CORROSION BEHAVIOR AND MECHANICAL-PROPERTIES OF PLATED ZN-ALLOYS

Zinc represents the most effective and economic corrosion protection e specially for steel base materials. Improved corrosion resistance can be achieved by alloying zinc with small amounts (around 1%) of transit ion metal elements like Co, Ni and Fe. ZnFe and Zn-Co alloys containin g 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 electrolyte s. The very homogeneous texture of the Zn-Fe-alloys proved by X-ray in vestigations obviously leads to the formation of a very uniform and pr otecting chromate layer in the chromatizing process. Corrosion tests i n the salt spray cabinet show a superior behaviour of chromated Zn-Fe- layers after heat treatment when compared to conventional chromated pu re zinc layers and other low alloyed zinc layers. These results are co nfirmed by outdoor exposure tests and electrochemical investigations. As zinc-alloys are supposed to replace cadmium for fastener applicatio ns mechanical properties like hardness as well as tribological data ar e of importance and were investigated. Further, the hydrogen embrittle ment 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 zi nc-alloys.