Studies On Corrosion Behaviour And Mechanism Of Galvannealed Coating

The galvannealed steel sheets (GA) have been wildly used in the home appliance, automobile and construction industry because its good corrosion resistant, better paintability and good weldability. In recent years, the demand for high corrosion resistance GA has increased up because the rapid development of automobile industry. The service situation of GA has been faced to severe challenges by the environmental pollution and greenhouse effect. Although a wealth of literature have reported that the corrosion processes and characteristic of the hot dip galvanized(GI) coating in different environment situation, there is little research in the corrosion resistance of GA, including the relationship of alloying process, composition and corrosion resistion, degree of deformation of GA and corrosion performance, and in situ corrosion.This article have studied the effect of structure and corrosion property of GA by the aluminum content in the liquid zinc and heating time. The GA could be made to corresponding components by undergoing deformation. Electrochemical corrosion resistance has been study on tensile and cup drawing samples. The dissolution features of GA have been concluded by chemical stripping experiments by means of observing the corrosion morphology changes. In-situ corrosion experiment under liquid has been take place on GA samples by atomic force microscopy, in order to research the corrosion mechanism. Some achievements had been obtained through the above study.(1) The GA with fully alloying and right proportion of Zn-Fe metal phase will show a better ability to resist corrosion. The equilibrium potential and thermodynamic stability would be better when the Fe element increase up in Zn-Fe metal phase. GA has a better corrosion resistance than GI.(2)There was a greater effect in cup drawing test than tensile deformation, it can be concluded that net cracks have greater impact than linear crack on the corrosion resistance of coating.(3) Due to the residual stress, these compression structures of GA and GI coating which were formed in the temper rolling process would first begin to corrosion in the corrosion medium.(4) The binary alloy phase of micron or nanoscale would take place corrosion on the tips of samples in the corrosion medium, therefore the surface roughness will become smaller. The metal corrosion was not only related to its equilibrium potential, but also the surface morphology. The dissolving tendency of flat and smooth surface morphology was smaller than the rough topography.