2 New Developments in the Theory and Practice of Pretreatment of Metals
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Prior to Electrophoretic Coating LESTER
STEINBRECHER
Amchem Products, Inc., Ambler, Pa. 19002
Any conversion coating system for metal parts to be electrocoated must be adapted system, and the optimum
to the entire ultimate paint operating
determined by experimentation.
conditions must
be
Zinc phosphate and iron
phosphate coatings are the two recommended systems for steel. Both suffer stripping during electrocoating, the spe cific amount of weight loss varying with the pH of the paint and the applied voltage. Weight losses of about 5-10% the original coating are not uncommon.
of
Poor "wet paint
adhesion," which on rare occasions is evidenced by removal of part of the electrocoat film during water rinsing, has been tied in with excessive coating weight loss. The weight of the zinc phosphate coatings applied can vary up to 500 mg/sq ft. Thin phosphate coatings are less corrosion resistant but allow better paint gloss and provide greater resistance to adhesion loss as in stone chipping. has been developed
A zinc phosphate coating system
which eliminates the problem of dis
coloration of white electrodeposited
paints.
" E a c h e l e c t r o p h o r e t i c p a i n t system m u s t b e c h e c k e d u n d e r v a r i o u s types of p r e t r e a t e d surfaces to ensure that t h e p a i n t system is c o m p a t i b l e w i t h t h e coated surface a n d p r o d u c e s o p t i m u m c o r r o s i o n resistance a n d p h y s i c a l properties. T h i s n o v e l m e t h o d of p a i n t a p p l i c a t i o n has m o t i v a t e d researchers i n t h e field of p r e p a i n t treatments of m e t a l surfaces to i n vestigate
existing a n d e n t i r e l y n o v e l processes w h i c h
offer
optimum
substrates f o r t h e m a n y different types o f electrophoretic p a i n t systems. F u r t h e r m o r e , solutions to p r o b l e m s w h i c h are specific to e l e c t r o p a i n t i n g 38
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
2.
STEINBRECHER
39
Pretreotment of Metals
has f o r c e d us i n some cases to r e q u i r e extremely close c o n t r o l of a l l p r e treatment stages a n d f o r m u l a t i o n of n e w p r o d u c t s to d e a l w i t h specific situations. T h i s p a p e r deals w i t h t h e m e t h o d s o f e v a l u a t i n g pretreatments of m e t a l parts to b e e l e c t r o p a i n t e d , p h o s p h a t e s t r i p p i n g d u r i n g electro deposition, o p t i m u m methods
of c l e a n i n g , w a t e r r i n s i n g , p h o s p h a t i n g ,
p a s s i v a t i o n r i n s i n g , effects of d r y i n g at different temperatures
prior to
e l e c t r o p a i n t i n g , a n d r i n s i n g after p a i n t i n g .
Evaluation Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch002
Before
'Procedure an optimum prepaint
treatment
recommendation
can be
offered, w e h a d to test t h e p a r t i c u l a r p a i n t i n q u e s t i o n i n o u r electropaint laboratory.
Panels of 4 " X 1 2 " c o m m e r c i a l g r a d e steel a n d g a l v a n i z e d
steel are treated, u t i l i z i n g sequences of c l e a n i n g a n d p h o s p h a t i z i n g , sub jected to different c o n d i t i o n s of final rinse, s u c h as n o rinse, D I w a t e r (DI =
d e i o n i z e d ) , c h r o m a t e a n d n o n - c h r o m a t e p a s s i v a t i n g rinses, a n d
p a s s i v a t i n g rinses f o l l o w e d b y D I water.
F u r t h e r m o r e , e a c h of these
variations is t h e n a i r d r i e d , b a k e d , or left w e t b e f o r e e n t e r i n g the elec tropaint bath.
T h e panels are t h e n e l e c t r o p a i n t e d i n t h e l a b o r a t o r y a p
paratus w h i c h consists of t h e f o l l o w i n g : a d c p o w e r s u p p l y w h i c h operates at a m a x i m u m of 1000 volts a n d 25 amps a n d a 15-liter stainless
steel
storage vessel f o r p a i n t w h i c h is m a i n t a i n e d at a constant t e m p e r a t u r e i n a water bath.
