TRANSITION-METAL EFFECTS IN THE CORROSION PROTECTION OF ELECTROPLATED ZINC ALLOY COATINGS

The surface segregation and corrosion protection effects of d-metal alloying components like nickel cobalt, or iron in zinc coatings without chromatizing treatment have been investigated by XPS, salt spray tests, potentiodynamic and galvanic current measurements. The corrosion inhibiting function of the alloy d-metals components in zinc coatings strongly depends on the pH of the contacting aqueous medium. In acidic environment, inhibition of self corrosion and sacrificial dissolution in 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 effects on the anodic dissolution. A dopant-vacancy interaction model is presented for a qualitative explanation. The corrosion current is reduced when the charge carriers, possibly mobile negatively charged Zn cation vacancies, are eliminated. The segregated alloy elements can combine with such vacancies, and scavenge further vacancies by forming immobile ion pairs.