Electrodeposition of Coatings

3 Changes in Zinc Phosphated Steel Surfaces during Electrodeposition

Downloaded by UNIV OF AUCKLAND on December 1, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch003

C. A. M A Y Shell Development Co., Emeryville, Calif. 94608

During the electrodeposition of a surface coating 30-40% of a phosphatized surface can be electrodissolved because of a decrease in the pH. Interestingly, this loss of the phosphatizing material does not reduce the corrosion protection, and scanning

electron microscope

studies show that the

phosphate crystal coating is not ruptured.

Previous studies

have shown that the most probable cause for reduced cor rosion protection

is the electrodissolved

occluded in the coating.

ions which are

Potentiodynamic anodic polariza

tion curves show that the water of hydration in the crystal structure also changes during electrodeposition.

This source

of reduced corrosion protection, however, depending

on the

phosphating treatment, can be corrected by heating.

~\J"ost of the vehicles u s e d i n t h e e l e c t r o d e p o s i t i o n of surface ly**

coatings

are c a r b o x y l a t e d versions o f c o n v e n t i o n a l c o a t i n g molecules. T h e

c a r b o x y l g r o u p is r e q u i r e d f o r aqueous s o l u b i l i t y b y n e u t r a l i z a t i o n w i t h v a r i o u s bases, g e n e r a l l y o r g a n i c amines, a n d t h e resultant i o n p r o v i d e s the necessary e l e c t r i c a l charge f o r a t t r a c t i o n to a n o d i c surfaces

during

the a p p l i c a t i o n o f t h e c o a t i n g . T h e c h e m i s t r y o f these a n o d i c depositions is q u i t e c o m p l e x a n d has b e e n w i d e l y d i s c u s s e d i n t h e literature. A l t h o u g h there is n o t c o m p l e t e agreement

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

events h a v e b e e n w e l l d e f i n e d . D u r i n g t h e c o a t i n g process t h e p H at the a n o d e is r e d u c e d ( I ) w h i c h results i n c o a g u l a t i o n o f t h e r e s i n o n t h e a n o d e surface. A d d i t i o n a l c o a g u l a t i o n of t h e r e s i n results f r o m the cations generated b y e l e c t r o d i s s o l u t i o n of t h e substrate (2)—e.g., i n t h e case o f a steel surface, ferrous ions. I n a d d i t i o n t h e c a r b o x y l a t e ions c a n u n d e r g o a K o l b e o x i d a t i o n {3,4,5).

T h e picture becomes even more complicated 47

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

48

ELECTRODEPOSITION

O F COATINGS

c o n s i d e r i n g that most steel substrates are p r e t r e a t e d b y some sort of phos p h a t i n g process p r i o r to c o a t i n g . M e n z e r ( 6 ) has stated that d e p e n d i n g o n t h e nature of the substrate a n d t h e e l e c t r o d e p o s i t i o n process, 4 0 - 6 0 % of a p h o s p h a t e d surface c a n b e d i s s o l v e d d u r i n g e l e c t r o d e p o s i t i o n . O u r o w n studies h a v e s h o w n that t h e m e t a l ions generated

during

the e l e c t r o d e p o s i t i o n process are o c c l u d e d i n t h e c o a t i n g rather

than

passed into the e l e c t r o d e p o s i t i o n b a t h ( 2 ) . W e w e r e s u b s e q u e n t l y able to demonstrate that these o c c l u d e d ions are r e s p o n s i b l e f o r r e d u c e d cor r o s i o n p r o t e c t i o n as m e a s u r e d b y salt s p r a y testing ( 7 ) . W e also f o u n d Downloaded by UNIV OF AUCKLAND on December 1, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch003

that i f a p h o s p h a t e d steel p a n e l w a s s o a k e d i n w a t e r f o r as l i t t l e as o n e h o u r , a l l o w e d to d r y , a n d c o a t e d b y solvent a p p l i c a t i o n , t h e salt s p r a y p r o t e c t i o n w a s also r e d u c e d . T h i s latter effect w a s s u b s e q u e n t l y t r a c e d to a n i n c r e a s e d w a t e r o f h y d r a t i o n of t h e p h o s p h a t e d surface b y a n a l o g y to t h e recent w o r k of C h a n c e a n d F r a n c e

(8).

T h e s e authors d e m o n

strated that i t w a s p o s s i b l e to p r e d i c t w h i c h p h o s p h a t e d surfaces w o u l d g i v e i n f e r i o r results b y t h e use of a n o d i c p o l a r i z a t i o n scans. U s i n g a s i m i l a r p o l a r i z a t i o n t e c h n i q u e w e s t u d i e d t h e effects that the e l e c t r o d e p o s i t i o n process h a d o n t h e h y d r a t i o n of p h o s p h a t e d sur faces.

T h i s a p p r o a c h p r o v e d u s e f u l n o t o n l y i n d e f i n i n g the n a t u r e of

v a r i o u s p h o s p h a t e d pretreatments b u t also f o r e s t i m a t i n g desirable b a k i n g schedules f o r e l e c t r o d e p o s i t e d coatings.

