13 Effect of Acid Rain on Exterior Anodized Aluminum Automotive Trim Gardner Haynes and Robert Baboian Downloaded by IMPERIAL COLLEGE LONDON on February 18, 2015 | http://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch013
Texas Instruments Incorporated, 34 Forest Street, Attleboro, M A 02703
Initial attempts to use aluminum for automotive trim were unsuccessful due to the corrosion behavior of the metal. It is therefore anodized for automotive trim applications to provide a protective oxide surface which acts as a barrier coating for corrosion protection. Aluminum and its alloys are susceptible to pitting and crevice corrosion in chloride containing environments. The corrosion resistance of anodized aluminum is therefore highly dependent on the quality of the anodized surface and the absence of scratches and other damage sites. The anodizing process consists of: 1) trim forming, 2) cleaning, 3) anodic electrolysis usually in sulfuric acid and, 4) sealing in an aqueous bath. Each of these steps are important, for example, the anodized oxide layer must be controlled to provide an optimum thickness. Sealing the anodized layer is also very important and must be complete. Corrosion problems associated with anodized aluminum trim include etching of the oxide film (known as "blush and bloom") and pitting of the aluminum. 1,2
P i t t i n g o f aluminum o c c u r s a t d e f e c t s i n t h e anodized c o a t i n g due t o poor p r o c e s s c o n t r o l o r m e c h a n i c a l f o r c e s i n s e r v i c e such as stone pecking.3,4 i c h a r a c t e r i z e d by a c e n t r a l p i t which i s surrounded w i t h a w h i t e h a l o o f c o r r o s i o n product. P i t t i n g i s more p r e v e l a n t i n h i g h c h l o r i d e c o n t a i n i n g environments. B l u s h a n d bloom a r e caused by the e t c h i n g o f t h e a n o d i z e d o x i d e which changes t h e s u r f a c e r e f l e c t i v i t y o f t h e t r i m material. P r o b l e m s o f a m i l k y w h i t e a p p e a r a n c e ( b l u s h and bloom) on t h e s u r f a c e o f a n o d i z e d aluminum t r i m have f r e q u e n t l y been r e p o r t e d . However, t h i s e f f e c t has been more pronounced i n r e c e n t y e a r s i n t h e N o r t h e a s t USA a n d C a n a d a . A l t h o u g h i n p r e v i o u s y e a r s t h e p r o b l e m h a s been r e l a t e d t o q u a l i t y v a r i a t i o n s , recent data i n d i c a t e s t h a t a c i d p r e c i p i t a t i o n enhances t h i s e f f e c t . T h i s i n v e s t i g a t i o n was conducted t o determine t h e r e l a t i v e s
0097-6156/86/0318-0213506.00/ 0 © 1986 American Chemical Society
In Materials Degradation Caused by Acid Rain; Baboian, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
M A T E R I A L S D E G R A D A T I O N C A U S E D BY A C I D RAIN
e f f e c t s o f a c i d p r e c i p i t a t i o n and road s a l t usage on c o r r o s i o n o f a n o d i z e d aluminum t r i m . A second o b j e c t i v e was t o d e v e l o p an a c c e l e r a t e d t e s t w h i c h c o u l d d u p l i c a t e the mechanisms o f c o r r o s i o n and s e p a r a t e the e f f e c t s o f a c i d p r e c i p i t a t i o n f r o m t h o s e o f road s a l t usage.
