s are engaged i n i n t e n s i v e studies r e l a t e d t o t h e ocean. One c a n m e n t i o n t h e o i l - a n d s u l f u r - m i n i n g i n d u s t r i e s w i t h t h e i r T e x a s t o w e r s , some of w h i c h a r e f a r o u t t o sea. N e w t y p e s of t e l e p h o n e a n d p o w e r cables a n d p i p e l i n e s are b e i n g p l a c e d o n t h e sea b o t t o m . S h o r e - b a s e d o p e r a t i o n s b y c h e m i c a l c o m p a n i e s a r e m a k i n g use o f sea w a t e r t o o b t a i n m a g n e s i u m , b r o m i n e , a n d sea s a l t . P h a r m a c e u t i c a l s a r e being extracted from sea-growing vegetation a n d f r o m marine animals. M a n y shoreb a s e d i n s t a l l a t i o n s use sea w a t e r f o r c o o l i n g a n d , i n some s p e c i a l cases, f o r s a n i t a t i o n o r for fire-protection services. T h i s i n c r e a s e d a c t i v i t y o n t h e sea, i n t h e sea, a n d u n d e r t h e sea focuses a t t e n t i o n o n t h e n e e d f o r k n o w l e d g e o f t h e b e h a v i o r o f m e t a l s i n these e n v i r o n m e n t s . I t has b e e n k n o w n f o r y e a r s t h a t sea w a t e r w i l l a t t a c k m o s t o f t h e c o m m o n m e t a l s . I n some service c o n d i t i o n s , c e r t a i n l i v i n g o r g a n i s m s i n t h e sea cause m a r i n e f o u l i n g o n t h e e x p o s e d m e t a l surfaces l e a d i n g t o c o r r o s i o n o r i n t e r f e r e n c e w i t h f u n c t i o n i n g of t h e e q u i p m e n t . 27

In SALINE WATER CONVERSION; Advances in Chemistry; American Chemical Society: Washington, DC, 1960.

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ADVANCES IN CHEMISTRY SERIES


Corrosîvîty of Sea Water A q u e o u s c o r r o s i o n i s e l e c t r o c h e m i c a l i n n a t u r e a n d i n v o l v e s t h e passage of e l e c t r i c c u r r e n t s . T h e c u r r e n t enters t h e s o l u t i o n a t l o c a l anodes a n d leaves t h e s o l u t i o n a t c a t h o d i c areas o n t h e m e t a l . I n t h e case o f steel, f e r r o u s i o n s e n t e r t h e s o l u t i o n a t t h e anode a n d hydrogen is deposited a t the cathode. V e r y p u r e w a t e r , because o f i t s h i g h r e s i s t i v i t y , m a k e s d i f f i c u l t t h e passage o f c o r r o s i o n c u r r e n t s t h r o u g h t h e s o l u t i o n f r o m a n o d i c t o c a t h o d i c areas. F o r a m b i e n t t e m p e r a t u r e s , h i g h l y p u r i f i e d w a t e r , free f r o m d i s s o l v e d o x y g e n , i s n o t c o r r o s i v e t o m e t a l s s u c h as steel. T h e a d d i t i o n o f s o d i u m c h l o r i d e t o w a t e r f o r m s a s o l u t i o n w h i c h tends t o b e c o r r o s i v e t o steel a n d o t h e r c o m m o n m e t a l s . H o w e v e r , i f t h e s o l u t i o n i s c o m p l e t e l y a i r free, i t s c o r r o s i v i t y m a y n o t b e m u c h g r e a t e r t h a n t h a t o f p u r e w a t e r . S e a w a t e r c o n t a i n s , i n a d d i t i o n : s u l f a t e , b i c a r b o n a t e , b r o m i d e , a n d fluoride i o n s . L a b o r a t o r y experience i n d i c a t e s t h a t , a t t h e same p H , s u l f a t e s o l u t i o n s t e n d t o b e less c o r r o s i v e t h a n s i m i l a r c h l o r i d e s o l u t i o n s . T h e presence o f b i c a r b o n a t e ions i n w a t e r , f r o m d i s s o l v e d C 0 , p r o m o t e s some c o r r o s i o n a t t a c k o n m a n y m e t a l s . I t also s h o u l d be m e n t i o n e d t h a t t h e p H o f s e a w a t e r n o r m a l l y ranges f o r m 7.50 t o 8.25, i n d i c a t i n g t h a t some free h y d r o x y l i o n s are a l w a y s p r e s e n t . B r o m i d e s a n d fluorides a r e v e r y c o r r o s i v e ; b u t , b e i n g p r e s e n t o n l y i n s m a l l a m o u n t s , t h e i r effects i n sea w a t e r p r o b a b l y a r e m a s k e d b y t h e v e r y h i g h c h l o r i d e c o n t e n t . I n e x p e r i m e n t a l w o r k , a c o n t i n u o u s s u p p l y o f f r e s h , c l e a n sea w a t e r i s essent i a l i f t h e effects o f s u c h m i n o r c o n s t i t u e n t s o n t h e c o r r o s i o n o f a m e t a l a r e t o b e evaluated. O n e m i g h t e x p e c t sea w a t e r t o b e c o r r o s i v e because t h e s o l u t i o n is a g o o d e l e c t r o l y t e , i t c o n t a i n s ions k n o w n f r o m experience t o p r o m o t e t h e a t t a c k o n c o m m o n m e t a l s , i t contains dissolved oxygen w h i c h promotes t h e attack—e.g., b y reacting w i t h nascent hydrogen formed a t t h e c a t h o d e — a n d i t contains a m u l t i t u d e of forms of organic life, some o f w h i c h affect m e t a l c o r r o s i o n . N a t u r a l processes, o p e r a t i n g b o t h a t t h e s u r f a c e a n d a t g r e a t d e p t h s , r e s u l t i n m o s t of t h e ocean b e i n g s t i r r e d c o n t i n u o u s l y . I t i s f o u n d t h a t t h e r e l a t i v e p r o p o r t i o n s o f d i s s o l v e d salts a r e v i r t u a l l y t h e same e v e r y w h e r e , a l t h o u g h t h e t o t a l salt c o n t e n t (salinity) m a y show appreciable variations w i t h geography. T h e more i m p o r t a n t ions f o u n d i n a sample of N o r t h Pacific Ocean surface water, as r e p o r t e d b y L y m a n a n d A b e l ( 5 ) , a r e t a b u l a t e d b e l o w . 2

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N a t u r a l vs. S y n t h e t i c S e a W a t e r . I t i s n o t a l w a y s r e a l i z e d t h a t n a t u r a l sea w a t e r i s a c o m p l e t e l y different m e d i u m f r o m s y n t h e t i c sea w a t e r f r o m t h e c o r r o s i o n s t a n d p o i n t . A p r i m a r y f a c t o r i s t h e presence o f b i o l o g i c a l effects i n n a t u r a l sea w a t e r t h a t are n o t p r e s e n t i n t h e s y n t h e t i c m i x t u r e . O n e effect i s t h a t b i o - f o u l i n g o f t h e m e t a l s u r f a c e o f t e n p r o m o t e s l o c a l i z e d a t t a c k . W h e n a b a r n a c l e o r m u s s e l a t t a c h e s i t s e l f t o a p l a t e o f stainless steel i n sea w a t e r , a differential aeration corrosion cell is formed. Intense local p i t t i n g results w h i c h m a y l e a d t o c o m p l e t e p e r f o r a t i o n i n a r e l a t i v e l y s h o r t t i m e . I n o n e case a 0 . 7 5 - i n c h t h i c k stainless steel p l a t e w a s p e r f o r a t e d b e n e a t h a b a r n a c l e base a f t e r 9 m o n t h s o f i m m e r s i o n in sea w a t e r .



