Electrodeposition of Coatings

1 Conversion and Electrodeposited Coatings:

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A Total Concept JAMES I. MAURER and ROBERT M . LACY The Parker Co., Oxy Metal Finishing Corp., Occidental Petroleum Corp., P. O. Box 201, Detroit, Mich. 48220

In the electrodeposition

of paint, part of the substrate be-

comes an integral part of the deposited film and can influence the coating properties.

This paper covers the effect

on paint quality of the cleaning of the metal surface, formation of the conversion coating, post treatment, deionized water rinsing, and dryoff conditions.

The system is evalu-

ated for salt spray and humidity resistance, adhesion, filiform

corrosion,

detergent

resistance,

paint film. For maximum selectivity conversion dryoff

coating,

a reactive

post

and

uniformity

of

of paint, the proper treatment, and

oven should be used. By carefully

the

matching the

paint formulation with the conversion coating, quality finishes can result, even if post treatment and the dryoff oven are eliminated.

Electrodeposited paints require a more uni-

form and complete coating than conventionally

deposited

paints.

T t is a c c e p t e d p r a c t i c e to c l e a n a n d to treat m e t a l surfaces to p r o d u c e o n t h e m a c o n v e r s i o n c o a t i n g before a p p l y i n g i n d u s t r i a l p a i n t (1,2).

finishes

C o n v e r s i o n coatings as a base for p a i n t h a v e b e e n p r o v e d v a l u a b l e

d u r i n g m a n y years o f field use, w h i c h h a v e s h o w n that t h e y p r o v i d e a s i m p l e , e c o n o m i c a l means o f s u b s t a n t i a l l y i n c r e a s i n g the o v e r a l l q u a l i t y of p a i n t e d p r o d u c t s . P r o p e r m e t a l p r e p a r a t i o n , i n c l u d i n g the f o r m a t i o n o f a surface c o n v e r s i o n c o a t i n g p r i o r to p a i n t i n g , c o n t r i b u t e s to p a i n t e d p r o d u c t d u r a bility by: ( 1 ) d e c r e a s i n g the s p r e a d o f c o r r o s i o n o f the substrate m e t a l at areas w h e r e the p a i n t film is b r o k e n , a n d i n this w a y m a t e r i a l l y r e d u c i n g the 7

In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

8

ELECTRODEPOSITION

OF

COATINGS

loss of p a i n t that w o u l d o r d i n a r i l y l i f t a n d p e e l a w a y as a result of the a c t i o n of the a l k a l i n e c o r r o s i o n p r o d u c t s . ( 2 ) p r e v e n t i n g or d e c r e a s i n g o n z i n c surfaces the r e a c t i o n of the z i n c m e t a l w i t h the p a i n t b y v i r t u e of the f a c t that the c o n v e r s i o n c o a t i n g is a n o n - m e t a l l i c , n o n - r e a c t i v e s e p a r a t i n g layer.


( 3 ) c o n t r o l l i n g the a c t i o n of m o i s t u r e w h i c h permeates the p a i n t film to a s u b s t a n t i a l degree. T h i s eliminates or m i n i m i z e s b l i s t e r i n g u n d e r h i g h h u m i d i t y c o n d i t i o n s , thus c o n t r i b u t i n g to p a i n t film i n t e g r i t y . ( 4 ) i m p r o v i n g the m e c h a n i c a l p a i n t a d h e s i o n b y i n c r e a s i n g the sur face area, a n d / o r p r o v i d i n g a c a p i l l a r y b e d ( 3 ) into w h i c h the o r g a n i c finish c a n penetrate. C o n v e r s i o n coatings

are p r o d u c e d b y the c h e m i c a l r e a c t i o n of a

c o a t i n g s o l u t i o n w i t h the m e t a l surface.

I n most cases, c o m p o n e n t s

of

the m e t a l surface react w i t h components of the c o a t i n g s o l u t i o n to p r o d u c e a t i g h t l y adherent, w a t e r - i n s o l u b l e i n o r g a n i c c o a t i n g o n the m e t a l . T h e m e t a l surface is thus r e n d e r e d n o n - m e t a l l i c . C l e a n i n g a n d c o n v e r s i o n c o a t i n g c a n b e c o m b i n e d into one Processes of this t y p e are g e n e r a l l y r e f e r r e d to as cleaner-coaters. greater

flexibility

step. Much

i n operation and usually higher quality can be

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

ob

conversion-coating

step a n d b y post t r e a t i n g the c o n v e r s i o n c o a t i n g to f u r t h e r e n h a n c e its a b i l i t y to h o l d p a i n t a n d m i n i m i z e corrosive attack of the m e t a l surface. A t y p i c a l p r o c e s s i n g sequence b e f o r e c o n v e n t i o n a l p a i n t a p p l i c a t i o n t o d a y w o u l d consist of: ( 1 ) C l e a n i n g the m e t a l , 6 0 - 9 0 seconds (2)

W a t e r rinse, 30 seconds

(3)

T r e a t m e n t to o b t a i n a c o n v e r s i o n c o a t i n g , 60 seconds

( 4 ) W a t e r rinse, 30 seconds ( 5 ) Post treatment or final rinse, 30 seconds (6)

Dryoff i n oven

T h e t w o b a s i c m e t h o d s of t r e a t i n g m e t a l surfaces are b y the i m m e r sion process a n d b y the s p r a y process (4, 5 ) .

T h e i m m e r s i o n process is

the o l d e r a n d the s i m p l e r a n d consists of d i p p i n g the p r o d u c t to

be

treated i n tanks c o n t a i n i n g the treatment s o l u t i o n . M o s t h i g h p r o d u c t i o n treatments of p r e f o r m e d m e t a l parts t o d a y , h o w e v e r use the b y the s p r a y process.

chemicals

T h e s p r a y process, i n this case, is not the a p p l i c a

t i o n of the s o l u t i o n u s i n g finely d i s p e r s e d particles, as is the case of the a p p l i c a t i o n of paints b y spray, b u t rather b y flooding the p r e f o r m e d parts b y i m p i n g i n g the s o l u t i o n onto the m e t a l surface t h r o u g h n o z z l e s that h a v e r e l a t i v e l y h i g h v o l u m e c a p a c i t y a n d w h i c h are d e s i g n e d to p r o d u c e a m i n i m u m b r e a k u p of s o l u t i o n . T h e s o l u t i o n d r a i n i n g f r o m the parts runs b a c k to a reservoir tank a n d is constantly r e c i r c u l a t e d onto the w o r k a n d c o n t i n u a l l y reused. C o n v e r s i o n coatings c a n b e p r o d u c e d b y b r u s h i n g or w i p i n g a t r e a t i n g s o l u t i o n o n the m e t a l surface.

Portable, heated spray


1.

MAURER

A N D LACY

9

Conversion and Electrodeposited Coatings

e q u i p m e n t or steam g e n e r a t i n g e q u i p m e n t c a n b e successfully u s e d to a p p l y c o n v e r s i o n coatings outdoors or w h e r e spray or i m m e r s i o n e q u i p m e n t is u n a v a i l a b l e . N o r m a l l y these latter methods are u s e d f o r l i m i t e d p r o d u c t i o n o r f o r l a r g e o r h e a v y items. O n e large use of p a i n t e d m e t a l is the p a i n t e d c o i l - p o s t f o r m e d a p p r o a c h to the p r o d u c t i o n of items s u c h as roof d e c k i n g , b u i l d i n g s i d i n g Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 9, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch001

a n d t r i m , a n d m a n y other a p p l i c a t i o n s w h e r e the p r o d u c t c a n b e f o r m e d after p a i n t i n g . Since the m e t a l is flat a n d i n c o i l f o r m , i t c a n b e p u l l e d t h r o u g h a s t r i p l i n e w h e r e e a c h o f t h e p r o c e s s i n g stages are separated b y squeegee rolls. T h i s process features v e r y short treatment times a n d v e r y h i g h l i n e speeds (6).

B o t h i m m e r s i o n a n d s p r a y processes are used.

A New Dimension U n t i l the i n t r o d u c t i o n of electrocoating (7, 8, 9 ) , the m e t h o d of a p p l i c a t i o n of the p a i n t film h a d l i t t l e i f a n y b e a r i n g o n the q u a l i t y of the finished

system. W i t h the i n t r o d u c t i o n of t h e e l e c t r o d e p o s i t e d p a i n t

film,

h o w e v e r , i t is necessary to t h i n k n o l o n g e r i n terms of a p a i n t a p p l i e d o n a substrate b u t rather a p a i n t film f o r m e d o n a substrate, w i t h components of the substrate b e c o m i n g a n i n t e g r a l part of the p a i n t film (10, 11, 12, 13,14,15). finish

W i t h e l e c t r o d e p o s i t e d p a i n t i n g i t is the i n t e r p l a y of t h e t o t a l

system that must b e c o n s i d e r e d to ensure

quality a n d economy.

o p t i m u m balance

of

T h i s p a p e r deals w i t h the role p l a y e d b y c o n

v e r s i o n coatings i n t h e t o t a l

finishing

system.

Current Practice V a r i o u s c o n v e r s i o n c o a t i n g processes are u s e d i n i n d u s t r y t o d a y . T h e t y p e u s e d depends o n t h e t y p e of m e t a l , the c o m b i n a t i o n of metals processed, a n d the q u a l i t y r e q u i r e m e n t s o f a g i v e n o p e r a t i o n .

T h e fol

l o w i n g s u m m a r i z e s the t y p e of c o n v e r s i o n coatings a v a i l a b l e : Steel: ( 1 ) I r o n p h o s p h a t e — a m i x t u r e of i r o n p h o s p h a t e a n d i r o n o x i d e ; c o n s i d e r e d to b e a m o r p h o u s . C o a t i n g w e i g h t s i n t h e range of 15 to 90 m g / s q ft. ( 2 ) Z i n c p h o s p h a t e — g r e y , c r y s t a l l i n e , essentially a m i x t u r e of z i n c a n d i r o n phosphates. C o a t i n g w e i g h t s i n t h e range of 100 to 600 m g / s q ft. ( 3 ) M o l y b d a t e / p h o s p h a t e — a m i x t u r e of i r o n p h o s p h a t e a n d m o l y b d a t e w i t h i r o n oxides; c o n s i d e r e d to b e a m o r p h o u s . C o a t i n g w e i g h t s i n t h e range o f 15 to 50 m g / s q f t . Z i n c Surfaces:

( H o t D i p p e d G a l v a n i z e d a n d E l e c t r o g a l v a n i z e d Steel)

( 1 ) Z i n c p h o s p h a t e — e s s e n t i a l l y z i n c p h o s p h a t e w i t h traces of n i c k e l ; grey, c r y s t a l l i n e . C o a t i n g w e i g h t range of 100 to 350 m g / s q ft.


