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Selecting a coating
5
Selecting a coating
T h e s e l e c t i o n of the b e s t c o a t i n g o r c o m b i n a t i o n of c o a t i n g s for a n y particular c o r r o s i o n c o n t r o l a p p l i c a t i o n necessitates consideration of all the f a c t o r s i n v o l v e d s o t h a t the m o s t e c o n o m i c c h o i c e m a y b e m a d e c o n s i s t e n t w i t h the p e r f o r m a n c e r e q u i r e d . T h e o r d e r in w h i c h t h e s e f a c t o r s s h o u l d b e c o n s i d e r e d is likely t o b e as f o l l o w s : 1. T h e e n v i r o n m e n t s ( s ) t h a t w i l l b e m e t in s e r v i c e 2. T h e s e r v i c e life r e q u i r e d 3.
D e c o r a t i v e a p p e a l a n d the d e g r e e o f d e t e r i o r a t i o n t h a t c a n b e tolerated
4. T h e substrate material 5.
T h e s h a p e a n d s i z e of the a r t i c l e
6. A n y s u b s e q u e n t 7.
fabrication
M e c h a n i c a l factors
H o w e v e r , in a n y p a r t i c u l a r c a s e the o r d e r o f i m p o r t a n c e m a y b e c h a n g e d to meet special c i r c u m s t a n c e s .
Environmental factors C o n s i d e r a t i o n o f the c o r r o s i v e e n v i r o n m e n t o r e n v i r o n m e n t s that w i l l b e m e t i n s e r v i c e g e n e r a l l y first results i n the e x c l u s i o n of u n s u i t a b l e c o a t i n g m a t e r i a l s , l e a v i n g a n u m b e r o f m a t e r i a l s o f g r e a t e r o r lesser m e r i t d e p e n d i n g o n the o t h e r r e q u i r e m e n t s of t h e a p p l i c a t i o n . T h u s a l u m i n i u m w o u l d b e r u l e d o u t o f c o n s i d e r a t i o n as a c o a t i n g m e t a l in strongly
alkaline
environments,
aluminium
and
lead
would
be
unsuitable for use in h i g h - c h l o r i d e e n v i r o n m e n t s , c o p p e r a n d z i n c w o u l d b e u n s u i t a b l e in a c i d i c e n v i r o n m e n t s — in all c a s e s o w i n g t o t h e i r e x c e s s i v e rates o f c o r r o s i o n i n t h e s e e n v i r o n m e n t s . A l u m i n i u m , c o p p e r , nickel and
tin are r e s i s t a n t t o a t m o s p h e r i c
environments;
a l u m i n i u m a n d nickel are also resistant to elevated temperatures but 125
126
Selecting a coating
are a t t a c k e d u n d e r c o n d i t i o n s of l i m i t e d a c c e s s of o x y g e n . N i c k e l , c o p p e r a n d tin are resistant t o p o t a b l e w a t e r s a n d s e a w a t e r
but
a l u m i n i u m is less resistant t o w a t e r s , p a r t i c u l a r l y w h e n the c h l o r i d e c o n t e n t is h i g h . C a d m i u m is p r e f e r a b l e t o z i n c in h u m i d e n v i r o n ments containing o r g a n i c v a p o u r s , a n d a l u m i n i u m , nickel a n d
tin
offer g o o d r e s i s t a n c e t o a c i d i c e n v i r o n m e n t s . L e a d g i v e s g o o d p e r f o r m a n c e in a t m o s p h e r i c , a c i d i c o r a q u e o u s e n v i r o n m e n t s , b u t n o t in the p r e s e n c e of h i g 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 s . C h r o m i u m
remains
b r i g h t a n d u n a t t a c k e d in m o s t e n v i r o n m e n t s , e x c e p t a c i d i c c h l o r i d e environments,
but coating discontinuities
may allow
preferential
a t t a c k o n u n d e r c o a t s o r s u b s t r a t e m e t a l s ; o n the o t h e r h a n d , z i n c a n d c a d m i u m ( w h i c h are e l e c t r o n e g a t i v e t o steel in the a t m o s p h e r e a n d in w a t e r s ) c a n p r o v i d e efficient sacrificial p r o t e c t i o n to s u i t a b l e s u b strates — n o t a b l y steel. S i l v e r , c o p p e r a n d , t o a lesser e x t e n t , n i c k e l are attacked b y sulphur c o m p o u n d s , w h i c h p r o d u c e unsightly
and
n o n - p r o t e c t i v e films o n their s u r f a c e s .
Service-life requirements and acceptable deterioration T h e s e r v i c e life r e q u i r e d affects b o t h the c h o i c e o f c o a t i n g m e t a l a n d a l s o its t h i c k n e s s , the latter b e i n g a l s o d e p e n d e n t o n the s e v e r i t y of t h e e n v i r o n m e n t t o w h i c h it is t o b e e x p o s e d . It w o u l d b e u n e c o n o m i c , f o r e x a m p l e , t o a p p l y a c o a t i n g of a h i g h l y resistant m e t a l t o a c o m p o n e n t that is r e q u i r e d t o h a v e o n l y a v e r y l i m i t e d s e r v i c e life, u n l e s s it is essential t o retain a n initial d e c o r a t i v e a p p e a r a n c e t h r o u g h o u t that life o r in c a s e s w h e r e a n y risk o f failure t h r o u g h d e f e c t s c o u l d n o t b e t o l e r a t e d for a n y r e a s o n — e . g . safety c o n s i d e r a t i o n s . The
w a y s in w h i c h c h o i c e of c o a t i n g m e t a l s a n d their t h i c k n e s s e s
are affected b y the s e r v i c e e n v i r o n m e n t a n d s e r v i c e life m a y b e illust r a t e d b y r e f e r e n c e t o Tables 5.1
and
5.2.
Thus
bright
n i c k e l is
u n a c c e p t a b l e for use o u t d o o r s in e x c e p t i o n a l l y s e v e r e c o r r o s i v e c o n d i t i o n s ; r e d u c t i o n of 1 5 - 5 0 p e r c e n t in m i n i m u m n i c k e l
thickness
r e q u i r e m e n t is a l l o w e d as the s e v e r i t y of the e n v i r o n m e n t is r e d u c e d t h r o u g h f o u r e n v i r o n m e n t a l c a t e g o r i e s ; 1 2 - 1 6 p e r c e n t r e d u c t i o n in n i c k e l t h i c k n e s s is a l s o a l l o w e d if m i c r o - d i s c o n t i n u o u s c h r o m i u m is u s e d i n s t e a d o f r e g u l a r c h r o m i u m in o u t d o o r s e r v i c e . S i m i l a r l y , s o m e w h a t g r e a t e r t h i c k n e s s e s of a l u m i n i u m o n steel t h a n of z i n c o n steel are
recommended
because
of
the
poorer
sacrificial
action
of
a l u m i n i u m ; t h i c k n e s s r e q u i r e m e n t s for b o t h t y p e s of c o a t i n g s m u s t b e i n c r e a s e d w i t h i n c r e a s i n g life r e q u i r e m e n t s o r i n c r e a s e s in the s e v e r i t y of the c o r r o s i v e e n v i r o n m e n t .
Selecting a coating T a b l e 5.1
127
TYPE AND THICKNESS REQUIREMENTS FOR NICKEL + CHROMIUM COATINGS ON STEEL Coating type and thickness
Service condition No.
