Corrosion behaviour and mechanical properties of plated Zn-alloys

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%) of transition metal elements like Co, Ni and Fe. ZnFe and Zn-Co alloys containing up to 1.5% of the transition metal were plated using an alkaline and acidic electrolyte respectively. Zn-Ni alloys with Ni-contents up to 14% 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 when compared to conventional chromated pure zinc layers and other low alloyed zinc layers. These results are confirmed by outdoor exposure tests and electrochemical investigations. As zinc-alloys are supposed to replace cadmium for fastener applications mechanical properties like hardness as well as tribological data are of importance and were investigated. Further, the hydrogen embrittlement of deposited high strength steel was also tested. On the basis of electro-chemical data a dopant-vacancy interaction model is presented for a qualitative explanation of the corrosion behaviour of plated zinc-alloys.