Dexter 1986 Aquacultural-Engineering

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AquaculmralEngineering 5 (1986i 333-3 45

M a t e r ia l s S c i e n c e in A q u a c u it u r al E n g i n e e r in g

Stephen C. Dexter

College of M arine Studies, University of D elaware. Lewes, Delaware 1995 8. USA

A B S T R A C T

The pert ine nt s trength an d &trabi l i~ ' propert ies o f metal l ic and

non m etal l ic m ater ials used in the construct ion o f aquacul tural sys tems

are ou tl ined . Re cen t advances in max imiz ing the per formance o f

mater ials fo r c om po ne nts such as nets, cages , f loats a nd screens use d in

f loat ing sy s tems are reviewed. Ma ter ials req uirements are discussed jbr

r ig id versus f l ex ib le sys tems and for min imiz ing prob lem s in mar ine

systems cau sed by biofouling. C om po ne nts o f f ix ed s tructures such as

water in take and supply l ines , pumps and pond sys tems are also con-s idered. Ma ter ials select ion fo r aquacu l tural sys tems is d iscussed in

terrns of t rade-off~ betw een durabi l i ty a nd perform anc e on th e one

hand and cap i ta l and main tenance cos t s on the o ther . New deve lop-

ments in bo th meta l l i c and nonmeta l l i c compos i t e mater ia l s and in

seawater resistant stainless steels may benefi t aquacultural engineering

in th e j~Lture.

I N T R O D U C T I O N

O v e r t h e p a st d e c a d e , t h e r e h as b e e n a t r e m e n d o u s g r o w t h o f i n te r e st

i n a v a r i e t y o f t y p e s o f m a r i c u l t u r e s y s t e m s . A l o n g w i t h t h i s g r o w t h ,

h o w e v e r , t h e r e h a s n o t b e e n a c o m m e n s u r a t e i n c r e a se i n t h e ap p l ic a -

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

s y s t e m s . T h e p u r p o s e o f t h is p a p e r is t o r e v i e w w h a t h a s b e e n

r e c o r d e d i n t h e r e c e n t m a r i c u l t u r e a n d r e l a t e d l i t e r a t u r e c o n c e r n i n g

m a t e r i a l s s e l e c t i o n a n d m a t e r i a l s p e r f o r m a n c e , g iv e a n o v e r v i e w o f t h e

p e r t i n e n t p r o p e r t i e s o f s t r u c t u r a l m a t e r i a l s o f i n t e r e s t i n t h e c o n s t r u c -

t i o n o f m a r i c u l t u r e s y s t e m s , a n d t o d i s c u s s s o m e o f th e i s su e s in v o l v e d

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

333

Aquacultural Engineering 0 1 4 4 -8 6 0 9 /8 6 /S 0 3 .5 0 - © E l sev ie r Ap plied Sc iencePublishers Ltd. England, 1986. Printed in Great Britain



334 S . C D e x t e r

Four principal journals were searched for articles discussing

materials and their selection for, and/or performance in, aquacultureand mariculture systems. These four journals were: Aquaculture(1976-84), Journal of Shellf ish Research and Journal of WorldMaricuhure Socieo' (1981-84) and Aquacuhural Engineering(1982-84). This search revealed that a wide variety of metallic and

nonmetallic structural materials are in common use in mariculture

systems. The new developments in advanced materials utilization have

been in the areas of materials used mostly for containing the

organisms being cultured, or for protecting them from predators, or

for providing a suitable substratum for attachment and growth of theorganisms, or a combination of these. The primary issues addressed

have been the durability of the materials in terms of their resistance to

corrosion and other types of degradation in the aquatic environment,

their compatibility with the cultured organisms and their relative costand maintenance requirements.

Neudecker (1982) has discussed the use of galvanized iron con-

tainers for growing oysters on square mesh plastic netting stapled tooak frames, as opposed to the earlier plastic tray system, in German

coastal waters. Heavy fouling encrustations were a major problem atsome of their test sites. Systems for the culture of Sydney Rock

Oysters in New South Wales estuarine waters have been described by

Wisely et al. (1979, 1983). They describe a galvanized steel cage

system fitted with either galvanized wire or high density polyethylene

plastic mesh for the floors and walls.Pfister and Romaire (1983) described experiments on the catch

efficiency of crawfish traps as a function of trap design and the use of

PVC plastic coated versus galvanized wire. They concluded that traps

made with plastic coated wire cost about $90 more than those made

with galvanized wire. The increase in commercial life of the trap from

one to two }'ears for the galvanized wire traps to three to five years for

the plastic coated wire traps, however, more than compensated for the

increase in cost.