Downloaded by IMPERIAL COLLEGE LONDON on February 18, 2015 | http://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch013
Experimental A n o d i z e d aluminum t e s t p a n e l s (4 i n c h by 6 i n c h ) were c u t from automotive r o c k e r p a n e l s purchased a t a l o c a l d e a l e r s h i p . B e f o r e t e s t i n g c u t e d g e s were masked w i t h a p o t t i n g compound t o e l i m i n a t e edge e f f e c t s . T e s t s o l u t i o n s were p r e p a r e d from r e a g e n t grade c h e m i c a l s and d i s t i l l e d w a t e r . A p p r o p r i a t e s a l t c o n c e n t r a t i o n s were o b t a i n e d u s i n g a m i x t u r e o f e q u a l w e i g h t s o f NaCl and C a C l . The pH o f the t e s t s o l u t i o n s was a d j u s t e d w i t h a m i x t u r e o f t h r e e p a r t s H SOi| and one p a r t HNOo. The r e s u l t i n g m a t r i x o f test s o l u t i o n s had a range o f t o t a l s a l t c o n c e n t r a t i o n o f 0 t o 20 p e r c e n t and a pH r a n g e o f 0.2 t o 7. Triplicate test p a n e l s were t e s t e d f o r each c o n d i t i o n w i t h i n t h e m a t r i x . The MIST t e s t c o n s i s t e d o f a s p r a y t o t h e h o r i z o n t a l t r i m t e s t p a n e l e v e r y two hours f o r 3 c y c l e s and s u b s e q u e n t d r y i n g f o r 18 h o u r s . The t e s t p a n e l s were e v a l u a t e d f o r degree o f b l u s h and bloom and p i t t i n g d a i l y . 2
2
R e s u l t s and D i s c u s s i o n R e s u l t s i n T a b l e I show t h a t MIST t e s t road s a l t s p r a y a l o n e d i d not produce b l u s h and bloom a l t h o u g h p i t t i n g was observed a t t h e h i g h e r s a l t c o n c e n t r a t i o n s . A c i d p r e c i p i t a t i o n had a v e r y p r o n o u n c e d a f f e c t a t o r b e l o w pH=4. T h i s pH r a n g e i s e a s i l y o b t a i n e d on a u t o m o b i l e s due t o a d r o p i n pH o f the s o l u t i o n as a r e s u l t o f e v a p o r a t i o n as w e l l a s t h e f a c t t h a t t h e pH o f p r e c i p i t a t i o n h a s b e e n l e s s t h a n 4 o n many o c c a s i o n s . The appearance o f t h e t e s t p a n e l s was s i m i l a r t o t h a t o b s e r v e d on v e h i c l e s a f t e r one or two y e a r s i n M o n t r e a l or a f t e r s e v e r a l y e a r s i n m i l d e r environments such a s D e t r o i t or Boston. The c o m b i n a t i o n o f a c i d p r e c i p i t a t i o n w i t h road s a l t s p r a y produced the worst e f f e c t s on a n o d i z e d aluminum. As t h e MIST t e s t pH became more a c i d i c and the amount o f s a l t i n c r e a s e d the time t o a c h i e v e an e q u i v a l e n t m i l k y w h i t e a p p e a r a n c e was r e d u c e d s i g n i f i c a n t l y ( T a b l e I ) . A t more n e u t r a l h i g h c h l o r i d e c o n c e n t r a t i o n s s e v e r e p i t t i n g o c c u r r e d w h i l e a t more acid high chloride c o n c e n t r a t i o n s b l u s h and bloom predominated. The l a t t e r e n v i r o n m e n t i s s i m i l a r t o t h a t e x i s t i n g i n t h e N o r t h e a s t USA and Canada and, t h e r e f o r e , the r e s u l t s can e x p l a i n the problems o f b l u s h and b l o o m i n t h e s e areas. C o r r o s i o n s u r v e y s by a u t o m o t i v e companies and t r i m p r o d u c e r s i n t h e s e a r e a s have shown t h a t b l u s h and b l o o m and p i t t i n g have become i n c r e a s i n g l y more s e v e r e over the l a s t t e n years.5 These problems have l e d t o a s h i f t away from a n o d i z e d aluminum as an automotive t r i m m a t e r i a l i n r e c e n t y e a r s .
In Materials Degradation Caused by Acid Rain; Baboian, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
HAYNES A N D BABOIAN Table Mist
I. Test
Exterior Anodized Aluminum Automotive Trim
Blush
and Bloom
o f Anodized
(Days t o E q u i v a l e n t
Milky
Aluminum
During
Appearance)
ω 7 . 0
th
ΓΗ
•H Ό Λ>1
4 . 0
Pitting
and Blush
and
Bloom
CO
Ο
ω
2 . 0
12
8
1 0
12
8
1 . 0
6
4
2
1
1
0 . 5
6
3
2
1
1
0 . 2
6
3
2
1
1
2
5
ίϋ •η T5
Downloaded by IMPERIAL COLLEGE LONDON on February 18, 2015 | http://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch013