Sometimes t h e p a n e l is l o w e r e d i n t o t h e p a i n t b a t h , a n d
the v o l t a g e is a p p l i e d s l o w l y , never a l l o w i n g t h e current to exceed 3 a m p s . A t other times the p a n e l is l o w e r e d into t h e b a t h at a c o n t r o l l e d rate w i t h p o w e r o n to simulate specific p r o d u c t i o n c o n d i t i o n s .
T h e p a n e l is r e
m o v e d f r o m the paint bath, rinsed w i t h D I water, air dried, a n d oven b a k e d at t h e suggested t e m p e r a t u r e a n d t i m e .
T h e p h o s p h a t i n g is per
f o r m e d i n the same l a b o r a t o r y so that a g i n g of t h e c o a t i n g does n o t b e c o m e a v a r i a b l e i n the testing a n d closely simulates l i n e c o n d i t i o n s b e t w e e n the t i m e of p h o s p h a t i n g a n d e l e c t r o p a i n t i n g . T h e p a i n t e d panels are t h e n tested f o r salt s p r a y resistance, properties, a n d gravelometer
h u m i d i t y , various
adhesion
resistance.
Phosphate Stripping during Electrodeposition Studies I n general, results i n d i c a t e the d i s s o l u t i o n o r p e e l i n g of z i n c phos phate coatings d u r i n g the e l e c t r o d e p o s i t i o n varies w i t h t h e p H of t h e p a i n t a n d the v o l t a g e a p p l i e d . P a i n t s o p e r a t i n g i n t h e l o w e r p H r a n g e of 7.4 to 7.8 have y i e l d e d highest c o a t i n g w e i g h t losses f r o m 25 to 5 0 % . H i g h e r p H systems, a b o v e 9.0, h a v e y i e l d e d t h e lowest losses o f 0 to 3 % . F o r a g i v e n p a i n t system, h a l v i n g the v o l t a g e r e d u c e d t h e a m o u n t of c o a t i n g lost b y a factor of 3. R e p r o d u c i b l e p h o s p h a t e c o a t i n g w e i g h t
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
40
ELECTRODEPOSITION
OF
COATINGS
losses w i l l b e o b t a i n e d o n l y i f a c e r t a i n p r o c e d u r e of testing is strictly followed. A f t e r s t u d y i n g m a n y p a i n t systems
o r i g i n a t i n g f r o m a l l over
the
w o r l d , it w a s f o u n d that the z i n c p h o s p h a t e w h i c h is d i s s o l v e d becomes a n i n t e g r a l p a r t of the p a i n t c o a t i n g itself.
C h e m i c a l analyses of u s e d
p a i n t baths i n o u r laboratories h a v e s h o w n the c o m p l e t e absence of z i n c p h o s p h a t e i n the p a i n t b a t h p r o p e r . O n rare occasions p a i n t s y s t e m s ' h a v e b e e n e n c o u n t e r e d w h i c h s h o w extremely p o o r a d h e s i o n to the z i n c p h o s p h a t e d surface w h e n the elec Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch002
t r o d e p o s i t e d film is w a t e r r i n s e d i m m e d i a t e l y after p a i n t i n g . T h e p a i n t is either c o m p l e t e l y or p a r t i a l l y r e m o v e d b y the s p r a y w a t e r rinse or c a n be easily r e m o v e d b y gentle r u b b i n g . W h e n this p h e n o m e n o n the z i n c p h o s p h a t e c o a t i n g is almost c o m p l e t e l y r e m o v e d .
occurs,
T h e p H of
the p a i n t system does not a p p e a r to b e a significant factor d a r i n g this severe s t r i p p i n g of the z i n c p h o s p h a t e c o a t i n g .
T h e theoretical
factors
c a u s i n g this p r o b l e m are at present u n k n o w n to us, b u t w e b e l i e v e that the e n t r a p m e n t of this large a m o u n t of z i n c p h o s p h a t e i n the p a i n t
film
itself is a p a r t i a l cause of this p o o r w e t a d h e s i o n . O u r laboratories h a v e f o u n d p r o c e d u r e s to m i n i m i z e or c o m p l e t e l y correct this p r o b l e m ( d i s cussed l a t e r ) . O c c a s i o n a l l y one comes across the statement i n the l i t e r a t u r e
that
i r o n p h o s p h a t e coatings are not d i s s o l v e d d u r i n g the e l e c t r o p a i n t i n g o p eration. If this w e r e true, i r o n phosphates c o u l d b e u s e d successfully as substrates to electrodeposit w h i t e paints. A c t u a l l y , i r o n
phosphate-coated
steel shows m o r e d i s c o l o r a t i o n i n a w h i t e electrodeposit t h a n z i n c phos p h a t e d steel.