A considerable amount of work has been done to improve the

corrosion and fouling resistance of fish cage systems. Biofouling canbe a particularly serious problem for marine aquaculture systems

operating on a continuous basis. Fouling on nets, cages, floats and

intake screens increases weight and drag, reduces buoyancy and water



Materials sc ience in aquac u lm ra l eng ineering 3 3 5

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

n u m b e r o f n e t s y s t e m f ai l u re s in l ar g e c o m m e r c i a l m a r i n e a q u a c u l t u r e

o p e r a t i o n s h a v e a l r e a d y o c c u r r e d , w i t h b i o fo u l i n g b e i n g a m a j o r c o n -

t r i b u t o r ( H u g u e n i n a n d A n s u i n i , 1 9 7 5 ).

H u g u e n i n a n d A n s u i n i ( 1 9 7 8 ) h a v e r e v i e w e d t h e t e c h n o l o g y ,

e c o n o m i c s a n d p r o b l e m s a s s o c i a t e d w i t h v a r i o u s c o m m o n l y u s e d fi sh

cage sys tems . The i r r ev iew covers the use o f : (1 ) f l ex ib le cages used

b o t h a s in d i v i d u a l u n i ts a n d a s n e s t e d s y s t e m s , m a d e m o s t ly f r o m

n y l o n a n d o t h e r s y n t h e t i c n e t t i n g a n d k n o t l e s s m e s h ; (2 ) r i n d c a g e

s y s t e m s m a d e w i t h g a l v a n i z e d , v i n y l - c o a t e d g a l v a n i z e d a n d

c o p p e r - n i c k e l a l l o y m e s h , a n d g a l v a n i z e d , w o o d , a l u m i n u m a n d f i b -e r g l a s s s t r u c t u r a l m e m b e r s ; a n d ( 3 ) v a r i o u s m o o r i n g a n d f l o a t a t i o n

m a t e r i a l s a n d c o n f i g u r a t i o n s . D e s i g n r e q u i r e m e n t s , i n c l u d i n g t h o s e

requ i rements fo r cor ros ion and fou l ing res i s tances , a re o f ten s i t e spe -

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

( river , e s tua ry , bay , l agoon o r ope n ocean) , wa te r qua l i ty and temp era -

t u re , p r o b a b i l it y o f s t o r m o c c u r r e n c e s a n d m a r g i n o f s a f e ty b a l a n c e d

a g a i n s t c o n s e q u e n c e s o f s y s t e m f a i l u r e . A s e x a m p l e s o f r e c u r r i n g

mate r ia l s - re la ted p rob lems wi th these types o f sys tems they c i t e : (1 )

b io fou l ing , wi th the consequen t inc rease in bo th we igh t and d rag ; (2 )cor ros ion o f the meta l l i c componen ts ; and (3 ) u l t ra -v io le t de te r io ra -

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

s y s t e m s e rv i c e l if e a n d i n c r e a s e m a i n t e n a n c e a n d r e p l a c e m e n t c o st s,

n o t t o m e n t i o n l o ss o f p r o d u c t a n d r e v e n u e s i n t h e c a s e o f la r ge s ca l e

sys tem fa i lu re .

I n o r d e r t o i n c r e a s e s y s t e m r e l ia b i l it y a n d p r o v i d e h i g h e r s t re n g t h

for hand l ing inc reas ed p ro du c t loads in the l a rge r cages , severa l types

of r ig id , fou l ing res i s tan t cages a re be ing deve loped . Ansu in i and

H u g u e n i n ( 1 9 7 8 ) h a v e d e s c r i b e d a p r o t o t y p e l o w - m a i n t e n a n c e c a g es y s t e m c o n s i s t i n g o f a f i b e r g l a s s f r a m e c o v e r e d w i t h 9 0 - 1 0

c o p p e r - n i c k e l e x p a n d e d m e t a l m e s h . T h e s e c a g e s w e r e f o u n d t o h a v e

b o t h o p e r a t i o n a l a n d e c o n o m i c a d v a n t a g e s o v e r t h e m o r e c o m m o n l y

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

m a i n t e n a n c e a n d r e p l a c e m e n t c o s ts , r e d u c e d p r o b a b i l i t y o f s y s t e m

s t ru c t u r a l f a il u re , a n d i n c r e a s e d p r o d u c t i v i t y f r o m i m p r o v e d w a t e r c ir -

cu la t ion due to a reduc t ion in ne t c logg ing by fou l ing . O th er fea tu res

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

ments used a t h igh s t ress po in t s o f the f ibe rg lass f rame , fa s teners o f



336 S. C De x t e r

copper-nickel or monel electrically isolated from the galvanized

components by plastic bushings to avoid galvanic corrosion problems,

and a floatation system of treated wood and polystyrene boxes

sheathed with 10 mils (254 k~m) of 90-10 copper-nickel to provide

fouling resistance and protection from slush ice.

