3 Changes in Zinc Phosphated Steel Surfaces during Electrodeposition
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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
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
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
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
o v e r the
50
ELECTRODEPOSITION
OF
COATINGS
PRETREATMENT A PRETREATMENT Β PRETREATMENT C
\
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.
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)
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
3.
M A Y
51
Zinc Phosphated Steel Surfaces
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.
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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.
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
52
ELECTRODEPOSITION
O F COATINGS
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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
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
-0.6
3.
M A Y
53
Zinc Phosphated Steel Surfaces
20,
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
.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
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
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
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
-0.6
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
3.
M A Y
Figure 9.
55
Zinc Phosphated Steel Surfaces
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
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
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.
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
3.
M A Y
57
Zinc Phosphated Steel Surfaces
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
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
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
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
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
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
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
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3.
M A Y
Figure 13.
59
Zinc Phosphated Steel Surfaces
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
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
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.
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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
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
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3.
M A Y
Zinc Phosphated Steel Surfaces
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.
In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.