Since i r o n phosphates h a v e c o a t i n g w e i g h t s o n the o r d e r
of o n l y 20 to 40 m g / s q ft, a 1 0 %
w e i g h t loss d u r i n g p a i n t i n g w o u l d
a m o u n t to o n l y 2 to 4 m g / s q ft c o m p a r e d w i t h a b o u t 15 to 30 m g / s q ft loss i n the case of z i n c p h o s p h a t e coatings.
T h u s , less p h o s p h a t e c o a t i n g
is i n c o r p o r a t e d i n the p a i n t film i n the case of i r o n phosphates. If a p a r t i c u l a r p a i n t system is o p e r a t e d b e l o w the r u p t u r e voltage, there does not a p p e a r to b e a n y c o r r e l a t i o n b e t w e e n the a m o u n t of coat i n g lost d u r i n g e l e c t r o d e p o s i t i o n a n d the c o r r o s i o n resistance of the p a i n t .
Cleaning T h e use of a p r o p e r cleaner is essential for q u a l i t y . T h e most i m p o r tant factor to consider is that the cleaner is c o m p a t i b l e w i t h the phos p h a t i n g processes a n d that t h e surface b e free of soil, o i l , a n d s o l i d dust particles so that it c a n a c c e p t a u n i f o r m a n d tight c o a t i n g . A n y i r r e g u larities i n the c o a t i n g a p p e a r a n c e
(streaks, o i l stains, or rust or
w i l l be m a g n i f i e d d u r i n g the e l e c t r o p a i n t i n g o p e r a t i o n .
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
dust)
2.
41
Pretreatment of Metals
STEiNBRECHER
W h e n aqueous strong a l k a l i n e solutions a r e u s e d f o r c l e a n i n g p r i o r to a z i n c p h o s p h a t e process, i t is essential f o r s u i t a b l e a p p e a r a n c e a n d h i g h q u a l i t y p e r f o r m a n c e that a g r a i n r e f i n i n g m a t e r i a l c o n t a i n i n g t i t a n i u m p h o s p h a t e b e u s e d i n the stage p r e c e d i n g t h e z i n c p h o s p h a t e b a t h so that a u n i f o r m fine c r y s t a l w i l l b e o b t a i n e d .
W h e r e m i l d e r c l e a n i n g at
a p H b e l o w 10 c a n b e u s e d , t i t a n i u m base m a y b e i n c o r p o r a t e d i n t h e cleaner p r o p e r .
S t r o n g a l k a l i c l e a n i n g , f o l l o w e d b y a separate r e f i n i n g
stage is p r e f e r r e d . I n t h e case of i r o n p h o s p h a t e processes strong a l k a l i n e cleaners are p r e f e r r e d a n d t i t a n a t i o n is unnecessary. A s a result of t h e strong a l k a l i treatment, p H 12 o r h i g h e r , n u m e r o u s Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch002
n u c l e i are i n a c t i v a t e d because t h e y are c o v e r e d b y oxides o r h y d r o x i d e s w h i l e s u l f u r i c o r h y d r o c h l o r i c a c i d p i c k l i n g m a y destroy u p to 9 0 % of the a c t i v e centers.
B y t r e a t i n g t h e m e t a l surface w i t h a t i t a n i u m p h o s
p h a t e s o l u t i o n , o r w i t h s i m i l a r a c t i v a t i n g solutions, n u m e r o u s n e w c r y s t a l n u c l e i are created at t h e l o c a l cathode, u p o n w h i c h the phosphates c a n t h e n g r o w . T h i s c r y s t a l g r o w t h proceeds p a r t i c u l a r l y easily u p o n c r y s t a l n u c l e i , w h i c h themselves consist of phosphates.
T h e larger the number
of n u c l e i , t h e less is t h e distance of t h e n u c l e i f r o m t h e m e t a l T h e crystals of t h e h e a v y m e t a l p h o s p h a t e
p a c k e d so that t h e n u m e r o u s crystals w i l l soon contact e a c h other. fore,
fine-grained
surface.
w i l l t h e n b e v e r y closely
a n d t h i n p h o s p h a t e coatings w i l l b e o b t a i n e d .