In their analysis of system economics, Ansuini and Huguenin

(1978) found that converting from a flexible cage system to modular

rigid cage system doubled the containment cost per unit volume, and

then upgrading from galvanized to copper alloy mesh added another

50% surcharge. Despite the higher acquisition cost, however, the

increased performance of the copper alloy cages resulted in aprojected yearly cost including maintenance and replacement of about

40% less for the rigid copper alloy cages as compared to the nylon net

cages. Moreover, they found that the greater fouling resistance of the

copper-nickel cages allowed a greater water flow into the cage

interior, and thus better utilization of the food available in the water.

A different approach to fouling control has been taken by Blair e t

a l . (1982). They utilized a partially submerged rectangular cage based

on a rigid polypropylene frame with mesh panels of either plastic or

copper-nickel held to the frame with strips of hardwood and nylonnuts and bolts. The cage could be rotated by inflation and deflation of

neoprene bladders located along each of the four corners of the

rectangle. At any given time, the mesh side currently out of the water

could be removed for maintenance and cleaning. Biofouling was thus

controlled with this cage system by stepwise 90 ° rotations at weekly

intervals. The total maintenance time for this setup was reduced by a

factor of about 25 compared with that for a conventional flexible net

system at the same location over a period of eight months in Dunstaff-

nage Bay, UK.Far less has been written in the aquaculture and mariculture litera-

ture about materials selection for fixed structures, such as water intake

and supply lines, pumps, pond systems and the like. Bouck (1981) has

contrasted the construction and maintenance of a seawater intake and

supply system built from PVC pipe with a more conventional system

of concrete-lined ductile iron pipe. Many references are made to the

use of PVC for pipes, lines and valves; fiberglass for building or lining

tanks and trays; and concrete for tanks and ponds (McVey, 1983).

Some papers mention the materials of construction in passing, othersmake no mention of them at all. Rarely is there given any rationale for



Materials science in aquaculmral engineering 3 3 7

s e l e c t i o n o f t h e m a t e r i a l s b e i n g u s e d , o r a c r it ic a l a n a ly s i s o f t h e p e r -

f o r m a n c e o f t h e s e m a t e r ia l s i n s er v ic e . T h i s l a ck o f i n f o r m a t i o n is

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

s y s te m s a r e r e l a ti v e ly s im p l e , a n d t h e m a i n p r o b l e m s f a c e d b y t h e

m a r i c u l t u r e i n d u s t r y h a v e b e e n b i o l o N c a l a n d b i o c h e m i c a l , r a t h e r

t h a n m a t e r i a l s r e l a t e d .

A g r e a t d e a l o f i n f o r m a t i o n , h o w e v e r , i s a v a i l a b l e i n t h e m a t e r i a l s

s c i e n c e li t e ra t u r e a b o u t t h e m a t e r i a ls t h a t w o u l d n o r m a l l y b e u s ed in

aquaculture/mariculture s y s t e m s . T h e t y p e o f i n f o r m a t i o n a v a i l a b l e

a n d t h e s o u r c e s f r o m w h i c h it c a n b e o b t a i n e d w i ll b e b r i ef ly r e v i e w e d

i n t he ne x t s e c t i on .

