Influence of chloride, sulfate and bicarbonate anions on the corrosion behavior of AZ31 magnesium alloy

Influence of chloride, sulfate and bicarbonate anions on the corrosion behavior of AZ31 magnesium alloys in dilute solutions was investigated by electrochemical measurements, whereby corrosion maps in terms of electrode potential and solution concentration were obtained. AZ31 alloy exhibited corrosion and passivation zones in those dilute solutions. The passivation zone was strongly influenced by the kinds of the solution used and the broadest zone was obtained in Na 2SO 4, followed by NaCl and NaHCO 3. AZ31 alloy had a better corrosion resistance in Na 2SO 4, attributing to the lowest attack by SO 4 2− and a thick corrosion product covering the surface which inhibited the corrosion attack. However, despite a much lower passivation zone in NaHCO 3 as compared to the other two solutions, the corrosion rate in NaHCO 3 was found to be the lowest. The values of open-circuit potential were in the passivation zone when [Cl −] was less than 0.2 mol/L in NaCl, and [SO 4 2−] and [HCO 3 −] were both less than 0.3 mol/L in Na 2SO 4 and NaHCO 3. In the passivation zone, the formation of surface films or corrosion products on the material surface retarded further corrosion in dilute solutions. In the corrosion zone, pits grew continuously in depth and diameter in NaCl and Na 2SO 4 resulting in the localized corrosion through the agglomeration of pits. The formation of crystalline-like MgCO 3 products exhibited the general corrosion in NaHCO 3.