Potentiostatic cleaning and conditioning of the cathodes used in electrodeposition cells
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loy which is t h e r m o d y n a m i c a l l y u n s t a b l e in a given e l e c t r o l y t e s o l u t i o n r e m a i n s v i s i b l y u n c h a n g e d for a p r o l o n g e d p e r i o d . It is m o s t c o m m o n l y o b s e r v e d with a c t i v e / p a s s i v e m e t a l s and a l l o y s ( n o r m a l l y the m e t a l f r e e l y c o r r o d e s at a n a c t i v e p o t e n t i a l but can be m a d e p a s s i v e ) . P a s s i v i t y g r e a t l y r e d u c e s the n o r m a l r a t e of c o r r o s i o n of the m a t e r i a l . O c c a s i o n a l l y , the p a s s i v e s t a t e can be u n s t a b l e ( g e n e r a l l y f r o m c h e m i c a l l y i n duced p a s s i v i t y ) , and o t h e r m e a n s such as e l e c t r o l y t i c a l l y i n d u c e d p a s s i v i t y m u s t be e x a m i n e d to p r o v i d e a m o r e p e r m a n e n t c o n t r o l l e d s t a t e of p a s s i v i t y . F i g . 1 s c h e m a t i c a l l y i l l u s t r a t e s the e s s e n t i a l b e h a v i o r of a t y p i c a l a c t i v e / p a s s i v e m e t a l . C o n s i d e r i n g f i r s t the h e a v i l y l i n e d curve, it is o b s e r v e d that the m e t a l r e s p o n d s m u c h like any of the n o n p a s s i v a t i n g m e t a l s and a l l o y s . As the p o t e n t i a l of the m e t a l is m a d e m o r e p o s i t i v e , the m e t a l follows what is e s s e n t i a l l y an a c t i v a t i o n p o l a r i z a t i o n c u r v e (anodic d i s s o l u tion r a t e i n c r e a s e s , M ~ M § + e-). T h i s zone of p o t e n t i a l s is called the a c t i v e r e g i o n . If the p o t e n t i a l of the metal is made m o r e positive, a sudden reduction in c u r r e n t is o b s e r v e d . The p o t e n t i a l at which this e v e n t o c c u r s is called p r i m a r y p a s s i v e p o t e n t i a l (Epp), and the c o r r e s p o n d i n g c u r r e n t is called c r i t i c a l c u r r e n t (Icrit). The r a n g e of p o t e n t i a l s which b e c o m e i n c r e a s i n g l y m o r e p o s i t i v e but c a u s e e s s e n t i a l l y no i n c r e a s e i n c u r r e n t , c o v e r s the r e g i o n c a l l e d p a s s i v e . A d d i t i o n a l i n c r e a s e in p o s i t i v e p o t e n t i a l f o r c e s the m e t a l into a r e g i o n c a l l e d t r a n s p a s s i v e . The p o t e n t i a l at that p o i n t is the c r i t i c a l p o t e n t i a l (Ecrit). W i t h i n the p a s s i v e r e g i o n t h e r e is g e n e r a l l y a wide r a n g e of p o t e n t i a l s which can be s e l e c t e d for a u t o m a t i c a l l y c o n t r o l l i n g the p a s s i v e s t a t e of the m e t a l . T h i s p o t e n t i a l is c a l l e d s e t - p o i n t p o t e n t i a l (Esp). C o n s i d e r i n g the fine lined c u r v e (M § + e- ~ M), the m e t a l t r u l y follows the a c t i v a t i o n p o l a r i z a t i o n c u r v e and the m o r e n e g a t i v e the a c t i v e / p a s s i v e m e t a l ' s pot e n t i a l b e c o m e s the g r e a t e r the r e d u c t i o n
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