FINK—CORROSION OF METALS IN SEA WATER

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A v a r i e t y o f a n i m a l s a n d p l a n t s , as w e l l as colonies of m i c r o o r g a n i s m s , m a y d e p o s i t f r o m n a t u r a l sea w a t e r o n t o t h e m e t a l s u r f a c e . T h e l i f e processes a n d d e c o m position products m a y contribute directly to attack o n the metal. F o u l i n g m a y o b s t r u c t flow i n h e a t e x c h a n g e r s a n d p i p e s , l e a d i n g t o s u c h c o r r o s i v e effects a s a r e c a u s e d b y overheating or i m p i n g e m e n t at local high-water velocities. S o m e o r g a n i s m s e n t e r a m a r i n e p i p i n g s y s t e m i n t h e i r l a r v a l stages, t h e n a n c h o r t h e m s e l v e s t o sites s u c h as i n t e r i o r t u b e w a l l s . I f these l a r v a e a r e a l l o w e d t o g r o w , t h e l o c a l t u r b u l e n c e m a y r e s u l t i n severe c o r r o s i o n . T h e o r g a n i s m s m a y b e k i l l e d b y c h l o r i n a t i o n , b y flooding t e m p o r a r i l y w i t h f r e s h w a t e r , b y h o t s e a w a t e r , o r b y t o x i c m a t e r i a l s s u c h as s o d i u m p e n t a c h l o r o p h e n o l . E v e n when t h e organisms are killed, t h e i r d e c o m p o s i n g b o d i e s m a y s t i l l c l o g t h e s e a - w a t e r s y s t e m a n d cause c o r r o s i o n . T w o facts concerning t h e corrosion behavior of metals i n sea w a t e r s t a n d o u t : A l t h o u g h there is v a r i a t i o n i n temperature, salinity, a n d content of l i v i n g o r g a n i s m s , t h e rates o f a t t a c k — e s p e c i a l l y f o r s t e e l — a r e o f t h e same o r d e r o f m a g n i t u d e a t m a n y different l o c a t i o n s t h r o u g h o u t t h e w o r l d . T h e r e l a t i v e o r d e r o f resistance o f m e t a l s as d e t e r m i n e d b y t h e i r c o r r o s i o n rates i n n a t u r a l sea w a t e r i s n o t r e a d i l y s i m u l a t e d w i t h s y n t h e t i c s e a - w a t e r p r e p a r a t i o n s i n the laboratory. F a c t o r s i n S a l t W a t e r C o r r o s i o n . Because of expediency, a n d i n spite of t h e s i t u a t i o n discussed a b o v e , m u c h w o r k h a s b e e n d o n e i n t h e l a b o r a t o r y w i t h saline s o l u t i o n s m a d e u p t o s i m u l a t e sea w a t e r . H a c h e (3) has c a r r i e d o u t s o m e v e r y i n t e r e s t i n g e x p e r i m e n t s c o n c e r n i n g t h e c o r r o s i o n o f steel i n s o d i u m c h l o r i d e s o l u t i o n s . I n F i g u r e 1, H a c h e s h o w s t h e effect of t e m p e r a t u r e o n t h e c o r r o s i o n o f i r o n b y a i r saturated a n d deaerated solutions containing 30 grams per liter of pure N a C l i n d o u b l y d i s t i l l e d w a t e r . T h e effect of o x y g e n i n i n c r e a s i n g c o r r o s i o n u p t o a b o u t 5 0 ° C . ( 1 2 2 ° F . ) i s seen. A b o v e t h i s t e m p e r a t u r e , t h e s o l u b i l i t y o f o x y g e n i s v e r y l i m i t e d . I t is f o r t h i s r e a s o n t h a t t h e c u r v e s b e c o m e t h e s a m e . I n t h e r a n g e f r o m 1 0 ° t o 5 0 ° C . (50° t o 1 2 2 ° F . ) , t h e corrosion increases at t h e rate o f a b o u t 5 % p e r ° C . S u c h a t e m perature dependence h a d been reported earlier b y P a l m a e r ( 7 ) .

Dissolved Iron, mg.

Figure I. Effect of temperature and aeration on the corrosion of iron in 3% sodium chloride solution F i g u r e 2 i l l u s t r a t e s c o r r o s i o n b e h a v i o r a n d o x y g e n s o l u b i l i t y as a f u n c t i o n o f t h e s a l i n i t y of t h e s o l u t i o n a t a fixed t e m p e r a t u r e . I t i s t o b e n o t e d t h a t a s m a l l v a r i a t i o n i n s a l i n i t y i n t h e n e i g h b o r h o o d o f t h e n o r m a l 3 . 5 % c o n t e n t o f sea w a t e r h a s n o effect o n o x y g e n s o l u b i l i t y a n d l i t t l e effect o n c o r r o s i o n r a t e .



30

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F a c t o r s i n S e a - W a t e r C o r r o s i o n . T h e effects o f t e m p e r a t u r e a n d s a l i n i t y i n sea w a t e r a r e b e l i e v e d t o b e s i m i l a r t o those f o u n d i n s a l t w a t e r , discussed a b o v e . G o o d q u a n t i t a t i v e i n f o r m a t i o n is n o t y e t r e a d i l y a v a i l a b l e . H a c h e (S) c o n c l u d e d t h a t l i g h t h a s a n i n h i b i t i n g effect o n c o r r o s i o n b y sea w a t e r a n d t h a t t h e r e s u l t s o f c o r r o s i o n tests c o n d u c t e d i n t o t a l d a r k n e s s were less p r o n e t o scatter.

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Variation of corrosion as a function of salinity at 24° C.

M a n y o t h e r p a r a m e t e r s t e n d t o influence t h e c o r r o s i o n o f m e t a l s i m m e r s e d i n sea w a t e r . W h e n t w o m e t a l s o f different p o t e n t i a l s a r e g a l v a n i c a l l y c o u p l e d , t h e a c c e l e r a t i o n of t h e a t t a c k o n t h e less n o b l e m e t a l o f t h e t w o i s o b s e r v e d f r e q u e n t l y . A s m a l l a r e a o f a n a n o d i c m e t a l c o u p l e d t o a l a r g e a r e a o f a second m e t a l t h a t i s c a t h o d i c can be p a r t i c u l a r l y dangerous. A useful guide t o help predict unfavorable c o m b i n a t i o n s i s t h e g a l v a n i c series of m e t a l s i n sea w a t e r (9). T h e reverse s i t u a t i o n — n a m e l y , a small cathode coupled t o a n anode that is large i n area—often proves satisfactory i n service. S o m e m e t a l s d e p e n d o n f o r m a t i o n o f a p r o t e c t i v e film f o r c o r r o s i o n resistance i n sea w a t e r . A f r e s h s u p p l y o f o x y g e n b r o u g h t t o t h e s u r f a c e o f t h e m e t a l t e n d s t o p r o m o t e t h e c o r r o s i o n r e a c t i o n i n some cases, a n d i n o t h e r s i t h e l p s f o r m d e s i r e d p r o t e c t i v e films. I f a c r i t i c a l v e l o c i t y o f flowing sea w a t e r i s exceeded, t h e film m a y b e e r o d e d a w a y . T h e v e l o c i t y f o r u s e f u l c o r r o s i o n resistance i s l o w f o r c o p p e r , h i g h e r f o r a l u m i n u m , c u p r o - n i c k e l s , a n d a l u m i n u m b r o n z e s , a n d h i g h e s t f o r stainless steels, Hastelloy C, and titanium. P r o t e c t i v e films o n m e t a l s also c a n b e d e s t r o y e d a n d c o r r o s i o n a c c e l e r a t e d b y t h e i m p i n g e m e n t o f a h i g h - v e l o c i t y s t r e a m o f sea w a t e r o n t o t h e m e t a l l i c s u r f a c e . T h e i n l e t e n d s o f condenser t u b e s , a r e f r e q u e n t l y a t t a c k e d ( F i g u r e 3,a). J e t tests h a v e been devised f o r r a n k i n g t h e susceptibility of metals to such impingement attack. A corrosion cell is f o r m e d between t h e bare surface d i r e c t l y under t h e j e t (anode) a n d the a d j a c e n t filmed s u r f a c e ( c a t h o d e ) . T h e s e j e t tests g i v e m o r e d e p e n d a b l e i n f o r m a t i o n , i f n a t u r a l sea w a t e r i s c o n t i n u o u s l y s u p p l i e d t o t h e e q u i p m e n t . U s u a l l y , t h e c o r r o s i o n - f a t i g u e l i m i t o f a m e t a l i n sea w a t e r i s l o w e r t h a n i n f r e s h w a t e r . R e p e a t e d flexing a l l o w s t h e c o r r o s i o n t o p r o c e e d a t t h e b r e a k s i n t h e p r o t e c t i v e film w h e r e a c t i v e m e t a l i s e x p o s e d . C a v i t a t i o n c o r r o s i o n i s c a u s e d b y r e p e a t e d p o u n d i n g r e s u l t i n g f r o m r a p i d collapse of v a p o r b u b b l e s . O n a m e t a l s u r f a c e w h e r e t h e r e i s a v i o l e n t flow o f w a t e r , s u c h a s t h a t o f a s h i p ' s p r o p e l l e r , t h e p r e s s u r e a t some a r e a m a y b e r e d u c e d t o w h e r e l o c a l i z e d boiling forms bubbles of vapor. A t a n o t h e r s i t e , these b u b b l e s s u d d e n l y c o l l a p s e .