10

ELECTRODEPOSITION

O F

COATINGS


( 2 ) Molybdate/phosphate—a mixture of zinc molybdate a n d phos phates; b l u i s h - g o l d e n i n c o l o r ; a m o r p h o u s . Aluminum: ( 1 ) Z i n c phosphate—a mixture of zinc a n d a l u m i n u m phosphates; c r y s t a l l i n e , g r e y i n color. C o a t i n g w e i g h t range o f 150 to 500 m g / s q f t . (2) Molybdate/phosphate—a mixture of a l u m i n u m molybdate a n d p h o s p h a t e ; g o l d e n - b l u e i n color. (3) C h r o m i c oxide—oxides of a l u m i n u m w i t h chromic compounds; colorless to g o l d e n ; c o n s i d e r e d to b e a m o r p h o u s . C o a t i n g w e i g h t range of 10 to 50 m g / s q ft. I n terms o f t o t a l m e t a l surface area treated, c o l d r o l l e d steel i s cer t a i n l y t h e most i m p o r t a n t m e t a l p a i n t e d b y e l e c t r o d e p o s i t i o n .

W e there

f o r e c o n s i d e r t w o o f t h e m o s t w i d e l y u s e d c o n v e r s i o n coatings f o r this m e t a l — t h e z i n c p h o s p h a t e a n d t h e i r o n p h o s p h a t e c o n v e r s i o n coatings.

A New Look at the Old Z i n c p h o s p h a t e coatings are, to t h e n a k e d eye, l i g h t grey, s c r a t c h a b l e coatings, w i t h a n o b v i o u s , fine c r y s t a l structure.

Under a light micro

scope, w e c a n see a definite c r y s t a l l i n e structure, b u t i t is d i f f i c u l t t o o b t a i n a r e a l l y g o o d v i e w o f t h e c o a t i n g because t h e z i n c

phosphate

crystals are translucent to l i g h t , a n d they a p p e a r r e l a t i v e l y coarse o n a m i c r o l e v e l . T h i s translucent p r o p e r t y , c o m b i n e d w i t h t h e s h a l l o w d e p t h of focus of a l i g h t m i c r o s c o p e gave the p h o t o g r a p h s i l l u s t r a t e d i n F i g u r e 1.

Figure 1. SAE 1010 cold rolled steel sur face coated with a zinc phosphate conver sion coating using a spray, nitrite-accelerated process. 200 X using a light microscope.



1.

M A U R E R A N D LACY


11

Figure 2. Same SAE as in Figure 1 but viewed at 200 X with a scanning electron microscope W i t h t h e a d v e n t o f t h e s c a n n i n g e l e c t r o n m i c r o s c o p e (17), w h i c h fea tures, a m o n g other things, a greater d e p t h o f focus a n d because t h e coat ings are essentially o p a q u e to t h e e l e c t r o n b e a m , c o n v e r s i o n coatings c a n n o w b e seen i n greater d e t a i l . F i g u r e 2 shows t h e same c o a t i n g as i n F i g u r e 1 b u t v i e w e d u s i n g a s c a n n i n g electron m i c r o s c o p e .

H i g h e r magnification viewing w i t h the

s c a n n i n g e l e c t r o n m i c r o s c o p e reveals f u r t h e r details o f a p h o s p h a t e c o n v e r s i o n c o a t i n g o n steel. F i g u r e 3 shows a t y p i c a l z i n c p h o s p h a t e c o a t i n g as w o u l d b e a p p l i e d to steel o r z i n c at a m a g n i f i c a t i o n o f 2000 χ . F i g u r e 4 shows a c a l c i u m m o d i f i e d , z i n c p h o s p h a t e process o n steel at 2000 χ . F i g u r e 5 shows a n i r o n p h o s p h a t e c o a t i n g , w h i c h has heretofore b e e n g e n e r a l l y c o n s i d e r e d a m o r p h o u s , at a m a g n i f i c a t i o n of 10,000 X . C o n v e r s i o n c o a t i n g a r e n o n - m e t a l l i c a n d are g e n e r a l l y as n o n - c o n d u c t i v e (18, 19). be

described

Since t h e e l e c t r o d e p o s i t e d p a i n t film c a n

a p p l i e d o n l y to a c o n d u c t i v e surface,

h o w c a n the conventional

c o n v e r s i o n coatings serve as a base f o r e l e c t r o d e p o s i t e d coatings? T h e a n s w e r is that w h i l e c o n v e r s i o n coatings are essentially n o n - c o n d u c t i v e , t h e y are d i s c o n t i n u o u s , a n d t h e e l e c t r o d e p o s i t i o n of p a i n t films begins i n the pores o f the c o n v e r s i o n c o a t i n g

(14,15,16).

A t y p i c a l processing sequence f o r t r e a t i n g metals b e f o r e t i o n a l p a i n t i n g consists of six stages.

conven

F o r electrodeposited paints, the

f o l l o w i n g sequence is u s e d :


12

ELECTRODEPOSITION

O F

COATINGS

( 1 ) C l e a n i n g the m e t a l ( 2 ) W a t e r rinse ( 3 ) T r e a t m e n t to o b t a i n a c o n v e r s i o n c o a t i n g ( 4 ) W a t e r rinse ( 5 ) Post treatment o r final rinse ( 6 ) D e i o n i z e d w a t e r rinse, 1 0 - 1 5 seconds


( 7 ) Dryoff i n oven E a c h step is d i s c u s s e d b e l o w . A t y p i c a l spray p h o s p h a t i z i n g u n i t is s h o w n i n F i g u r e 6.

Figure 3. SAE cold rolled steel treated with a spray nitrite-accelerated, zinc phos phate process modified by nickel and flu oride

Cleaners T h e first step i n f o r m i n g a c o n v e r s i o n c o a t i n g is attack of t h e m e t a l surface b y the c o a t i n g s o l u t i o n . T o o b t a i n a u n i f o r m i n i t i a l attack, a n d thus a u n i f o r m final c o n v e r s i o n c o a t i n g , a l l u n w a n t e d soils m u s t b e r e m o v e d f r o m the surface of the m e t a l . A l m o s t a l l m e t a l u s e d i n i n d u s t r y is c o a t e d w i t h a t h i n film of o i l b y the p r o d u c e r to protect i t d u r i n g s h i p p i n g a n d storage.

T o i d e n t i f y various grades of m e t a l , most m i l l s a p p l y

a p r i n t e d i d e n t i f i c a t i o n o n their m e t a l sheets o r coils, u s u a l l y g i v i n g t h e i r t r a d e n a m e a n d t y p e of m e t a l . M o r e o v e r , d u r i n g a n n e a l i n g a n d c o l d r o l l i n g , s m u t - l i k e soils are f o r m e d , w h i c h consist of mixtures of p a r t i a l l y b u r n t r o l l i n g oils, finely d i v i d e d m e t a l particles, a n d oxides of t h e m e t a l .



1.

MAURER A N D L A C Y


13

Figure 4. SAE 1010 cold rolled steel treated with a spray zinc phosphate process, calcium modified, ferrous ion present. 200 X using a scanning electron microscope. T o f a b r i c a t e m e t a l into t h e d e s i r e d shape, i t is often necessary t o a p p l y pressing o r d r a w i n g l u b r i c a n t s .

T h e s e a r e u s e f u l because they

adhere

t e n a c i o u s l y to t h e m e t a l , r e d u c e f r i c t i o n a n d w e a r b e t w e e n t h e d i e a n d the m e t a l , a n d thus e l i m i n a t e s c o r i n g , s c r a t c h i n g , a n d g a l l i n g .

Figure 5. SAE 1010 cold rolled steel treated with a chlorate accelerated, spray, iron phosphate process. 10,000 X using a scanning electron microscope.


During

14

ELECTRODEPOSITION

O F

COATINGS

storage a n d d u r i n g their journeys t h r o u g h f a b r i c a t i o n plants t h e m e t a l parts p i c k u p shop d i r t , c h a l k , w a x , a n d i n k w h i c h are i n s p e c t i o n aids o r identification markings.

T h e s e soils o n t h e m e t a l surface c a n seriously

affect the subsequent m e t a l processing steps a n d m u s t b e r e m o v e d . T h e cleaners f o r m u l a t e d a n d selected m u s t b e able to r e m o v e t h e v a r i o u s soils d e s c r i b e d above, a n d after t h e w a t e r rinse stage t h e y m u s t


p r o d u c e a surface that is c o n d u c i v e to t h e c o n v e r s i o n c o a t i n g

process.

ELEVATION

L^-OAMPER

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V LONGITUDINAL

SECTION

A«

|> t6A 4 6 44i0| » «» ° «"» «j ' -7 REMOVABLE SCREENS'

I OVERFLOW

CUTTER PLAN

S C H E D U L E ZONE PARCO*CLEANE R WATER RINSE PARCO CLEANER WATER RINSE

BONDER 1 T E ®

TREATMENT

WATER RINSE PARCOLENE® TREATMENT OEIONIZED WATER RINSE F R E S H DEIONIZED RINSE

TIME CYCLE

45 S E C

45 SEC 45 SEC 45 SEC

OPERATING T E M P . »F N U M B E R 150-180 150-180 150-180 130 - 1 8 0

6 0 SEC

130-180

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NO HEAT NO HEAT

15 S E C

NO HEAT

1 3 5 1 3 5

1 35 1 35 105 • 12 105 84 48 24

NOZZLES TYPE PRESS 15 PSI H 3/8 U15070 H 3/8 U15070 15 P S I H 3/8 U15070 15 PSI H 3/8 U15070 15 PSI 3/8 BSS 50-50.1 IOPSI IOPSI 3/8KSS 30 IOPSI H 3/8 U15070 IOPSI 3/8 KS S 30 IOPSI 3/8KSS30 I/8KSS 10

IOPSI

" F I R S T SET OF SPRAYS IN THE BONDERITE

Figure 6.

