Threshold Chloride Concentration for Passivity Breakdown of Mg-Zn-Gd-Nd-Zr Alloy （EV31A） in Basic Solution

Mg-Zn-Gd-Nd-Zr alloy （EV31A） is a heat-treatable magnesium （Mg） cast alloy that can be used up to 200 ℃ for automobile and aerospace applications. This alloy has excellent mechanical properties （ultimate tensile strength： 280 MPa at room temperature, and -230 MPa at 200 ℃） and improved corrosion resistance. Electrochemical corrosion studies were conducted on this alloy under different heat treatment conditions in 0.1 M NaOH solution with the addition of 0-1000 ppm of chloride. The alloy showed excellent passivity in the 0.1 M NaOH solution. The passive potential range typically extended to more than 1.2 VAg/AgCl. The transpassive potential was observed to be dependent on heat treatment condition of the alloy. More than 80 ppm of chloride was required to induce passivity breakdown in any heat treatment condition. Peak aging at 200 ℃ for 16 h imparted better resistance for localized corrosion than other heat-treated con- ditions. The alloy in the as-received condition showed the highest passivation kinetics due to its smaller grain size that possibly increased the diffusion of reactive elements to form protective oxide. The passive film of the EV31A alloy showed n-type semiconductivity with a charge carrier density of - 2 × 10^21 cm^- 3 with no chloride addition. The charge carrier density increased with chloride addition in the electrolyte which could be correlated with the susceptibility to localized corrosion.