TRANSITION-METAL EFFECTS IN THE CORROSION PROTECTION OF ELECTROPLATEDZINC ALLOY COATINGS

The surface segregation and corrosion protection effects of d-metal al loying components like nickel cobalt, or iron in zinc coatings without chromatizing treatment have been investigated by XPS, salt spray test s, potentiodynamic and galvanic current measurements. The corrosion in hibiting function of the alloy d-metals components in zinc coatings st rongly depends on the pH of the contacting aqueous medium. In acidic e nvironment, inhibition of self corrosion and sacrificial dissolution i n contact with steel is negligible. In unbuffered neutral and alkaline contact, however, alloyed passive layers with segregated d-metals are formed. In this case, the transition metal dopants show inhibiting ef fects on the anodic dissolution. A dopant-vacancy interaction model is presented for a qualitative explanation. The corrosion current is red uced when the charge carriers, possibly mobile negatively charged Zn c ation vacancies, are eliminated. The segregated alloy elements can com bine with such vacancies, and scavenge further vacancies by forming im mobile ion pairs.