ZONE

Details of


1.

MAURER


AND LACY

15

T h i s means that the cleaners must also c o n d i t i o n or treat the surface of the m e t a l to r e n d e r it r e c e p t i v e to the c o n v e r s i o n c o a t i n g step. W e there fore speak i n i n d u s t r y of c l e a n i n g a n d c o n d i t i o n i n g the surfaces. C l e a n i n g has a l w a y s been i m p o r t a n t i n p r e p a r i n g m e t a l for p a i n t i n g .

With

the

i n t r o d u c t i o n of e l e c t r o d e p o s i t e d paints, h o w e v e r , the necessity of o b t a i n i n g u n i f o r m c o n v e r s i o n coatings has b e c o m e e v e n m o r e i m p o r t a n t .

With


c o n v e n t i o n a l paints, a reasonable degree of u n i f o r m i t y m u s t b e m a i n -

SECTION

A-A

FRESH DEIONIZEO RINSE

Λ

FRESH WATER RINSE

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DEIONIZER UNIT

PUMPS G Ρ Μ / Ε A HEAD PRESS G PM

5.0 3.0 3.5 3.0 3.0 1.0

f

bdOOO

SETTLING TANK

4.3 4.3 4.3 4.3

,

45 45 45 45

FT FT FT FT

35 FT

35FT 35FT 35FT

60 0 60 0 60 0 600 600 375 250 225

SOL. TANK CAP. G A L S , 18 0 0 18 0 18 0 180 180

0 0 0 0

1000 750 750

Η Ρ 10

MOTORS R Ρ M 17 5 0

10 10 10 10

17 5 0 17 5 0 1750 1750

7 1/2

1750 1750 1750

3 3

BTU/HR

NET

2.1 0 0 , 0 0 0

1,800,00 0 1.800 . 0 0 0 1,800,00 0 1 ,800,000

spray phosphatizing unit


16

ELECTRODEPOSITION

t a i n e d t o p r e v e n t d i f f e r e n t i a l gloss of the p a i n t

film.

OF

COATINGS

T h i s p r o b l e m is

a p p a r e n t w h e n a one-coat p a i n t system is a p p l i e d over a m e t a l

part

h a v i n g w i d e l y v a r y i n g c r y s t a l l i n e forms of the c o n v e r s i o n c o a t i n g .

This

p r o b l e m has not, h o w e v e r , b e e n v e r y serious i n the i n d u s t r y , a n d coatings h a v i n g v i s u a l differences i n r e g a r d to streak-like discolorations or pat terns of different c o l o r e d c o n v e r s i o n coatings are satisfactorily p a i n t e d


w i t h o u t a n y sacrifice i n a p p e a r a n c e or q u a l i t y . T h e differences, v i s u a l l y apparent, i n m a n y c o n v e r s i o n c o a t i n g lines w h e n the c l e a n i n g is m a r g i n a l or w e a k reflects different size crystals a n d w i t h e l e c t r o d e p o s i t e d

paints

h a v e b e e n f o u n d to reflect differences i n a p p a r e n t c o n d u c t i v i t y of t h e surface.

Generally speaking, w i t h a zinc phosphate

c o a t i n g , the

finer

the c r y s t a l structure, the less the porosity i n the c o a t i n g . C o n v e r s e l y , the larger the c r y s t a l structure, the h i g h e r the porosity. T h e difference i n the surface c o n d u c t i v i t y c a u s e d b y differences i n p o r o s i t y means that a p a i n t film

is a p p l i e d b y e l e c t r o d e p o s i t i o n at a different rate over one t y p e of

c o a t i n g structure t h a n another; this c a n result i n v i s u a l differences c a n b e u n d e s i r a b l e , p a r t i c u l a r l y w i t h one-coat p a i n t systems.

that

It is thus

i n c r e a s i n g l y i m p o r t a n t that the cleaner f o r m u l a t i o n s be c a r e f u l l y c o n s i d e r e d w i t h respect to the s o i l r e m o v a l a n d their a b i l i t y to c o n d i t i o n the surface to o b t a i n a u n i f o r m c o n v e r s i o n c o a t i n g . W i t h c a l c i u m m o d i f i e d , z i n c p h o s p h a t e processes, a u n i f o r m , h a r d , dense, u s e f u l c o a t i n g c a n b e o b t a i n e d after strong a l k a l i cleaners.

With

m a n y other processes, p a r t i c u l a r l y those u s e f u l for t r e a t i n g m i x e d p r o d u c t i o n of z i n c a n d steel or z i n c , steel, a n d a l u m i n u m , i t is necessary to use a surface a c t i v a t i n g c o m p o u n d b a s e d o n t i t a n i u m p h o s p h a t e

(20).

Since the t i t a n i u m p h o s p h a t e activator is not stable i n h i g h l y a l k a l i n e solutions, it is necessary i n some operations to c l e a n i n t w o stages, sepa r a t e d b y a w a t e r rinse.

I n the first stage s p e c i a l l y f o r m u l a t e d , strong

cleaners are u s e d to r e m o v e the b u l k of the soil f r o m the surface. r e l a t i v e l y c l e a n surface

is treated i n a m i l d e r cleaner

The

containing

the

a c t i v a t i n g agent, w h i c h i n t u r n p r o d u c e s a surface h i g h l y c a p a b l e accepting a u n i f o r m conversion coating.

of

A n o t h e r w a y of h a n d l i n g this

s i t u a t i o n w h e n the e q u i p m e n t is l i m i t e d to a single c l e a n i n g stage is to use a r e l a t i v e l y strong cleaner i n the first stage a n d t h e n inject c o n t i n u o u s l y into the w a t e r rinse, a s l u r r y of the t i t a n i u m p h o s p h a t e a c t i v a t i n g compound. Since i n d u s t r y has b e c o m e c r i t i c a l a b o u t the u n i f o r m i t y of the c o n v e r s i o n c o a t i n g , the synthetic surfactant systems u s e d i n the a l k a l i cleaner h a v e a strong b e a r i n g o n the r e c e p t i v i t y of the surface to the c o n v e r s i o n c o a t i n g s o l u t i o n , a n d a n u m b e r of cleaners h a v e b e e n f o r m u l a t e d that h a v e r e s u l t e d i n i m p r o v e d u n i f o r m i t y i n the c o n v e r s i o n c o a t i n g . I n electrocoating,

there is a strong t e n d e n c y for the i n i t i a l p a i n t

d e p o s i t e d to m i g r a t e to the outermost s k i n of the finished surface.


With

1.


MAURER AND LACY

17

this m i g r a t i o n , inerts, s m u t - l i k e soils, c h a l k m a r k i n g s , m i l l i d e n t i f i c a t i o n inks, etc., t e n d to s h o w u p o n the finished surface.

I n t h e case of p r i m e r s ,

this l i f t i n g effect m a y n o t b e significant. H o w e v e r , w i t h one-coat systems, p a r t i c u l a r l y l i g h t colors, this c a n result i n i r r e g u l a r color patterns or i n readable

markings.

N o r m a l c l e a n i n g techniques

m a y not adequately

r e m o v e a l l these s m u t - l i k e soils, a n d i t is sometimes necessary to resort Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 9, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch001

to m e c h a n i c a l effort ( 21 ) s u c h as h a n d w i p i n g , m e c h a n i c a l b r u s h i n g , o r s p e c i a l solvent t o u c h u p s . F o r h o t r o l l e d steel, z i n c , a l u m i n u m , or steel that has c o r r o d e d , c o n v e n t i o n a l a l k a l i n e cleaners r a r e l y d o a satisfactory job.

I n t h e case of steel, i t m a y b e necessary to r e m o v e the scale a n d

rust b y c o n v e n t i o n a l a c i d p i c k l i n g .

W i t h a l u m i n u m , d e o x i d i z i n g steps

s u c h as n i t r i c a c i d or c h r o m a t e - f l u o r i d e treatment m a y b e necessary to r e m o v e t h e oxides. T h e best s o l u t i o n to corrosion is its p r e v e n t i o n . T h e r e f o r e , i m p r o v e d i n - p l a n t storage is strongly r e c o m m e n d e d .

W i t h conventional paint prac

tices, i t is p o s s i b l e to cover u p a v a r i e t y of defects; this is n o t t h e case with

electrocoating.

A l m o s t a l l of t h e cleaners u s e d p r i o r to c o n v e r s i o n coatings are a l k a l i n e . T h e f o r m u l a t i o n s v a r y w i d e l y , a n d most are p r o p r i e t a r y . T h e f o r m u l a t i o n b e l o w is t y p i c a l of a h e a v y d u t y a l k a l i c l e a n e r : 10 to 2 0 % s o d i u m carbonate 15 to 2 5 % t e t r a s o d i u m p y r o p h o s p h a t e 50 to 7 0 % s o d i u m metasilicate 2 to 8 % n o n - i o n i c , l o w f o a m i n g , surfactant system A cleaner s u c h as this w o u l d b e u s e d at concentrations f r o m 0.5 to 2 ounces p e r g a l l o n at 6 6 ° - 7 7 °

C (150°-170°

F ) f o r 1 to IV2 m i n u t e s

b y spray a p p l i c a t i o n . W h e n p r o c e s s i n g a l u m i n u m , i t is sometimes

desir

able to use cleaners f o r m u l a t e d p r i m a r i l y of s o d i u m h y d r o x i d e to e t c h the surface of the m e t a l .

F o r a l u m i n u m extrusions or stampings

that

h a v e b e e n scratched or a b r a d e d b y the f o r m i n g o p e r a t i o n , the e t c h i n g t y p e a l u m i n u m cleaners w i l l t e n d to s m o o t h o u t a n d g e n e r a l l y i m p r o v e the a p p e a r a n c e of t h e part. W h a t e v e r c h e m i c a l f o r m u l a t i o n is u s e d , u n i f o r m , c o m p l e t e c l e a n i n g is necessary since the nature of the c l e a n i n g has a strong b e a r i n g o n t h e c o n v e r s i o n c o a t i n g step.

Water Rinses G e n e r a l l y , t h e c h e m i c a l s i n t h e cleaner, c o n v e r s i o n c o a t i n g , a n d post treatment

stages of a p h o s p h a t i z i n g u n i t are n o t c o m p a t i b l e .