Description of environment
Ni
4
Exceptionally severe outdoor
40 μπι duplex 30 μηι duplex
0.3 μιτι regular 0.3 μπι microdiscontinuous
3
Normal outdoor
40 μηι bright 30 μηι bright
0.3 μπι regular 0.3 μπι microdiscontinuous 0.3 μιτι regular 0.3 μπι microdiscontinuous
30 μπι duplex 25 μπι duplex
Cr
2
Indoor with condensation
20 μηι bright or duplex
0.3 μπι regular or micro-discontinuous
1
Indoor dry
10 μπι bright or duplex
0.3 μιτι regular or micro-discontinuous
Abstracted from BS 1224:1970 Electroplated coatings of nickel and chromium
Effect of substrate material T h e particular substrate material that has to b e p r o t e c t e d b y a metal c o a t i n g i n f l u e n c e s b o t h t h e c h o i c e of c o a t i n g s a n d a l s o p o s s i b l y t h e m e t h o d s b y w h i c h they are to b e a p p l i e d . Z i n c a n d c a d m i u m
are
h i g h l y e f f e c t i v e c o a t i n g m e t a l s f o r steel, s i n c e t h e y a r e a n o d i c t o steel a n d p r o v i d e s a c r i f i c i a l p r o t e c t i o n t o t h e s u b s t r a t e at d i s c o n t i n u i t i e s in the c o a t i n g . C o a t i n g s that are c a t h o d i c to a substrate metal m u s t b e a p p l i e d a n d m a i n t a i n e d free f r o m d e f e c t s t h a t w o u l d e x p o s e t h a t s u b s t r a t e . T o e n s u r e this t h e c o a t i n g t h i c k n e s s m u s t b e sufficient t o p r e vent c o r r o s i o n p e n e t r a t i o n w i t h i n the r e q u i r e d lifetime of the c o m p o n e n t . A l t e r n a t i v e l y , c a t h o d i c c o a t i n g s m a y b e u s e d w i t h e x p o s u r e of t h e s u b s t r a t e p r o v i d e d t h a t t h e s u b s t r a t e c o r r o s i o n sites w i l l r a p i d l y p a s s i v a t e b y t h e f o r m a t i o n of i n s o l u b l e c o r r o s i o n p r o d u c t s o r t h a t t h e r a t e o f a t t a c k o n t h e s u b s t r a t e is i n s u f f i c i e n t t o affect a d v e r s e l y
the
s e r v i c e life of t h e a r t i c l e . C o n t r o l c a n a l s o b e e x e r c i s e d b y t h e u s e of multi-layer c o a t i n g systems (for e x a m p l e , c o p p e r o r nickel u n d e r c o a t s w i t h g o l d c o a t i n g s o r n i c k e l u n d e r c o a t s w i t h c h r o m i u m c o a t i n g s ) , in w h i c h c a s e the a n o d i c / c a t h o d i c r e l a t i o n s h i p
of i m m e d i a t e
impor-
t a n c e is t h a t b e t w e e n a d j a c e n t c o a t i n g l a y e r s . H o w e v e r , as the p e r i o d
128
Selecting a coating
T a b l e 5.2
THICKNESS R E Q U I R E M E N T S FOR ON
Environment
ZINC OR
ALUMINIUM
COATINGS
STEEL
Coating thickness (μπι) for various service lives
Outdoor industrial
Outdoor rural
Outdoor marine
Indoor wet
Indoor dry
5 years
15 years
50 years
30 Zn 50 Al
125 Zn 125 Al
—
7 Zn 25 Al
30 Zn 37 Al
—
15 Zn 50 Al
30 Zn 75 Al
—
15 Zn 50 Al
30 Zn 75 Al
—
7 Zn 37 Al
10 Zn 50 Al
15 Zn 75 Al
A b s t r a c t e d from D D 2 4 : 1 9 7 3 Recommendations for methods of protection against corrosion on light section steel used in building
of e x p o s u r e o f a c o m p o s i t e s y s t e m i n c r e a s e s a n d c o r r o s i o n p e n e t r a t e s t h r o u g h s u c c e s s i v e c o a t i n g layers to the substrate, c o m p l e x e l e c t r o c h e m i c a l r e l a t i o n s h i p s m a y b e set u p a n d o n e o r m o r e c o m p o n e n t o f t h e s y s t e m m a y b e a t t a c k e d at a n e n h a n c e d r a t e . Aluminium
may
be applied
t o steel b y h o t - d i p p i n g s i n c e
the
m e l t i n g p o i n t of steel is sufficiently g r e a t e r t h a n t h a t of a l u m i n i u m , b u t if a l u m i n i u m a l l o y s h a v e t o b e p r o t e c t e d b y p u r e a l u m i n i u m c o a t ings they must chromium
be applied by metal spraying or cladding. W h e n
is t o b e a p p l i e d as a c o a t i n g m e t a l , e l e c t r o d e p o s i t i o n
d i r e c t l y o n t o the s u b s t r a t e g e n e r a l l y p r o d u c e s a c o a t i n g w i t h i n a d e q u a t e a d h e s i o n a n d / o r i n a d e q u a t e p r o t e c t i o n of the substrate. W i t h steel s u b s t r a t e s n i c k e l m a y b e a p p l i e d d i r e c t l y as a n u n d e r c o a t for c h r o m i u m , but with z i n c substrates an u n d e r c o a t of c o p p e r must be a p p l i e d b e n e a t h the n i c k e l a n d w i t h a l u m i n i u m s u b s t r a t e s it is n e c e s s a r y first t o a p p l y a c h e m i c a l z i n c a t e t r e a t m e n t f o l l o w e d b y c o p p e r a n d n i c k e l u n d e r c o a t s . W i t h p l a s t i c s s u b s t r a t e s it is first n e c e s s a r y t o a p p l y a n e l e c t r o l e s s c o p p e r o r n i c k e l d e p o s i t in o r d e r t o m a k e
the
s u b s t r a t e c o n d u c t i n g for e l e c t r o p l a t i n g ; t h i c k d u c t i l e u n d e r c o a t s a r e f r e q u e n t l y n e c e s s a r y t o e n s u r e the r e t e n t i o n o f a d h e s i o n b e t w e e n t h e plastics
substrate and
effects o f differential composites.
the n i c k e l a n d
c h r o m i u m layers w h e n
t h e r m a l e x p a n s i o n c a u s e stress in the
the
plated
Selecting a coating
129
Effect of shape and size of articles T h e s h a p e a n d size of the a r t i c l e t o b e c o a t e d h a s little, if a n y , effect o n the c h o i c e of c o a t i n g m e t a l , e x c e p t in s o far as e c o n o m i c c o n s i d e r a t i o n s m a y l i m i t the s i z e o f a r t i c l e w h i c h c a n b e c o a t e d w i t h a g i v e n , c o s t l y , m a t e r i a l . H o w e v e r , s h a p e a n d size i n f l u e n c e c o n s i d e r a b l y the c h o i c e of m e t h o d b y w h i c h the c o a t i n g m a y b e a p p l i e d . V e r y s m a l l a r t i c l e s m a y b e difficult o r i m p o s s i b l e t o j i g for n o r m a l e l e c t r o p l a t i n g ; they m a y b e c o a t e d b y barrel plating techniques, b y h o t - d i p p i n g o r p e r h a p s b y v a c u u m m e t a l l i s i n g . S i m i l a r l y , e x c e s s i v e l y l a r g e articles m a y e x c e e d the c a p a c i t y o f b o t h e l e c t r o p l a t i n g o r h o t - d i p p i n g t a n k s — t h o u g h s o m e l a t i t u d e is p o s s i b l e w i t h the latter p r o c e s s b y u s i n g d o u b l e - e n d d i p p i n g t e c h n i q u e s . I n t h e s e c a s e s the o n l y p r a c t i c a l s o l u t i o n is t o a p p l y the c o a t i n g s b y m e t a l s p r a y i n g t e c h n i q u e s o r t o r e d e s i g n the a r t i c l e in s e v e r a l s m a l l e r c o m p o n e n t p a r t s that c a n b e c o a t e d before assembly. Intricately
shaped
c o m p o n e n t s (particularly
those with deeply
r e c e s s e d r e g i o n s ) a r e difficult t o e l e c t r o p l a t e w i t h c o a t i n g s of e v e n t h i c k n e s s b e c a u s e o f the l i m i t e d t h r o w i n g p o w e r o f p l a t i n g s o l u t i o n s (see C h a p t e r 3 ) — a l t h o u g h s o m e a m e l i o r a t i o n of this p r o b l e m c a n b e a c h i e v e d , at a d d i t i o n a l c o s t , b y u s i n g a u x i l i a r y a n d c o n f o r m i n g - s h a p e a n o d e s t o e v e n o u t the c u r r e n t d e n s i t y d i s t r i b u t i o n o n the being
plated.
Similarly,
article
electrodeposits covering completely
the
i n s i d e of s m a l l - b o r e h o l l o w s e c t i o n s c a n b e o b t a i n e d o n l y b y u s i n g i n t e r n a l l y p l a c e d a n o d e s . H o t d i p p i n g m a y p r o v i d e b e t t e r c o v e r a g e in t h e s e c a s e s , a l t h o u g h t h i c k e n i n g of the c o a t i n g in r e c e s s e d a r e a s m a y mar detailed shape and small-bore holes m a y b e c o m e clogged with coating metal. M e t a l spraying techniques can c o p e well with irregularly s h a p e d a r t i c l e s b u t c o a t i n g s c a n n o t b e m e t a l s p r a y e d i n s i d e n a r r o w b o r e s . C h e m i c a l ( e l e c t r o l e s s ) p l a t i n g , h o w e v e r , w i l l c o a t the m o s t c o m p l e x shapes with even thickness b o t h internally a n d extern a l l y . P r o b a b l y the b e s t w a y of h a n d l i n g b o t h c o m p l e x - s h a p e d a n d e x c e s s i v e l y l a r g e c o m p o n e n t s in o r d e r t o a c h i e v e the b e s t c o a t i n g s is t o r e d e s i g n t h e m s o as t o s i m p l i f y the a p p l i c a t i o n of the c h o s e n c o a t i n g by
the d e s i r e d m e t h o d . I n d e e d it is a f u n d a m e n t a l
p r i n c i p l e in
a c h i e v i n g b o t h the b e s t a p p l i c a t i o n a n d the b e s t p e r f o r m a n c e of c o a t e d m e t a l c o m p o n e n t s that the c o a t i n g r e q u i r e m e n t s b e t a k e n i n t o a c c o u n t fully at the o r i g i n a l d e s i g n s t a g e .
