New Developments in the Theory and Practice of Pretreatment of

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


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 .


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.


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


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


2.

STEINBRECHER

43


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


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


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


2.

STEiNBRECHER

45


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 .


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.


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


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.


be

New Developments in the Theory and Practice of Pretreatment of

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