There Coarse
g r a i n e d z i n c p h o s p h a t e coatings w h i c h result after strong a l k a l i n e c l e a n i n g or a c i d p i c k l i n g a n d n o t i t a n i u m a c t i v a t i o n a l w a y s have a greater area of pores t h a n fine g r a i n e d z i n c p h o s p h a t e coatings.
A greater n u m b e r of
pores present i n a coarse z i n c p h o s p h a t e c o a t i n g p r o d u c e s a m o r e c o n d u c t i v e surface a n d c o n s e q u e n t l y a h i g h e r p a i n t film thickness t h a n that produced on a
fine-grained
surface.
If a steel p a n e l is c l e a n e d i n a strong
a l k a l i n e s o l u t i o n a n d the b o t t o m h a l f is treated i n a d i l u t e t i t a n i u m phos p h a t e s o l u t i o n , z i n c p h o s p h a t e d , a n d t h e n e l e c t r o p a i n t e d , t h e interface b e t w e e n the coarse a n d r e f i n e d area w i l l b e easily seen a n d t h e p a i n t film
thickness w i l l b e greater i n t h e coarse area.
Thus, a non-uniform
z i n c p h o s p h a t e c o a t i n g w i l l result i n a n o n - u n i f o r m e l e c t r o p a i n t
film.
W o r k w h i c h has b e e n p r e v i o u s l y d e r u s t e d i n areas w i t h a p h o s p h o r i c a c i d deruster is left w i t h a v e r y l i g h t i r o n p h o s p h a t e film i n these areas. If this w o r k is t h e n passed t h r o u g h a p o w e r spray w a s h e r a n d z i n c phos p h a t e d , the i r o n p h o s p h a t e areas resist t h e z i n c p h o s p h a t e s o l u t i o n . T h e r e s u l t i n g p h o s p h a t e d part appears w i t h patches of a m o r p h o u s i r o n phos p h a t e s u r r o u n d e d b y z i n c p h o s p h a t e crystals. If this p a r t is n o w electrop a i n t e d , the areas of i r o n p h o s p h a t e w i l l s h o w a m u c h h i g h e r film t h i c k ness t h a n t h e s u r r o u n d i n g z i n c p h o s p h a t e d surface a n d m i g h t a c t u a l l y s h o w signs o f r u p t u r e i n these areas.
Therefore,
non-phosphoric acid
derusters s h o u l d b e u s e d i f t h e p a r t is to b e s u b s e q u e n t l y z i n c p h o s p h a t e d and
electropainted.
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
42
ELECTRODEPOSITION
OF
COATINGS
Phosphating T h e c h o i c e b e t w e e n z i n c p h o s p h a t e , i r o n p h o s p h a t e , or no p h o s p h a t e is g o v e r n e d b y the e n d use of the article, a n d i n g e n e r a l f o l l o w s the same p a t t e r n as for c o n v e n t i o n a l p a i n t i n g . I n general, z i n c phosphate-treated surfaces
p r o d u c e the best c o r r o s i o n resistance.
O u r laboratories
have
f o u n d i r o n phosphates to be suitable i n a p p l i c a t i o n s w h e r e less c o r r o s i o n resistance is p e r m i t t e d a n d w h e n one-coat systems are u s e d , t h e
latter
p r i m a r i l y because of gloss considerations. Z i n c p h o s p h a t e c o n v e r s i o n coatings w h i c h h a v e a n extremely
fine
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c r y s t a l structure, p r o v i d e the best substrate for electropaints w i t h respect to surface a p p e a r a n c e , a d h e s i o n , a n d c o r r o s i o n resistance.
Although a
c a l c i u m - z i n c process p r o d u c e s a n almost a m o r p h o u s t y p e of c o a t i n g o n steel, it is u n s u i t a b l e for t r e a t i n g g a l v a n i z e d steel surfaces.