31


T h e r e s u l t a n t h a m m e r i n g m a y , i n t i m e , cause t h e s u r f a c e l a y e r t o f a i l a n d pieces o f m e t a l t o f l a k e off. T h e a c t i v e m e t a l sites, t h u s exposed, m a y i n t u r n b e r a p i d l y a t t a c k e d b y sea w a t e r . I t h a s b e e n f o u n d d i f f i c u l t t o d e v e l o p a l a b o r a t o r y test t h a t w i l l a c c u r a t e l y s i m u l a t e c a v i t a t i o n a t t a c k as i t o c c u r s i n a c t u a l m a r i n e s e r v i c e .

Crevice corrosion

Figure 3.

Corrosion in sea water

σ. Inlet impingement attack due to excessive turbulence b. Concentration cells T h e i m p o r t a n c e of bio-fouling t o corrosion has already been mentioned. Marine f o u l i n g i s l i k e l y t o o c c u r i n a l l t h e oceans f r o m t h e A r c t i c t o t h e T r o p i c s , e s p e c i a l l y d u r i n g t h e breeding period. I n n o r t h e r n waters, fouling occurs o n l y d u r i n g t h e s u m m e r , w h e r e a s i n t h e T r o p i c s i t i s p r a c t i c a l l y c o n t i n u o u s . C o p p e r a n d some of i t s a l l o y s , high i n copper, n o r m a l l y are n o t susceptible. H o w e v e r , a n y treatment t h a t prevents t h e t o x i c c o p p e r ions f r o m f o r m i n g m a y r e s u l t i n a t e n d e n c y f o r c o p p e r t o f o u l . C o r r o s i v e a t t a c k o n some m e t a l s a c t u a l l y m a y b e s t i f l e d b y f o u l i n g . F o r m a n y m e t a l s , l o c a l i z e d a t t a c k c a n b e m o r e serious u n d e r f o u l e d areas ( F i g u r e 3 , 6 ) . A t v e l o c i t i e s of 2 t o 3 feet p e r second o r m o r e , m e t a l s u s u a l l y a r e n o t a p t t o f o u l . O n c e t h e o r g a n i s m s b e c o m e a t t a c h e d , as d u r i n g a s h u t - d o w n o r o f f - p e r i o d o f a p l a n t , e x t r e m e l y h i g h v e l o c i t i e s m a y be needed t o dislodge t h e m at t h e s t a r t - u p . D i f f e r e n t i a l a e r a t i o n c a n p l a y a n i m p o r t a n t role i n p r o d u c i n g a c c e l e r a t e d a t t a c k o n m e t a l s i n s e a w a t e r — f o r e x a m p l e , a t c r e v i c e s . T h e s e crevices m a y b e f o r m e d b y l o o s e l y a t t a c h e d m a r i n e o r g a n i s m s , b y d e b r i s , o r b y c l o s e - f i t t i n g surfaces t o w h i c h s e a w a t e r h a s r e s t r i c t e d access ( F i g u r e 3 , 6 ) . T h e c o r r o s i o n b e h a v i o r tends t o f o l l o w m u c h t h e same p a t t e r n i n c l e a n sea w a t e r t h e w o r l d o v e r . I f t h e r e i s p o l l u t i o n , i t m a y cause a n u n b a l a n c e o f t h e p H , c h a n g e t h e m a r i n e o r g a n i s m c o n c e n t r a t i o n , decrease t h e d i s s o l v e d o x y g e n , o r a l t e r t h e i o n i c b a l a n c e of sea w a t e r . P o l l u t i o n n o r m a l l y r e s u l t s i n saline w a t e r s ' b e c o m i n g c o n s i d e r a b l y m o r e aggressive t o o n e o r m o r e m e t a l s . Specific tests m u s t b e m a d e t o d e t e r m i n e t h e a p p l i c a b i l i t y of a m e t a l i n each t y p e of p o l l u t e d water. Sulfides, w h i c h are often found m s u c h w a t e r s , t e n d t o p r o m o t e c o r r o s i o n of b o t h f e r r o u s a n d n o n f e r r o u s m e t a l s .


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Scale F o r m a t i o n b y S e a W a t e r . A s s e a w a t e r i s h e a t e d ( j i ) , i t s s c a l e - f o r m i n g constituents t e n d t o become supersaturated. Scale crystals w i l l be deposited a t t h e p o i n t o f highest t e m p e r a t u r e , w h i c h m a y b e t h e m e t a l s u r f a c e t h r o u g h w h i c h heat i s being introduced. T h e bicarbonate i o n is p a r t l y transformed t o carbonate i o n a n d t o c a r b o n d i o x i d e . T h e c a r b o n a t e i o n reacts w i t h c a l c i u m , w h i c h p r e c i p i t a t e s as c a l c i u m c a r b o n a t e . A s t h e sea w a t e r i s h e a t e d s t i l l f u r t h e r , t h e r e m a i n i n g c a r b o n a t e i o n i n t h e w a t e r decomposes t o free c a r b o n d i o x i d e a n d h y d r o x y l i o n s . T h e l a t t e r c o m b i n e w i t h m a g n e s i u m w h i c h p r e c i p i t a t e s as m a g n e s i u m h y d r o x i d e . B o t h o f these s c a l e - f o r m i n g r e a c t i o n s c a n b e i n h i b i t e d b y s l i g h t l y a c i d i f y i n g the i n c o m i n g sea w a t e r . A s t h e sea w a t e r i s n e a t e d s t i l l h i g h e r , a p o i n t i s r e a c h e d a t w h i c h c a l c i u m s u l f a t e w i l l p r e c i p i t a t e . T h i s r e a c t i o n i s m u c h less r e a d i l y p r e v e n t e d t h a n t h e first t w o , a n d i t is r e p o r t e d t o o c c u r o n l y a b o v e 325° F . i n sea w a t e r h e a t e d u n d e r p r e s s u r e . T h u s , t h e r e a r e t h r e e m a j o r scale c o m p o n e n t s i n sea w a t e r : calcium carbonate, magnesium hydroxide, a n d calcium sulfate. F r o m t h e s t a n d p o i n t o f c o r r o s i o n p r o t e c t i o n , a s l i g h t a m o u n t o f m i n e r a l scale f o r m a t i o n — i f the thickness can be controlled—is desirable. T h i s helps t o e x p l a i n w h y , i n some cases, sea w a t e r i s f o u n d t o b e less c o r r o s i v e t h a n a b r i n e of s o d i u m c h l o r i d e w i t h t h e same s a l i n i t y . H o w e v e r , i n a p p l i c a t i o n s w h e r e heat is b e i n g t r a n s f e r r e d across the m e t a l w a l l , t h e presence of scale m a r k e d l y affects t h e efficiency. I n distillation units, a m e t h o d of corrosion control based o n i n t e n t i o n a l mineral-scale f o r m a t i o n w o u l d thus lead t o greater fuel consumption. T h e s e scales also c a n b e p r o d u c e d e l e c t r o l y t i c a l l y b y a p p l y i n g c a t h o d i c c u r r e n t t o t h e m e t a l s u r f a c e i m m e r s e d i n sea w a t e r . A s a r e s u l t o f t h e h y d r o x y l ions f o r m e d a t t h e c a t h o d e , t h e film o f sea w a t e r a t t h e m e t a l s u r f a c e b e c o m e s h i g h l y a l k a l i n e a n d scale is p r e c i p i t a t e d .