A water-

rinse stage is therefore p l a c e d b e t w e e n e a c h stage to r e m o v e t h e u n r e a c t e d chemicals f r o m the m e t a l a n d thus m i n i m i z e d r a g - i n of c h e m i c a l s f r o m one stage to another. S i n c e this is p r i m a r i l y a d i l u t i o n process, s u c h concepts as t h e c o u n t e r f l o w of the rinse solutions, b r e a k i n g u p t h e rinse


18

ELECTRODEPOSITION

O F

COATINGS

stage i n t o t w o parts, a n d i n t r o d u c i n g the f r e s h w a t e r to t h e stage t h r o u g h nozzles at the exit of the w a t e r rinse stage are u s e d to i m p r o v e efficiency.

Conversion Coatings T h e pioneers

i n the d e v e l o p m e n t

o f electrodeposited

paints h a d


h o p e d that this r e v o l u t i o n a r y means of a p p l y i n g a p a i n t film w o u l d re q u i r e o n l y s u p e r f i c i a l l y degreased surfaces a n d w o u l d n o t r e q u i r e conver sion coatings f o r h i g h q u a l i t y m e t a l

finishing

A s more knowledge

(22).

a n d experience w e r e o b t a i n e d , h o w e v e r , it b e c a m e clear that p r o p e r m e t a l preparation was not only important, b u t i n many ways even more critical t h a n w i t h c o n v e n t i o n a l m e t h o d s of a p p l y i n g p a i n t . d o o r exposure resistance,

detergent

resistance,

N o t o n l y w e r e out

filiform

c o r r o s i o n resist

ance, salt f o g resistance, h u m i d i t y resistance, a n d p h y s i c a l tests i n f l u e n c e d b y the m e t a l p r e p a r a t i o n b u t also the a p p e a r a n c e b o t h u n i f o r m i t y a n d i n color.

of the p a i n t film i n

S t i l l another factor that is i n f l u e n c e d b y

the m e t a l treatment is w e t film a d h e s i o n , w h i c h is the a b i l i t y of the electt r o d e p o s i t e d film to resist the w a t e r r i n s i n g that i t is u s u a l l y g i v e n after the d e p o s i t i o n of t h e p a i n t film, p r i o r to c u r i n g . A l l of the c o m m e r c i a l l y u s e d electrocoating p a i n t systems a p p l y the p a i n t at the anode.

W h e n steel is p a i n t e d b y the a n o d i c

process, H , 0 , a n d F e +

process.

2

2 +

electrocoat

are f o r m e d at t h e a n o d e b y the e l e c t r o l y t i c

T h e conventional

conversion

coatings

are essentially n o n -

c o n d u c t i v e , a n d this means that the i n i t i a l d e p o s i t i o n of the p a i n t occurs i n the areas b e t w e e n the c r y s t a l structure

(14).

T h i s i m p l i e s a n inter

r e l a t i o n s h i p b e t w e e n the structure of t h e c o n v e r s i o n c o a t i n g , the p a i n t , a n d the a p p l i c a t i o n parameters of a g i v e n f o r m u l a t i o n . T h a t is, d e p e n d i n g u p o n the c o n d u c t i v i t y characteristics of a g i v e n p a i n t system, w e w i l l h a v e either h i g h e r or l o w e r c u r r e n t densities, w h i c h w i l l i n t u r n m a k e f o r greater or lesser quantities of h y d r o g e n , o x y g e n , a n d ferrous i r o n . a d d i t i o n to t h e o v e r a l l quantities of these materials

In

p r o d u c e d i n the

d e p o s i t i o n of a film, the rate at w h i c h they are p r o d u c e d c a n also h a v e a b e a r i n g o n the o v e r a l l results.

I n t u r n , the nature of the c o n v e r s i o n

c o a t i n g influences b o t h the rate a n d w a y the deposit occurs.

The migra

t i o n o f the m e t a l d i s s o l v e d at the anode also is i n f l u e n c e d b y t h e n a t u r e of the c o n v e r s i o n c o a t i n g . The

interplay between

t h e a n o d i c d i s s o l u t i o n w i t h the c o n c o m -

m i t a n t f o r m a t i o n of gases a n d m e t a l ions a n d the characteristics of the d e p o s i t e d p a i n t film also result i n a p o r t i o n of the c o n v e r s i o n c o a t i n g being eroded away a n d distributed i n the resulting

film.

This

means

that the p a i n t film is d e r i v e d n o t o n l y f r o m the r e s i n a n d p i g m e n t of the p a i n t f o r m u l a t i o n b u t also f r o m the d i s s o l v e d substrate a n d the conver sion c o a t i n g c o m p o n e n t s .

T h e extent o f the c o n v e r s i o n c o a t i n g


loss

1.

M A U R E R

19


A N D LACY

d e p e n d s o n the nature of t h e c o a t i n g itself a n d o n a p a r t i c u l a r p a i n t formulation.

T h e u p p e r p a r t of T a b l e I illustrates t h e m a g n i t u d e

of

this loss. T h e l o w e r p a r t of T a b l e I shows t h e salt f o g c o r r o s i o n resistance of t h e finishing systems (see also F i g u r e 7 ) . O b v i o u s l y there are great differences

i n t h e a m o u n t of c o n v e r s i o n c o a t i n g loss b e t w e e n

various

p a i n t systems, a n d this loss is a f u n c t i o n o f t h e p a r t i c u l a r c o n v e r s i o n Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 9, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch001

c o a t i n g . A l s o e v i d e n t is t h e l a c k of c o r r e l a t i o n b e t w e e n c o a t i n g loss a n d c o r r o s i o n resistance.

T h i s specific i n t e r d e p e n d e n c e

between

the con

v e r s i o n c o a t i n g a n d a specific electropaint f o r m u l a t i o n is o n e of t h e inter esting characteristics of t h e e l e c t r o d e p o s i t e d p a i n t i n g process. A c c o r d i n g to M a y a n d S m i t h (10), Table I.

t h e c o n v e r s i o n c o a t i n g r e m o v e d d u r i n g electro-

Coating Loss and Corrosion Resistance

Conversion Coating

Initial Average Coaling Weight Prior to Painting mg/sq ft

A" Β C D Ε F

308 198 256 65 155· 129

Percent Conversion Coating Loss Electropaint Numbers

10 43 44 14 75 79

δ% Salt Fog Corrosion Resistance *, « AA p a i n t „creepage 0A face r u s t i n g 2 Β 0 p a i n t creepage face r u s t i n g 1 C 0 p a i n t creepage 2 face r u s t i n g D 0-1 p a i n t creepage face r u s t i n g 1 Ε p a i n t creepage 0.5-1 1 face r u s t i n g F p a i n t creepage 0-1.5 2 face r u s t i n g

5 0.6 4 0 10 1

5 5 7 0 11 12

6 30 11 3 21 12

11 16 6 1.5 18 18

1.5-2.5 2 1-1 2 1-1 1 78% Ρ 7 1-2 7s 3 1.5-2.5 6

2-3.5 2 1.5-2 1 0-1 1 87% Ρ 3 1.5-2 3 1.52.5 2

1-1 3 1-1 3 1-1 3 1.5-3.5 2 1-1 3 1-1 3

0-1.5 7 1-1 6 1-1 4 95% Ρ 7 1-1 7 1-1 7

b

α

Legend: A—zinc phosphate process, nitrite accelerated. Β—zinc phosphate process, calcium modified. C—zinc phosphate process, nickel and fluoride modified, nitrite ac celerated. D—iron phosphate process, chlorate accelerated. Ε—zinc phosphate process, chlorate accelerated. F—zinc phosphate process, coating weights controlled by using a di basic, dihydroxy acid, nickel and fluoride modified, nitrite accelerated.

336 hours exposure to 5% salt spray (ASTM B117-64), average two panels, paint creepage is the loss of paint from a scribe in 1/16 inch increments or percent. Face rusting is a spot type corrosion of the painted surface; a rating of 1 is perfect; 8 is 100% rusing. See Figure 7 for examples of salt spray failure. 6


20

ELECTRODEPOSITION

OF

COATINGS

d e p o s i t i o n is u n i f o r m l y d i s t r i b u t e d t h r o u g h o u t the cross-section p a i n t film. C h e e v e r a n d W o j t k o o w i a k ( 1 5 )

of

the

also f o u n d that the z i n c f r o m

the z i n c p h o s p h a t e c o a t i n g w a s d i s t r i b u t e d u n i f o r m l y o n a m a c r o scale t h r o u g h o u t the p r i m e r that t h e y s t u d i e d . I n contrast to the above, w o r k b y S i m p s o n ( 3 ) , u s i n g a n electron p r o b e m i c r o a n a l y s i s t e c h n i q u e , c l u d e d that there was no major d i f f u s i o n or m i x i n g of the

con

conversion


c o a t i n g i n the p a r t i c u l a r p r i m e r film he s t u d i e d b u t that the r e m o v e d z i n c phosphate

c o a t i n g r e m a i n e d near the p r i m e r - m e t a l interface.

Simpson

p r o d u c e d a z i n c p h o s p h a t e c o a t i n g o n S A E 1010 c o l d r o l l e d steel, u s i n g the c o n v e r s i o n c o a t i n g process C ( T a b l e I ), a p p l y i n g a p r i m e r b y electro d e p o s i t i o n a n d c u r i n g at 1 9 7 ° C ( 3 9 5 ° F ) for 25 m i n u t e s . S m a l l strips of the s a m p l e w e r e then m o u n t e d i n a s o l u t i o n of p o l y methacrylate,

w h i c h , after h a r d e n i n g , was p o l i s h e d to expose a cross-

section of the p a i n t a l u m i n a i n water.

film/steel

substrate. T h e final p o l i s h was a 1 - m i c r o n

F o l l o w i n g the p o l i s h i n g , the specimens w e r e c a r e f u l l y

w a s h e d , p r i o r to v a c u u m d e p o s i t i o n of c a r b o n , w h i c h was u s e d to d i s s i -

Figure 7.

Example of salt spray test results (see Table I)

Left: salt fog creepage for 4-inch wide test panel; creepage 2-4. Right: "face rusting" with "5" rating.



1.


Figure 8.