General Experimental Approach A g e n e r a l d i s c u s s i o n of the e x p e r i m e n t a l a p p r o a c h at this p o i n t a p pears a d v i s a b l e f o r a better u n d e r s t a n d i n g of t h e d a t a w h i c h f o l l o w s . If a m e t a l substrate is p l a c e d i n a n aqueous s o l u t i o n of a n i n o r g a n i c salt a n d c o n n e c t e d to a s t a n d a r d c a l o m e l e l e c t r o d e ( S C E ) t h r o u g h a n i n - l i n e v o l t m e t e r , a p o t e n t i a l is n o t e d . I n t h e case of a p h o s p h a t e d steel surface this is a r o u n d —0.6 v o l t . I f a potentiostat is p l a c e d i n this system a n d a v o l t a g e is a p p l i e d i n a p o s i t i v e ( a n o d i c )

direction, electrodissolution

takes p l a c e a n d a c u r r e n t is d e v e l o p e d . T h e m a g n i t u d e of t h e c u r r e n t is a m e a s u r e of the rate of d i s s o l u t i o n of t h e substrate at t h e a p p l i e d voltage. B y i n c r e a s i n g t h e v o l t a g e at a u n i f o r m rate, a p l o t of t h e a m p e r a g e as a f u n c t i o n of t h e v o l t a g e c a n b e o b t a i n e d . If t h e rate o f voltage increase is k e p t constant

f o r a l l experiments, t h e p o l a r i z a t i o n scans are q u i t e

r e p r o d u c i b l e f o r a g i v e n set of e x p e r i m e n t a l c o n d i t i o n s .

Results and Discussion W e d e c i d e d to e x a m i n e three types of p h o s p h a t e d steel

surfaces

u s i n g this a p p r o a c h to ascertain h o w v a r i o u s m e t a l pretreatments m a y i n f l u e n c e t h e e l e c t r o d e p o s i t i o n process.

T h e three surfaces u s e d w e r e


3.

M A Y

49

Zinc Phosphated Steel Surfaces

typical commercial products (Parker Rustproofmg D i v . , H o o k e r C h e m . ) , A - B o n d e r i t e 100, B - B o n d e r i t e E P - 2 , a n d C - B o n d e r i t e E P - 8 9 : A . A s t a n d a r d z i n c p h o s p h a t e process, n i t r i l e accelerated. B . Z i n c p h o s p h a t e process, n i c k e l a n d fluoride m o d i f i e d , n i t r i l e ac celerated. C . Z i n c p h o s p h a t e process, c a l c i u m m o d i f i e d . P r e t r e a t m e n t A is a c o m m o n l y u s e d m e t a l p r e t r e a t m e n t f o r q u a l i t y finishes.

Pretreatment

Β is a m o d i f i e d v e r s i o n of t h e z i n c

phosphate

process r e c o m m e n d e d as a g o o d starting p o i n t f o r t h e e v a l u a t i o n of elec Downloaded by UNIV OF AUCKLAND on December 1, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch003

t r o d e p o s i t i o n coatings.

P r e t r e a t m e n t C is r e c o m m e n d e d f o r l i g h t c o l o r e d

electrocoats. Table I.

Metal Concentration Found in Electrodeposited Coatings on Various Phosphated Surfaces

Pretreatment

A

Iron, % w Zinc, % w Nickel, % w Calcium, ppm

0.07 0.95 — —

B 0.06 0.57 0.45 34

C 0.03 0.24 — 280

A n e l e c t r o d e p o s i t e d c o a t i n g o n e a c h t y p e of substrate w a s first ex a m i n e d f o r the c o n c e n t r a t i o n o f t h e v a r i o u s o c c l u d e d , e l e c t r o d i s s o l v e d metals.

T h e results are g i v e n i n T a b l e I.

T h e electrocoating

vehicle

u s e d i n this d e t e r m i n a t i o n a n d t h r o u g h o u t t h e i n v e s t i g a t i o n w a s a m a l e i n i z e d , o i l m o d i f i e d , e p o x y ester of the b i s p h e n o l t y p e ( 9 ) . A s seen, pretreatments metal.

A a n d Β gave a p p r o x i m a t e l y t h e same c o n c e n t r a t i o n of

T h e nickel modified pretreatment y i e l d e d a lower zinc concen

t r a t i o n , b u t this w a s c o m p e n s a t e d f o r b y the o c c l u d e d n i c k e l ions. T h e c a l c i u m p h o s p h a t e m o d i f i c a t i o n ( C ) y i e l d e d m u c h less m e t a l , p a r t i c u l a r l y electrodissolved iron.

T h e latter fact p r o b a b l y accounts f o r the better

c o l o r of electrocoatings the b a k i n g of t h e

o n this surface since i r o n salts d i s c o l o r d u r i n g

finish.