C H A R A C T E R I S T I C S O F C A N D I D A T E M A T E R I A L S

A s u m m a r y o f m a n y o f t h e p e r t i n e n t p r o p e r t i e s o f b o t h m e t al li c a n d

n o n m e t a l l i c s t r u c t u r a l m a t e r i a l s o f i n t e r e s t i n m a r i c u l t u r a l s y s t e m s i s

g i ve n i n T a b l e s 1 , 2 , a n d 3 . A l a r ge vo l um e o f l i t e r a t u r e e x i s ts f r om

w h i c h th e d a t a i n t h e s e ta b l es w e r e t ak e n . A n u m b e r o f s u m m a r y

v o l u m e s , w h i c h t h e r e a d e r s h o u l d c o n s u l t fo r f u r th e r i n f o r m a t i o n o nt h e p r o p e r t i e s o f t h e s e m a t e r i a l s a n d t h e i r r e s i s t a n c e to d e t e r i o r a t i o n

in n a t u r a l a q u e o u s e n v i r o n m e n t s , in c l u d e : L a Q u e , 1 9 7 5 ; F i n k a n d

B o y d , 1 9 7 8 ; D e x t e r , 1 9 7 9 ; S c h u m a c h e r . 1 9 7 9 ; A m e r i c a n S o c ie t y f o r

M e t a ls ( A S M ) , 1 9 8 1 ; C o b u r n , 1 9 8 4 ; a n d U h l ig a n d R e v i e , 1 9 8 5 .

T h e m e t a l s a n d a l lo y s l is te d in T a b l e 1 r a n g e f r o m s t r u c t u r a l s te e ls

a n d a l u m i n u m a l l o y s t h a t a r e i n e x p e n s i v e , b u t r e q u i r e p r o t e c t i v e

m e a s u r e s a g a i n s t e l e c t r o c h e m i c a l c o r r o s i o n i n o r d e r to o b t a in a r e a so n -

a b l e s e r v i c e li f et im e , to c o p p e r a l l o y s t h a t a re m o r e c o r r o s i o n r e s is t a n t

a n d h a v e s o m e a n t i f o u l i n g p r o p e r t i e s , t o t h e s t a i n l e s s s t e e l s a n d s u p e r -

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

h i g h e r i n i t i a l c o s t s . L o w c o s t s t r u c t u r a l s t e e l c o r r o d e s m o r e o r l e s s

u n i f o r m l y o v e r its e n t i r e e x p o s e d s u r f a c e , b u t i t d o e s s o in a p r e d i c t -

a b l e m a n n e r t h a t c a n u s u a l ly b e a c c o u n t e d f o r i n th e d e s ig n o f a

s y s t e m .

T h e c o r r o s i o n r a t e o f s te e l i n a e r a t e d n a t u r a l w a t e rs d e p e n d s

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

o n t h e t e m p e r a t u r e a n d s a li n it y o f t h e w a t e r. T h e e f fe c ts o f f r e sh a n d

s e a w a t e r c h e m i s t r ie s o n t h e c o r r o s i o n t y p e a n d r a te o f s te e l a n d o t h e r

s t r u c t u r a l a l l o y s h a s b e e n r e v i e w e d b y D e x t e r a n d C u l b e r s o n ( 1 9 8 0 ) .



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3-*0 S.C. Dexter

I n d e a e r a t e d e n v i r o n m e n t s c o n t a i n i n g su lf a te s , t h e c o r r o s i o n r a te o f

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

b a c t e r i a ( s e e U h l i g a n d R e v i e , 1 9 8 5 ).

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

a c c e p t a b l e l im i t s b y t h e u s e o f c o a t i n g s t o g e t h e r w i t h c a t h o d i c p r o t e c -

t io n . C o a t i n g s f u n c t i o n b y i s o l a t in g t h e u n d e r l y i n g m e t a l f r o m t h e c o r -

r o s iv e e n v i r o n m e n t . C a t h o d i c p r o t e c t i o n i s a n e l e c t r o c h e m i c a l

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

r o d i n g s t r u c t u r a l m e t a l (s t ee l i n th is c a s e ) b y m e a n s o f a p o w e r s u p p l y

a n d a n a u x i li ar y e l e c t ro d e . T h e c a t h o d i c p r o t e c t i o n c u r r e n t is a p p l i e d

i n a d i r e c t i o n s o a s t o o p p o s e , a n d n e g a t e , t h e n a t u r a l ly o c c u r r i n g c o r -r o s i o n c u r r e n t . C o a t i n g s u s e d t o g e t h e r w i t h c a t h o d i c p r o t e c t i o n

r e d u c e t h e s u r fa c e a r e a o f b a r e m e t a l t o b e p r o t e c t e d . T h i s r e d u c e s

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

c a t h o d i c p r o t e c t i o n s y s t em , t h u s m a k i n g it m o r e e c o n o m i c to o p e r a te .