Effect of subsequent fabrication Fabrication
that m u s t
t a k e p l a c e after a p p l i c a t i o n o f a n y
metal
c o a t i n g s h o u l d a l w a y s b e c o n s i d e r e d w h e n b o t h the c o a t i n g m e t a l a n d
130
Selecting a coating
its m e t h o d o f a p p l i c a t i o n a r e c h o s e n . O b v i o u s l y a n y c u t t i n g o r t r i m m i n g that h a s t o b e d o n e after a p p l y i n g the c o a t i n g w i l l d a m a g e the c o a t i n g a n d e x p o s e the s u b s t r a t e m e t a l . A n o d i c c o a t i n g s m a y w e l l b e a b l e to c o p e w i t h the e x p o s e d a r e a of s u b s t r a t e b y p r o v i d i n g sacrificial p r o t e c t i o n , p r o v i d e d that the a r e a in q u e s t i o n is n o t t o o l a r g e , b u t the i n c r e a s e d rate of c o n s u m p t i o n of the c o a t i n g m e t a l that results f r o m the p r e s e n c e o f e x p o s e d s u b s t r a t e m a y w e l l m a r k e d l y r e d u c e the ultim a t e effective life of the c o a t e d a r t i c l e c o m p a r e d w i t h that w h i c h w o u l d h a v e b e e n a c h i e v e d if e x p o s u r e h a d b e e n a v o i d e d . I n the c a s e of c a t h o d i c c o a t i n g s , h o w e v e r , a n y s u b s t r a t e m e t a l e x p o s e d as a result of c u t t i n g after c o a t i n g w i l l itself b e p r e f e r e n t i a l l y a t t a c k e d ; p r o v i s i o n t h e r e f o r e h a s t o b e m a d e t o p r o v i d e l o c a l p r o t e c t i o n in t h e e x p o s e d a r e a o r to r e p a i r the c o a t i n g b e f o r e the a r t i c l e is p l a c e d in s e r v i c e . T h e o n l y c o a t i n g p r o c e s s that c a n b e r e a d i l y a p p l i e d in situ t o a l i m i t e d a r e a of a l a r g e s t r u c t u r e , t h u s r e p a i r i n g a n y d a m a g e d c o a t i n g s , is metal spraying
( a l t h o u g h it m a y b e p o s s i b l e w i t h c e r t a i n
coating
m e t a l s s u c h as tin, l e a d a n d t h e i r a l l o y s t o effect l o c a l i s e d r e p a i r s b y soldering or brazing techniques). P o s t - c o a t i n g w e l d i n g o p e r a t i o n s d e s t r o y the c o a t i n g in the w e l d z o n e , a n d in p a r t o r p e r h a p s the w h o l e of the h e a t - a f f e c t e d z o n e , s o l o c a l r e p a i r o r p r o t e c t i o n is r e q u i r e d s i m i l a r t o that n e c e s s a r y w h e r e c u t t i n g has t a k e n p l a c e . I n a d d i t i o n , the c o a t i n g m e t a l m a y w e l l affect the w e l d i n g p r o c e s s b y a l l o y i n g , c a u s i n g u n s o u n d o r e m b r i t t l e d w e l d m e n t s . A further h a z a r d d u r i n g w e l d i n g c a n a r i s e f r o m the p r o d u c t i o n of t o x i c v a p o u r s p r o d u c e d f r o m the c o a t i n g m e t a l s ; for this r e a s o n c a d m i u m s h o u l d never b e c h o s e n as a c o a t i n g m e t a l for steel that m u s t be subsequently welded. A s s e m b l y of c o a t e d c o m p o n e n t s m a y p r o d u c e c r e v i c e d r e g i o n s , s u c h as in b o l t e d - u p l a p j o i n t s o r b e n e a t h the h e a d s o f fasteners, a n d the s u s c e p t i b i l i t y of the c h o s e n c o a t i n g m e t a l t o c r e v i c e c o r r o s i o n m u s t b e b o r n e in m i n d . S i m i l a r l y , b i - m e t a l l i c c o n t a c t c a n o c c u r o n a s s e m b l y ; i d e a l l y , s u c h c o n t a c t s s h o u l d b e d e s i g n e d o u t of the s t r u c ture, o r a s s e m b l y m a d e w i t h n o n - m e t a l l i c ( i n s u l a t i n g ) s e p a r a t o r s in the j o i n t , b u t w h e r e t h e s e m e t h o d s c a n n o t b e f o l l o w e d the c o a t i n g m e t a l c h o s e n m u s t b e as c o m p a t i b l e as p o s s i b l e w i t h the d i s s i m i l a r c o n t a c t i n g m e t a l . F o r e x a m p l e , w h e r e steel a n d a l u m i n i u m m u s t b e in c o n t a c t the steel s h o u l d b e c o a t e d w i t h c a d m i u m , s i n c e c a d m i u m a n d a l u m i n i u m w h e n in c o n t a c t d o n o t l e a d t o b i - m e t a l l i c c o r r o s i o n of t h e latter m e t a l . Mating and threaded c o m p o n e n t s must be designed and p r o d u c e d s o as t o a l l o w for d i m e n s i o n a l c h a n g e s o c c u r i n g d u r i n g c o a t i n g , a n d the c o a t i n g t h i c k n e s s a n d m e t h o d of a p p l i c a t i o n m u s t b e c h o s e n to a c h i e v e the b e s t c o m p r o m i s e b e t w e e n a d e q u a t e
fit a n d
p r o t e c t i o n w i t h the m i n i m u m of p o s t - c o a t i n g m a c h i n i n g .
adequate
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Selecting a coating
T h e i n t e r n a l stress, d u c t i l i t y a n d b r i t t l e n e s s o f c o a t i n g m e t a l s ( a n d , w h e r e a p p r o p r i a t e , of a l l o y l a y e r s ) m u s t b e t a k e n i n t o a c c o u n t w h e n c h o o s i n g a c o a t i n g m e t a l a n d its m e t h o d o f a p p l i c a t i o n for a c o m p o n e n t that m u s t b e d e f o r m e d d u r i n g f a b r i c a t i o n o r in s e r v i c e . E l e c t r o d e p o s i t s s u c h as c h r o m i u m a n d s o m e n i c k e l s c a n w i t h s t a n d o n l y a small
amount
of d e f o r m a t i o n
without
cracking or spalling;
the
d e v e l o p m e n t of excessively thick alloy layers d u r i n g h o t - d i p p i n g also e m b r i t t l e s the c o a t i n g a n d l e a d s t o failure o n d e f o r m a t i o n . T h e h a r d ness, ductility a n d frictional properties of a c o a t i n g metal m a y b e of c o n s i d e r a b l e c o n s e q u e n c e in p o s t - f a b r i c a t i o n . A v e r y soft c o a t i n g s u c h as l e a d , o r t o lesser e x t e n t a l u m i n i u m , c a n d e f o r m r e a d i l y u n d e r l o a d ; this m a y l e a d t o m o r e efficient e l i m i n a t i o n of s o m e c r e v i c e s b u t m a y a l s o c a u s e l o c a l i s e d t h i n n i n g o f the c o a t i n g o r e v e n e x p o s u r e o f the s u b s t r a t e . S p r a y e d z i n c o r a l u m i n i u m c o a t i n g s o n steel a r e of e s p e c i a l v a l u e in a p p l i c a t i o n s w h e r e f r i c t i o n g r i p b o l t i n g is i n v o l v e d . S l i p f a c t o r s of the o r d e r of 0 . 4 5 - 0 . 5 5 a r e r e a d i l y o b t a i n e d w i t h s p r a y e d z i n c c o a t i n g s , a n d in the c a s e o f s p r a y e d a l u m i n i u m c o a t i n g s the s l i p f a c t o r c a n rise as h i g h as 0 . 7 . G a l v a n i s e d steel in the 'as g a l v a n i s e d ' c o n d i t i o n has a s o m e w h a t l o w e r slip factor than sprayed zinc, o w i n g t o the s m o o t h n e s s of the d e p o s i t , b u t in s e r v i c e u n d e r loading
a
hysteresis
loading cycle
occurs which
dynamic
produces
self-
r o u g h e n i n g of the f a y i n g s u r f a c e s w i t h a c o n s e q u e n t l o c k i n g a c t i o n w h i c h p r e v e n t s a n y s l i p t a k i n g p l a c e . C o n v e r s e l y , the l o w t o r q u e r e s i s t a n c e of c a d m i u m m a k e s it the b e s t c h o i c e of c o a t i n g m e t a l f o r steel b o l t s for s t r u c t u r e s that m u s t b e a s s e m b l e d a n d d i s m a n t l e d frequently. O n e p o i n t n o t d i r e c t l y c o n c e r n e d w i t h f a b r i c a t i o n b u t a l l i e d t o it — a n d a m a t t e r that is f r e q u e n t l y o v e r l o o k e d — is t o e n s u r e that all c o m p o n e n t s in a c o m p o s i t e s t r u c t u r e h a v e c o m p a r a b l e effective service lives. T h u s h o t - d i p p e d galvanised c o m p o n e n t s h a v i n g a c o a t i n g t h i c k n e s s of s o m e 5 0 μ π ι o r m o r e m a y b e a s s e m b l e d u s i n g fasteners that h a v e b e e n e l e c t r o p l a t e d w i t h z i n c t o a t h i c k n e s s of p e r h a p s o n l y 10-20
μ η ι . I n c a s e s s u c h as this t h e life o f the fasteners w i l l b e o n l y
2 0 - 4 0 p e r c e n t that of the rest of t h e s t r u c t u r e ( s i n c e the life of a z i n c c o a t i n g is p r o p o r t i o n a l t o its t h i c k n e s s ) a n d u n s i g h t l y r u s t i n g , o r perhaps even collapse, will o c c u r
prematurely.