Metals Used în Sea Water F e r r o u s - B a s e M e t a l s . S T E E L . S t e e l finds w i d e use i n sea w a t e r , e s p e c i a l l y as a s t r u c t u r a l m a t e r i a l s u c h as f o r s h i p s , off-shore d r i l l i n g t o w e r s , p i l i n g f o r p i e r s a n d d o c k s , a n d sea w a l l s . I t i s also u s e d as p i p i n g t o h a n d l e sea w a t e r . C o m p a r e d t o o t h e r m e t a l s , steel i s one o f t h e least e x p e n s i v e m a t e r i a l s of c o n s t r u c t i o n . Steel is the basic m a t e r i a l of construction for m a n y sea-water plants. E v e n though i t m a y c o r r o d e i n some sections o f a p l a n t , s u c h as a d i s t i l l a t i o n u n i t , i t s l o w i n i t i a l cost m a y r e s u l t i n l o w e r o v e r - a l l costs i f g o o d design a n d g o o d c o r r o s i o n - c o n t r o l p r a c t i c e s are f o l l o w e d . T h e r a t e o f a t t a c k f o r i m m e r s e d c o n d i t i o n s i s f a i r l y u n i f o r m i n u n p o l l u t e d sea w a t e r a n d averages a b o u t 5 m i l s p e r y e a r . G e n e r a l a t t a c k , w h e n i t o c c u r s , h a s b e e n observed t o be a linear f u n c t i o n of t i m e — t h a t is, i t continues a t a f a i r l y u n i f o r m rate, despite a n a c c u m u l a t i o n of corrosion products o r m a r i n e g r o w t h . O f t e n , h o w e v e r , steel i s s u b j e c t t o p i t t i n g a t t a c k b y sea w a t e r . T h e deepest a t t a c k o n steel i s r e p o r t e d t o b e a b o u t 10 t o 15 m i l s p e r y e a r . T h e presence o f m i l l scale on the steel—especially w h e n i t covers a large p o r t i o n of the exposed area—significantly increases t h e r a t e o f p i t t i n g , a n d p e n e t r a t i o n s o f a b o u t 2 0 m i l s p e r y e a r c a n b e e x p e c t e d . T h i s i s a r e s u l t o f t h e m i l l scale s e r v i n g as l a r g e c a t h o d e s t o t h e s m a l l , b a r e a n o d i c areas. P i t t i n g a t t a c k o f t e n t a p e r s off w i t h c o n t i n u e d e x p o s u r e . A recent p a p e r b y F o r g e s o n , S o u t h w e l l , a n d A l e x a n d e r (2) gives t h e r e s u l t s o f a n 8 - y e a r exposure o f s t r u c t u r a l steels t o t r o p i c a l sea w a t e r . I t w a s f o u n d t h a t s m a l l a d d i t i o n s o f c o p p e r d i d n o t a l t e r t h e c o r r o s i o n o f m i l d steel. Steels c o n t a i n i n g 2 a n d 5% of n i c k e l were d e f i n i t e l y i n f e r i o r t o m i l d steel o n t h e basis o f b o t h g e n e r a l a n d l o c a l a t t a c k . Steels c o n t a i n i n g 3 a n d 5% o f c h r o m i u m w e r e m o r e r e s i s t a n t t h a n m i l d steel f o r t h e first 3 y e a r s , b u t t h e n c o r r o d e d s i g n i f i c a n t l y m o r e t h a n m i l d steel d u r i n g t h e r e m a i n i n g 5 y e a r s . T h e a m o u n t o f p i t t i n g also w a s g r e a t e r f o r c h r o m e steels d u r i n g t h e l a t t e r p e r i o d . F o r c o m p l e t e i m m e r s i o n , m i l d steel i s p r e f e r r e d . I n o t h e r e x p e r i m e n t s c o n d u c t e d i n S a n D i e g o H a r b o r , C a l i f o r n i a , t h e r a t e o f p i t t i n g f o r m i l d steel w a s f o u n d t o b e as h i g h as 60 m i l s p e r y e a r (8).




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W R O U G H T I R O N . W r o u g h t i r o n has f o u n d c o n s i d e r a b l e a p p l i c a t i o n i n m a r i n e - b a s e d s t r u c t u r e s a n d i n p i p i n g . W h i l e w r o u g h t i r o n , a c c o r d i n g t o s o m e sources, a p p e a r s t o be m o r e r e s i s t a n t t h a n m i l d steel t o g e n e r a l a n d l o c a l i z e d c o r r o s i o n b y sea w a t e r , t h e r e is n o g e n e r a l a c c e p t a n c e o f t h i s m a t e r i a l b y o p e r a t o r s o f m a r i n e p l a n t s . A d v a n t a g e is s o m e t i m e s t a k e n o f i t s d i r e c t i o n a l p h y s i c a l p r o p e r t i e s w h e r e c o r r o s i o n resistance also is needed. I t s h o u l d b e m e n t i o n e d t h a t f o r t h e same i n i t i a l cost a m u c h t h i c k e r w a l l c a n b e p u r c h a s e d i n a cast i r o n o r a steel p i p e t h a n f o r one o f w r o u g h t i r o n . Until directly comparable engineering d a t a are developed i n t h e economic use of w r o u g h t i r o n vs. steel o r cast i r o n i n s e a - w a t e r p l a n t s , m a n y e x p e r i e n c e d m a r i n e o p e r a t o r s h e s i t a t e t o s p e c i f y i t i n p l a c e o f o t h e r less e x p e n s i v e f e r r o u s m a t e r i a l s . A n o l d e r f o r m o f w r o u g h t i r o n , as p r o d u c e d i n E n g l a n d , c o n t a i n e d s l a g l a y e r s . T h e s e l a y e r s , w h e n s u i t a b l y o r i e n t e d , w e r e f o u n d i n some c o r r o s i o n e n v i r o n m e n t s t o prevent p i t t i n g attack f r o m progressing deeply into t h e m e t a l . W h i l e this behavior a c c o u n t s f o r t h e l o n g e r l i f e t i m e of t h e a n c i e n t f o r m o f t h e m a t e r i a l , i t does n o t f o l l o w (according t o m a r i n e specialists) t h a t t h e m o d e r n fibrous f o r m of w r o u g h t i r o n w i l l g r e a t l y o u t l a s t steel i n i d e n t i c a l s e a - w a t e r s e r v i c e . C A S T I R O N . T h e i r o n phase i n cast i r o n i s r e a d i l y a t t a c k e d b y sea w a t e r , as i s t h e case f o r m i l d steel. I f t h e l a y e r of g r a p h i t e l e f t w i t h t h e c o r r o s i o n p r o d u c t i s dense a n d compact, f u r t h e r corrosion tends t o be stifled. I f the layer is porous, corrosion m a y be accelerated b y t h e galvanic action between t h e graphite a n d t h e i r o n beneath. T h e a t t a c k t h e n a p p r o a c h e s a r a t e s i m i l a r t o t h a t f o u n d f o r t h e p i t t i n g o f m i l d steel. A l u m i n u m a n d A l u m i n u m A l l o y s . A l u m i n u m c a n b e e m p l o y e d i n sea w a t e r as a resistant m a t e r i a l of construction. Experiments at Fort Belvoir, Virginia, a n d elsewhere, i n d i c a t e t h a t b y p r o p e r c o r r o s i o n - c o n t r o l p r a c t i c e s , a l u m i n u m c a n b e u s e d f o r a n e n t i r e p l a n t w h i c h processes s e a w a t e r . T h e sea water entering t h e plant s h o u l d b e free o f a l l m e t a l l i c i o n s , e s p e c i a l l y c o p p e r o r n i c k e l . I t i s e s s e n t i a l , i n s u c h a p l a n t , t h a t n o c o p p e r - b a s e a l l o y s b e u s e d a t a l l a n d t h a t g a l v a n i c couples t o m o s t other metals be avoided. T o o b t a i n longer lifetime i n sea-water service, a l u m i n u m - c l a d t u b i n g is r e c o m m e n d e d . P r o p e r l y chosen, t h e c l a d d i n g acts as a s a c r i f i c i a l m e t a l a n d t h e a t t a c k w i l l n o t p e n e t r a t e i n t o t h e base m e t a l u n t i l m o s t o f t h e c l a d d i n g h a s b e e n c o n s u m e d b y c o r r o s i o n . T h e a l u m i n u m u s u a l l y chosen f o r c l a d d i n g i s b o t h m o r e c o r r o s i o n r e s i s t a n t a n d s l i g h t l y a n o d i c t o t h e base a l l o y . T h e f a c t t h a t a m e t a l i s m o r e a n o d i c t h a n a n a d j a c e n t o n e i n t h e g a l v a n i c series f o r s e a w a t e r does n o t n e c e s s a r i l y i m p l y i t w i l l corrode a t a higher rate w h e n exposed u n c o u p l e d . A l u m i n u m a l l o y s h a v e b e e n u s e d t o f a b r i c a t e h e a t i n g coils f o r t a n k e r s i n w h i c h c r u d e o i l a n d s e a - w a t e r b a l l a s t a r e a l t e r n a t e cargoes. Savings i n weight a n d i n losses due t o c o r r o s i o n a r e r e p o r t e d t o b e v e r y f a v o r a b l e c o m p a r e d w i t h m i l d steel. Stainless Steels. I n s p e c i a l c i r c u m s t a n c e s , u n d e r c a r e f u l l y c o n t r o l l e d c o n d i t i o n s , some grades o f stainless steel h a v e g i v e n g o o d service i n sea w a t e r . M o s t a u t h o r i t i e s s t r o n g l y r e c o m m e n d c a u t i o n i n t h e use o f stainless steel f o r s e r v i c e i n sea w a t e r . I n heat e x c h a n g e r s e r v i c e , stainless steels c a n b e u s e d a t m u c h h i g h e r v e l o c i t i e s t h a n most c o m m o n metals. T h e r e i s , nevertheless, t h e ever-present possibility t h a t s t r e s s - c o r r o s i o n c r a c k i n g m a y t a k e p l a c e . T h i s b e c o m e s m u c h m o r e l i k e l y as t h e m e t a l t e m p e r a t u r e i s r a i s e d . A n y p o i n t s a t w h i c h h i g h stress a n d h i g h c h l o r i d e c o n c e n t r a t i o n s o c c u r are p a r t i c u l a r l y p r o n e t o t h i s t y p e o f f a i l u r e . S t a i n l e s s steel also i s s u s c e p t i b l e t o c r e v i c e c o r r o s i o n a n d d e p o s i t a t t a c k . D i f f e r e n t i a l a e r a t i o n cells, f o r m e d b e t w e e n s t a g n a n t a n d w e l l - a e r a t e d areas o n t h e m e t a l surface, m a y promote r a p i d attack. It is found that T y p e 316, containing m o l y b d e n u m , is more resistant t o t h e i n i t i a t i o n o f p i t s . H o w e v e r , once a p i t i s s t a r t e d , t h e r a t e o f p e n e t r a t i o n m a y b e o f t h e same o r d e r as f o r a m o l y b d e n u m - f r e e a l l o y . S i n c e t h e process side i n a h e a t e x c h a n g e r m a y d e m a n d stainless steel, as i n t h e case o f a m a r i n e - b a s e d c h e m i c a l p l a n t t h a t has b e e n f o r c e d t o use sea w a t e r f o r c o o l i n g p u r p o s e s , i t i s necessary t o c o n s i d e r m e a s u r e s f o r o b t a i n i n g o p t i m u m s e r v i c e . T h e c o o l i n g w a t e r s h o u l d b e free f r o m solids w h i c h c o u l d d e p o s i t o n t h e t u b e s . T h e