C o n v e r s e l y , as t h e c o a t i n g d e t e r i o r a t e s o v e r t i m e , th e c a t h o d i c p r o t e c -

t i o n s y s t e m a u t o m a t i c a l l y p r o t e c t s t h e m e t a l a t d e v e l o p i n g f la w s in t h e

coa t i ng .

T h e c o p p e r a n d c o p p e r - n i c k e l a ll o y s i n T a b l e 1 a ls o t e n d t o

c o r r o d e i n a u n i f o r m a n d p r e d i c t a b l e m a n n e r . I n a d d i t i o n , a s t h e yc o r r o d e , t h e y r e l e a s e c u p r o u s i o ns i n t o t h e b o u n d a r y l a y er o f w a t e r i n

w h i c h t h e y a re i m m e r s e d . I n s u f fi c ie n t c o n c e n t r a t io n s , t h e s e c u p r o u s

ion s a re tox ic to m ar in e fou lim,~ or~anisms,~ g iv ing r i s e to the an t i -

f o u l i n g p r o p e r t i e s o f t h e s e a l l o y s . I t w a s e s t i m a t e d b y L a Q u e i n t h e

1 9 3 0 s ( se e L a Q u e , 1 9 7 5 ) th a t th e m i n i m u m c o r r o s i o n r a t e t ha t w o u l d

r e le a s e e n o u g h c o p p e r i o n s to m a k e t h e a l lo y a n t if o u l in g w a s 5 m g p e r

1 0 0 c m - " o f e x p o s e d s u r f a c e a r e a p e r d ay ' o f e x p o s u r e . T h i s c o r r e -

s p o n d s t o a u n i f o r m s u r fa c e p e n e t r a t i o n o f a b o u t 0 - 0 0 0 8 in (0 "0 2 m m )

y e a r - j . I f f o r a n y r e a s o n t h e c o r r o s i o n r at e o f e v e n p u r e c o p p e r fa llsbe l ow t ha t r a te , t he m e t a l w i l l l o s e i ts an t i f ou l i ng p r ope r t i e s . T h us , i f a

c o p p e r - n i c k e l a l l o y i s c h o s e n f o r a p a r t i c u l a r a p p l i c a t i o n b e c a u s e o f

i t s a n t i f o u l i n g p r o p e r t i e s , i t w i l l b e c o u n t e r p r o d u c t i v e t o a t t e m p t t o

r e d u c e i ts c o r r o s i o n r a te b y u s e o f c o a ti n g s o r c a t h o d i c p r o t e c t i o n .

T h e v a r i o u s r e q u i r e m e n t s o f a n,,' l a rg e e n g i n e e r i n g s y s t e m u s u a l l y

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

s ary . T h u s d i f f e re n t m e t a l s a re o f t e n p u r p o s e l y jo i n e d t o g e t h e r i n e l e c-

t r i ca l con t ac t . T h i s g i ves r i s e t o t he pos s i b i l i t y t ha t one o f t he me t a l s

w i l l b e d a m a g e d b y g a l v a n i c c o r r o s i o n . T h i s p r o b l e m c a n u s u a l l y b e

s o l v e d b y r e c o g n i z i n g w h e r e i t m a y o c c u r , p r e d i c t i n g w h i c h m e t a l w il l



Materials science in aqu acu lmr al engineering

TABLE 2Galva mc Ser i e s o f M eta ls an d Al loys in Seawa te r ( a f t e r Dexte r . 1 979)

341

Allov Potential range on

saturated calomel scale (l')

Gra phi t e (o r g raph i t e - con t a in ing p l ast ic ) + 0 -30 t o + 0 -20

Plat inum + 0"35 to + 0.20

Ha ste l loy C- 27 6 + 0" 10 to - 0 .04

Una l loyed t i tan ium + 0" 06 t o - 0 -05

304 and 31 6 s ta inless s tee l (pass ive) 0-00 to - 0 .15

M one l 400 - 0" 04 t o - 0 . 14

9 0 - 1 0 c o p p e r - n i c k e l - 0 "2 1 t o - 0 . 28

C op pe r - 0"28 to - 0"36

304 and 316 s ta inless s tee l (ac t ive) - 0 .35 to - 0 .57

AISI 1020 s teel - 0 .60 to - 0 .70

Alum inum a l loys - 0 . 70 t o - 0 . 90

Z i n c - 0 - 9 8 t o - 1 .0 3

M agnesium - I '60 to - 1"63

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

m e a s u r e . P r e d i c t i o n o f w h i c h m e t a l w i l l b e d a m a g e d w h e n d i s s i m i la rm e t a l s a r e j o i n e d t o g e t h e r i s d o n e b y m e a n s o f a g a l v a n i c s e r i e s , s u c h

a s t h e o n e s h o w n in T a b l e 2 f o r m e t a ls a n d a l lo y s in s e a w a t e r . W h e n

t w o m e t a l s f r o m t h a t t a b le a r e p l a c e d in c o n t a c t i n s e a w a t e r , t h e o n e