Mechanical factors M e c h a n i c a l f a c t o r s that m u s t b e c o n s i d e r e d w h e n c h o o s i n g a c o a t i n g a r e m o s t l y t h o s e of stress d u r i n g s e r v i c e — e i t h e r d y n a m i c o r static. T h e a p p l i c a t i o n o f h e a t d u r i n g h o t - d i p p i n g p r o c e s s e s , a n d t o a lesser e x t e n t d u r i n g m e t a l s p r a y i n g , c a n a d v e r s e l y affect t h e m e c h a n i c a l
132
Selecting a coating
p r o p e r t i e s of the s u b s t r a t e m e t a l b y p a r t i a l o r c o m p l e t e a n n e a l i n g d u r i n g c o a t i n g . If this o c c u r s the s t r e n g t h of the c o m p l e t e d c o m p o n e n t m a y b e i n a d e q u a t e for its a p p l i c a t i o n , o r the c o m p o n e n t m a y b e d i s t o r t e d d u r i n g c o a t i n g s o that s u b s e q u e n t a s s e m b l y is difficult o r even impossible. W h e n c o a t e d c o m p o n e n t s are stressed d u r i n g a s s e m b l y o r in serv i c e , failure c a n o c c u r if the c o a t i n g m e t a l is s u s c e p t i b l e t o stress c o r r o s i o n — for e x a m p l e , stressed c o p p e r o r c o p p e r a l l o y s e x p o s e d t o a m m o n i a c a l environments. Alternatively, a substrate metal susceptible t o stress c o r r o s i o n m a y b e c o m p l e t e l y p r o t e c t e d b y m e a n s of a suitable
metal
c o a t i n g — for e x a m p l e , h i g h - s t r e n g t h
aluminium
alloys coated with sprayed pure a l u m i n i u m o r with zinc. D y n a m i c stressing d u r i n g s e r v i c e m a y p r o d u c e f l e x i n g of a c o m p o n e n t , a n d in these c a s e s if the c o a t i n g is brittle it m a y c r a c k a n d e x p o s e the s u b strate w i t h c o n s e q u e n t loss of p r o t e c t i o n ; a n e x a m p l e of this m a y b e s e e n in the c a s e of thick ' c r a c k - f r e e ' c h r o m i u m d e p o s i t s , w h i c h fracture t h r o u g h b r i t t l e n e s s w h e n f l e x e d (as in m o t o r c a r b u m p e r - b a r s o r h u b - d i s c s ) , the c r a c k s t h e n p r o p a g a t i n g t h r o u g h the n i c k e l u n d e r c o a t to e x p o s e the steel s u b s t r a t e . C o a t i n g b y e l e c t r o d e p o s i t i o n , b y the n a t u r e of the p r o c e s s , freq u e n t l y p r o d u c e s c a t h o d i c h y d r o g e n at the m e t a l s u r f a c e , a n d this h y d r o g e n m a y b e a b s o r b e d in the c o a t i n g a n d / o r in the s u b s t r a t e . T h e p r e s e n c e of this h y d r o g e n in c e r t a i n m e t a l s c a n result in e m b r i t t l e m e n t , s u c h as is the c a s e w i t h h i g h - t e n s i l e steels, l e a d i n g t o brittle f r a c t u r e w h e n stressing o c c u r s in s e r v i c e . P r o v i s i o n t o m a d e in the appropriate
standard
specifications to carry
o u t stress-relief
an-
n e a l i n g t r e a t m e n t s t o r e m o v e o r m i n i m i s e these effects. T h u s B S 1 2 2 4 : 1 9 7 0 {Electroplated coatings of nickel and chromium) s p e c i f i e s stressrelief a n n e a l i n g b e f o r e p l a t i n g for 1 h o u r at 1 3 0 - 2 1 0 ° C a c c o r d i n g t o the t y p e of steel, o r after p l a t i n g for 5 h o u r s at 1 9 0 - 2 1 0 ° C o r for 15 h o u r s at 1 7 0 ° C if the h i g h e r t r e a t m e n t t e m p e r a t u r e w o u l d b e h a r m f u l t o the m e c h a n i c a l p r o p e r t i e s of t h e steel. S i m i l a r p r o v i s i o n s are m a d e for z i n c o r c a d m i u m p l a t e d h i g h - s t r e n g t h steel c o m p o n e n t s , b u t in s p e c i a l c a s e s it m i g h t b e d e s i r a b l e t o a v o i d c o a t i n g b y e l e c t r o d e p o s i t i o n a n d use e i t h e r m e t a l s p r a y e d z i n c o r v a c u u m m e t a l l i s e d z i n c o r c a d m i u m coating processes, thus a v o i d i n g e x p o s u r e to h y d r o g e n . S p r a y e d m e t a l c o a t i n g s m a y a l s o b e p r e f e r a b l e in
applications
w h e r e fatigue l o a d i n g is i n v o l v e d , s i n c e the c o m p r e s s i v e stressing of the s u r f a c e l a y e r s of the s u b s t r a t e b y the g r i t - b l a s t i n g p r e t r e a t m e n t m a y i m p r o v e fatigue p r o p e r t i e s . I n a p p l i c a t i o n s w h e r e fretting of b o l t e d j o i n t s c a n o c c u r , the r o u g h i r r e g u l a r i t i e s p r e s e n t o n the s u r f a c e of a s p r a y e d m e t a l c o a t i n g c a n i n c r e a s e f r i c t i o n in the j o i n t a n d s o r e d u c e fretting c o r r o s i o n . M e t a l - c o a t e d plastics materials are s u b j e c t e d to particular
hazards
133
Selecting a coating
t h r o u g h m e c h a n i c a l f o r c e s d u r i n g s e r v i c e . T h e p r i n c i p a l of these is the risk o f r u p t u r e b e t w e e n the c o a t i n g a n d the s u b s t r a t e t h r o u g h stresses a r i s i n g d u r i n g t e m p e r a t u r e c h a n g e s b e c a u s e of the g r e a t d i s p a r i t y b e t w e e n the c o e f f i c i e n t s o f t h e r m a l e x p a n s i o n o f m e t a l s a n d p l a s t i c s . I n p r a c t i c e it m a y b e n e c e s s a r y t o i n c o r p o r a t e a sufficient t h i c k n e s s o f a d u c t i l e u n d e r c o a t s u c h as c o p p e r , w h i c h w i l l p r e v e n t r u p t u r e w h e n differential e x p a n s i o n a n d c o n t r a c t i o n o c c u r s . C a s e s h a v e a l s o o c c u r r e d in p r a c t i c e w h e r e p l a s t i c s c o m p o n e n t s p l a t e d w i t h n i c k e l a n d c h r o m i u m h a v e failed u n d e r stress in s e r v i c e b y f r a c t u r e in a sharply angled recess, although similar plastics c o m p o n e n t s used u n p l a t e d h a v e p e r f o r m e d s a t i s f a c t o r i l y . T h i s m o d e of failure is c a u s e d by
stress c o n c e n t r a t i o n
in
the
n o t c h o f the
recess c r a c k i n g
c h r o m i u m c o a t i n g , the c r a c k t h e n p r o p a g a t i n g t h r o u g h the
the
metal
u n d e r c o a t s a n d t h r o u g h the p l a s t i c s s u b s t r a t e . I n s u c h c a s e s the o n l y r e m e d y lies in the r e d e s i g n of t h e c o m p o n e n t t o e l i m i n a t e the n o t c h effect. M e c h a n i c a l f a c t o r s o t h e r t h a n stress that m u s t b e c o n s i d e r e d a r e those
involving movement.
Relative
movement
between
coated
c o m p o n e n t s a n d o t h e r c o m p o n e n t s in a n a s s e m b l y o c c a s i o n s w e a r , a n d the w e a r r e s i s t a n c e o f the c o a t i n g m e t a l is t h e n the
primary
c o n s i d e r a t i o n . I n g e n e r a l the h a r d n e s s of the m e t a l c o a t i n g is the p r i n c i p a l f a c t o r o f c o n c e r n h e r e , s i n c e n o r m a l l y a softer m a t e r i a l w e a r s w h e n in m o v i n g c o n t a c t w i t h a h a r d e r m a t e r i a l ( a l t h o u g h t h e r e a r e e x c e p t i o n s to this). Diffusion c o a t i n g s a n d e l e c t r o d e p o s i t e d c o a t i n g s of h a r d m e t a l s s u c h as c h r o m i u m a n d n i c k e l are p r i n c i p a l l y u s e d for wear-resistant applications, but metal sprayed coatings subsequently m a c h i n e d a n d p e r h a p s a l s o h e a t - t r e a t e d a r e often e m p l o y e d . A s e c o n d a r y p r o p e r t y o f m e t a l c o a t i n g s o f i m p o r t a n c e f o r w e a r r e s i s t a n c e is the a d e q u a c y of its a d h e s i o n t o the s u b s t r a t e . W h e r e a d h e s i o n is p o o r , r u b b i n g a c t i o n c a n c a u s e l o c a l i s e d r u p t u r e at the
coating/substrate
m e t a l i n t e r f a c e , l e a d i n g t o b l i s t e r i n g o r e v e n c o m p l e t e s p a l l i n g off of the c o a t i n g m e t a l .
Effect of environmental movement M o v e m e n t o f the e n v i r o n m e n t m a y a l s o n e e d t o b e c o n s i d e r e d . I n applications involving e x p o s u r e to m o v i n g liquids or gases erosion c a n o c c u r . T h i s m a y b e insufficient t o e r o d e the m e t a l itself
but
n e v e r t h e l e s s sufficient t o r e m o v e the p r o t e c t i v e films l o c a l l y a n d t h u s set
up
a n o d i c areas
where
enhanced
c o r r o s i o n will o c c u r
(for
e x a m p l e , i m p i n g e m e n t c o r r o s i o n of c o p p e r a n d its a l l o y s i m m e r s e d in m o v i n g w a t e r ) , o r it m a y b e o f sufficent m a g n i t u d e
to
damage
m e c h a n i c a l l y the m e t a l itself (as in c a v i t a t i o n c o r r o s i o n ) . I n e i t h e r
134
Selecting a coating
case premature localised penetration through a coating m a y occur, c a u s i n g e x p o s u r e of the s u b s t r a t e w i t h c o n s e q u e n t loss of p r o t e c t i o n , o r e v e n s t r i p p i n g the c o a t i n g c o m p l e t e l y f r o m c o n s i d e r a b l e a r e a s o f the c o m p o n e n t as c o r r o s i o n u n d e r c u t s the c o a t i n g a n d t u r b u l e n c e is i n c r e a s e d in the m o v i n g l i q u i d . T h e r e m e d y in t h e s e c a s e s lies in the s e l e c t i o n o f a c o a t i n g m a t e r i a l ( e . g . n i c k e l o r n i c k e l a l l o y ) that offers i m p r o v e d r e s i s t a n c e to e r o s i o n , o r in the r e d e s i g n of the c o m p o n e n t s o that the e r o s i v e effects are r e d u c e d .