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v e l o c i t y s h o u l d n o t b e a l l o w e d t o d r o p m u c h b e l o w 5 feet p e r second. C r e v i c e areas a n d s t a g n a n t areas s h o u l d b e m i n i m i z e d o r e l i m i n a t e d b y d e s i g n . R e s i d u a l a n d service stresses s h o u l d b e h e l d t o t h e l o w e s t possible l e v e l s . U n d e r s u c h c a r e f u l l y c o n t r o l l e d c o n d i t i o n s , stainless steel c o u l d g i v e g o o d s e r v i c e . S t a i n l e s s steel p u m p i m p e l l e r s are g i v i n g excellent service i n sea w a t e r . T h e same m a y b e s a i d f o r stainless steel s h i p p r o p e l l e r s n o w i n u s e o n c o a s t a l vessels. I n b o t h cases, i t i s essential t h a t t h e stainless steel n o t b e l e f t f o r l o n g p e r i o d s b e t w e e n r u n s i n s t a g n a n t sea w a t e r . S t a i n l e s s steel g e n e r a l l y w i t h s t a n d s p o l l u t e d sea w a t e r a n d p o l l u t e d b r a c k i s h w a t e r b e t t e r t h a n c o p p e r - b a s e a l l o y s . S u b s t i t u t i n g a n a u s t e n i t i c stainless screen f o r s i l i c o n b r o n z e t r a s h r a c k s h a s r e s u l t e d i n g r e a t l y i m p r o v e d service a t a west coast p o w e r p l a n t . N o r m a l l y stainless steel screens, because o f t h e crevices i n v o l v e d ( w h e r e t h e w i r e s c r o s s ) , a r e n o t r e c o m m e n d e d f o r use i n sea w a t e r . T h i s a l t e r a t i o n o f t h e u s u a l c o r r o s i o n m e c h a n i s m , p r e s u m a b l y r e l a t e d t o t h e h y d r o g e n sulfide c o n t e n t o f p o l l u t e d sea w a t e r , needs t o b e s t u d i e d . M o n e l . E x t e n s i v e use f o r h a n d l i n g sea w a t e r i s p r o v i d e d b y M o n e l . I t has g i v e n excellent service i n heat e x c h a n g e r s , p i p i n g , s h e a t h i n g t o p r o t e c t steel a t t h e h a l f - t i d e zone, v a l v e s , p u m p i m p e l l e r s , a n d fittings i n g e n e r a l . M o n e l has b e e n u s e d i n f a b r i c a t i n g d i s t i l l a t i o n u n i t s . I n o n e t y p e , a M o n e l heat e x c h a n g e r " b a s k e t " c a n b e m a d e t o flex i n o r d e r t h a t t h e s e a - w a t e r scale w i l l d r o p off, t h u s r e s t o r i n g i t s t h e r m a l efficiency. T h i s service i s c o n s i d e r e d m u c h t o o severe f o r m o s t m e t a l s . M o n e l s h a f t s , s u c h as t h e h a r d e r Κ M o n e l , a r e r e s i s t a n t t o c o r r o s i o n f a t i g u e i n s e a - w a t e r service. M a r i n e e x p e r i e n c e , i n g e n e r a l , considers M o n e l o n e of t h e best a l l - a r o u n d m e t a l s f o r h a n d l i n g sea water. C o p p e r - B a s e A l l o y s . T h e r e i s a w i d e r a n g e of c o p p e r - b a s e a l l o y s t h a t h a v e g i v e n g o o d service i n sea w a t e r . A d m i r a l t y b r a s s , 7 0 C u - 2 9 Z n - 1 S n , p l u s a n i n h i b i t o r s u c h as a r s e n i c , h a s f o u n d w i d e use as condenser t u b e s i n m a r i n e - b a s e d p l a n t s u s i n g sea w a t e r f o r c o o l i n g . W h i l e i t i s n o t so r e s i s t a n t as t h e c u p r o - n i c k e l s , i t o f t e n seems t o b e p r e f e r r e d because o f t h e l o w e r i n i t i a l cost. A n o t h e r w i d e l y u s e d a l l o y i n c o o l i n g - w a t e r service i s a l u m i n u m b r a s s , 76 C u - 2 2 Z n - 2 A l . T h e a l u m i n u m c o n t e n t i m p r o v e s t h e resistance t o v e l o c i t y a n d i m p i n g e m e n t . T h i s a l l o y seems t o b e m o r e s u s c e p t i b l e t o p i t t i n g t h a n some o f t h e o t h e r c o p p e r a l l o y s i n s t a g n a n t sea w a t e r . A r s e n i c is a d d e d as a n i n h i b i t o r of d e z i n c i f i c a t i o n . T h e h i g h - t i n b r o n z e s — e . g . , 9 0 C u - 1 0 S n — a r e k n o w n t o h a v e excellent l i f e t i m e i n sea w a t e r as condenser t u b e s . U n d o u b t e d l y , t h e i r cost has r e s t r i c t e d t h e w i d e r u s e o f these r e s i s t a n t a l l o y s . O r d i n a r y copper is n o t recommended f o r sea-water plants i f t h e water velocities are m u c h g r e a t e r t h a n 2 feet p e r second. I t remains to be demonstrated whether c o p p e r c a n b e e m p l o y e d i n c o n t a c t w i t h c o m p l e t e l y d e a e r a t e d sea w a t e r a t m u c h higher velocities. CUPRO-NICKELS. A t t h e p r e s e n t t i m e , m o s t m a r i n e o p e r a t o r s agree t h a t t h e c u p r o - n i c k e l s are t h e m o s t u s e f u l m a t e r i a l o f c o n s t r u c t i o n f o r s e a - w a t e r p l a n t s . S i n c e W o r l d W a r I I , t h e 9 0 C u - 1 0 N i a l l o y m o d i f i e d w i t h a b o u t 1.5% o f i r o n h a s b e c o m e well established. T h e l o w e r n i c k e l c o n t e n t r e s u l t s i n a cost a d v a n t a g e o v e r t h e 70 C u - 3 0 N i a l l o y . A l l o y s c o n t a i n i n g 7 0 C u - 3 0 N i o r 8 0 C u - 2 0 N i , e a c h w i t h a d d e d i r o n , are p r e f e r r e d b y some designers t o w i t h s t a n d m o r e severe c o n d i t i o n s . T h e c u p r o - n i c k e l s h a v e b e e n w i d e l y a c c e p t e d as t h e best a v a i l a b l e a l l o y f o r c o n d e n s e r t u b e s . F o r h a n d l i n g h o t sea w a t e r , t h e r e has been s o m e f a v o r a b l e e x p e r i e n c e w i t h c u p r o - n i c k e l for p u m p s a n d heat exchangers. T i t a n i u m . U n l i k e o t h e r m e t a l s , t i t a n i u m n o r m a l l y does n o t p i t , i s n o t s u s c e p t i b l e t o stress c o r r o s i o n , i s free f r o m l o c a l c o r r o s i o n u n d e r f o u l i n g o r g a n i s m s , i s free f r o m impingement a n d c a v i t a t i o n attack a t velocities w h i c h attack copper-base alloys, a n d is n o t s u s c e p t i b l e t o sulfide a t t a c k i n c o n t a m i n a t e d s e a w a t e r . Experiments with w a t e r v e l o c i t i e s a t 20 t o 50 feet p e r second s h o w n o a t t a c k o n t i t a n i u m . T i t a n i u m a n d i t s a l l o y s a r e s a i d t o b e less s u s c e p t i b l e t o m i n e r a l s c a l i n g i n sea w a t e r t h a n m o s t o t h e r m e t a l s . ( T h e c o r r o s i o n p r o d u c t s o n a m e t a l , s u c h as steel, p r o b -



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a b l y h e l p t o a n c h o r sea-scale deposits.) Although thermal conductivity is low, the o v e r - a l l efficiency, t a k i n g f u l l a d v a n t a g e o f t h e v e r y h i g h v e l o c i t i e s p e r m i t t e d , c a n b e greater t h a n w i t h m a t e r i a l used a t present i n sea-water applications i n v o l v i n g h i g h rates o f h e a t t r a n s f e r .