T h e three p h o s p h a t e surfaces w e r e first e x a m i n e d as r e c e i v e d f r o m the s u p p l i e r . P r i o r to s a m p l i n g , e a c h substrate w a s h e a t e d f o r three m i n utes at 190 ° C to ensure the u n i f o r m i t y of the s t a r t i n g surface.

T h i s is a n

i m p o r t a n t p r e c a u t i o n since t h e w a t e r of h y d r a t i o n w i l l increase s l o w l y o n storage u n d e r n o r m a l l a b o r a t o r y c o n d i t i o n s ( 7 ). E a c h p r e t r e a t m e n t gives a characteristic p o l a r i z a t i o n c u r v e as s h o w n i n F i g u r e 1. T h e r e p r o d u c i b i l i t y of the d a t a is q u i t e g o o d , a n d h e n c e the curves c o u l d b e u s e d f o r i d e n t i f i c a t i o n purposes i n m u c h the same m a n n e r that i n f r a r e d spectra are u s e d to i d e n t i f y o r g a n i c c o m p o u n d s .

T h e d o t t e d p o r t i o n s of the

curves are anomalies w h i c h sometimes a p p e a r i n t h e a n o d i c scans, p r o b a b l y as a result of v a r i a t i o n s i n the p h o s p h a t e p r e t r e a t m e n t


o v e r the

50

ELECTRODEPOSITION

OF

COATINGS

PRETREATMENT A PRETREATMENT Β PRETREATMENT C

\


.

1

12

j 1 01 0.2

I ,• —Λ

{

1

\ .ι

0.1

Figure 1.

ιI 0

1

"

J

ι — -0.1 1

'

I1

1,

-0.2 ' VOLTS v$ SCE

I1

-0.3

1.

1 1 -0.4

1.

11 -0.5

1

^

-0.6

Anodic polarization curves for various phosphated surfaces

surface tested. T h e s t a n d a r d z i n c p h o s p h a t e ( A ) a n d t h e n i c k e l m o d i f i e d z i n c p h o s p h a t e ( B ) surfaces g i v e s i m i l a r results, b u t t h e c a l c i u m phos p h a t e surface is q u i t e different. T h e s e differences are also seen b y m i c r o s c o p i c e x a m i n a t i o n of the three substrates.

F i g u r e 2 shows 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 p h o t o

m i c r o g r a p h s of e a c h surface at 1000 X ( t i m e s ) m a g n i f i c a t i o n . A s m i g h t b e e x p e c t e d pretreatments A a n d Β a p p e a r q u i t e s i m i l a r , b u t C , the c a l c i u m p h o s p h a t e m o d i f i c a t i o n , is q u i t e different. I n t h e latter case t h e p h o s p h a t e crystals a p p e a r to b e m u c h less i n t i m a t e l y associated w i t h the steel substrate.

Figure 2.

Phosphated surfaces before electrodeposition (X 450)


3.

M A Y

51


T h e effect of e l e c t r o d e p o s i t i o n ( E / D ) o n t h e three p h o s p h a t e d sur faces is s h o w n i n F i g u r e s 3, 4, a n d 5. T h e samples w e r e p r e p a r e d b y e l e c t r o d e p o s i t i n g t h e e p o x y ester o n the substrate i n q u e s t i o n , w a s h i n g the film off w i t h t e t r a h y d r o f u r a n , d r y i n g at r o o m t e m p e r a t u r e , p u n c h i n g out the test s p e c i m e n , a n d d e t e r m i n i n g t h e a n o d i c p o l a r i z a t i o n charac teristics.


20

AFTER E/D

-

AS SUPPLIED O R AFTER E/D + 3 M I N . A T 190 C

\

e

-

0.2

1

0.1

,

1

Figure 3.

0

,

1

^

-0.1

ι

.

-0.2 VOLTS v$ SCE

ι

-0.3

.

ι

ι,

-0.4

P^r

-0.5

5 5 8

^

-0.6

Effect of electrodeposition on pretreatment A

A s F i g u r e s 3 a n d 4 s h o w , the s t a n d a r d z i n c p h o s p h a t e

pretreatment

a n d t h e n i c k e l a n d fluoride m o d i f i c a t i o n b e h a v e i n a s i m i l a r f a s h i o n . A f t e r e l e c t r o d e p o s i t i o n the a n o d i c excursions gave peaks at a p p r o x i m a t e l y —0.37 a n d + 0 . 1 3 v o l t i n e a c h case. B y h e a t i n g either p r e t r e a t m e n t f o r three m i n u t e s at 190 ° C after i t h a d b e e n t h r o u g h t h e e l e c t r o d e p o s i t i o n process, t h e o r i g i n a l p o l a r i z a t i o n scan w a s restored. p h o s p h a t e m o d i f i e d pretreatment

W i t h the calcium

the results are m o r e c o m p l e x .