Selecting a coating
T h e s e l e c t i o n of the b e s t c o a t i n g o r c o m b i n a t i o n of c o a t i n g s for a n y particular c o r r o s i o n c o n t r o l a p p l i c a t i o n necessitates consideration of all the f a c t o r s i n v o l v e d s o t h a t the m o s t e c o n o m i c c h o i c e m a y b e m a d e c o n s i s t e n t w i t h the p e r f o r m a n c e r e q u i r e d . T h e o r d e r in w h i c h t h e s e f a c t o r s s h o u l d b e c o n s i d e r e d is likely t o b e as f o l l o w s : 1. T h e e n v i r o n m e n t s ( s ) t h a t w i l l b e m e t in s e r v i c e 2. T h e s e r v i c e life r e q u i r e d 3.
D e c o r a t i v e a p p e a l a n d the d e g r e e o f d e t e r i o r a t i o n t h a t c a n b e tolerated
4. T h e substrate material 5.
T h e s h a p e a n d s i z e of the a r t i c l e
6. A n y s u b s e q u e n t 7.
fabrication
M e c h a n i c a l factors
H o w e v e r , in a n y p a r t i c u l a r c a s e the o r d e r o f i m p o r t a n c e m a y b e c h a n g e d to meet special c i r c u m s t a n c e s .
Environmental factors C o n s i d e r a t i o n o f the c o r r o s i v e e n v i r o n m e n t o r e n v i r o n m e n t s that w i l l b e m e t i n s e r v i c e g e n e r a l l y first results i n the e x c l u s i o n of u n s u i t a b l e c o a t i n g m a t e r i a l s , l e a v i n g a n u m b e r o f m a t e r i a l s o f g r e a t e r o r lesser m e r i t d e p e n d i n g o n the o t h e r r e q u i r e m e n t s of t h e a p p l i c a t i o n . T h u s a l u m i n i u m w o u l d b e r u l e d o u t o f c o n s i d e r a t i o n as a c o a t i n g m e t a l in strongly
alkaline
environments,
aluminium
and
lead
would
be
unsuitable for use in h i g h - c h l o r i d e e n v i r o n m e n t s , c o p p e r a n d z i n c w o u l d b e u n s u i t a b l e in a c i d i c e n v i r o n m e n t s — in all c a s e s o w i n g t o t h e i r e x c e s s i v e rates o f c o r r o s i o n i n t h e s e e n v i r o n m e n t s . A l u m i n i u m , c o p p e r , nickel and
tin are r e s i s t a n t t o a t m o s p h e r i c
environments;
a l u m i n i u m a n d nickel are also resistant to elevated temperatures but 125
126
Selecting a coating
are a t t a c k e d u n d e r c o n d i t i o n s of l i m i t e d a c c e s s of o x y g e n . N i c k e l , c o p p e r a n d tin are resistant t o p o t a b l e w a t e r s a n d s e a w a t e r
but
a l u m i n i u m is less resistant t o w a t e r s , p a r t i c u l a r l y w h e n the c h l o r i d e c o n t e n t is h i g h . C a d m i u m is p r e f e r a b l e t o z i n c in h u m i d e n v i r o n ments containing o r g a n i c v a p o u r s , a n d a l u m i n i u m , nickel a n d
tin
offer g o o d r e s i s t a n c e t o a c i d i c e n v i r o n m e n t s . L e a d g i v e s g o o d p e r f o r m a n c e in a t m o s p h e r i c , a c i d i c o r a q u e o u s e n v i r o n m e n t s , b u t n o t in the p r e s e n c e of h i g 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 s . C h r o m i u m
remains
b r i g h t a n d u n a t t a c k e d in m o s t e n v i r o n m e n t s , e x c e p t a c i d i c c h l o r i d e environments,
but coating discontinuities
may allow
preferential
a t t a c k o n u n d e r c o a t s o r s u b s t r a t e m e t a l s ; o n the o t h e r h a n d , z i n c a n d c a d m i u m ( w h i c h are e l e c t r o n e g a t i v e t o steel in the a t m o s p h e r e a n d in w a t e r s ) c a n p r o v i d e efficient sacrificial p r o t e c t i o n to s u i t a b l e s u b strates — n o t a b l y steel. S i l v e r , c o p p e r a n d , t o a lesser e x t e n t , n i c k e l are attacked b y sulphur c o m p o u n d s , w h i c h p r o d u c e unsightly
and
n o n - p r o t e c t i v e films o n their s u r f a c e s .
Service-life requirements and acceptable deterioration T h e s e r v i c e life r e q u i r e d affects b o t h the c h o i c e o f c o a t i n g m e t a l a n d a l s o its t h i c k n e s s , the latter b e i n g a l s o d e p e n d e n t o n the s e v e r i t y of t h e e n v i r o n m e n t t o w h i c h it is t o b e e x p o s e d . It w o u l d b e u n e c o n o m i c , f o r e x a m p l e , t o a p p l y a c o a t i n g of a h i g h l y resistant m e t a l t o a c o m p o n e n t that is r e q u i r e d t o h a v e o n l y a v e r y l i m i t e d s e r v i c e life, u n l e s s it is essential t o retain a n initial d e c o r a t i v e a p p e a r a n c e t h r o u g h o u t that life o r in c a s e s w h e r e a n y risk o f failure t h r o u g h d e f e c t s c o u l d n o t b e t o l e r a t e d for a n y r e a s o n — e . g . safety c o n s i d e r a t i o n s . The
w a y s in w h i c h c h o i c e of c o a t i n g m e t a l s a n d their t h i c k n e s s e s
are affected b y the s e r v i c e e n v i r o n m e n t a n d s e r v i c e life m a y b e illust r a t e d b y r e f e r e n c e t o Tables 5.1
and
5.2.
Thus
bright
n i c k e l is
u n a c c e p t a b l e for use o u t d o o r s in e x c e p t i o n a l l y s e v e r e c o r r o s i v e c o n d i t i o n s ; r e d u c t i o n of 1 5 - 5 0 p e r c e n t in m i n i m u m n i c k e l
thickness
r e q u i r e m e n t is a l l o w e d as the s e v e r i t y of the e n v i r o n m e n t is r e d u c e d t h r o u g h f o u r e n v i r o n m e n t a l c a t e g o r i e s ; 1 2 - 1 6 p e r c e n t r e d u c t i o n in n i c k e l t h i c k n e s s is a l s o a l l o w e d if m i c r o - d i s c o n t i n u o u s c h r o m i u m is u s e d i n s t e a d o f r e g u l a r c h r o m i u m in o u t d o o r s e r v i c e . S i m i l a r l y , s o m e w h a t g r e a t e r t h i c k n e s s e s of a l u m i n i u m o n steel t h a n of z i n c o n steel are
recommended
because
of
the
poorer
sacrificial
action
of
a l u m i n i u m ; t h i c k n e s s r e q u i r e m e n t s for b o t h t y p e s of c o a t i n g s m u s t b e i n c r e a s e d w i t h i n c r e a s i n g life r e q u i r e m e n t s o r i n c r e a s e s in the s e v e r i t y of the c o r r o s i v e e n v i r o n m e n t .
Selecting a coating T a b l e 5.1
127
TYPE AND THICKNESS REQUIREMENTS FOR NICKEL + CHROMIUM COATINGS ON STEEL Coating type and thickness
Service condition No.
