The statistical evaluation of susceptibility of AZ91 magnesium alloy to pitting corrosion

In this paper susceptibilities to pitting corrosion of the as-cast AZ91 magnesium alloy and the homogenized and quenched AZ91 alloy were compared. The heat treatment of AZ91 alloy, i.e. homogenization, resulted in dissolution of an intermetallic compound Al12Mg17 (beta-phase) in a solid solution of aluminum in magnesium (alpha-phase). Subsequent quenching of the homogenized alloy caused formation of non-equilibrium, supersaturated solid solution of aluminum in magnesium. Electrochemical measurements were performed in a solution of 0,1 mol . dm(-3) sodium chloride saturated with Mg(OH)(2). Statistical methods revealed that the pitting potentials and the differences between pitting and corrosion potentials were significantly different for AZ91 magnesium alloy in two investigated states. A higher value of the pitting potential for AZ91 magnesium alloy in the as-cast state comparing to the pitting potential of the homogenized and quenched alloy indicated that the as-cast alloy is less susceptible to the pitting corrosion than the homogenized one. The similar conclusion was reached upon the analysis of the differences between pitting and corrosion potentials. The observed differences in pitting potentials can be explained assuming that the passive layer forming on the as-cast alloy is much more robust than the passive layer existing on the homogenized and quenched alloy.