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

r o s i o n . T h e d e g r e e o f a c c e l e r a t i o n d e p e n d s o n t h e r e l a t i v e e x p o s e d

a r e a s o f t h e t w o m e t a l s a n d o n t h e i r d i s t a n c e o f s e p a r a t i o n o n t h e

c h a r t . T h u s , a n a l u m i n u m a l lo y w i l l b e s e v e r e l y d a m a g e d b y c o n t a c t

w i th a c o p p e r a l l o y , a n d e v e n m o r e s o b y c o n t a c t w i th a g r a p h i t e -

c o n t a i n i n g p l a s t i c , w h i l e o n l y m o d e r a t e l y d a m a g e d b y c o n t a c t w i th t h e

s a m e e x p o s e d a r e a o f s te e l. F o r d e t a i l e d i n s t r u c t i o n s in h o w t o u s e t h e

g a l v a n i c s e r i e s s e e D e x t e r ( t 9 7 9 ) o r U h l i g a n d R e v i e ( 1 9 8 5) . G a l v a n i c

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

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

c o a t i n g s o n s t e e l ( g a l v a n i z i n g ) p e r f o r m t hi s f u n c t i o n . T h e z i n c

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

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

T h e a l u m i n u m , s t a i n le s s s t e e l a n d s u p e r a l l o y s i n T a b l e 1 d o n o t

c o r r o d e u n i f o r m l y i n a q u e o u s e n v i r o n m e n t s . W h e n t h e y c o r r o d e a t

a ll , th e y d o s o b y l o c a l i z e d f o r m s o f a t t a c k s u c h a s p i t t i n g , c r e v i c e c o r -



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Materials science in aquacultural engineering 343

r o s i o n a n d s tr e ss c o r r o s i o n c r a c k i n g . T h e s e ~ ' p e s o f c o r r o s i o n a r e

m o r e d a n g e r o u s t h a n i s u n i f o r m c o r r o s i o n b e c a u s e t h e y a r e m o r e

d i f f ic u l t t o de t e c t a n d c a n l e a d t o f a i l u r e o f a s t r u c t u r e w i t h li tt le w a r n -

i n g a n d w i t h l i t t l e o v e r a l l l o s s o f m e t a l . T h e b e s t d e s i g n p r o c e d u r e i n

t he s e c a s e s i s t o a vo i d u s i ng a l l oys t ha t a r e s u s c e p t i b l e t o l oc a l i z e d

c o r r o s i o n i n a g i v e n e n v i r o n m e n t . E x a m p l e s a r e t h e s u b s t i t u t i o n o f

s t ai n le s s s te e l ty p e 3 1 6 L f o r t y p e 3 t 6 t o a v o i d i n t e r g r a n u l a r c o r r o s i o n

o f w e l d m e n t s , a n d t h e u s e o f t y p e 2 9 - 4 o r a s u p e r a l l o y in p la c e o f 3 0 4

a nd 316 i n c a s e s w he r e t he l a t t e r t w o a l l oys ha ve i n s u f f i c i e n t r e s i s t -

a n c e t o p it ti n g o r c r e v i c e c o r r o s i o n .

S e v e r a l n o n m e t a l l i c m a t e r i a l s c o m m o n l y u s e d i n m a r i c u l t u r es y s te m s a r e s h o w n i n T a b l e 3 . D e g r a d a t i o n o f s u c h m a t e ri a ls i n n a tu r a l

a q u e o u s e n v i r o n m e n t s n o r m a l ly t ak e s p l a ce b y o n e o f t w o m e c h a n i s m s ,