Description of environment
Ni
4
Exceptionally severe outdoor
40 μπι duplex 30 μηι duplex
0.3 μιτι regular 0.3 μπι microdiscontinuous
3
Normal outdoor
40 μηι bright 30 μηι bright
0.3 μπι regular 0.3 μπι microdiscontinuous 0.3 μιτι regular 0.3 μπι microdiscontinuous
30 μπι duplex 25 μπι duplex
Cr
2
Indoor with condensation
20 μηι bright or duplex
0.3 μπι regular or micro-discontinuous
1
Indoor dry
10 μπι bright or duplex
0.3 μιτι regular or micro-discontinuous
Abstracted from BS 1224:1970 Electroplated coatings of nickel and chromium
Effect of substrate material T h e particular substrate material that has to b e p r o t e c t e d b y a metal c o a t i n g i n f l u e n c e s b o t h t h e c h o i c e of c o a t i n g s a n d a l s o p o s s i b l y t h e m e t h o d s b y w h i c h they are to b e a p p l i e d . Z i n c a n d c a d m i u m
are
h i g h l y e f f e c t i v e c o a t i n g m e t a l s f o r steel, s i n c e t h e y a r e a n o d i c t o steel a n d p r o v i d e s a c r i f i c i a l p r o t e c t i o n t o t h e s u b s t r a t e at d i s c o n t i n u i t i e s in the c o a t i n g . C o a t i n g s that are c a t h o d i c to a substrate metal m u s t b e a p p l i e d a n d m a i n t a i n e d free f r o m d e f e c t s t h a t w o u l d e x p o s e t h a t s u b s t r a t e . T o e n s u r e this t h e c o a t i n g t h i c k n e s s m u s t b e sufficient t o p r e vent c o r r o s i o n p e n e t r a t i o n w i t h i n the r e q u i r e d lifetime of the c o m p o n e n t . A l t e r n a t i v e l y , c a t h o d i c c o a t i n g s m a y b e u s e d w i t h e x p o s u r e of t h e s u b s t r a t e p r o v i d e d t h a t t h e s u b s t r a t e c o r r o s i o n sites w i l l r a p i d l y p a s s i v a t e b y t h e f o r m a t i o n of i n s o l u b l e c o r r o s i o n p r o d u c t s o r t h a t t h e r a t e o f a t t a c k o n t h e s u b s t r a t e is i n s u f f i c i e n t t o affect a d v e r s e l y
the
s e r v i c e life of t h e a r t i c l e . C o n t r o l c a n a l s o b e e x e r c i s e d b y t h e u s e of multi-layer c o a t i n g systems (for e x a m p l e , c o p p e r o r nickel u n d e r c o a t s w i t h g o l d c o a t i n g s o r n i c k e l u n d e r c o a t s w i t h c h r o m i u m c o a t i n g s ) , in w h i c h c a s e the a n o d i c / c a t h o d i c r e l a t i o n s h i p
of i m m e d i a t e
impor-
t a n c e is t h a t b e t w e e n a d j a c e n t c o a t i n g l a y e r s . H o w e v e r , as the p e r i o d
128
Selecting a coating
T a b l e 5.2
THICKNESS R E Q U I R E M E N T S FOR ON
Environment
ZINC OR
ALUMINIUM
COATINGS
STEEL
Coating thickness (μπι) for various service lives
Outdoor industrial
Outdoor rural
Outdoor marine
Indoor wet
Indoor dry
5 years
15 years
50 years
30 Zn 50 Al
125 Zn 125 Al
—
7 Zn 25 Al
30 Zn 37 Al
—
15 Zn 50 Al
30 Zn 75 Al
—
15 Zn 50 Al
30 Zn 75 Al
—
7 Zn 37 Al
10 Zn 50 Al
15 Zn 75 Al
A b s t r a c t e d from D D 2 4 : 1 9 7 3 Recommendations for methods of protection against corrosion on light section steel used in building
of e x p o s u r e o f a c o m p o s i t e s y s t e m i n c r e a s e s a n d c o r r o s i o n p e n e t r a t e s t h r o u g h s u c c e s s i v e c o a t i n g layers to the substrate, c o m p l e x e l e c t r o c h e m i c a l r e l a t i o n s h i p s m a y b e set u p a n d o n e o r m o r e c o m p o n e n t o f t h e s y s t e m m a y b e a t t a c k e d at a n e n h a n c e d r a t e . Aluminium
may
be applied
t o steel b y h o t - d i p p i n g s i n c e
the
m e l t i n g p o i n t of steel is sufficiently g r e a t e r t h a n t h a t of a l u m i n i u m , b u t if a l u m i n i u m a l l o y s h a v e t o b e p r o t e c t e d b y p u r e a l u m i n i u m c o a t ings they must chromium
be applied by metal spraying or cladding. W h e n
is t o b e a p p l i e d as a c o a t i n g m e t a l , e l e c t r o d e p o s i t i o n
d i r e c t l y o n t o the s u b s t r a t e g e n e r a l l y p r o d u c e s a c o a t i n g w i t h i n a d e q u a t e a d h e s i o n a n d / o r i n a d e q u a t e p r o t e c t i o n of the substrate. W i t h steel s u b s t r a t e s n i c k e l m a y b e a p p l i e d d i r e c t l y as a n u n d e r c o a t for c h r o m i u m , but with z i n c substrates an u n d e r c o a t of c o p p e r must be a p p l i e d b e n e a t h the n i c k e l a n d w i t h a l u m i n i u m s u b s t r a t e s it is n e c e s s a r y first t o a p p l y a c h e m i c a l z i n c a t e t r e a t m e n t f o l l o w e d b y c o p p e r a n d n i c k e l u n d e r c o a t s . W i t h p l a s t i c s s u b s t r a t e s it is first n e c e s s a r y t o a p p l y a n e l e c t r o l e s s c o p p e r o r n i c k e l d e p o s i t in o r d e r t o m a k e
the
s u b s t r a t e c o n d u c t i n g for e l e c t r o p l a t i n g ; t h i c k d u c t i l e u n d e r c o a t s a r e f r e q u e n t l y n e c e s s a r y t o e n s u r e the r e t e n t i o n o f a d h e s i o n b e t w e e n t h e plastics
substrate and
effects o f differential composites.
the n i c k e l a n d
c h r o m i u m layers w h e n
t h e r m a l e x p a n s i o n c a u s e stress in the
the
plated
Selecting a coating
129
Effect of shape and size of articles T h e s h a p e a n d size of the a r t i c l e t o b e c o a t e d h a s little, if a n y , effect o n the c h o i c e of c o a t i n g m e t a l , e x c e p t in s o far as e c o n o m i c c o n s i d e r a t i o n s m a y l i m i t the s i z e o f a r t i c l e w h i c h c a n b e c o a t e d w i t h a g i v e n , c o s t l y , m a t e r i a l . H o w e v e r , s h a p e a n d size i n f l u e n c e c o n s i d e r a b l y the c h o i c e of m e t h o d b y w h i c h the c o a t i n g m a y b e a p p l i e d . V e r y s m a l l a r t i c l e s m a y b e difficult o r i m p o s s i b l e t o j i g for n o r m a l e l e c t r o p l a t i n g ; they m a y b e c o a t e d b y barrel plating techniques, b y h o t - d i p p i n g o r p e r h a p s b y v a c u u m m e t a l l i s i n g . S i m i l a r l y , e x c e s s i v e l y l a r g e articles m a y e x c e e d the c a p a c i t y o f b o t h e l e c t r o p l a t i n g o r h o t - d i p p i n g t a n k s — t h o u g h s o m e l a t i t u d e is p o s s i b l e w i t h the latter p r o c e s s b y u s i n g d o u b l e - e n d d i p p i n g t e c h n i q u e s . I n t h e s e c a s e s the o n l y p r a c t i c a l s o l u t i o n is t o a p p l y the c o a t i n g s b y m e t a l s p r a y i n g t e c h n i q u e s o r t o r e d e s i g n the a r t i c l e in s e v e r a l s m a l l e r c o m p o n e n t p a r t s that c a n b e c o a t e d before assembly. Intricately
shaped
c o m p o n e n t s (particularly
those with deeply
r e c e s s e d r e g i o n s ) a r e difficult t o e l e c t r o p l a t e w i t h c o a t i n g s of e v e n t h i c k n e s s b e c a u s e o f the l i m i t e d t h r o w i n g p o w e r o f p l a t i n g s o l u t i o n s (see C h a p t e r 3 ) — a l t h o u g h s o m e a m e l i o r a t i o n of this p r o b l e m c a n b e a c h i e v e d , at a d d i t i o n a l c o s t , b y u s i n g a u x i l i a r y a n d c o n f o r m i n g - s h a p e a n o d e s t o e v e n o u t the c u r r e n t d e n s i t y d i s t r i b u t i o n o n the being
plated.
Similarly,
article
electrodeposits covering completely
the
i n s i d e of s m a l l - b o r e h o l l o w s e c t i o n s c a n b e o b t a i n e d o n l y b y u s i n g i n t e r n a l l y p l a c e d a n o d e s . H o t d i p p i n g m a y p r o v i d e b e t t e r c o v e r a g e in t h e s e c a s e s , a l t h o u g h t h i c k e n i n g of the c o a t i n g in r e c e s s e d a r e a s m a y mar detailed shape and small-bore holes m a y b e c o m e clogged with coating metal. M e t a l spraying techniques can c o p e well with irregularly s h a p e d a r t i c l e s b u t c o a t i n g s c a n n o t b e m e t a l s p r a y e d i n s i d e n a r r o w b o r e s . C h e m i c a l ( e l e c t r o l e s s ) p l a t i n g , h o w e v e r , w i l l c o a t the m o s t c o m p l e x shapes with even thickness b o t h internally a n d extern a l l y . P r o b a b l y the b e s t w a y of h a n d l i n g b o t h c o m p l e x - s h a p e d a n d e x c e s s i v e l y l a r g e c o m p o n e n t s in o r d e r t o a c h i e v e the b e s t c o a t i n g s is t o r e d e s i g n t h e m s o as t o s i m p l i f y the a p p l i c a t i o n of the c h o s e n c o a t i n g by
the d e s i r e d m e t h o d . I n d e e d it is a f u n d a m e n t a l
p r i n c i p l e in
a c h i e v i n g b o t h the b e s t a p p l i c a t i o n a n d the b e s t p e r f o r m a n c e of c o a t e d m e t a l c o m p o n e n t s that the c o a t i n g r e q u i r e m e n t s b e t a k e n i n t o a c c o u n t fully at the o r i g i n a l d e s i g n s t a g e .