Before

e l e c t r o d e p o s i t i o n t h e a n o d i c p o l a r i z a t i o n c u r v e looks q u i t e s i m i l a r to t h e other t w o p h o s p h a t e coatings after e l e c t r o d e p o s i t i o n . T h e c o a t i n g process changes t h e c u r v e to a p o s i t i o n m o r e n e a r l y s i m i l a r to the o r i g i n a l curves f o r pretreatments

A a n d B . W h e n this substrate w a s h e a t e d to 190 ° C

after e l e c t r o d e p o s i t i o n , t h e i n i t i a l p o l a r i z a t i o n characteristics a l w a y s restored.

A l t h o u g h t h e results w i t h this substrate

were not

were incon

sistent, t h e general shape o f t h e p o l a r i z a t i o n c u r v e f o l l o w i n g the poste l e c t r o d e p o s i t i o n h e a t i n g is as s h o w n i n F i g u r e 5. T h u s , there a p p e a r e d to h a v e b e e n some p e r m a n e n t d a m a g e to this pretreatment electrodeposition

caused b y the

process.


52

ELECTRODEPOSITION

O F COATINGS


20

0.2

0.1

0

Figure 4.

-0.1

-0.2 VOLTS v$ SCE

-0.3

-0.4

-0.5

-0.6

Effect of electrodeposition on pretreatment Β

A s stated earlier, p a r t of t h e p h o s p h a t e d surface is e l e c t r o d i s s o l v e d d u r i n g coating deposition.

T h e results thus f a r l e a d t o t h e c o n c l u s i o n

that r e d u c i n g t h e a m o u n t of p h o s p h a t i n g also does n o t change t h e p o l a r i z a t i o n characteristics i f t h e substrate is p r o p e r l y h e a t e d f o l l o w i n g elec-

-^AS 16

1

SUPPLIED

—

Hp

"

Ζ oc et

3

V

,

4 AFTER E/D +3 M I N . A T I W C 0 0.2

•

J

0.1

,

Figure 5.

ι 0

,

\

J-0.1 1

" Γ 1 I I I I κ. . . . . I

;

I

.

-0.2 VOLTS v$ SCE

V I

.

-0.3

I

-0.4

_

i

1—

-0.5

Effect of electrodeposition on pretreatment C


-0.6

3.

M A Y

53


20,


.6 h

0.2

0.1

Figure 6.

0

-0.1

-0.2 VOLTS vs SCE

-0.3

-0.4

-0.5

-0.6

Effect of soaking in Ε/D bath on pretreatment Β

t r o d e p o s i t i o n a n d film r e m o v a l . H o w e v e r , i n k e e p i n g w i t h t h e

findings

of C h a n c e a n d F r a n c e , t h e e l e c t r o d e p o s i t i o n process o r contact w i t h t h e aqueous b a t h appears to h a v e i n c r e a s e d t h e w a t e r of h y d r a t i o n i n t h e p h o s p h a t e c r y s t a l structure.

I n v e s t i g a t i o n of t h e latter p o i n t p r o v e d i n

teresting i n that i t p o i n t e d o u t f u r t h e r differences b e t w e e n t h e p h o s p h a t e d 20 !

—

,

VOLTS v$ SCE

Figure 7.

Effect of soaking in Ε/D hath on pretreatment C


54

E L E C T R O D E P O S I T I O N O F COATINGS

surfaces.

D u r i n g o u r l a b o r a t o r y operations a m a x i m u m of five m i n u t e s

is r e q u i r e d f r o m t h e t i m e a p a n e l is p l a c e d i n t h e e l e c t r o d e p o s i t i o n b a t h u n t i l t h e c o a t i n g is r e a d y to b e p l a c e d i n a n o v e n f o r b a k i n g . A c c o r d i n g l y , panels w i t h e a c h surface p r e t r e a t m e n t w e r e s o a k e d i n t h e electro d e p o s i t i o n b a t h f o r five m i n u t e s , r i n s e d w i t h t e t r a h y d r o f u r a n , a i r d r i e d , a n d e x a m i n e d b y a n o d i c p o l a r o g r a p h y . F i g u r e s 6 a n d 7 s h o w that t h e n i c k e l m o d i f i e d z i n c p h o s p h a t e ( Β ) a n d the c a l c i u m p h o s p h a t e m o d i f i c a t i o n ( C ) are essentially u n a f f e c t e d b y this p r o c e d u r e . T h e s t a n d a r d z i n c p h o s p h a t e ( A ) , o n t h e other h a n d ( F i g u r e 8 ) , p a r t i a l l y a p p r o a c h e s the Downloaded by UNIV OF AUCKLAND on December 1, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch003

characteristics of a p a n e l w h i c h has b e e n subjected to t h e e l e c t r o d e p o s i t i o n process. T h i s i n a sense explains w h y t h e n i c k e l a n d f l u o r i d e m o d i f i e d z i n c p h o s p h a t e m a y h a v e b e e n t h e r e c o m m e n d e d p r e t r e a t m e n t f o r elec trocoating.