T h e thinner,
m o r e densely p a c k e d c o a t i n g p r o d u c e s o p t i m u m a d h e s i o n a n d c o h e s i o n of the a p p l i e d o r g a n i c film since less c o n t a m i n a n t i n the f o r m of z i n c p h o s p h a t e is i n t r o d u c e d i n t o the p a i n t film p r o p e r d u r i n g the d e p o s i t i o n process.
electro
Z i n c p h o s p h a t e coatings i n w e i g h t range of 1 5 0 - 2 0 0
m g / s q ft offer w i t h m a n y p a i n t systems the best c o m p r o m i s e for o p t i m u m c o r r o s i o n resistance a n d a d h e s i o n p e r f o r m a n c e . I m p o r t a n t n e w p h o s p h a t i n g systems for p r e t r e a t i n g steel
surfaces
p r i o r to the e l e c t r o d e p o s i t i o n of one-coat w h i t e paints h a v e b e e n d e v e l o p e d . W h e n a steel surface c o a t e d w i t h c o n v e n t i o n a l i r o n , z i n c , or c a l c i u m - z i n c phosphates is e l e c t r o p a i n t e d w i t h a w h i t e film, the r e s u l t i n g w o r k appears y e l l o w - o f f w h i t e w i t h r e d stains, blotches, a n d v a r i o u s types of blemishes s h o w i n g t h r o u g h the w h i t e film. It is b e l i e v e d that the m a i n source of this d i s c o l o r a t i o n is c a u s e d b y the o x i d a t i o n of m e t a l l i c i r o n at the anode to v a r i o u s types of i r o n hydrates w h i c h b e c o m e p a r t of the paint
film.
T h e i r o n hydrates e n t r a p p e d i n the p a i n t film are d a r k a n d
t e n d to d i s c o l o r the film. I n a d d i t i o n , the hydrates f o r m i r o n oxides w h e n the p a i n t film is subjected to heat d u r i n g the c u r i n g o p e r a t i o n . T h e s e i r o n oxides because of t h e i r d a r k c o l o r t h e n s h o w t h r o u g h the w h i t e p a i n t If m e t a l l i c c o p p e r is p a i n t e d w h i t e e l e c t r o p h o r e t i c a l l y , the
film.
copper
is also o x i d i z e d at the a n o d e to c u p r i c ions. T h e c u p r i c hydrates w h i c h b e c o m e p a r t of the w h i t e p a i n t film are b l u e a n d thus d i s c o l o r the w h i t e paint
film.
F o r e x a m p l e , i n a n a m i n e - s o l u b i l i z e d system the c u p r i c ions
c o m b i n e w i t h the amines i n the p a i n t system to f o r m the intense blue* copper a m m o n i u m complex, C u ( N H ) . 3
4
P h o s p h a t i n g systems h a v e b e e n f o r m u l a t e d b y i n c o r p o r a t i n g s m a l l quantities of c o p p e r i n the c o a t i n g solutions.
T h e resulting phosphate
coatings i n c l u d e traces of c o p p e r w h i c h d u r i n g the e l e c t r o p h o r e t i c p a i n t i n g is o x i d i z e d to the characteristic
b l u e hydrates, w h i c h f u n c t i o n to
c a n c e l the u n d e s i r e d colors of the i r o n hydrates or oxides. T h e result is
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
2.
STEINBRECHER
43
Pretreatment of Metals
an u n b l e m i s h e d p a i n t film that is a w h i t e r - w h i t e color o r a b l u e - t i n t e d w h i t e color, d e p e n d i n g o n t h e a m o u n t of c o p p e r that w a s d e p o s i t e d o n the surface.
T h e a m o u n t d e p o s i t e d is r e g u l a t e d b y c o n t r o l l i n g the c o p p e r
content of the system.
T h i s pretreatment
system i n c o n j u n c t i o n
with
w h i t e electropaints presently i n use s h o u l d e n a b l e t h e p a i n t i n d u s t r y to use w h i t e f o r m u l a t i o n s successfully i n e v e r y d a y p r o d u c t i o n .
Passivation Acidulated Rinse W h e n z i n c p h o s p h a t e d parts enter the electropaint b a t h either a i r Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch002
d r y o r w e t , the u s e of a p a s s i v a t i n g rinse c o n t a i n i n g b o t h hexavalent a n d t r i v a l e n t c h r o m i u m enhances salt spray results. u t i l i z e d p r i o r t o t h e electropaint
W h e n a dryoff o v e n is
tank, a c h r o m a t e
nor h i n d e r s the final c o r r o s i o n resistance.
rinse neither
helps
O n e m a y o r m a y n o t use a
c h r o m a t e rinse i f a d r y o f f o v e n is u s e d a n d o b t a i n the same result.