Effect of subsequent fabrication Fabrication
that m u s t
t a k e p l a c e after a p p l i c a t i o n o f a n y
metal
c o a t i n g s h o u l d a l w a y s b e c o n s i d e r e d w h e n b o t h the c o a t i n g m e t a l a n d
130
Selecting a coating
its m e t h o d o f a p p l i c a t i o n a r e c h o s e n . O b v i o u s l y a n y c u t t i n g o r t r i m m i n g that h a s t o b e d o n e after a p p l y i n g the c o a t i n g w i l l d a m a g e the c o a t i n g a n d e x p o s e the s u b s t r a t e m e t a l . A n o d i c c o a t i n g s m a y w e l l b e a b l e to c o p e w i t h the e x p o s e d a r e a of s u b s t r a t e b y p r o v i d i n g sacrificial p r o t e c t i o n , p r o v i d e d that the a r e a in q u e s t i o n is n o t t o o l a r g e , b u t the i n c r e a s e d rate of c o n s u m p t i o n of the c o a t i n g m e t a l that results f r o m the p r e s e n c e o f e x p o s e d s u b s t r a t e m a y w e l l m a r k e d l y r e d u c e the ultim a t e effective life of the c o a t e d a r t i c l e c o m p a r e d w i t h that w h i c h w o u l d h a v e b e e n a c h i e v e d if e x p o s u r e h a d b e e n a v o i d e d . I n the c a s e of c a t h o d i c c o a t i n g s , h o w e v e r , a n y s u b s t r a t e m e t a l e x p o s e d as a result of c u t t i n g after c o a t i n g w i l l itself b e p r e f e r e n t i a l l y a t t a c k e d ; p r o v i s i o n t h e r e f o r e h a s t o b e m a d e t o p r o v i d e l o c a l p r o t e c t i o n in t h e e x p o s e d a r e a o r to r e p a i r the c o a t i n g b e f o r e the a r t i c l e is p l a c e d in s e r v i c e . T h e o n l y c o a t i n g p r o c e s s that c a n b e r e a d i l y a p p l i e d in situ t o a l i m i t e d a r e a of a l a r g e s t r u c t u r e , t h u s r e p a i r i n g a n y d a m a g e d c o a t i n g s , is metal spraying
( a l t h o u g h it m a y b e p o s s i b l e w i t h c e r t a i n
coating
m e t a l s s u c h as tin, l e a d a n d t h e i r a l l o y s t o effect l o c a l i s e d r e p a i r s b y soldering or brazing techniques). P o s t - c o a t i n g w e l d i n g o p e r a t i o n s d e s t r o y the c o a t i n g in the w e l d z o n e , a n d in p a r t o r p e r h a p s the w h o l e of the h e a t - a f f e c t e d z o n e , s o l o c a l r e p a i r o r p r o t e c t i o n is r e q u i r e d s i m i l a r t o that n e c e s s a r y w h e r e c u t t i n g has t a k e n p l a c e . I n a d d i t i o n , the c o a t i n g m e t a l m a y w e l l affect the w e l d i n g p r o c e s s b y a l l o y i n g , c a u s i n g u n s o u n d o r e m b r i t t l e d w e l d m e n t s . A further h a z a r d d u r i n g w e l d i n g c a n a r i s e f r o m the p r o d u c t i o n of t o x i c v a p o u r s p r o d u c e d f r o m the c o a t i n g m e t a l s ; for this r e a s o n c a d m i u m s h o u l d never b e c h o s e n as a c o a t i n g m e t a l for steel that m u s t be subsequently welded. A s s e m b l y of c o a t e d c o m p o n e n t s m a y p r o d u c e c r e v i c e d r e g i o n s , s u c h as in b o l t e d - u p l a p j o i n t s o r b e n e a t h the h e a d s o f fasteners, a n d the s u s c e p t i b i l i t y of the c h o s e n c o a t i n g m e t a l t o c r e v i c e c o r r o s i o n m u s t b e b o r n e in m i n d . S i m i l a r l y , b i - m e t a l l i c c o n t a c t c a n o c c u r o n a s s e m b l y ; i d e a l l y , s u c h c o n t a c t s s h o u l d b e d e s i g n e d o u t of the s t r u c ture, o r a s s e m b l y m a d e w i t h n o n - m e t a l l i c ( i n s u l a t i n g ) s e p a r a t o r s in the j o i n t , b u t w h e r e t h e s e m e t h o d s c a n n o t b e f o l l o w e d the c o a t i n g m e t a l c h o s e n m u s t b e as c o m p a t i b l e as p o s s i b l e w i t h the d i s s i m i l a r c o n t a c t i n g m e t a l . F o r e x a m p l e , w h e r e steel a n d a l u m i n i u m m u s t b e in c o n t a c t the steel s h o u l d b e c o a t e d w i t h c a d m i u m , s i n c e c a d m i u m a n d a l u m i n i u m w h e n in c o n t a c t d o n o t l e a d t o b i - m e t a l l i c c o r r o s i o n of t h e latter m e t a l . Mating and threaded c o m p o n e n t s must be designed and p r o d u c e d s o as t o a l l o w for d i m e n s i o n a l c h a n g e s o c c u r i n g d u r i n g c o a t i n g , a n d the c o a t i n g t h i c k n e s s a n d m e t h o d of a p p l i c a t i o n m u s t b e c h o s e n to a c h i e v e the b e s t c o m p r o m i s e b e t w e e n a d e q u a t e
fit a n d
p r o t e c t i o n w i t h the m i n i m u m of p o s t - c o a t i n g m a c h i n i n g .
adequate
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Selecting a coating
T h e i n t e r n a l stress, d u c t i l i t y a n d b r i t t l e n e s s o f c o a t i n g m e t a l s ( a n d , w h e r e a p p r o p r i a t e , of a l l o y l a y e r s ) m u s t b e t a k e n i n t o a c c o u n t w h e n c h o o s i n g a c o a t i n g m e t a l a n d its m e t h o d o f a p p l i c a t i o n for a c o m p o n e n t that m u s t b e d e f o r m e d d u r i n g f a b r i c a t i o n o r in s e r v i c e . E l e c t r o d e p o s i t s s u c h as c h r o m i u m a n d s o m e n i c k e l s c a n w i t h s t a n d o n l y a small
amount
of d e f o r m a t i o n
without
cracking or spalling;
the
d e v e l o p m e n t of excessively thick alloy layers d u r i n g h o t - d i p p i n g also e m b r i t t l e s the c o a t i n g a n d l e a d s t o failure o n d e f o r m a t i o n . T h e h a r d ness, ductility a n d frictional properties of a c o a t i n g metal m a y b e of c o n s i d e r a b l e c o n s e q u e n c e in p o s t - f a b r i c a t i o n . A v e r y soft c o a t i n g s u c h as l e a d , o r t o lesser e x t e n t a l u m i n i u m , c a n d e f o r m r e a d i l y u n d e r l o a d ; this m a y l e a d t o m o r e efficient e l i m i n a t i o n of s o m e c r e v i c e s b u t m a y a l s o c a u s e l o c a l i s e d t h i n n i n g o f the c o a t i n g o r e v e n e x p o s u r e o f the s u b s t r a t e . S p r a y e d z i n c o r a l u m i n i u m c o a t i n g s o n steel a r e of e s p e c i a l v a l u e in a p p l i c a t i o n s w h e r e f r i c t i o n g r i p b o l t i n g is i n v o l v e d . S l i p f a c t o r s of the o r d e r of 0 . 4 5 - 0 . 5 5 a r e r e a d i l y o b t a i n e d w i t h s p r a y e d z i n c c o a t i n g s , a n d in the c a s e o f s p r a y e d a l u m i n i u m c o a t i n g s the s l i p f a c t o r c a n rise as h i g h as 0 . 7 . G a l v a n i s e d steel in the 'as g a l v a n i s e d ' c o n d i t i o n has a s o m e w h a t l o w e r slip factor than sprayed zinc, o w i n g t o the s m o o t h n e s s of the d e p o s i t , b u t in s e r v i c e u n d e r loading
a
hysteresis
loading cycle
occurs which
dynamic
produces
self-
r o u g h e n i n g of the f a y i n g s u r f a c e s w i t h a c o n s e q u e n t l o c k i n g a c t i o n w h i c h p r e v e n t s a n y s l i p t a k i n g p l a c e . C o n v e r s e l y , the l o w t o r q u e r e s i s t a n c e of c a d m i u m m a k e s it the b e s t c h o i c e of c o a t i n g m e t a l f o r steel b o l t s for s t r u c t u r e s that m u s t b e a s s e m b l e d a n d d i s m a n t l e d frequently. O n e p o i n t n o t d i r e c t l y c o n c e r n e d w i t h f a b r i c a t i o n b u t a l l i e d t o it — a n d a m a t t e r that is f r e q u e n t l y o v e r l o o k e d — is t o e n s u r e that all c o m p o n e n t s in a c o m p o s i t e s t r u c t u r e h a v e c o m p a r a b l e effective service lives. T h u s h o t - d i p p e d galvanised c o m p o n e n t s h a v i n g a c o a t i n g t h i c k n e s s of s o m e 5 0 μ π ι o r m o r e m a y b e a s s e m b l e d u s i n g fasteners that h a v e b e e n e l e c t r o p l a t e d w i t h z i n c t o a t h i c k n e s s of p e r h a p s o n l y 10-20
μ η ι . I n c a s e s s u c h as this t h e life o f the fasteners w i l l b e o n l y
2 0 - 4 0 p e r c e n t that of the rest of t h e s t r u c t u r e ( s i n c e the life of a z i n c c o a t i n g is p r o p o r t i o n a l t o its t h i c k n e s s ) a n d u n s i g h t l y r u s t i n g , o r perhaps even collapse, will o c c u r
prematurely.