T h e standard zinc phosphate

finish

changes s o m e w h a t b y

m e r e l y b e i n g e x p o s e d to t h e e l e c t r o d e p o s i t i o n b a t h ; t h e c a l c i u m p h o s p h a t e m o d i f i e d finish is n o t c h a n g e d b y s o a k i n g , b u t the e l e c t r o d e p o s i t i o n process changes t h e p r e t r e a t m e n t to t h e extent t h a t t h e d a m a g e m a y b e permanent.

O n t h e other h a n d , the n i c k e l m o d i f i e d z i n c p h o s p h a t e

changes o n l y 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 recovers w h e n r e h e a t e d .

It

w o u l d thus a p p e a r that this finish a n d t h e s t a n d a r d z i n c p h o s p h a t e s h o u l d r e c o v e r the p r o p e r degree of h y d r a t i o n i f t h e e l e c t r o d e p o s i t e d c o a t i n g is b a k e d at a sufficiently h i g h t e m p e r a t u r e . O u r efforts w e r e next e x t e n d e d to a n i n v e s t i g a t i o n of t h e p r o p e r b a k i n g temperatures f o r t h e v a r i o u s pretreatments.

A preliminary check

20

AFTER 5 MIN.SOAKJ

4

0 0.2

0.1

0

-0.1

-0.2

-0.3

-0.4

-0.5

VOLTS v i SCE

Figure 8.

Effect of soaking in Ε/D bath on pretreatment A


-0.6


3.

M A Y

Figure 9.

55


Effect of baking polystyrene film on pre-electrodeposited pretreat ment A

s h o w e d (see F i g u r e 9 ) that i f t h e u n c u r e d , e l e c t r o d e p o s i t e d film w a s solvent w a s h e d off o f a p a n e l , i n this case p r e t r e a t m e n t A , h e a t i n g at the r e l a t i v e l y m i l d c o n d i t i o n s of one-half h o u r at 120 ° C restored t h e o r i g i n a l p o l a r i z a t i o n characteristics.

W e suspected, h o w e v e r , that t h e

presence of the c o a t i n g w o u l d r e t a r d t h e " r e c o v e r y " because of d i f f u s i o n l i m i t a t i o n s . A s w i l l b e s h o w n , this p r o v e d to b e t h e case. B e c a u s e of o u r suspicions, subsequent experiments w e r e d e s i g n e d as n e a r l y as p o s s i b l e to s i m u l a t e a n e l e c t r o d e p o s i t i o n p r o c e d u r e . T h e first attempt to s i m u l a t e t h e e l e c t r o d e p o s i t i o n process

consisted

of e l e c t r o c o a t i n g w i t h the m a l e i n i z e d e p o x y ester, b a k i n g at v a r i o u s temperatures, a n d t h e n r e m o v i n g t h e film b y s w e l l i n g i n a strong o r g a n i c solvent. I n every case the c u r r e n t peaks of t h e a n o d i c p o l a r i z a t i o n scans w e r e greatly d i m i n i s h e d i n d i c a t i n g that some of the c o a t i n g c o u l d n o t b e r e m o v e d . A c c o r d i n g l y , this a p p r o a c h w a s a b a n d o n e d . T h e problem was then approached b y electrodepositing the coating, w a s h i n g off the u n c u r e d c o a t i n g w i t h t e t r a h y d r o f u r a n , d r y i n g , a n d i m m e d i a t e l y a p p l y i n g a c o a t i n g of p o l y s t y r e n e f r o m t o l u e n e s o l u t i o n i n a thickness w h i c h w o u l d b e o b t a i n e d f r o m t h e e l e c t r o d e p o s i t e d

coating.

T h e p o l y s t y r e n e c o a t e d p a n e l w a s t h e n b a k e d at t h e d e s i r e d t e m p e r a t u r e , the c o a t i n g w a s h e d off w i t h toluene, a n d t h e p o l a r i z a t i o n scan

deter

m i n e d . A s seen f r o m F i g u r e 9 w i t h pretreatment A a p a r t i a l " r e c o v e r y " was o b t a i n e d after a b a k e of one-half h o u r at 120 ° C .

Since

complete

r e c o v e r y w a s o b t a i n e d u n d e r t h e same c o n d i t i o n s i n t h e absence of a


56

ELECTRODEPOSITION

OF

COATINGS

c o a t i n g , t h e c o a t i n g d e f i n i t e l y slows d o w n the r e m o v a l of t h e w a t e r of h y d r a t i o n f r o m t h e p h o s p h a t e c r y s t a l structure.

W h e n the polystyrene

c o a t i n g w a s b a k e d at 150 ° C , h o w e v e r , difficulties w e r e

encountered.

E v e n s o a k i n g o v e r n i g h t i n t h e solvent d i d n o t r e m o v e a l l of t h e p o l y styrene.