Methods of Protection Protective Coatings. A v a r i e t y o f p r o t e c t i v e coatings i s a v a i l a b l e f o r steel i n sea-water service. F o r such applications as ships' hulls, n o r m a l practice i s t o a p p l y compatible antifouling paint over t h e corrosion-resistant primer coating system. F i g u r e 4 s h o w s p a n e l s w h i c h h a v e r e s i s t e d f o u l i n g f o r a y e a r o r m o r e as c o m p a r e d w i t h a d j a c e n t p a n e l s w h e r e t h e e x p e r i m e n t a l c o a t i n g is n o t p r o t e c t i v e .

Figure 4.

Experiments at Battel le's Daytona Beach Marine Station

σ. Effect of cathodic protection on paint systems 6. Antifouling paint studies Cathodic Protection. Steel c a n be protected b y cathodic current, supplied e i t h e r f r o m s a c r i f i c i a l anodes o r a n e x t e r n a l d i r e c t c u r r e n t source. T h e m e t h o d is effective f o r c o m p l e t e l y i m m e r s e d s t e e l — i . e . , f o r surfaces o n s t r u c t u r e s b e l o w t h e low-tide level. C u r r e n t consumption c a n be greatly reduced b y a p p l y i n g a suitable p a i n t s y s t e m t o t h e steel b e f o r e i t i s i m m e r s e d i n s e a w a t e r . S u c h a p a i n t s y s t e m should be alkali resistant.


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A l u m i n u m c a n be c a t h o c l i c a l l y p r o t e c t e d , b u t c a u t i o n m u s t b e e m p l o y e d . I f t h e r e is t o o m u c h a l k a l i d e v e l o p e d as a r e s u l t of too h i g h a c a t h o d i c c u r r e n t , t h e a l u m i n u m w i l l b e a t t a c k e d . F i g u r e 4 shows a c a t h o d i c - p r o t e c t i o n e x p e r i m e n t b e i n g c o n d u c t e d a t Battelle's marine station. W a t e r Treatment. M a n y years have been spent i n developing corrosion i n h i b i t o r s for o r d i n a r y waters. O n l y a v e r y few such treatments have been studied f o r use i n sea w a t e r . F o r a o n c e - t h r o u g h p l a n t , c o r r o s i o n p r e v e n t i o n b y t h e u s e o f i n h i b i t o r s u s u a l l y is f o u n d t o b e t o o c o s t l y . I f sea w a t e r i s first d e a e r a t e d , o n l y a s m a l l a m o u n t of c o r r o s i o n i n h i b i t o r , i f a n y , p r o b a b l y w o u l d b e n e e d e d t o p r e v e n t a t t a c k o n steel, o r c o p p e r - b a s e a l l o y s . F o r a l u m i n u m , o x y g e n i s n e e d e d t o p r o m o t e a p r o t e c t i v e film. M o s t of t h e i n t e r e s t , so f a r , h a s been i n d e v e l o p i n g w a t e r t r e a t m e n t s f o r scale prevention, not corrosion prevention.

Design Consideration in Heat Exchangers I n choosing materials f o r sea-water, heat exchanger, o r evaporator service, m a n y f a c t o r s — s u c h as cost, a v a i l a b i l i t y , efficiency, as w e l l as c o r r o s i o n — m u s t be c o n s i d e r e d . M a t e r i a l s a v a i l a b l e i n w r o u g h t f o r m s , s u c h as t u b i n g a n d sheet, h a v e b e e n h i g h l i g h t e d i n t h i s r e v i e w . A l l o f these m e t a l s c a n b e f a b r i c a t e d b y w e l l - e s t a b l i s h e d m e t h o d s including welding. W h i l e the t h e r m a l conductivity for t h e alloys varies considerably, t h e p r o p e r t y o f i n t e r e s t t o t h e designer i s t h e o v e r - a l l r a t e o f h e a t t r a n s f e r i n s e r v i c e . F o r efficiency, t h i n p r o t e c t i v e films o n t h e s u r f a c e a r e t o b e p r e f e r r e d t o h e a v y d e p o s i t s of scale a n d c o r r o s i o n p r o d u c t s . W h e r e s p a c e a n d w e i g h t a r e i m p o r t a n t , s u c h as o n b o a r d s h i p , a n e x p e n s i v e m a t e r i a l ( s u c h as t i t a n i u m ) m i g h t cost less i n t h e l o n g r u n because o f i t s f r e e d o m f r o m m a i n t e n a n c e expense. H e a t Exchangers U s i n g C o o l i n g W a t e r . T u b u l a r - t y p e heat exchangers used i n m a r i n e service are a m o n g the m o s t critical i n a distillation p l a n t . One troublesome a r e a i n a t u b e a n d s h e l l - t y p e h e a t e x c h a n g e r i s a t t h e i n l e t ends o f t h e t u b e s . I m p i n g e m e n t c o r r o s i o n , w h i c h i s c a u s e d b y h i g h - v e l o c i t y t u r b u l e n t flow, i s e n c o u n t e r e d m o s t f r e q u e n t l y o v e r t h e first 6 i n c h e s of t h e i n l e t ends o f t h e t u b e s ( F i g u r e 3 , a ) . Beyond t h i s p o i n t , t h e flow u s u a l l y t e n d s t o b e l a m i n a r unless t h e r e i s some o b s t r u c t i o n i n t h e tube. I m p i n g e m e n t a t t a c k a t t h e i n l e t ends c a n b e m i n i m i z e d b y p r o v i d i n g a s u i t a b l e a m o u n t o f c a t h o d i c c u r r e n t . W h i l e t h i s c u r r e n t does n o t e n t e r t h e t u b e ends t o a n y g r e a t d e p t h , i t p r o t e c t s t h e first few i n c h e s . T h e c u r r e n t m a y b e p r o v i d e d b y s a c r i f i c i a l anodes made f r o m i r o n , zinc, a l u m i n u m , o r m a g n e s i u m . I f s a c r i f i c i a l anodes a r e u s e d i n c o n d e n s e r w a t e r b o x e s , i t i s d e s i r a b l e t o select a t y p e t h a t w i l l last f o r several years. P u r e zinc o r specially f o r m u l a t e d zinc anodes h a v e r e g a i n e d f a v o r i n r e c e n t y e a r s w i t h some o p e r a t o r s . H i g h - p u r i t y z i n c i s s a i d t o be s e l f - r e g u l a t i n g a n d does n o t r e q u i r e a r e s i s t a n c e t o c o n t r o l t h e c u r r e n t . I n s o m e cases, t h e t u b e sheet a n d t u b e ends also a r e c o a t e d t o r e d u c e t h e c u r r e n t d r a i n o n t h e anodes. A n o t h e r m e t h o d o f p r o v i d i n g p r o t e c t i v e c u r r e n t i s t o use t h e heads o r w a t e r b o x e s as s a c r i f i c i a l anodes. T h e s e w a t e r boxes, w h e n m a d e o f h e a v y steel o r o f cast i r o n , p r o v i d e g a l v a n i c p r o t e c t i o n t o t h e t u b e ends a n d t u b e sheet as t h e y c o r r o d e . I n c i d e n t a l l y , t h e s m a l l a m o u n t of i r o n i n t r o d u c e d i n t o sea w a t e r b y s u c h corrosion, o r b y i n t e n t i o n a l chemical a d d i t i o n , i s considered beneficial b y some a u t h o r i ties f o r p r o m o t i n g p r o t e c t i v e films o n c o p p e r - b a s e a l l o y s . R e d u c e d a t t a c k c a n also be a c c o m p l i s h e d b y flaring t h e t u b e ends t o f a c i l i t a t e s t r e a m l i n e flow. I t i s essential t h a t the cross-over area i n the head o r channel b e larger t h a n the cross-sectional area of t h e t u b e s t o r e d u c e t u r b u l e n c e . M u n r o (6) r e c o m m e n d s 1 2 5 % f o r t h e c r o s s - o v e r area i n water boxes f o r sea-water service. A l s o f r o m t h e s t a n d p o i n t of turbulence, side e n t r y i s p r e f e r r e d t o a x i a l e n t r y a t t h e f r o n t e n d o f t h e c o n d e n s e r . I f t h e f e r r o u s heads o r w a t e r b o x e s i n a c o n d e n s e r a r e l i n e d w i t h a p r o t e c t i v e c o a t i n g , o r w i t h a m e t a l s u c h a s M o n e l , t h e i n l e t t u b e ends w i l l n o l o n g e r r e c e i v e