21

Optical photomicrograph of section of painted steel

p a t e electric charges d u r i n g the p r o b e o p e r a t i o n .

F i g u r e 8 shows a n

o p t i c a l p h o t o m i c r o g r a p h of a t y p i c a l cross section at 800 X m a g n i f i c a t i o n . I n t h e e l e c t r o n p r o b e m i c r o a n a l y s i s , a 1 - m i c r o n d i a m e t e r b e a m of electrons w a s s c a n n e d across t h e surface; the resultant a n d characteristic x-rays w e r e m o n i t o r e d b y s t a n d a r d x-ray

fluorescence

techniques.

The

specimens w e r e thus s c a n n e d across the interface f r o m steel to the m o u n t i n g m e d i a , w i t h measurements iron, phosphorus, a n d zinc.

o f the characteristic x-ray intensities f o r

T w o typical intensities-distance

plots are

s h o w n i n F i g u r e s 9 a n d 10. F r o m the d a t a i n these figures, f o r c o n v e r s i o n c o a t i n g C , w i t h this p a r t i c u l a r p r i m e r , S i m p s o n c o n c l u d e d that there w a s n o major d i f f u s i o n or m i x 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 c a u s e d b y either t h e a n o d i c d i s s o l u t i o n o r t h e t u r b u l e n t m i x i n g that m i g h t h a v e o c c u r r e d d u r i n g t h e heat c u r i n g of the p a i n t . O u r w o r k i n this area, a l t h o u g h l i m i t e d , thus disagrees w i t h t h e conclusions of M a y / S m i t h a n d C h e e v e r / W o j t k o w i a k . P o s s i b l y some e l e c t r o d e p o s i t e d f o r m u l a t i o n s result i n m o r e u n i f o r m d i f f u s i o n o f the r e m o v e d c o n v e r s i o n coatings t h a n others.

Further work

needs to b e done. T h e q u a l i t y of i n d u s t r i a l l y p a i n t e d m e t a l is e v a l u a t e d i n different ways. place

O f course, t h e i d e a l w a y to evaluate the

finished

a

finishing

p r o d u c t i n its n o r m a l exposure

e x a m i n e i t over a lapse o f t i m e .

system is to

environment a n d

U n f o r t u n a t e l y , w h i l e this

approach

p r o v i d e s a l o n g range e v a l u a t i o n , i t is n o t a p r a c t i c a l means of q u a l i t y c o n t r o l o r a means of d e v e l o p i n g n e w p r o d u c t s f o r i n d u s t r i a l use. H e n c e ,


22

ELECTRODEPOSITION


paint film

I

O F COATINGS

steel

Figure 9. Electron probe scans across painted section. For Fe, scale X 1000 cpm; Zn and Ρ scale X 100 cpm. l a b o r a t o r y tests h a v e b e e n d e v i s e d t o a i d i n m o r e r a p i d e v a l u a t i o n f o r research a n d d e v e l o p m e n t a n d q u a l i t y c o n t r o l . A m o n g t h e tests w h i c h are i n f l u e n c e d b y c o n v e r s i o n coatings a r e t h e 5 % salt f o g test B-117),

a l k a l i n e detergent

reistance

test

(ASTM

(ASTM

D-2248), humidity

resistance test ( A S T M D - 2 2 4 7 , 1 0 0 % r e l a t i v e h u m i d i t y ) , a n d t h e fili f o r m c o r r o s i o n (24) resistance test.

I n a d d i t i o n , v a r i o u s p h y s i c a l tests

are u s e d to d e t e r m i n e the a d h e s i o n o f t h e o r g a n i c c o a t i n g system to t h e m e t a l , s u c h as k n i f e a d h e s i o n , d e f o r m a t i o n tests, t h e c o n i c a l m a n d r e l , a n d i m p a c t tests. W e must also consider b y v i s u a l i n s p e c t i o n the u n i f o r m i t y a n d a n y c o l o r changes that m i g h t h a v e resulted f r o m t h e c o n v e r s i o n coating o n the electrodeposited

film.

T o illustrate h o w c o n v e r s i o n coatings c a n a n d d o influence a n u m b e r o f these q u a l i t y aspects o f a finished part, w e present selected

data

c o v e r i n g e l e c t r o d e p o s i t e d films o n c o l d r o l l e d steel, g a l v a n i z e d steel, a n d a l u m i n u m . F i g u r e 11 ( t o p ) illustrates t h e degree o f difference that c a n b e o b t a i n e d i n salt f o g exposure b e t w e e n a c l e a n e d b u t u n c o a t e d c o l d r o l l e d steel surface vs. a steel surface treated to f o r m a z i n c p h o s p h a t e c o a t i n g . T h e u n t r e a t e d steel surface has almost c o m p l e t e l y lost t h e elec t r o d e p o s i t e d p r i m e r whereas t h e z i n c p h o s p h a t e treated c o l d r o l l e d steel surface has r e t a i n e d almost a l l o f the p r i m e r . T h e l o w e r r o w o f panels ( F i g u r e 11) shows salt s p r a y c o r r o s i o n resistance o f c o l d r o l l e d steel


1.

M A U R E R A N D L A C Y

23


surfaces t r e a t e d w i t h either a n i r o n p h o s p h a t e ( r i g h t p a n e l ) o r a z i n c p h o s p h a t e ( l e f t p a n e l ) process.

M a n y p e o p l e h a v e t a u g h t that a n i r o n

p h o s p h a t e c o a t i n g is i n f e r i o r to a z i n c p h o s p h a t e c o a t i n g , b u t w i t h specific electropaints a n d t h e p r o p e r i r o n p h o s p h a t e process results c a n b e o b t a i n e d that are e q u i v a l e n t to t h e z i n c p h o s p h a t e process. T h e r e are m a n y p r o d u c t i o n lines i n o p e r a t i o n t o d a y u s i n g e l e c t r o d e p o s i t e d p a i n t s

that


p r e p a r e c o l d r o l l e d steel surfaces b y u s i n g a n i r o n p h o s p h a t e process. F i g u r e 12 illustrates t h e c o r r o s i o n resistance of a w h i t e

electro

d e p o s i t e d p a i n t o n a l u m i n u m . T h e test s p e c i m e n o n the left w a s c l e a n e d a n d e t c h e d whereas that o n t h e r i g h t w a s c l e a n e d , e t c h e d , a n d g i v e n a chromic oxide conversion coating.

A f t e r 7,000 hours i n 5 % salt s p r a y

there is essentially n o difference i n test results. T h e r e are a f e w p i n - p o i n t l i k e blisters o n t h e u n t r e a t e d a l u m i n u m , b u t f o r a l l p r a c t i c a l purposes t h e c l e a n e d o n l y a l u m i n u m surface b e h a v e d as w e l l as d i d t h e o n e w i t h t h e c o n v e r s i o n c o a t i n g . T h i s illustrates that w i t h m a n y alloys of a l u m i n u m there is n o n e e d f o r a c o n v e r s i o n c o a t i n g w h e n u s i n g e l e c t r o d e p o s i t e d paints. O n e e x p l a n a t i o n is that there is a n in situ a n o d i z i n g o f t h e a l u m i -

Figure 10. Electron probe scans across painted section. For Fe, scale X 1000 cpm; Zn and Ρ scale X 100 cpm.


24

ELECTRODEPOSITION

O F

COATINGS

n u m d u r i n g t h e i n i t i a l phases o f e l e c t r o d e p o s i t i o n . W h a t e v e r t h e m e c h a n i s m , h o w e v e r , i n m o s t cases a c o n v e r s i o n c o a t i n g is n o t n e e d e d o n a l u m i n u m w h e n using electrodeposited paint

finishes.

A possible excep

t i o n w o u l d b e i n lines t r e a t i n g a m i x t u r e o f a l u m i n u m w i t h other m e t a l surfaces.

I f t h e other m e t a l surfaces, s u c h as c o l d r o l l e d steel o r h o t

d i p p e d g a l v a n i z e d steel, a r e t r e a t e d to p r o d u c e a c o n v e r s i o n c o a t i n g o n


t h e i r surfaces, t h e c o n d u c t i v i t y w i l l b e s i g n i f i c a n t l y different f r o m that of c l e a n e d a n d e t c h e d a l u m i n u m . I t m a y b e necessary, u n d e r these c i r c u m stances to u s e a c o n v e r s i o n c o a t i n g process that w i l l coat a l l three metals so that t h e a m o u n t o f e l e c t r o d e p o s i t e d film w i l l b e t h e same o n a l l three metals.

I f t h e c o n d u c t i v i t y is n o t t h e same, t h e p a i n t film o r its gloss

w i l l b e different. O n e i n s i d i o u s t y p e o f c o r r o s i o n that c a n o c c u r o n t h e surface o f p a i n t e d c o l d r o l l e d steel, i n relative h u m i d i t i e s f r o m 5 0 to 9 5 % , is a t h r e a d - l i k e c o r r o s i o n w h i c h has b e e n n a m e d

filiform

cororsion.

v e r s i o n coatings o n steel surfaces w i l l n o t stop

filiform

c o r r o s i o n , b u t as

Con

i l l u s t r a t e d i n F i g u r e 13 w i l l s u b s t a n t i a l l y decrease i t . T h e p a n e l o n t h e left w a s c l e a n e d a n d p a i n t e d ; t h e p a n e l o n t h e r i g h t w a s c l e a n e d , z i n c

Figure 11. Salt spray corrosion test. Top row: zinc phosphate coating vs. untreated steel; bottom row: zinc phosphate coating vs. iron phosphate coating on steel.



1.

MAURER

AND LACY


Figure 12. Untreated vs. treated aluminum after salt spray exposure

Figure 13. Filiform corrosion. Untreated vs. zinc phosphate coating on steel.