A p p a r e n t l y , d u r i n g the b a k i n g o p e r a t i o n at t h e h i g h e r tem

perature, the p o l y m e r penetrates t h e p h o s p h a t e c r y s t a l structure to a degree that i t cannot b e easily r e m o v e d . T h e n e t result is a p o l a r i z a t i o n c u r v e f o r t h e 150 ° C e x p e r i m e n t as s h o w n i n F i g u r e 9. A s j u d g e d b y t h e t e r m i n a t i o n of t h e a n o d i c d i s s o l u t i o n at —0.15 v o l t , t h e p r o p e r degree Downloaded by UNIV OF AUCKLAND on December 1, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch003

of h y d r a t i o n has b e e n restored since this is characteristic of t h e p h o s p h a t e surface b e f o r e e l e c t r o d e p o s i t i o n . H o w e v e r , t h e d o u b l e peaks,

character

istic of a p h o s p h a t e d surface after e l e c t r o d e p o s i t i o n , casts some d o u b t o n this c o n c l u s i o n . 20 AFTER E/D

—CONTROLS

*

Ε

12

-

Ζ

200°ς

60° C

. X

^ . B E F O R E E/D

s

150°C or 175°C

^120°C 4

1

0.2

Figure 10.

ι 0.1

t

1 0

/

ι

ι -0.1

;

-0.2

VOLTS vs SCE

I . -0.3

I

. -0.4

l ^ T ^ f c -0.5 -0.6

Effect of paraffin heating on pre-electrodeposited pretreatment A

T h e most satisfactory p r o c e d u r e consisted of e l e c t r o d e p o s i t i o n , re m o v a l of the u n c u r e d c o a t i n g w i t h a solvent, a n d h e a t i n g t h e s a m p l e f o r the d e s i r e d t i m e a n d t e m p e r a t u r e u n d e r a p o o l of m o l t e n paraffin w a x . Samples thus o b t a i n e d c o u l d b e r e a d i l y c l e a n e d b y s o a k i n g f o r a f e w m i n utes i n benzene.

T h e results s h o w e d that this r e c o v e r y t e m p e r a t u r e is

a b o u t 50 ° C h i g h e r t h a n w o u l d b e r e q u i r e d i n a c t u a l p r a c t i c e w h e r e a t h i n c o a t i n g is o n t h e surface.

H o w e v e r , t h e d a t a p e r m i t a n e s t i m a t i o n of t h e

m i n i m u m d e s i r a b l e b a k i n g t e m p e r a t u r e a n d also t h e t e m p e r a t u r e differ ences w h i c h m a y b e r e q u i r e d f o r p r o p e r d e h y d r a t i o n of different phos phate

treatments.


3.

M A Y

57


S h o w n i n F i g u r e 10 a r e t h e results o b t a i n e d w i t h t h e s t a n d a r d z i n c p h o s p h a t e pretreatment ( A ) . T h e h e a t i n g p e r i o d i n e a c h case w a s oneh a l f h o u r . A t 1 5 0 ° a n d 175 ° C a p r o n o u n c e d c h a n g e occurs i n t h e p o l a r i z a t i o n scan, b u t c o m p l e t e r e c o v e r y is n o t n o t e d u n t i l t h e s p e c i m e n is h e a t e d at 2 0 0 ° C .

C o m b i n i n g this d a t a w i t h t h e results i n F i g u r e 9, it

w o u l d a p p e a r that the p r o p e r b a k i n g f o r a c o a t i n g e l e c t r o d e p o s i t e d o n this substrate is less t h a n 200 ° C a n d m o r e l i k e l y close to 150 ° C . U n d o u b t e d l y i t is m u c h m o r e difficult f o r t h e w a t e r to escape t h r o u g h t h e


r e l a t i v e l y " t h i c k " p a r a f f i n l a y e r t h a n 0 . 5 - 1 m i l of surface c o a t i n g .

AFTER E/D

χ

-

-

CONTROLS

BEFORE E/D Ε

\ \NS.

Ζ

225 ° C - —

\^

ί

|200 C

^ ^ j - .

e

:'• v O ^ y ' 2 0 o r 6 0 ° c s

0.2

-4

Figure 11.

0.1

—'

0

.

-0.1

L

1

ι

-0.2 VOLTS vs SCE

>s^ii^..

1 .

-0.3

1 i

-0.4

ΐ"^*!^·· -0.6

-0.5

Effect of paraffin heating on pre-electrodeposited pretreatment Β

E x a m i n a t i o n of F i g u r e 11 reveals f u r t h e r differences b e t w e e n t h e nickel a n d fluoride modified zinc phosphate ( B ) a n d the standard pre t r e a t m e n t ( A ) . M o s t i m p o r t a n t is t h e f a c t that c o m p l e t e r e c o v e r y does n o t o c c u r u n t i l 225 ° C , 25 ° C h i g h e r t h a n that o b s e r v e d w i t h t h e s t a n d a r d z i n c p h o s p h a t e . T h u s i f a c o m p a r i s o n w e r e m a d e b e t w e e n pretreatments A a n d Β w i t h r e g a r d to salt s p r a y p e r f o r m a n c e , i t w o u l d b e e x p e c t e d that coatings b a k e d at 150 ° C w o u l d g i v e s i m i l a r results, a n d a n y differences w o u l d o n l y a p p e a r w h e n b a k i n g temperatures of 175 ° C o r h i g h e r w e r e u s e d . T h e results also s h o w that i n a d d i t i o n to the h i g h e r r e c o v e r y t e m p e r a t u r e r e q u i r e d , t h e p a r t i a l changes o b s e r v e d at t h e l o w e r b a k i n g temperatures w i t h p r e t r e a t m e n t ( a ) are n o t as a p p a r e n t i n this case. T h e calcium phosphate modification ( C ) behaved i n a markedly different m a n n e r f r o m t h e other t w o b o n d c o a t s ( F i g u r e 1 2 ) .