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c a t h o d i c p r o t e c t i o n f r o m the i r o n t o c o p p e r a l l o y c o u p l e . I m p i n g e m e n t a t t a c k a t t h e t u b e ends m a y b e e x p e c t e d t o b e m o r e severe. S a c r i f i c i a l anodes, as a l r e a d y d e s c r i b e d , p r o b a b l y w i l l b e needed. F o r p a i n t e d w a t e r boxes, t h e a n o d e d e s i g n a n d p l a c e m e n t m u s t be c a r e f u l l y designed. T o o m u c h c u r r e n t m a y r e s u l t i n b l i s t e r s o n t h e p a i n t c o a t i n g a n d u n d e s i r a b l e m i n e r a l scale o n the t u b e i n l e t s e c t i o n . T h e r e a r e r e p o r t s t h a t pieces of d i s l o d g e d p a i n t c o a t i n g h a v e caused s u d d e n p l u g g i n g o f condenser t u b e s w i t h s e r i o u s loss of c o o l i n g c a p a c i t y . I n c e r t a i n c h e m i c a l p l a n t s , t h e process s o l u t i o n b e i n g c o o l e d i s u n d e r pressure o r i s v e r y c o r r o s i v e . I t i s f o u n d e x p e d i e n t i n some cases t o p u t t h e Ιολν-pressure s e a w a t e r o n the s h e l l side o f t h e h e a t e x c h a n g e r . U n d e r these c o n d i t i o n s , t h e steel s h e l l w i l l suffer m o r e r a p i d a t t a c k because o f g a l v a n i c c o u p l i n g t o t h e c o p p e r - b a s e a l l o y tubing. H o w e v e r , only the outer tubes are " s e e n " b y the shell i n this couple. Never theless, t h i s r e p r e s e n t s a l a r g e c a t h o d e . F o r e a c h c o n d e n s e r - t u b e a l l o y , t h e r e i s a r e c o m m e n d e d m a x i m u m as w e l l as m i n i m u m v e l o c i t y i n s e a - w a t e r s e r v i c e . S i l t o r o t h e r fine solids i n t h e c o o l i n g w a t e r i s a c o m p l i c a t i n g f a c t o r . I t m a y b e h e l p f u l t o raise t h e m i n i m u m v e l o c i t y t o t h e p o i n t w h e r e t h e s i l t is a l w a y s m a i n t a i n e d i n s u s p e n s i o n . I n b r a c k i s h w a t e r s , t h e same g e n e r a l c o r r o s i o n p r i n c i p l e s m a y a p p l y as f o r sea w a t e r . E x p e r i e n c e has also s h o w n t h a t t h e r e m a y b e c o n s i d e r a b l e v a r i a t i o n f r o m p l a n t t o p l a n t i n t h e p e r f o r m a n c e o f m e t a l s , e v e n w h e r e t h e c o o l i n g w a t e r comes o u t o f t h e same g e n e r a l source s u c h as a deep b a y o r e s t u a r y . T h e r e a r e c o a s t a l sites w h e r e t h e s a l i n i t y m a y s h o w v e r y m a r k e d seasonal f l u c t u a t i o n . P r o t e c t i v e film f o r m a t i o n i s essential f o r l o n g l i f e t i m e i n condenser t u b i n g . T u b e s i n s t a l l e d a t t h e season o f t h e y e a r w h e n c o n d i t i o n s are m o s t f a v o r a b l e t o f o r m p r o t e c t i v e films t e n d t o g i v e l o n g e r s e r v i c e . T h e use o f s a c r i f i c i a l anodes f o r p r o t e c t i o n o f t h e heads o f t h e h e a t e x c h a n g e r m a y n o t b e so effective i n b r a c k i s h w a t e r . T h e w a t e r m a y b e v e r y c o r r o s i v e because o f p o l l u t i o n o r c o n t a m i n a t i o n , e v e n w h e n t h e s a l i n i t y i s r e l a t i v e l y l o w . M o r e anodes, w i t h closer s p a c i n g , n o r m a l l y a r e needed t o g i v e sufficient p r o t e c t i o n i n b r a c k i s h w a t e r t h a n i n sea w a t e r . E v a p o r a t o r s a n d H e a t E x c h a n g e r s for H a n d l i n g H o t Sea W a t e r . R e l a t i v e l y l i t t l e i n f o r m a t i o n is a v a i l a b l e o n m a t e r i a l s o f c o n s t r u c t i o n f o r e l e v a t e d - t e m p e r a t u r e s e r v ice w i t h sea w a t e r . U s u a l l y , o n e also m u s t c o n s i d e r t h e a t t a c k b y flue gas, s t e a m , o r o t h e r m e d i u m o n the o t h e r side o f t h e m e t a l b a r r i e r . C u p r o - n i c k e l s are r e c o m m e n d e d f o r c o n s i d e r a t i o n i n e v a p o r a t o r s e r v i c e , e s p e c i a l l y if sea w a t e r i s t o b e h a n d l e d a t 2 0 0 ° F . o r h i g h e r . M o n e l s h o u l d g i v e g o o d s e r v i c e . T h e r e i s some i n t e r e s t i n u s i n g e i t h e r T y p e 316 stainless steel o r A l l o y 20. B e c a u s e o f the d a n g e r o f stress c o r r o s i o n , p r e c a u t i o n s m u s t b e t a k e n w i t h these l a t t e r t w o m a t e r i a l s . H a s t e l l o y C is s a i d t o b e v e r y r e s i s t a n t t o h o t sea w a t e r . F r o m a corrosion viewpoint, the m e t a l w i t h the most outstanding promise f o r seaw a t e r heat exchangers i s t i t a n i u m . L a b o r a t o r y e x p e r i m e n t s s t a r t i n g w i t h s e a w a t e r , a n d h e a t i n g u n d e r pressure t o a b o v e t h e c r i t i c a l t e m p e r a t u r e , h a v e i n d i c a t e d t i t a n i u m t o be g r e a t l y s u p e r i o r t o H a s t e l l o y C a n d M o n e l , u n d e r these c o n d i t i o n s ( 4 ) . A l l i n d i c a tions suggest t h a t t i t a n i u m is the m o s t r e s i s t a n t o f t h e c o m m e r c i a l l y a v a i l a b l e m e t a l s to sea w a t e r a t t e m p e r a t u r e s u p t o 7 5 0 ° F . A t t h e p r e s e n t t i m e , i t i s r e c o m m e n d e d t h a t t h e h e a t - t r a n s f e r surfaces o f e v a p o r a t o r s a n d t h e h e a t exchangers f o r h a n d l i n g h o t sea w a t e r b e m a d e o f 7 0 % c o p p e r , 3 0 % n i c k e l a l l o y w i t h 0 . 7 % i r o n . T h i s a l l o y has g i v e n excellent service i n heat e x c h a n g e r s a b o a r d ships under a wide v a r i e t y of service conditions. P u m p s . M a t e r i a l s of construction f o r p u m p s v a r y w i t h the t y p e of service. A t l o w e r v e l o c i t i e s , sea w a t e r has b e e n h a n d l e d s u c c e s s f u l l y i n p u m p s u s i n g casings m a d e f r o m cast i r o n . C a s t i r o n c o n t a i n i n g a f e w p e r cent o f n i c k e l i s r e p o r t e d t o b e finer g r a i n e d a n d t o h a v e s m a l l e r g r a p h i t e flakes. T h i s m a y e x p l a i n i t s s u p e r i o r p e r f o r m a n c e f o r s e a - w a t e r service c o m p a r e d w i t h o r d i n a r y cast i r o n . A f t e r a y e a r o r t w o i n sea w a t e r , a r e s i d u a l l a y e r o f g r a p h i t e f o r m s o n t h e c a s t i r o n surface. I f a cast-iron p u m p casing becomes g r a p h i t i z e d , the p o t e n t i a l relationship is r e v e r s e d a n d t h e c a s i n g m a y b e c o m e c a t h o d i c t o t h e b r o n z e i m p e l l e r . T h e l a t t e r