How
ever, a c h r o m a t e rinse is also d e s i r e d to m i n i m i z e t h e r u s t i n g of parts d u r i n g l i n e stops.
F u r t h e r m o r e , i t has b e e n g e n e r a l l y f o u n d that i f the
w o r k is o v e n d r i e d w i t h o u t a c h r o m e rinse, the results w i l l b e better t h a n a i r - d r i e d m e t a l even w h e n f o l l o w e d b y a n o p t i m u m c h r o m a t e rinse sys tem. O v e n - d r i e d w o r k w i t h or w i t h o u t a c h r o m a t e final rinse w i l l a l w a y s p r o d u c e better c o r r o s i o n resistance t h a n a n a i r - d r i e d system f o l l o w e d b y a c h r o m a t e rinse. N o n - c h r o m a t e final rinses h a v e b e e n d e v e l o p e d w h i c h p r o d u c e e q u i v alent c o r r o s i o n resistance to s t a n d a r d hexavalent rinses w h e n p a i n t e d w i t h electrophoretic I n the case o f i r o n p h o s p h a t e
chromium containing
primers.
coatings, a n after-treatment w i t h a
p a s s i v a t i n g s o l u t i o n c o n t a i n i n g b o t h hexavalent a n d t r i v a l e n t c h r o m i u m c o m p o u n d s is a b s o l u t e l y necessary f o r suitable c o r r o s i o n resistance. T h e use of a dryoff o v e n has n o effect o n t h e u l t i m a t e q u a l i t y w h e n these amorphous-coated
surfaces are e l e c t r o p a i n t e d .
A p a r t i a l l y r e d u c e d c h r o m a t e after-rinse leads t o a r e d u c t i o n i n the area o c c u p i e d b y t h e press. T h e t r i v a l e n t a n d h e x a v a l e n t c h r o m i u m rinse a c t u a l l y leads to a " c l o g g i n g " of the pores b y the f o r m a t i o n of a c h r o m i u m chromate gel.
Deionized Water Rinsing A d e i o n i z e d w a t e r rinse is a l w a y s r e c o m m e n d e d as the final step i n the treatment of the m e t a l , w h e t h e r or n o t a n o v e n dryoff is u s e d .
This
prevents c a r r y - o v e r of f o r e i g n ions into the p a i n t b a t h . It has b e e n s h o w n that w h e n certain concentrations
of c h l o r i d e or c h r o m a t e are r e a c h e d i n
the p a i n t b a t h , r u p t u r i n g o f t h e p a i n t occurs a n d t h e p a i n t q u a l i t y falls off d r a s t i c a l l y . A b o v e 60 p p m of c h l o r i d e ions t h e salt spray resistance decreases, a n d at 100 p p m scribe f a i l u r e is severe.
H o w e v e r , i t is n o t
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
44
ELECTRODEPOSITION
OF
COATINGS
chromâtes or c h l o r i d e s per se w h i c h affect e l e c t r o p a i n t i n g baths b u t a n y electrolyte w h i c h m i g h t elevate the c o n d u c t i v i t y of the p a i n t b a t h . D e i o n i z e d w a t e r a p p l i c a t i o n after the p a s s i v a t i n g c h r o m a t e r i n s e w i l l , i n general, not alter the salt spray resistance of a n electropaint p r i m e r . T h e t i m e b e t w e e n the a p p l i c a t i o n of the c h r o m a t e rinse a n d the d e i o n i z e d rinse was not significant w h e n d w e l l times w e r e v a r i e d b e t w e e n 10 a n d 300
seconds. T h e p u r i t y of the rinse stages has b e e n f o u n d to b e r e l a t e d to the
w e t a d h e s i o n p r o b l e m p r e v i o u s l y discussed. W h e n a c e r t a i n p a i n t f o r m u
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l a t i o n is m a r g i n a l i n its w e t a d h e s i o n properties, a s m a l l increase i n the c o n d u c t i v i t y of the D I rinse w i l l t e n d to p r o d u c e m u c h poorer a d h e s i o n of the w e t film. T h u s , close c o n t r o l of the rinse stages becomes essential w h e n this p r o b l e m is at h a n d . T h e presence of specific ions i n the passi v a t i n g rinse s o l u t i o n also tends to m i n i m i z e or c o m p l e t e l y e l i m i n a t e the problem.