Excluding

the a f o r e m e n t i o n e d inconsistencies, w h i c h w e r e also e v i d e n t i n these



58

ELECTRODEPOSITION

Figure 12.

O F COATINGS

Effect of paraffin heating on pre-electrodeposited pretreatment C

experiments, r e v e r s i o n to t h e i n i t i a l p o l a r i z a t i o n scan c o u l d o c c u r at a n y t e m p e r a t u r e f r o m 120 ° C u p w a r d s . V i s u a l e x a m i n a t i o n of the three p h o s p h a t e surfaces

after

electro

d e p o s i t i o n w i t h 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 makes the b e h a v i o r of pretreatment C more understandable.

T h e p h o t o m i c r o g r a p h s are s h o w n

i n F i g u r e 13. T h e s t a n d a r d z i n c p h o s p h a t e p r e t r e a t m e n t

( A ) has n o t

c h a n g e d m a r k e d l y . Some of t h e f r a g m e n t a r y c r y s t a l structure, h o w e v e r , appears to h a v e b e e n r e m o v e d . C o n s i d e r i n g t h e n i c k e l m o d i f i c a t i o n ( Β ), a n a p p a r e n t flaw o r c r a c k w a s d i s c o v e r e d at 1000 X m a g n i f i c a t i o n . O n closer e x a m i n a t i o n (3000 X ) , h o w e v e r , e v e n i n this area t h e steel sub strate is n o t v i s i b l e a n d t h e p r e t r e a t m e n t appears intact. phosphate modified pretreatment

( C ) is a different story.

T h e calcium Microscopic

e x a m i n a t i o n at 1000 X i n d i c a t e d that there m a y w e l l b e areas of bare ( u n p h o s p h a t e d ) m e t a l s h o w i n g . T h i s is q u i t e e v i d e n t at 3000 X

mag

nification. T h e r i d g e s w h i c h are characteristic of t h e u n t r e a t e d , p o l i s h e d steel substrate are r e a d i l y apparent. M a n y s u c h areas c a n b e seen i n this picture.

T h e e l e c t r o d e p o s i t i o n process has a c t u a l l y r u p t u r e d t h e phos

phate coating.

It thus appears that i f t h e b o n d coat is r u p t u r e d , the

a n o d i c p o l a r i z a t i o n scans are erratic, r e c o v e r y is n o t a l w a y s

consistent

a n d d e p e n d s o n t h e area selected f o r testing. It is also p o s s i b l e that flaws are present i n t h e o r i g i n a l c a l c i u m p h o s p h a t e p r e t r e a t m e n t a n d are e n l a r g e d b y the e l e c t r o d e p o s i t i o n .

Based o n the analytical data presented

i n T a b l e I, since less of t h e pretreatment is e l e c t r o d i s s o l v e d d u r i n g t h e



3.

M A Y

Figure 13.

59


Photomicrographs on the phosphated surfaces after electrodepo sition

Magnification: upper left, X 500; lower left, X 1500; middle, X 500; upper right, X 500; lower right, X 1500

c o a t i n g process, t h e ease of r u p t u r e w o u l d i n d i c a t e a t h i n n e r p h o s p h a t e coating.

Conclusions A n o d i c p o l a r i z a t i o n t e c h n i q u e s a p p e a r to b e a u s e f u l t o o l f o r s t u d y i n g changes i n t h e w a t e r of h y d r a t i o n of p h o s p h a t e d surfaces d u r i n g t h e e l e c t r o d e p o s i t i o n of a surface coating. of i d e n t i f y i n g v a r i o u s p h o s p h a t e w h i c h change

T h e y n o t o n l y p r o v i d e a means

surfaces

t h e nature o f t h e surface

b u t indicate the conditions d u r i n g the electrodeposition

process a n d h o w these changes m a y b e rectified. T h e n i c k e l a n d fluoride m o d i f i c a t i o n of t h e p h o s p h a t i n g process has the advantage of r e t a r d i n g changes i n t h e p h o s p h a t e c r y s t a l structure r e s u l t i n g f r o m exposure to t h e aqueous p a i n t system. T h e changes w h i c h are b r o u g h t about b y e l e c t r o d e p o s i t i o n , h o w e v e r , c a n o n l y b e r e c t i f i e d b y b a k i n g the coated m e t a l at 25 ° C h i g h e r t h a n that r e q u i r e d f o r a s t a n d a r d z i n c p h o s p h a t e treatment.