25

26

ELECTRODEPOSITION

O F

COATINGS

p h o s p h a t e treated, a n d p a i n t e d . T h e test specimens s h o w n i n F i g u r e 14 w e r e exposed f o r 30 days i n a r e l a t i v e h u m i d i t y of 8 7 % after first activat i n g t h e test area b y e x p o s i n g t h e panels f o r 4 h o u r s i n a 5 % salt f o g chamber. O n e of t h e best c o n v e r s i o n c o a t i n g f o r steel, w h e r e detergent

resist

a n c e is of p r i m e c o n c e r n , is t h e c a l c i u m m o d i f i e d , z i n c p h o s p h a t e process. Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 9, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch001

N o r m a l l y a n i c k e l - f l u o r i d e m o d i f i e d , n i t r i t e accelerated

zinc phosphate

process is n o t as g o o d as a c a l c i u m m o d i f i e d , z i n c p h o s p h a t e w h e n c o m p a r e d i n the detergent

resistance test.

p o s i t e d p a i n t f o r m u l a t i o n s d e s i g n e d f o r detergent

process

W i t h the electrode resistance,

w e have

f o u n d t h e reverse to b e t r u e — t h e n i c k e l - f l u o r i d e , n i t r i t e accelerated z i n c p h o s p h a t e processes g i v e better detergent resistance t h a n d o t h e c a l c i u m m o d i f i e d , z i n c p h o s p h a t e coatings.

T h i s is i l l u s t r a t e d i n F i g u r e 14.

F i g u r e s 15 a n d 16 s h o w that i t is necessary to select a p a r t i c u l a r process w i t h i n a g i v e n class.

F i g u r e 15 shows t w o different z i n c phos

p h a t e processes f o r h o t d i p p e d g a l v a n i z e d steel surfaces; b o t h g i v e excel lent test results w i t h c o n v e n t i o n a l paints b u t s h o w a s u b s t a n t i a l difference i n salt s p r a y corrosion resistance w i t h t h e p a r t i c u l a r

electrodeposited

Figure 14. Detergent test results. Calcium modified zinc phosphate (shown on left) vs. nickel-fluoride modified zinc phosphate coat ings (shown on right).



1.



27

Figure 15. Salt spray resistance. Two types of zinc phosphate coatings on hot dip galvan ized steel.

paint formulation used.

T h e same t y p e of difference is i l l u s t r a t e d i n

F i g u r e 16 w h i c h compares t w o different z i n c p h o s p h a t e treatments o n c o l d r o l l e d steel.

A g a i n w e see a difference w i t h the e l e c t r o d e p o s i t e d

p a i n t f o r m u l a t i o n , whereas w i t h a c o n v e n t i o n a l p a i n t w e w o u l d e x p e c t to see little i f a n y difference b e t w e e n these t w o processes. T h e s e illustrations s h o w that i t is i m p o s s i b l e to m a k e g e n e r a l r e c o m m e n d a t i o n s as to the p r o p e r c o n v e r s i o n c o a t i n g process

to use f o r a

m e t a l surface w i t h o u t a s t u d y of the p a r t i c u l a r e l e c t r o d e p o s i t e d

paint

f o r m u l a t i o n that w i l l b e used.

Post Treatment, Deionized Water Rinse, and Dryoff s Conventional Paints. N o r m a l l y , c o n v e r s i o n coatings f o r c o n v e n t i o n a l paints are post treated w i t h a d i l u t e c h r o m a t e rinse a n d t h e n d r i e d . T h e c h r o m a t e post treatment c a n b e as s i m p l e as a 0 . 1 % c h r o m i c a c i d s o l u t i o n or as c o m p l e x as c o m b i n a t i o n s of c a l c i u m chromate, c a l c i u m phos phate, or c h r o m i c c h r o m a t e rinses w i t h c o n t r o l l e d p H ' s . T h e c h r o m a t e


28

ELECTRODEPOSITION

OF

COATINGS

rinses r e d u c e the t e n d e n c y of paints b l i s t e r u n d e r exposure to

high

h u m i d i t y c o n d i t i o n s a n d h e l p to i m p r o v e the c o r r o s i o n resistance resistance as m e a s u r e d b y the salt f o g test a n d o u t d o o r exposure. tunately, h o w e v e r , i n areas w h e r e the c h r o m a t e rinses a c c u m u l a t e as the b o t t o m edges of parts, crevices, a n d a r o u n d holes)

as

Unfor (such

a p o i n t is

r e a c h e d w h e r e the b l i s t e r i n g occurs, either i n spite of or because of too


h i g h a c o n c e n t r a t i o n of c h r o m a t e salts. T o e l i m i n a t e the p r o b l e m of salt a c c u m u l a t i o n , the p r o d u c t i o n items are r i n s e d w i t h a n essentially ion-free w a t e r

(deionized) water.

The

d e i o n i z e d w a t e r rinse washes off the c h r o m a t e a n d w a t e r salts, thus i n c r e a s i n g the h u m i d i t y resistance of the

finish.

H o w e v e r , until recently,

the c o r r o s i o n resistance w a s decreased b y this rinse.

T h e p r o b l e m of

decreased c o r r o s i o n resistance r e s u l t i n g f r o m the use of a final d e i o n i z e d w a t e r rinse w a s r e c e n t l y s o l v e d b y the i n t r o d u c t i o n of the c h r o m i c c h r o m a t e post treatment

( U . S . patents 3,222,226 a n d 3,279,958).

c h r o m i c - c h r o m a t e post treatment, o p e r a t e d as t a u g h t i n the

The

reference

patents, a l l o w s the r e m o v a l of the s o l u b l e salts f r o m the surface b y the d e i o n i z e d w a t e r rinse w i t h o u t d i m i n u t i o n of the c o r r o s i o n

resistance.

W i t h c o n v e n t i o n a l paints, l i t t l e i f a n y difference i n q u a l i t y is o b t a i n e d as a result of the t y p e of d r y i n g i n the range of r o o m t e m p e r a t u r e u p to a n o v e n t e m p e r a t u r e of 2 6 0 ° C , a n d f r o m 5 to 10 m i n u t e s . T h e essential r e q u i r e m e n t w i t h c o n v e n t i o n a l solvent-based systems is that the p a r t b e free of surface w a t e r before the finish is a p p l i e d . Electrodeposited Coatings.

Electrodeposited

finishes

b a s e d system that is sensitive to electrolyte content.

use a w a t e r -

S i n c e the electro-

paints are w a t e r based, there s h o u l d b e n o reason w h y the w a r e has to be d r y p r i o r to e n t r y i n t o the p a i n t . T h e o b v i o u s advantages of g o i n g i n w e t , w i t h the e l i m i n a t i o n of costly d r y o f f ovens, the cost of f u e l to operate the d r y o f f ovens, a n d the e l i m i n a t i o n of the floor space f o r b o t h the o v e n itself a n d the c o o l i n g area necessary to r e d u c e the t e m p e r a t u r e of the p a r t p r i o r to entry i n t o the p a i n t tank h a v e p r o m p t e d extensive s t u d y into the i n t e r r e l a t i o n s h i p a m o n g post treatment, dryoff, a n d the paint formulations. Since

electrodeposited

p a i n t systems

are

sensitive

to

electrolyte

content, it is almost a l w a y s necessary to rinse the c o n v e r s i o n coatings w i t h d e i o n i z e d w a t e r to decrease the electrolyte d r a g i n to the p a i n t tank to a p o i n t w h e r e it does n o t cause a n y l o n g t e r m p r o b l e m s . A t first, it was suggested that the c h r o m a t e salts f r o m the post treatments w e r e the biggest p r o b l e m to the p a i n t .

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

chromâtes, h o w e v e r , it is a p p a r e n t that chromâtes per se are not the p r i n c i p a l cause of p a i n t p r o b l e m s , except f o r t h e i r c o n t r i b u t i o n to the t o t a l electrolyte content of the p a i n t .


1.

MAURER A N D L ACY


29

T h e exact a m o u n t of electrolyte that c a n b e safely c a r r i e d i n t o t h e e l e c t r o d e p o s i t i o n p a i n t tank w i l l d e p e n d u p o n t h e rate of t u r n o v e r of t h e p a i n t , t h e r e l a t i v e d r a g - o u t of the p a i n t , w h i c h is a f u n c t i o n of t h e p a r t shape a n d d r a i n t i m e , a n d the nature of t h e p a i n t f o r m u l a t i o n .

Close

c o o p e r a t i o n is necessary b e t w e e n t h e p a i n t s u p p l i e r a n d the operator of the p l a n t to ensure

adequate

deionized water rinsing i n commercial


practice. W e b e l i e v e t h e most efficient a p p r o a c h is to p r o v i d e a t h o r o u g h rinse b y a r e c i r c u l a t i n g d e i o n i z e d w a t e r zone, f o l l o w e d b y a fresh d e i o n i z e d w a t e r spray.

F i g u r e 6 shows a final d e i o n i z e d w a t e r stage as r e c o m -

Figure 16. Salt spray resistance. Two types of zinc phosphate coating on steel.

mended b y T h e Parker C o . , O x y M e t a l Finishing Corp.

Since t h e

effectiveness of the r i n s i n g step depends u p o n the q u a n t i t y a n d q u a l i t y of t h e w a t e r as w e l l as the effectiveness of s p r a y i n g i t to r e a c h a l l parts of t h e w o r k , i t is a d v i s a b l e to d e t e r m i n e the electrolyte content o f t h e w a t e r a c t u a l l y c a r r i e d into the p a i n t . T h e c o l l e c t i o n of the rinse d r i p p i n g s f r o m the w o r k a n d t h e continuous m e a s u r e m e n t of t h e i r c o n d u c t i v i t y b y a c a r r y o u t m o n i t o r (see F i g u r e 6 ) p r o v i d e a s i m p l e , effective means o f e n s u r i n g that a p r e d e t e r m i n e d m a x i m u m l e v e l of electrolyte i n p u t w i l l n o t b e exceeded.


30

ELECTRODEPOSITION

OF

COATINGS

W h i l e the c o n d u c t i v i t y range of rinse d r i p p i n g s w i l l v a r y w i t h the p a i n t t u r n o v e r a n d t y p e of p a i n t , a u s e f u l p o i n t of d e p a r t u r e has b e e n to adjust the d e i o n i z e d w a t e r r i n s i n g s u c h that the d r i p p i n g s h a v e a m a x i mum and

c o n d u c t i v i t y (13)

of 60 m i c r o m h o s c m " . If w e a c c e p t this v a l u e 1

the n e e d for the d e i o n i z e d w a t e r rinse to ensure c o n t i n u e d h i g h

p e r f o r m a n c e f r o m the p a i n t , w e m i g h t ask i f a n y v a l u e is d e r i v e d f r o m a


post treatment w i t h a c h r o m a t e - c o n t a i n i n g s o l u t i o n w h e n a p p l y i n g elec trodeposited U.S.

finishes.

