Influence of Cl^- on the stability of surface films on the β phase model alloy of magnesium-aluminum alloys

The stability and semiconductive property of surface films formed on the β phase model alloy were investigated by means of electrochemical impendence spectroscopy(EIS) and Mott-Schottky approach in solutions containing Cl^-.The results indicate that the increase of Cl^-concentration may accelerate the formation and active dissolution of the films on the surface of β phase,leading to the deterioration of film stability.This phenomenon can be attributed to the fact that the semiconductive type of the films of β phase is P-type in case of solutions with low Cl^-concentration.As P-type semiconducting films are cationic conductor films,it is difficult for Cl^-to penetrate in a way of transportation and diffusion.When the Cl^-concentration increases,the semiconductive type of the films may turn into N-type where Cl-migrates or diffuses in the films,penetrates through the bottom of the films,causes continuous corrosion of the substrate and finally the surface films break down.