T h e i m p o r t a n t p o i n t to e m p h a s i z e is that

evaluations of a pretreatment

f o r corrosion protection, w h e n using the

e l e c t r o d e p o s i t i o n process,

s h o u l d b e b a s e d o n coatings

b a k e d over a


60

ELECTRODEPOSITION

b r o a d range of temperatures.

O F COATINGS

T h e differences w h i c h are o b s e r v e d m a y

not b e the result of a greater degree of c u r e of t h e c o a t i n g at a h i g h e r t e m p e r a t u r e b u t rather a c h a n g e i n t h e n a t u r e of t h e p h o s p h a t e c r y s t a l structure.


Experimental A schematic of t h e e x p e r i m e n t a l setup is s h o w n i n F i g u r e 14. T h e a p p a r a t u s consists of a s t a n d a r d c a l o m e l reference electrode ( S C E ) , a p l a t i n u m counter electrode, a n d the s p e c i m e n w h i c h are c o n n e c t e d t o a potentiostat. T h e potentiostat is i n t u r n c o n n e c t e d to a n X - Y r e c o r d e r to o b t a i n t h e voltage a n d a m p e r a g e measurements.

POTENTIOSTAT S

1

1 r

· !

X-Y RECORDER PLATINUM REFERENCE ELECTRODE SOLUTION STANDARD CALOMEL ELECTRODE

SPECIMEN HOLDER Figure 14.

JACKETED TEMPERATURE CONTROLLED CONTAINER

Schematic for anodic polarization equipment

A f t e r t r e a t i n g a p a r t i c u l a r p h o s p h a t e d surface i n the d e s i r e d f a s h i o n , specimens w e r e p u n c h e d f r o m the p a n e l , a n d o n e w a s m o u n t e d i n a s p e c i a l h o l d e r (10). T h e h o l d e r is s h o w n i n F i g u r e 15. M i c r o s c o p i c e x a m i n a t i o n s h o w e d that the shear p u n c h i n g o p e r a t i o n d i d n o t d a m a g e the p h o s p h a t e crystals i n the test area. F u r t h e r , the p r e p a r e d specimens r e p r o d u c i b l y r e t a i n e d t h e i r p o l a r i z a t i o n characteristics over a p e r i o d of several days. T h e t i m e b e t w e e n s a m p l e p r e p a r a t i o n a n d testing w a s thus not a factor. A f t e r m o u n t i n g the s p e c i m e n i n t h e h o l d e r , i t w a s i m m e r s e d i n t h e e l e c t r o l y t e s o l u t i o n , 0 . 6 M a m m o n i u m nitrate. T h e s o l u t i o n w a s m a i n t a i n e d o x y g e n free b y a h i g h p u r i t y n i t r o g e n sparge a n d t h e r m o s t a t i c a l l y h e l d at 25.0 ± 0.1 ° C S t a r t i n g f r o m a rest p o t e n t i a l of —0.61 v o l t vs. S C E the p o t e n t i a l w a s i n c r e a s e d at a rate of 1.2 volts p e r h o u r i n the



3.

M A Y


Figure 15.

61

Specimen holder

p o s i t i v e ( a n o d i c ) d i r e c t i o n . T h e p o l a r i z a t i o n c u r v e w a s thus o b t a i n e d b y r e c o r d i n g t h e v o l t a g e - a m p e r a g e r e l a t i o n s h i p o n t h e X - Y recorder.

Acknowledgments T h e a u t h o r wishes t o t h a n k J . I. M a u r e r a n d Κ. I. S a a d f o r t h e i r v a l u a b l e t e c h n i c a l discussions, G . J . M c C l u r g a n d M . M . N o t t a g e f o r their part i n the experimental work, a n d R . G . Meisenheimer for the s c a n n i n g electron p h o t o m i c r o g r a p h s .

Literature Cited 1. Nakamura, Y., Komata, K., Nozaki, H . , Bull. Chem. Soc. Japan (1970) 43, 663. 2. May, C. Α., Smith, G . , J. Paint Technol. (Nov. 1968) 40 (526), 494. 3. LeBras, L . R., J. Paint Technol. (Feb. 1965) 38, 85. 4. Smith, G . , May, C. Α., ADVAN. CHEM. S E R . (1970) 92, 140-149. 5. Giboz, J.-P., Lahaye, J., J. Paint Technol. (Sept. 1970) 42 (548), 501. 6. Menzer, W., Product Finishing (June 1968) 21 (6), 92. 7. May, C . Α., J. Paint Technol. (Jan. 1971) 43 (552), 43. 8. Chance, R. L . , France, W . P. Jr., Corrosion-Nace (Aug. 1969) 25 (8), 329. 9. Preliminary Technical Information, R E S : 68:15, Epikote Resin Ester DX-31, Shell Chemicals U.K. Ltd., London, SE 1. 10. France, W . D . Jr., J. Electrochem. Soc. (1967) 114, 818. RECEIVED

May

28,

1971.


Electrodeposition of Coatings

Recommend Documents