W i t h post treatments, other t h a n those c o v e r e d b y

patents 3,222,226 a n d 3,279,958 ( t h a t is, rinses not o p e r a t e d

t r i v a l e n t c h r o m i u m a n d w i t h p H ' s outside the range of 3.8 to 6 ), w e

with find

l i t t l e i f any q u a l i t y i m p r o v e m e n t results f r o m their use. T h e n e e d of or benefit d e r i v e d f r o m the post treatments c o v e r e d b y the subject patents are a f u n c t i o n of the p a i n t systems, the presence of or the l a c k of a dryoff step, a n d the t y p e of c o n v e r s i o n c o a t i n g over w h i c h they are u s e d . T h e c h r o m i c - c h r o m a t e rinses u s e d as taught u n d e r the subject patents w i l l hereafter b e t e r m e d reactive

post-treatments.

T h e degree of d r y i n g has l i t t l e , i f any, effect o n c o n v e n t i o n a l solventb a s e d p a i n t systems.

Figure 17.

W i t h e l e c t r o d e p o s i t e d paints the n a t u r e of the d r y -

Effect of chromate post treatment on salt spray results when painted wet

Test conditions reading from left to right: (1) steel, zinc phosphate, deionized water, painted wet; (2) steel, zinc phosphate, reactive chromate, deionized water, painted wet; (3) hot dipped galvanized steel, zinc phosphate, deionized water, painted wet; (4) hot dipped galvanized steel, zinc phosphate, reactive chromate, deionized water, painted wet



1.

M A U R E R A N D

Figure 18.

LACY


31

Effect of chromâte post treatment on salt spray results when painted after oven drying

Test conditions reading from left to right: (1) steel, zinc phosphate, deionized water, oven dried; (2) steel, zinc phosphate, reactive chrornate, deionized water, oven dried; (3) hot dipped galvanized steel, zinc phosphate, deionized water, oven dried; (4) hot dipped galvanized steel, zinc phosphate, reactive chr ornate, deionized water, oven dried off c a n h a v e a significant effect o n the e n d q u a l i t y . T h e i n t e r p l a y of post treatment, dryoff o v e n , a n d p a i n t is most a p p a r e n t w i t h salt s p r a y corro sion resistance, w e t film a d h e s i o n , a n d whiteness of w h i t e , one-coat p a i n t s . I n general, a l l e l e c t r o d e p o s i t e d paints are i m p r o v e d b y r e a c t i v e post treatment a n d a d r y o f f o v e n .

T h e degree of i m p r o v e m e n t varies tre

m e n d o u s l y , h o w e v e r , w i t h the p a i n t f o r m u l a t i o n a n d the t y p e of c o n version coating. F i g u r e s 17, 18, 19, a n d 20 s h o w the salt s p r a y c o r r o s i o n resistance of a n u m b e r of electropaints as a f u n c t i o n of the post treatment a n d dryoff. T o i l l u s t r a t e the differences that c a n be o b t a i n e d , d a t a h a v e b e e n selected u s i n g z i n c p h o s p h a t e o n c o l d r o l l e d a n d h o t d i p p e d g a l v a n i z e d steels. F i g u r e 17 is a n e x a m p l e of the effect o n salt f o g c o r r o s i o n resistance u s i n g a z i n c p h o s p h a t e c o a t i n g w i t h a n d w i t h o u t a r e a c t i v e post treat m e n t w i t h a specific e l e c t r o p a i n t f o r m u a l t i o n . T h e t w o panels o n the left are c o l d r o l l e d steel; the t w o panels o n the r i g h t are h o t d i p p e d g a l v a n i z e d steel. W i t h the c o l d r o l l e d steel, there is essentially n o dif ference i n c o r r o s i o n resistance as a f u n c t i o n of post treatment, w h e r e a s w i t h the g a l v a n i z e d steel, there is a significant difference i n c o r r o s i o n


32

ELECTRODEPOSITION

O F COATINGS

resistance; t h e test s p e c i m e n w i t h o u t t h e r e a c t i v e post treatment is m u c h w e a k e r t h a n t h e test s p e c i m e n treated w i t h t h e r e a c t i v e

post-treatment.

I n contrast to t h e results i n F i g u r e 17, F i g u r e 18 shows a s i m i l a r test w i t h a different electropaint.

I n this case w e see n o difference i n t h e

c o r r o s i o n resistance o f t h e h o t d i p p e d g a l v a n i z e d steel w i t h o r w i t h o u t


a reactive post treatment w h i l e w e d o see a difference i n t h e c o r r o s i o n

Figure 19. Salt spray test results with given electropaint as function of heating of con version coating Left: steel, zinc phosphate, dried at 149°C (300°F) Right: steel, zinc phosphate, dried at 260°C (500°F) resistance o f t h e c o l d r o l l e d steel surfaces. that c a n o c c u r i n c o r r o s i o n resistance

F i g u r e 19 shows t h e changes

w h e n t h e c o n v e r s i o n c o a t i n g is

h e a t e d to d r i v e off three of t h e f o u r waters o f h y d r a t i o n . W i t h this p a r t i c u l a r electropaint, w e o b t a i n e d a n i m p r o v e m e n t e v e n at this r e l a t i v e l y h i g h d r y o f f c o n d i t i o n . F i g u r e 20 illustrates that the effect o f dryoff varies w i t h t h e c o n v e r s i o n c o a t i n g . H e r e , a l l f o u r panels h a v e b e e n c o a t e d w i t h the same e l e c t r o p a i n t e d finish; h o w e v e r , the u p p e r r o w o f panels has o n e t y p e o f z i n c p h o s p h a t e c o a t i n g o n i t , t h e l o w e r r o w a different z i n c phos-


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33

p h a t e c o a t i n g . W e see l i t t l e d i f f e r e n c e b e t w e e n t h e c o r r o s i o n resistance w h e n t h e coatings a r e o v e n d r i e d p r i o r to t h e a p p l i c a t i o n o f t h e p a i n t film film

b u t a c o n s i d e r a b l e d i f f e r e n c e i n c o r r o s i o n resistance w h e n t h e p a i n t is a p p l i e d o n these w e t c o n v e r s i o n coatings. T h e reason f o r a c h a n g e i n t h e characteristics o f a n e l e c t r o d e p o s i t e d

p a i n t film b e c a u s e of h e a t i n g o f t h e c o n v e r s i o n c o a t i n g is n o t f u l l y u n d e r


stood.

O n e p o s s i b l e e x p l a n a t i o n appears to b e f o u n d i n a s t u d y o f t h e

d e h y d r a t i o n o f t h e z i n c p h o s p h a t e c o a t i n g as a f u n c t i o n o f t e m p e r a t u r e a n d t i m e . Z i n c p h o s p h a t e coatings h a v e as t h e i r p r i n c i p a l constituents two

minerals,

phosphophyllite ( Z n F e ( P 0 ) 2 2

( Z n ( P 0 ) 2 ' 4 H 0 ) (25). 3

4

2

4

' 4H 0) 2

a n d hopeite

A n extensive s t u d y o f t h e d e h y d r a t i o n of z i n c

Figure 20. Effect of dryoff. Top: one type of zinc phosphate coating; bottom: another type of zinc phosphate coating. Upper left: steel, zinc phosphate, reactive chro mate, deionized water rinse, painted wet Upper right: steel, zinc phosphate, reactive chro mate, deionized water rinse, oven dried at 149°C (300° F) prior to painting Lower left: steel, zinc phosphate, reactive chro mate, deionized water rinse, painted wet Lower right: steel, zinc phosphate, reactive chro mate, deionized water rinse, oven dried at 149°C (300° F) prior to painting


34

ELECTRODEPOSITION

O F

COATINGS

p h o s p h a t e c o n v e r s i o n coatings w a s c a r r i e d out i n o u r laboratories i n 1961. F i g u r e 21 shows t y p i c a l d a t a o b t a i n e d f r o m this s t u d y (26).

T h e process

of d e h y d r a t i o n is at least p a r t i a l l y reversible, a n d t h e u n p a i n t e d 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 i l l r e g a i n lost w a t e r f r o m the atmosphere. W h e t h e r zero, one, t w o , o r three 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 n e e d b e r e m o v e d f o r m a x i m u m c o r r o s i o n resistance w i l l d e p e n d u p o n a Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 9, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch001

g i v e n p a i n t a n d t h e p a r t i c u l a r t y p e of z i n c p h o s p h a t e c o n v e r s i o n c o a t i n g . I n general, r e m o v a l of t w o 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 , w h i c h w o u l d o c c u r i n m a n y s t a n d a r d d r y o f f ovens, is a d e q u a t e .

H o w e v e r , this

d e p e n d s o n t h e p a i n t a n d the c o n v e r s i o n c o a t i n g ; some paints i m p r o v e b y r e m o v a l of a t h i r d m o l e c u l e of w a t e r ; others g i v e satisfactory results when painted wet. 4

ο u ο ni W

ο

2

4

6

8

10

12

14

D r y i n g T i m e - Minutes

Figure 21.

Dryoff vs. water of crystallizationfossas function of temperature

In E u r o p e a n d i n J a p a n , because of i n c r e a s e d pressures to m i n i m i z e p o l l u t i o n , the t r e n d has b e e n to e l i m i n a t e the post-treatment p r e p a r i n g m e t a l p r i o r to e l e c t r o d e p o s i t e d paints.

step i n

T h i s has b e e n

done

because of t h e i r l a c k of d i s p o s a l facilities to h a n d l e c h r o m a t e c o n t a i n i n g wastes o r t h e i r disinterest i n p r o v i d i n g t h e m . I n b o t h E u r o p e a n d J a p a n , energy to p r o d u c e heat has a l w a y s b e e n at a p r e m i u m a n d therefore they h a v e b e e n a m o n g the first to e l i m i n a t e t h e d r y o f f o v e n step.