T h e f ir st is w e a t h e r in g . T h i s i n v o lv e s a c o m b i n a t i o n o f w e t / d r y ( a n d

s o m e t i m e s f r e e z e / t h a w ) c y c li n g a n d u l tr a - v io l e t ir r a d i a t io n f r o m s u n -

lig h t. T h e s e c o n d i n v o lv e s w a t e r a b s o r p t i o n a n d t h e c o n s e q u e n t

c h a n g e s i n d i m e n s i o n s a n d p h y s i ca l a n d m e c h a n i c a l p r o p e r t ie s . S u c h

c h a n g e s i n p r o p e r t i e s a r e u s u a l ly , b u t n o t a l w a y s , d e t r i m e n t a l , a n d c a n

r a n g e f r o m i n s ig n i fi ca n t t o a 3 0 - 5 0 % r e d u c t i o n . M a n y o f t h e s e

m a t e r i a l s h a v e s p e c i a l p r o p e r t i e s t h a t m a k e t h e m a t t r a c t i v e f o r c e r t a i na p p l ic a t io n s . T h e f o a m s , w o o d s a n d p o l y p r o p y l e n e a r e li g ht er t h a n

w a t e r a n d c a n s e r v e a s fl o a t a t io n m a t e r i a ls . P o l y e t h y l e n e a n d t e f lo n

a r e c h e m i c a l l y i n e rt , n o n t o x i c a n d h a v e v e r y l o w w a t e r a b s o r p t i o n .

N y l o n , o n t h e o t h e r h a n d , ab s o r b s 7 - 1 0 % w a t e r o v e r a p e ri o d o f

s e v e r a l m o n t h s i m m e r s i o n . A s t h i s a b s o r p t i o n o c c u r s , t h e m a t e r i a l

s w e l ls a n d t h e m e c h a n i c a l p r o p e r t i e s d e c r e a s e . S w e l li n g c a n b e a

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

t a i n e d , b u t it c a n a l so b e b e n e f i c i a l. S w e l l in g o f n y l o n f a s t e n e r s u n d e r

w a t e r c a n h e l p p r e v e n t t h e i r l o o s e n i n g u n d e r v i b r a t i o n a l l o a d i n g .K e v l a r f ib e r s h a v e t h e h i g h e s t st r e n g th - a n d m o d u l u s - t o - w e i g h t r at io s

o f a n y m a t e r i a l a v a i l a b le f o r m a k i n g c a b l e s . C o n c r e t e is a h i g h ly v e r s a -

t il e m a t e r i a l a n d is m a d e e v e n m o r e s o b y v i r t u e o f its c o m p a t i b i li t y

w i t h t h e s t e e l b a r s u s e d f o r r e i n f o r c e m e n t . S t e e l a n d c o n c r e t e h a v e

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

n o t c o r r o d e i n t h e al k a li n e e n v i r o n m e n t o f w a t e r s a t u r a te d c o n c r e t e

( p H 1 2 ) . O n l y w h e n t h e c o n c r e t e b e g i n s t o d e t e r i o r a t e , o r t h e p o r e

w a t e r b e c o m e s c o n t a m i n a t e d w i th h ig h c o n c e n t r a t i o n s o f c h l o r i d e

i o n s ( f r o m s e a w a t e r o r r o a d d e i c i n g s a lt s) d o e s c o r r o s i o n o f t h e r ei n -

f o r c i ng s t e e l be g i n .



344 ~ C D e x ~ r

M A T E R L A L S S E L E C T I O N A N D N E W D E V E L O P N L E N T S

T h e r e a r e a l w a y s t r a d e - o f f s i n v o l v e d i n th e p r o c e s s o f m a t e r ia l s s e le c -

t io n . T h e m a t e r ia l s t h a t a r e t h e m o s t c o r r o s i o n r e s i s t a n t a r e u s u a ll y t h e

m o s t e x p e n s i v e . A n i m p o r t a n t a s p e c t o f m a t e r i a ls s e l e c ti o n t h at is n o t

o f t e n g i v e n s e ri o u s c o n s i d e r a t i o n is t h e q u a l it y o f t h e p e r s o n n e l t h a t

w i ll b e u s i n g a n d m a i n t a i n i n g t h e s t r u c t u r e d u r i n g i ts s e r v ic e l if e ti m e .

I n g e n e r a l , i t m a y b e s a i d t h a t t h e m o r e t h e s e p e o p l e k n o w a b o u t

p o t e n t i a l c o r r o s i o n p r o b l e m s a n d h o w t o s o l v e t h e m , t h e m o r e f r e e -

d o m o n e h a s t o s e l e c t l e s s t h a n i d e a l m a t e r i a l s c o m b i n a t i o n s f r o m a

c o r r o s i o n p o i n t o f v ie w . C o n v e r s e ly , if t h e s e p e o p l e k n o w a n d / o r c a r el it tl e a b o u t m a t e r i a l s a n d c o r r o s i o n t h e n t h e d e s i g n e r is w e l l a d v i s e d t o

e l i m i n a t e t h e p o t e n t i a l p r o b l e m s i n t h e d e s i g n s ta g e b y s p e c i ~ 'i n g h ig h

q u a li ty , c o r r o s i o n r e s i s t a n t a n d c o m p a t i b l e m a t e r i a l s sy s t e m s .