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can n o w be expected t o corrode a t a higher rate. M a n y operators prefer a more r e s i s t a n t m a t e r i a l f o r t h e c a s i n g , s u c h as M o n e l , a b r o n z e , o r a c u p r o - n i c k e l . T h i s a v o i d s t h e a b o v e p r o b l e m w i t h cast i r o n . A l t h o u g h i m p e l l e r s a r e c o m m o n l y m a d e o f one o f t h e b r o n z e s , m a n y o p e r a t o r s p r e f e r M o n e l a n d S M o n e l because o f t h e l o n g e r service o b t a i n e d . S e v e r a l i n s t a n c e s o f c o r r o s i o n c r a c k i n g o f a u s t e n i t i c stainless steel p u m p shafts h a v e b e e n r e p o r t e d . I t i s f o u n d t h a t p u m p s h a f t s of Κ M o n e l g i v e excellent s e r v i c e i n sea water. T h e lifetime of the p u m p , p a r t i c u l a r l y the impeller, is greatly extended i f i t is o p e r a t e d b e l o w i t s r a t e d c a p a c i t y f o r f r e s h w a t e r w h e n i t i s u s e d f o r sea w a t e r . F o r h a n d l i n g h o t sea w a t e r , C a r p e n t e r 2 0 i m p e l l e r a n d casings h a v e b e e n u s e d w i t h success. C u p r o - n i c k e l also h a s b e e n u s e d f o r p u m p s h a n d l i n g h o t sea w a t e r . I m p i n g e m e n t o r E r o s i o n - C o r r o s i o n . A c o n s i d e r a b l e n u m b e r of f a i l u r e s i n s e a w a t e r service t e n d t o o c c u r o n t h e d o w n s t r e a m side o f v a l v e s , fittings, b r a n c h c o n n e c t i o n s , e t c . E x c e s s i v e l y h i g h s e a - w a t e r v e l o c i t i e s a n d t u r b u l e n c e were i n v o l v e d . I t i s n o t c o n s i d e r e d g o o d p r a c t i c e t o c o n t r o l r a t e s of flow b y t h r o t t l i n g a t t h e v a l v e , since this introduces turbulence just b e y o n d t h e v a l v e . V a l v e s should be used for isolation s e r v i c e . G o o d h y d r a u l i c d e s i g n i s essential f o r l o n g l i f e t i m e i n a p l a n t f o r h a n d l i n g sea w a t e r . V e l o c i t i e s i n pipe's, etc., s h o u l d b e i n t h e r a n g e r e c o m m e n d e d f o r t h e m a t e r i a l i n sea water. Intake t o P l a n t . C l e a n sea w a t e r ( o r b r a c k i s h w a t e r ) free f r o m s u s p e n d e d m a t t e r is essential f r o m a n o p e r a t i o n a l s t a n d p o i n t . S h e l l fish, t r a s h , etc., i f a l l o w e d i n t o t h e w a t e r s y s t e m i n t h e p l a n t , m a y b e c o m e l o d g e d a t c r i t i c a l p o i n t s , s u c h as h e a t e x c h a n g e r tubing.

Conclusions F o r e c o n o m y i n c o n s t r u c t i o n a n d o p e r a t i o n of p l a n t s f o r h a n d l i n g s a l i n e w a t e r , particular attention m u s t be p a i d to corrosion factors. Steel can be used i n m a n y a p plications, i f i t i s protected b y the use of coatings o r is given cathodic protection. A l l o w a n c e also c a n b e m a d e i n t h e d e s i g n f o r t h e c o r r o s i o n o f steel. I f i t does n o t c o n f l i c t w i t h o t h e r d e s i g n f a c t o r s , i n c o m i n g sea w a t e r s h o u l d b e d e a e r a t e d t o c o n t r o l steel c o r r o s i o n . C o r r o s i o n i n h i b i t o r s a n d c h e m i c a l t r e a t m e n t m a y b e e m p l o y e d t o reduce t h e attack, b u t frequently this method is too costly. A l a r g e n u m b e r o f c o p p e r - b a s e a n d n i c k e l - b a s e a l l o y s ( s u c h as c u p r o - n i c k e l s , M o n e l , a n d a l u m i n u m b r a s s ) h a v e b e e n u s e d i n s e a - w a t e r service w i t h success. S p e c i a l m a t e r i a l s s u c h as H a s t e l l o y C , I l l i u m , a n d t i t a n i u m a r e a v a i l a b l e f o r e x t r e m e l y c o r r o s i v e s i t u a t i o n s . T h e e v i d e n c e , so f a r , i n d i c a t e s t i t a n i u m t o b e o u t s t a n d i n g a n d t o r a n k a b o v e o t h e r c o m m e r c i a l l y a v a i l a b l e m e t a l s i n c o r r o s i o n resistance u n d e r c o n d i t i o n s i n v o l v i n g h i g h temperature, velocity, a n d other adverse e n v i r o n m e n t a l conditions. C h l o r i n a t i o n t o a residual of 0.5 p . p . m . is w i d e l y used. I t is r e q u i r e d t o prevent f o u l i n g o f t h e M o n e l o r b r o n z e i n t a k e screens, t h e d u c t s , p i p e s , e t c . , t h r o u g h o u t t h e s y s t e m . E x p e r i e n c e h a s s h o w n t h a t c h l o r i n a t i o n , unless c a r e f u l l y c o n t r o l l e d t o l o w r e s i d u a l s , t e n d s t o increase t h e c o r r o s i v i t y o f saline w a t e r s . S a c r i f i c i a l anodes f r e q u e n t l y a r e u s e d t o p r o t e c t t h e r o t a t i n g steel f r a m e w h i c h s u p p o r t s the M o n e l screens a t t h e p l a n t i n t a k e . R e c e n t l y , a l u m i n u m has been demonstrated t o be a v e r y useful m a t e r i a l of c o n s t r u c t i o n f o r s e a - w a t e r p l a n t s . S p e c i a l care m u s t b e t a k e n w i t h a l u m i n u m t o a v o i d c o n t a c t w i t h m o s t o t h e r m e t a l s . H e a v y m e t a l ions i n t h e i n c o m i n g s e a w a t e r , s u c h as c o p p e r , m u s t b e r e m o v e d b e f o r e t h e w a t e r enters a n a l u m i n u m i n s t a l l a t i o n t o p r e v e n t l o c a l a t t a c k s u c h as p i t t i n g . W i t h greatly increasing interest i n t h e ocean for m a n y purposes, t h e corrosion experts w i l l be increasingly active i n h e l p i n g t o design corrosion-resistant a p p a r a t u s a n d plants for sea-water service.



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Acknowledgment T h e a u t h o r is indebted t o a large n u m b e r of specialists who generously c o n t r i b u t e d t i m e , experience, a n d ideas t o t h i s p a p e r . T h e author v e r y m u c h appreciates t h e s u p p o r t of t h e Office o f S a l i n e W a t e r o n a p r o j e c t c l o s e l y r e l a t e d t o t h e s u b j e c t o f t h i s paper.


Literature Cited (1) B a d g e r , W. L., a n d Associates, Inc., Ann Arbor, Mich., "Critical R e v i e w of L i t e r a t u r e o n F o r m a t i o n a n d P r e v e n t i o n of Scale," Research a n d D e v e l o p m e n t P r o g r . R e p t . N o . 25, Office of Saline W a t e r , July 1959. (2) Forgeson, B. W . , S o u t h w e l l , C . R., A l e x a n d e r , A. L., Corrosion 16, No. 3 (1960). (3) H a c h e , Α., Rev. mét. 53, No. 1 (1956). (4) L a s e r s o n , G. L., et al., "Extraction of F r e s h W a t e r f r o m Sea W a t e r in the S u p e r c r i t i c a l S t a t e , " t o the Office of Saline W a t e r , by N u c l e a r D e v e l o p m e n t Associates, Inc., W h i t e P l a i n s , N. Y., NDA 32-2, p. 36, July 31, 1954. (5) L y m a n , J o h n , Abel, R. Β., J. Chem. Educ. 35, 113-5 (1958). (6) M u n r o , J. D., Proc. Am. Petrol. Inst. 34, 19 (1954). (7) P a l m a e r , W . , J. lutte contre corrosion, 18-24 (1938). (8) P e t e r s o n , M. H., W a l d r o n , L. J., "An I n v e s t i g a t i o n of t h e C o r r o s i o n R a t e of Mild Steel in San D i e g o H a r b o r , " Natl. Assoc. C o r r o s i o n E n g r s . , 16th A n n . C o n f . , D a l l a s , T e x . , M a r c h 14-18, 1960. (9) U h l i g , H. H., ed., " C o r r o s i o n H a n d b o o k , " p . 416T, E l e c t r o c h e m i c a l S o c i e t y , 1948. RECEIVED for review July 15, 1960. A c c e p t e d July 28, 1960.


SALINE WATER CONVERSION

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