Drying of Phosphate Coatings Prior to Electropainting W i t h most p a i n t systems tested, o v e n d r y i n g of the z i n c p h o s p h a t e c o a t i n g b e f o r e p a i n t i n g i n the g e n e r a l range of 1 5 0 ° - 2 0 5 ° C for times of 5 - 1 0 m i n u t e s greatly increases the salt s p r a y p e r f o r m a n c e c o m p a r e d w i t h parts w h i c h w e r e p a i n t e d air d r y . If the o v e n t e m p e r a t u r e greatly exceeds 250°—i.e., 2 5 0 ° - 3 0 0 ° C times of 6 - 1 0
m i n u t e s — c o r r o s i o n resistance d r o p s a p p r e c i a b l y .
for
Recent
w o r k o n the t h e r m a l b e h a v i o r of z i n c p h o s p h a t e crystals s c r a p e d f r o m a steel surface a n d o n the steel surface itself, u t i l i z i n g t h e r m o g r a v i m e t r i c a n d d i f f e r e n t i a l t h e r m a l a n a l y t i c a l t e c h n i q u e s , h a v e s h o w n the exact de gree of d e h y d r a t i o n of the z i n c c r y s t a l at v a r i o u s temperatures.
The
c h a n g e i n the degree of h y d r a t i o n of the z i n c p h o s p h a t e c r y s t a l is f o l l o w e d b y a c h a n g e i n the c o r r o s i o n resistance. thermogravimetric
analysis
(TGA)
F i g u r e 1 illustrates the results of
and
differential
( D T A ) of z i n c p h o s p h a t e s c r a p e d f r o m steel surfaces.
thermal
analysis
Thermogravimetric
analysis is a m e a s u r e m e n t b y a t h e r m o b a l a n c e of the a c t u a l w e i g h t c h a n g e of a sample as a f u n c t i o n of t i m e or temperature.
A s a m p l e is w e i g h e d
b e f o r e a n d after h e a t i n g to a d e s i r e d t e m p e r a t u r e for a d e s i r e d t i m e . D i f f e r e n t i a l t h e r m a l analysis demonstrates calorific phenomena—i.e., disassociation, a l l o t r o p i e changes,
a n d phase changes.
D T A and T G A
are o f t e n u s e d together, as w e d i d i n the z i n c p h o s p h a t e s t u d y , to d e m o n strate d e c o m p o s i t i o n .
T h u s , as the T G A c u r v e s h o w e d a w e i g h t
loss
c a u s e d b y w a t e r e v o l u t i o n , the D T A c u r v e s h o w e d a n e n d o t h e r m i c reac t i o n t a k i n g p l a c e at the d e c o m p o s i t i o n t e m p e r a t u r e or t e m p e r a t u r e w h e r e w a t e r w a s d r i v e n off. T h e first t w o m o l e c u l e s of w a t e r of h y d r a t i o n are lost at 180 ° C w h i l e the last traces of w a t e r d i s a p p e a r at 320 ° C .
The corresponding D T A
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
2.
STEiNBRECHER
45
Pretreatment of Metals
c u r v e shows e n d o t h e r m i c peaks at 1 8 0 ° C a n d 3 2 0 ° C also. T h e increase i n c o r r o s i o n resistance o n u s i n g a dryoff o v e n is c a u s e d b y the loss of t w o m o l e c u l e s of w a t e r of h y d r a t i o n f r o m t h e h o p e i t e a n d p h o s p h o p h y l l i t e , each c r y s t a l l i z i n g w i t h f o u r m o l e c u l e s of w a t e r of c r y s t a l l i z a t i o n .
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TGA
\ r
'VI 1
1
100
200
1
1
1
1
1
1
300
400
500
600
700
800
900
TEMPERATURE ° C
Figure 1.
TGA and DTA of zinc phosphate scraped from steel surfaces
T h e r m o g r a v i m e t r i c analysis of a z i n c p h o s p h a t e c o a t i n g o n the sur face of steel g e n e r a l l y s h o w e d the same d e c o m p o s i t i o n temperatures as that of t h e s c r a p e d z i n c p h o s p h a t e crystals.
