Zinc-Alloy-Plating for Corrosion Protection

Zinc represents the most effective and economic corrosion protection especially for steel base materials. Improved corrosion resistance can be achieved by alloying zinc with small amounts (around 1 Yo) of transition metal elements like Co, Ni and Fe. Zn-Fe and Zn-Co alloys containing up to 1.5 Yo of the transition metal were plated using an alkaline and acidic electrolyte respectively. Zn-Ni alloys with Nicontents up to 14 YO were deposited on steel substrates using both types of electrolytes. The very homogeneous texture of the Zn-Fe-alloys proved by X-ray investigations obviously leads to the formation of a very uniform and protecting chromate layer in the chromatizing process. Corrosion tests in the salt spray cabinet show a superior behaviour of chromated Zn-Fe-layers after heat treatment i f compared to conventional chromated pure zinc layers and other low alloyed zinc layers. These results are confirmed by outdoor exposure tests and electrochemical investigations. On the basis of electrochemical data a dopant-vacancy interaction model is presented for a qualitative explanation. The corrosion current of plated Zn-alloys is reduced when the charge carriers, possibly mobile negatively charged Zn cation vacancies, are eleminated. The segregated alloy elements can combine with such vacancies, and scavenge further vacancies by forming immobile ion pairs.