Under

p r o d u c t i o n operations,

nor a

u s i n g neither

a reactive

post-treatment

dryoff o v e n , t h e y h a v e e x p e r i e n c e d p o o r , w e t film a d h e s i o n . B y this w e m e a n a loss o f t h e p a i n t d u r i n g t h e w a t e r r i n s i n g of t h e electrodeposited p a i n t film p r i o r t o c u r i n g . T h i s loss of a d h e s i o n c a n b e q u i t e spotty o n a g i v e n p a r t a n d a p p a r e n t l y occurs c u r r e n t density.

g e n e r a l l y i n t h e areas of lowest

T h e p r o b l e m of p o o r , w e t film a d h e s i o n is d e f i n i t e l y


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35


r e l a t e d to t h e p a i n t f o r m u l a t i o n a n d the a m o u n t of electrolyte F i g u r e 22 illustrates this p r o b l e m .

content.

H o w e v e r , w i t h e v e r y t h i n g else h e l d

constant, the p r o b l e m b e c o m e s m o r e p r e v a l e n t w h e n n e i t h e r a r e a c t i v e post treatment or a dryoff o v e n is used. I n most cases, t h e u s e of either the reactive post treatment or a dryoff o v e n w i l l s i g n i f i c a n t l y i m p r o v e o r


e l i m i n a t e the p r o b l e m .

Figure 22. Example of poor wet film adhesion W h i l e p o l l u t i o n pressures h a v e f o r c e d some to b e m o r e

concerned

w i t h p o l l u t i o n of t h e e n v i r o n m e n t t h a n w e h a v e been, w e are a w a k e n i n g to the p r o b l e m , a n d there is n o d o u b t that t h e most c o m p e t i t i v e

total

system u s i n g e l e c t r o d e p o s i t i o n w o u l d b e o n e that p r o d u c e s a c o n v e r s i o n coating,

that w h e n m a t c h e d

with

an electrodeposited

paint

a c c e p t a b l e q u a l i t y w i t h t h e e l i m i n a t i o n of t h e post-treatment

provides stage a n d

the d r y o f f stage. T h i s p a p e r is c o n c e r n e d

w i t h the effect of e l i m i n a t i n g t h e post-

t r e a t m e n t a n d the dryoff o v e n as i t relates to the c o n v e r s i o n system.

coating

T h e r e are other reasons to e l i m i n a t e t h e dryoff o v e n or to a v o i d

the e l i m i n a t i o n of the dryoff o v e n , b a s e d o n considerations s u c h as t h e presence o f t h e s o u n d d e a d e n e r bodies.

pads i n t h e case o f t h e a u t o m o t i v e

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

i n t o the e l e c t r o d e p o s i t e d paints f u l l of w a t e r a n d therefore d o n o t p i c k u p a n y significant a m o u n t of p a i n t , thus r e d u c i n g o v e r a l l costs. I n some


36

ELECTRODEPOSITION

O F

COATINGS

cases, m o i s t u r e or w a t e r i n crevices m a y result i n a d i l u t i o n , i n l o c a l areas, of t h e e l e c t r o d e p o s i t e d

paints, r e s u l t i n g i n less t h a n d e s i r a b l e

coating

deposition. T h e e l e c t r o d e p o s i t e d one-coat w h i t e systems o v e r a l u m i n u m o r g a l v a n i z e d steel surfaces, i f p r o p e r l y c l e a n e d a n d treated, d o n o t present a serious p r o b l e m insofar as o b t a i n i n g a u n i f o r m w h i t e surface is c o n c e r n e d .


H o w e v e r , t h e e l e c t r o d e p o s i t i o n o f one-coat systems o n steel is another matter.

B e c a u s e o f t h e a n o d i c d i s s o l u t i o n o f t h e substrate, a d e g r e e o f

y e l l o w i n g o r t a n d i s c o l o r a t i o n o f t h e p a i n t film is o b t a i n e d f r o m t h e i r o n salts. E v e n w i t h the best c o n v e r s i o n coatings for steel, t h e r e is a t e n d e n c y to d i s c o l o r t h e p a i n t i f t h e r e a c t i o n post treatment a n d t h e d r y o f f o v e n are o m i t t e d . It appears that t h e p a s s i v i t y o b t a i n e d b y t h e reactive c h r o m a t e treatment goes a l o n g w a y to m i n i m i z e either t h e d i f f u s i o n o f t h e d i s s o l v e d i r o n t h r o u g h t h e p a i n t film o r its a b i l i t y to react w i t h t h e p a i n t components.

W h a t e v e r t h e m e c h a n i s m , w e h a v e f o u n d that t h e r e a c t i v e

post-treatments h e l p assure t h e p r o d u c t i o n o f a m o r e u n i f o r m a n d w h i t e , e l e c t r o d e p o s i t e d p a i n t systems.

Conclusions It is necessary to r e c o g n i z e the i n t e r d e p e n d e n c e o f t h e options a v a i l able i n m e t a l treatment maximum

flexibility

a n d electrodeposited

paint formulations. F o r

w i t h respect t o t h e selection

of electrodeposited

paints, the p r o p e r c o n v e r s i o n c o a t i n g , r e a c t i v e post treatment, a n d d r y o f f o v e n s h o u l d b e u s e d . B y m a t c h i n g the e l e c t r o d e p o s i t e d p a i n t f o r m u l a t i o n w i t h t h e c o n v e r s i o n c o a t i n g , q u a l i t y finishing c a n b e o b t a i n e d e v e n w i t h the

e l i m i n a t i o n o f t h e reactive

post

treatment

a n d the dryoff oven.

T i g h t e r c o n t r o l of t h e e l e c t r o d e p o s i t e d p a i n t a n d p a i n t i n g c o n d i t i o n s are r e q u i r e d w h e n these t w o steps are not u s e d . Present

m e t a l treatments result i n h i g h q u a l i t y

finishing

systems.

T h e m e t a l treatment i n d u s t r y cannot b e c o m p l a c e n t , h o w e v e r , because of t h e i n c r e a s e d awarness a n d c o n c e r n f o r e l i m i n a t i n g t h e p o l l u t i o n o f o u r e n v i r o n m e n t , i t is essential that m e t a l systems

treatment—electrodeposition

b e d e v e l o p e d that p r o v i d e q u a l i t y

finishes

without pollution.

D e v e l o p m e n t o f s u c h systems w i l l r e q u i r e the c o o p e r a t i o n a n d f u l l u n d e r s t a n d i n g o f a l l parties c o n c e r n e d — t h o s e

working i n metal

treatment,

p a i n t f o r m u l a t i o n , a n d e q u i p m e n t d e s i g n a n d finally t h e user h i m s e l f .

Literature Cited 1. "Metals Handbook," Vol. 2, 8th ed., p. 529-547, American Society For Metals. 2. Cavanagh, W., Gibson, R., "Phosphate Coating of Metal Surfaces For Industrial Use," Plating (June 1955).



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3. Cheever, G . D . , "Wetting of Phosphate Interfaces by Polymer Liquids," "Interface Conversion For Polymer Coatings," Weiss, P., Cheever, G . D . , Eds., Elsevier, New York. 4. Maurer, J. I., "Preparation of Metal Surfaces for Organic Finishes," Ameri can Society of Tool and Manufacturing Engineers, paper FC68-652. 5. Maurer, J. I., "Surface Preparation for Organic Finishes," Ind. Finishing (April 1969). 6. Ellis, I. W . , Maurer, J. I., "Metal Preparation for The Coil Coating Indus try," J. Paint Technol. (July 1967) 39, 460-463. 7. Bogart, H . N., Burnside, G . L . , Brewer, G . E . F., "The Concept and De velopment of The Ford Electrocoating System," Society of Automotive Engineers, paper 988A. 8. Revelt, P. Α., Goodbye To Rust and Corrosion," Ward's Auto World (1970) 6 (2). 9. LeBras, L . R., "Electrodeposition—Theory and Mechanisms," J. Paint Tech nol. (Feb. 1966) 38, (493). 10. May, C. Α., Smith, G . , "Dissolution of The Anode During The Electro deposition of Surface Coatings," J. PaintTechnol.(Nov. 1968) 40 (526). 11. "Conversion Coatings Ready For Electropainting," Steel Magazine (Sept. 1966) 46, 47. 12. Maurer, J. I., Saad, Κ. I., "Pre-Cleaning Determines Electropaint Finishes," Canadian Paint Finishing (July 1967). 13. Saad, Κ. I., "How to Prepare Metal Surfaces for Electropainting," Product Finishing (May 1969) 46-55. 14. Hays, D . R., White, C. S., "Electrodeposition of Paint: Deposition Param eters," J. Paint Technol. (Aug. 1969) 41 (535). 15. Cheever, G . D . , Wojtkowiak, J. J., "Instrumental Studies of The Surfaces and Internal Composition of Paint Films," J. PaintTechnol.(July 1970) 42 (546). 16. Menzer, W . , "Chemische Oberflächenbehandlung von Metallen vor der Electrotauchlackierung," Internationale Tagung für Oberflächentechnik der Metalle, Hannover, May 5-8, 1968. 17. Kimoto, S., Russ, J. C., "The Characteristics and Application of The Scan ning Microscope," Mater. Res. Standards (Jan. 1969). 18. Maher, J. F., "Phosphate Coatings," Metal Finishing, 1970 Guidebook, pp. 594, 596. 19. Machu, W., "The Kinetics of The Formation of Phosphate Coatings," "Inter face Conversion For Polymer Coatings," Weiss, P., Cheever, G . D . , Eds., p. 130, Elsevier, New York. 20. U.S. Patents 2,310,239 and 2,874,081. 21. Adams, M., "Cleaning and Phosphating of Assembled Bodies," Society of Automotive Engineers, Paper 668A. 22. Ellinger, M . L . , "Electrophoretic Deposition of Paints in Further Fields of Metal Finishing," J. PaintTechnol.(March 1969) 41 (530). 23. Simpson, V . P., Hooker Chemical Research Center, Grand Island, Ν. Y., private communication. 24. Van Loo, M., Laiderman, D . D., Bruhn, R. R., "Filiform Corrosion," (Aug. 1953) 9 (8). 25. Laukonis, J. V., "The Role of Oxide Films in The Zinc Phosphating of Steel Surfaces," "Interface Conversion for Polymer Coatings," Weiss, P., Cheever, G . D., Eds., Elsevier, New York. 26. Maurer, J. I., Saad, Κ. I., "Study of Coatings Prior to Electrodeposition of Paint," Parker International Conference, Detroit, Mich., Oct. 1966. RECEIVED May 27,

1971.


Electrodeposition of Coatings

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