H o w e v e r , above 4 0 0 ° C a
w e i g h t g a i n w a s o b s e r v e d i n the T G A c u r v e . A s t h e t e m p e r a t u r e w a s g r a d u a l l y i n c r e a s e d ( h e a t i n g rate w a s 1 0 ° C / m i n . ), the increase i n w e i g h t c o n t i n u e d at a m u c h greater rate. M i c r o s c o p i c studies of t h e crystals w e r e also p e r f o r m e d after sub j e c t i n g t h e m e t a l t o v a r i o u s e l e v a t e d temperatures.
A t extremely h i g h
temperatures a b o v e 430 ° C the a n h y d r o u s c r y s t a l a c t u a l l y shrinks to f o r m m o r e v o i d s i n the m e t a l surface.
M i c r o s c o p i c pictures h a v e s h o w n a
s h r i n k a g e i n the c r y s t a l above 4 0 0 ° C .
T h u s , the T G A analysis i n c o n
j u n c t i o n w i t h p h o t o m i c r o g r a p h s indicates that as the z i n c p h o s p h a t e d surface is exposed to temperatures
above 400 ° C , the c o a t i n g shrinks
a n d o x i d a t i o n of the m e t a l takes over r a p i d l y , c a u s i n g a sharp increase in weight.
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
46
ELECTRODEPOSITION
OF
COATINGS
Rinsing after Painting Some p a i n t systems are sensitive to tap w a t e r r i n s i n g even i f f o l l o w e d b y a final d e i o n i z e d w a t e r rinse.
M o s t p a i n t systems, h o w e v e r , c a n be
r i n s e d w i t h tap w a t e r if the final rinse b e f o r e d r y i n g is d e i o n i z e d water. If t a p w a t e r is u s e d as a final rinse after p a i n t i n g , s p o t t i n g occurs o n the surface either b y r e a c t i o n of the tap w a t e r salts w i t h the u n c u r e d p a i n t or e v a p o r a t i o n of the tap w a t e r itself. A trace q u a n t i t y of w e t t i n g agent
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is a d d e d to the final d e i o n i z e d rinse b y some p a i n t m a n u f a c t u r e r s .
Pretreatment of Zinc and Aluminum T h e pretreatment of z i n c , i n general, f o l l o w s the same rules as the pretreatment of steel. F o r p a s s i v a t e d g a l v a n i z e d steel sheets a z i n c phos p h a t e s o l u t i o n c o n t a i n i n g f l u o r i d e w i l l be h e l p f u l i n f o r m i n g the c o a t i n g . T h e most i m p o r t a n t steps i n t r e a t i n g a l u m i n u m p r i o r to electropaint i n g are a l k a l i n e c l e a n i n g a n d d e o x i d i z i n g the surface.
If the surface has
scratches a n d other i m p e r f e c t i o n s , e t c h i n g the a l u m i n u m i n strong a l k a l i solutions f o l l o w e d b y a d e s m u t t i n g o p e r a t i o n is o f t e n desirable. recent research w o r k has s h o w n that c h r o m a t e
Our
c o a t e d a l u m i n u m per
f o r m e d better i n a c i d salt spray after e l e c t r o p a i n t i n g t h a n a l k a l i e t c h e d a n d d e o x i d i z e d a l u m i n u m . T h e use of acetic a c i d to l o w e r the p H of salt solutions u s e d i n accelerated test p r o g r a m s has s h o w n that i t is a u s e f u l t o o l i n d i s t i n g u i s h i n g b e t w e e n a g o o d composite system a n d one w h i c h is i n f e r i o r . F o r a l u m i n u m parts w h i c h are c o n t i g u o u s to steel, a n d the parts are z i n c p h o s p h a t e d , c o n v e n t i o n a l treatments w i l l coat o n l y the steel.
Thus,
the p a r t w i l l be z i n c p h o s p h a t e d steel a n d b a r e a l u m i n u m . Since there is a large difference i n c o n d u c t i v i t y b e t w e e n z i n c p h o s p h a t e d steel a n d b a r e a l u m i n u m , there is a p o s s i b i l i t y that r u p t u r i n g of the
electropaint
o n the m o r e c o n d u c t i v e a l u m i n u m surface w i l l occur. T h e r e f o r e , a treat ment w h i c h w i l l zinc phosphate
b o t h a l u m i n u m a n d steel w o u l d
r e c o m m e n d e d i n this case.
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
be