Mechanical factors M e c h a n i c a l f a c t o r s that m u s t b e c o n s i d e r e d w h e n c h o o s i n g a c o a t i n g a r e m o s t l y t h o s e of stress d u r i n g s e r v i c e — e i t h e r d y n a m i c o r static. T h e a p p l i c a t i o n o f h e a t d u r i n g h o t - d i p p i n g p r o c e s s e s , a n d t o a lesser e x t e n t d u r i n g m e t a l s p r a y i n g , c a n a d v e r s e l y affect t h e m e c h a n i c a l
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Selecting a coating
p r o p e r t i e s of the s u b s t r a t e m e t a l b y p a r t i a l o r c o m p l e t e a n n e a l i n g d u r i n g c o a t i n g . If this o c c u r s the s t r e n g t h of the c o m p l e t e d c o m p o n e n t m a y b e i n a d e q u a t e for its a p p l i c a t i o n , o r the c o m p o n e n t m a y b e d i s t o r t e d d u r i n g c o a t i n g s o that s u b s e q u e n t a s s e m b l y is difficult o r even impossible. W h e n c o a t e d c o m p o n e n t s are stressed d u r i n g a s s e m b l y o r in serv i c e , failure c a n o c c u r if the c o a t i n g m e t a l is s u s c e p t i b l e t o stress c o r r o s i o n — for e x a m p l e , stressed c o p p e r o r c o p p e r a l l o y s e x p o s e d t o a m m o n i a c a l environments. Alternatively, a substrate metal susceptible t o stress c o r r o s i o n m a y b e c o m p l e t e l y p r o t e c t e d b y m e a n s of a suitable
metal
c o a t i n g — for e x a m p l e , h i g h - s t r e n g t h
aluminium
alloys coated with sprayed pure a l u m i n i u m o r with zinc. D y n a m i c stressing d u r i n g s e r v i c e m a y p r o d u c e f l e x i n g of a c o m p o n e n t , a n d in these c a s e s if the c o a t i n g is brittle it m a y c r a c k a n d e x p o s e the s u b strate w i t h c o n s e q u e n t loss of p r o t e c t i o n ; a n e x a m p l e of this m a y b e s e e n in the c a s e of thick ' c r a c k - f r e e ' c h r o m i u m d e p o s i t s , w h i c h fracture t h r o u g h b r i t t l e n e s s w h e n f l e x e d (as in m o t o r c a r b u m p e r - b a r s o r h u b - d i s c s ) , the c r a c k s t h e n p r o p a g a t i n g t h r o u g h the n i c k e l u n d e r c o a t to e x p o s e the steel s u b s t r a t e . C o a t i n g b y e l e c t r o d e p o s i t i o n , b y the n a t u r e of the p r o c e s s , freq u e n t l y p r o d u c e s c a t h o d i c h y d r o g e n at the m e t a l s u r f a c e , a n d this h y d r o g e n m a y b e a b s o r b e d in the c o a t i n g a n d / o r in the s u b s t r a t e . T h e p r e s e n c e of this h y d r o g e n in c e r t a i n m e t a l s c a n result in e m b r i t t l e m e n t , s u c h as is the c a s e w i t h h i g h - t e n s i l e steels, l e a d i n g t o brittle f r a c t u r e w h e n stressing o c c u r s in s e r v i c e . P r o v i s i o n t o m a d e in the appropriate
standard
specifications to carry
o u t stress-relief
an-
n e a l i n g t r e a t m e n t s t o r e m o v e o r m i n i m i s e these effects. T h u s B S 1 2 2 4 : 1 9 7 0 {Electroplated coatings of nickel and chromium) s p e c i f i e s stressrelief a n n e a l i n g b e f o r e p l a t i n g for 1 h o u r at 1 3 0 - 2 1 0 ° C a c c o r d i n g t o the t y p e of steel, o r after p l a t i n g for 5 h o u r s at 1 9 0 - 2 1 0 ° C o r for 15 h o u r s at 1 7 0 ° C if the h i g h e r t r e a t m e n t t e m p e r a t u r e w o u l d b e h a r m f u l t o the m e c h a n i c a l p r o p e r t i e s of t h e steel. S i m i l a r p r o v i s i o n s are m a d e for z i n c o r c a d m i u m p l a t e d h i g h - s t r e n g t h steel c o m p o n e n t s , b u t in s p e c i a l c a s e s it m i g h t b e d e s i r a b l e t o a v o i d c o a t i n g b y e l e c t r o d e p o s i t i o n a n d use e i t h e r m e t a l s p r a y e d z i n c o r v a c u u m m e t a l l i s e d z i n c o r c a d m i u m coating processes, thus a v o i d i n g e x p o s u r e to h y d r o g e n . S p r a y e d m e t a l c o a t i n g s m a y a l s o b e p r e f e r a b l e in
applications
w h e r e fatigue l o a d i n g is i n v o l v e d , s i n c e the c o m p r e s s i v e stressing of the s u r f a c e l a y e r s of the s u b s t r a t e b y the g r i t - b l a s t i n g p r e t r e a t m e n t m a y i m p r o v e fatigue p r o p e r t i e s . I n a p p l i c a t i o n s w h e r e fretting of b o l t e d j o i n t s c a n o c c u r , the r o u g h i r r e g u l a r i t i e s p r e s e n t o n the s u r f a c e of a s p r a y e d m e t a l c o a t i n g c a n i n c r e a s e f r i c t i o n in the j o i n t a n d s o r e d u c e fretting c o r r o s i o n . M e t a l - c o a t e d plastics materials are s u b j e c t e d to particular
hazards
133
Selecting a coating
t h r o u g h m e c h a n i c a l f o r c e s d u r i n g s e r v i c e . T h e p r i n c i p a l of these is the risk o f r u p t u r e b e t w e e n the c o a t i n g a n d the s u b s t r a t e t h r o u g h stresses a r i s i n g d u r i n g t e m p e r a t u r e c h a n g e s b e c a u s e of the g r e a t d i s p a r i t y b e t w e e n the c o e f f i c i e n t s o f t h e r m a l e x p a n s i o n o f m e t a l s a n d p l a s t i c s . I n p r a c t i c e it m a y b e n e c e s s a r y t o i n c o r p o r a t e a sufficient t h i c k n e s s o f a d u c t i l e u n d e r c o a t s u c h as c o p p e r , w h i c h w i l l p r e v e n t r u p t u r e w h e n differential e x p a n s i o n a n d c o n t r a c t i o n o c c u r s . C a s e s h a v e a l s o o c c u r r e d in p r a c t i c e w h e r e p l a s t i c s c o m p o n e n t s p l a t e d w i t h n i c k e l a n d c h r o m i u m h a v e failed u n d e r stress in s e r v i c e b y f r a c t u r e in a sharply angled recess, although similar plastics c o m p o n e n t s used u n p l a t e d h a v e p e r f o r m e d s a t i s f a c t o r i l y . T h i s m o d e of failure is c a u s e d by
stress c o n c e n t r a t i o n
in
the
n o t c h o f the
recess c r a c k i n g
c h r o m i u m c o a t i n g , the c r a c k t h e n p r o p a g a t i n g t h r o u g h the
the
metal
u n d e r c o a t s a n d t h r o u g h the p l a s t i c s s u b s t r a t e . I n s u c h c a s e s the o n l y r e m e d y lies in the r e d e s i g n of t h e c o m p o n e n t t o e l i m i n a t e the n o t c h effect. M e c h a n i c a l f a c t o r s o t h e r t h a n stress that m u s t b e c o n s i d e r e d a r e those
involving movement.
Relative
movement
between
coated
c o m p o n e n t s a n d o t h e r c o m p o n e n t s in a n a s s e m b l y o c c a s i o n s w e a r , a n d the w e a r r e s i s t a n c e o f the c o a t i n g m e t a l is t h e n the
primary
c o n s i d e r a t i o n . I n g e n e r a l the h a r d n e s s of the m e t a l c o a t i n g is the p r i n c i p a l f a c t o r o f c o n c e r n h e r e , s i n c e n o r m a l l y a softer m a t e r i a l w e a r s w h e n in m o v i n g c o n t a c t w i t h a h a r d e r m a t e r i a l ( a l t h o u g h t h e r e a r e e x c e p t i o n s to this). Diffusion c o a t i n g s a n d e l e c t r o d e p o s i t e d c o a t i n g s of h a r d m e t a l s s u c h as c h r o m i u m a n d n i c k e l are p r i n c i p a l l y u s e d for wear-resistant applications, but metal sprayed coatings subsequently m a c h i n e d a n d p e r h a p s a l s o h e a t - t r e a t e d a r e often e m p l o y e d . A s e c o n d a r y p r o p e r t y o f m e t a l c o a t i n g s o f i m p o r t a n c e f o r w e a r r e s i s t a n c e is the a d e q u a c y of its a d h e s i o n t o the s u b s t r a t e . W h e r e a d h e s i o n is p o o r , r u b b i n g a c t i o n c a n c a u s e l o c a l i s e d r u p t u r e at the
coating/substrate
m e t a l i n t e r f a c e , l e a d i n g t o b l i s t e r i n g o r e v e n c o m p l e t e s p a l l i n g off of the c o a t i n g m e t a l .
Effect of environmental movement M o v e m e n t o f the e n v i r o n m e n t m a y a l s o n e e d t o b e c o n s i d e r e d . I n applications involving e x p o s u r e to m o v i n g liquids or gases erosion c a n o c c u r . T h i s m a y b e insufficient t o e r o d e the m e t a l itself
but
n e v e r t h e l e s s sufficient t o r e m o v e the p r o t e c t i v e films l o c a l l y a n d t h u s set
up
a n o d i c areas
where
enhanced
c o r r o s i o n will o c c u r
(for
e x a m p l e , i m p i n g e m e n t c o r r o s i o n of c o p p e r a n d its a l l o y s i m m e r s e d in m o v i n g w a t e r ) , o r it m a y b e o f sufficent m a g n i t u d e
to
damage
m e c h a n i c a l l y the m e t a l itself (as in c a v i t a t i o n c o r r o s i o n ) . I n e i t h e r
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Selecting a coating
case premature localised penetration through a coating m a y occur, c a u s i n g e x p o s u r e of the s u b s t r a t e w i t h c o n s e q u e n t loss of p r o t e c t i o n , o r e v e n s t r i p p i n g the c o a t i n g c o m p l e t e l y f r o m c o n s i d e r a b l e a r e a s o f the c o m p o n e n t as c o r r o s i o n u n d e r c u t s the c o a t i n g a n d t u r b u l e n c e is i n c r e a s e d in the m o v i n g l i q u i d . T h e r e m e d y in t h e s e c a s e s lies in the s e l e c t i o n o f a c o a t i n g m a t e r i a l ( e . g . n i c k e l o r n i c k e l a l l o y ) that offers i m p r o v e d r e s i s t a n c e to e r o s i o n , o r in the r e d e s i g n of the c o m p o n e n t s o that the e r o s i v e effects are r e d u c e d .