Investigation of microcracks on conversion film of AZ80 Mg alloy

Abstract

Chemical conversion films are widely used to improve the corrosion resistance of Mg alloys. However, abundant microcracks can be found on the surface of conversion film using scanning electron microscopy (SEM) observation. The phosphate conversion film was dried by three methods in this study, thereafter the formation reasons of microcracks and their effect on the corrosion resistance were investigated using SEM and optical morphology observations, and electrochemical measurements. The results indicate that the conversion film dried at high temperature or vacuuming during SEM measurements can accelerate the formation of microcracks. Especially, the microcracks observed using SEM can be attributed to the effect of vacuuming instead of showing the original status. Corrosive mediums are susceptible to preferentially absorb on the microcracks, further to form localised corrosion. It is better to dry the conversion films at room temperature instead of high temperature during the actual applications.