E x p e r i e n c e h a s t a u g h t th a t h ig h l y i n a p p r o p r i a t e m a t e r ia l s c o m b i n a -

t i o n s s u c h a s a l u m i n u m a l l o y s i n con t a c t w i t h l a r ge ba r e a r ea s o f s t a i n -

l e s s s t e e l a l l o y s i n s e a w a t e r c a n b e m a d e t o w o r k f o r a n a c c e p t a b l y

l o n g ti m e i f t h e p e o p l e i n c h a r g e o f m a i n t e n a n c e k n o w w h a t t h e p ro b -

l e m s a re a n d w i ll b e m e t i c u l o u s i n t h e i r d e t a il e d m a i n t e n a n c e p r o c e -

d u r e s . I f t h i s t y p e o f p e r s o n n e l i s n o t a v a i l a b l e , h o w e v e r , s u c h am a t e r ia l s c o m b i n a t i o n s h o u l d b e a v o i d e d . A n y t im e t h a t ex t e n s iv e

m a i n t e n a n c e p r o c e d u r e s w i ll b e d i f fi cu l t o r u n d e s i r a b l e , o r t h e p r o p e r

p e r s o n n e l w i l l n o t b e a v a i l a b l e , o r t h e c o n s e q u e n c e s o f a s t r u c t u r a l

f a i l u r e w i l l b e u n a c c e p t a b l e , t h e d e s i g n e r s h o u l d c h o o s e t o o p t i m i z e

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

m a t e r i a l ( to avo i d ga l van i c e f fec t s ) t ha t i s a s co r r o s i o n r e s i s t an t as pos -

s ib le .

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

d e a l i n g w i t h m a r i c u l t u r e s y s te m s s h o u l d b e a w a r e o f n e w d e v e l o p -m e n t s i n t h e f i e ld o f m a t e r i a l s t e c h n o l o g y . T w o o f t h e s e i n p a r ti c u l a r

a r e t h e r a p id d e v e l o p m e n t o f n o n m e t a l l i c c o m p o s i t e m a t e ri al s a n d t h e

r e c e n t c o m m e r c i a l a v a i l a b i l i t y o f s e v e r a l n e w s t a i n l e s s s t e e l a l l o y s

h a v i ng e n h a n c e d s e a w a t e r c o r r o s i o n r es is ta n c e. N o n m e t a ll ic c o m -

p o s i te s c a n b e f o r m u l a t e d w i th h i g h l y f a v o r a b l e m e c h a n i c a l p r o p -

e r ti e s , i n c l u d i n g h i g h s t r e n g t h - t o - w e i g h t r at io s , a n d t h e y a r e n o t

s u s c e p t ib l e t o e l e c t r o c h e m i c a l c o r r o s i o n . T h e p r o p e r t i e s a n d

i m p r o v e d c o r r o s i o n b e h a v i o r o f th e n e w s ta i n le s s al lo y s (s e e t y p e 2 9 - 4

i n T a b l e 1 ) h a v e b e e n s u m m a r i z e d b y S t r e i c h e r ( 1 9 8 3 ) . T h e s e n e w

a l lo y s h a v e c o s t s s im i l a r to t h e c o n v e n t i o n a l 3 0 0 s e r ie s s t ai n le s s st e e ls



Materials science in aquacultural engineering 34 5

c o u p l e d w i t h c o r r o s i o n r e s i s t a n c e s c l o s e to t h o s e o f th e s u p e r a l l o y s .

S u c h d e v e l o p m e n t s m a y p r o v e a d v a n t a g e o u s t o t he m a r i c u l u re

i n d u s tr y , in t h e f u t u r e .

R E F E R E N C E S

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H u g u en i n , J . E . & A n s u i n i , F . J . ( 1 9 7 8 ) . A r ev i ew o f t h e t ech n o l o g y an deco n o m i cs o f mar i n e f i sh cag e s y st ems . A q u a cu l t u r e , 15, 151.

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