The Preparation Of The Vacuum Plating Zinc Magnesium Alloy Strip And Corrosion Mechanism Research

Corrosion resistance is an important research field in the steel industry. To plate a protective coating on steel is a major approach to resist corrosion. As a new type of zinc-base alloys coatings, zinc-magnesium alloys coating has drawn more and more attention. Because coatings prepared by vacuum plating have better adhesion, compactivity, machinability and corrosion resistance than those prepared by traditional wet plating and hot-dip plating, vacuum plating is becoming a competitive and promising preparation method. Therefore, studies on vacuum plating and properties of coating prepared by this means have great scientific significance and huge potential application value.Vacuum evaporation was applied to deposit magnesium on galvanized steel in this dissertation. Fast annealing was then used to prepare Zn-Mg alloys. The research approach is as below:components and organization structures of Zn-Mg alloys coating were studied; the alloying process under thermal treatment was analyzed; the corrosion resistant properties were tested; and the corrosion mechanism was explored. The main conclusions are:(1) Conclusion of the studies on the thermal treatment:Zn-Mg alloys cannot be formed at a relatively low temperature; with the increase of the temperature, more Zn-Mg alloys are formed; Fe-Zn alloys can be formed at a relatively high temperature. The annealing time dependency follows the similar rule, that is, as the annealing proceeds, alloy types and grain orientations increase, and Fe-Zn alloys are formed.(2) Conclusion of the studies on diffusion:magnesium is enriched within zinc layer during the diffusion. Zn-Mg alloys are formed during the mutual diffusion. The magnesium enrichment has barrier effect, which prohibits inward diffusion of magnesium and outward diffusion of iron.(3) Conclusion of the salt spray test:there is an optimal annealing time, which results in approximately7-fold hours for the formation of the red rust; when annealing time is relatively long, with the increase of the temperature, the red rust is formed at a faster rate due to more Fe-Zn alloys at sub-surface.(4) Conclusion of the electrochemical experiment:corrosion process of Zn-Mg alloys coating consists of three stages-the formation of the passive layer, the dissolution of the passive layer and the dissolution of the zinc layer. When substantial magnesium exists on the surface, there is a stage of the dissolution of the magnesium before the other three stages. The studies on corrosion mechanism illustrate that the passive layer consists of magnesium oxide, magnesium hydroxide and zinc hydroxide, and the depletion of Zn-Mg alloys causes the decomposition of the passive layer. The main reason of the enhanced corrosion resistance is the formation of the passive layer.