Sheradizing Process And Its Effect On Microstructure And Corrosion Resistance

Sheradizing is a promising new coating preparation method in recent years, the corrosion resistance of which is excellent. The thickness of sheradizing layer is uniform and easy to control. In this paper, a new kind of penetrating agent was obtained. The effect of sheradizing temperature, holding time and penetrating agent on the thickness and the microstructure of the sheradizing layer were studied. And the corrosion resistance and mechanism of sheradizing and zinc-aluminized were also studied.The penetrating agent was determined through many tests. The results showed that the sheradizing layer thickness which is uniform and bright is increased with this new agent.It is found that the sheradizing coating thickness increases with the increase of temperature and holding time. The effect of the temperature was greater than the holding time. The grain of sheradizing surface, the microstructure of which is coarse granular, grew up with the increase of temperature and holding time. And a more uniform and compact microstructure was obtained. The phase composition from inside to the outside were Γ(Fe3Zn10), Γ1(Fe11Zn40) and δ(FeZn7) respectively.The reaction speed of zinc-aluminized was much higher than the sheradizing. The surface microstructure of zinc-aluminized was granule and flake. Fe2Al5 was formed as a phase with temperature increase. The phase composition from inside to the outside were Γ(Fe3Zn10), Γ1(Fe11Zn40) and delta(FeZn7) respectively, while FeAl and Fe2Al5 uniformly dispersed in the coating.The corrosion products of sheradizing coating after the neutral salt spray test were ZnO, Zn(OH)2, Zn5(OH)8Cl2H2O and FeZn7. A new corrosion product Fe OCl, which can accelerate the corrosion of sheradizing coating, was formed. The corrosion products of zinc-aluminized after the neutral salt spray test were ZnO, Zn(OH)2, Zn5(OH)8Cl2H2O,ZnAl2O4, FeOCl and FeZn7. The zinc-aluminized coating has better corrosion resistance than sheradizing due to the formation of a dense ZnAl2O4 layer on the outside of the coating which can slow down the corrosion rate.