ELECTROCHEMICAL POLARIZATION BEHAVIOUR OF Mg-Al ALLOYS IN NEAR-NEUTRAL SOLUTIONS

Abstract

<p>A study has been conducted in mildly aggressive saline solutions to indentify subtle yet important difference in the anodic and cathodic process of three Mg-Al alloys, AZ31B, AM30 and AM60B in a partially passive state and a localized corrosion state. The influence of metallurgical factors and environment variables on the corrosion resistance and surface film breakdown process has been investigated using potentiodynamic and potentiostatic tests combined with optical microscopy.</p> <p>All three Mg-Al alloys corroded in a partially protected state under open circuit conditions in the test solution and the surface film formed on each exhibited a similar breakdown potential. This indicates that metallurgical factors such as alloying additions and the presence of the β-phase (Mg₁₇Al₁₂) did not significantly influence the surface film breakdown process. AM60B exhibited improved corrosion resistance at potentials below the breakdown potential due to the formation of a more protective surface film. The β- phase, however, did not strongly influence either the anodic process at potentials above the breakdown potential or the cathodic process. It was determined that increasing the alloy Al content increases the corrosion potential of Mg-Al alloys, but also increases the risk of localized corrosion. The similar anodic and cathodic polarization behaviour exhibited by AZ31B and AM30 indicates that a 1 wt % Zn alloying addition does not strongly influence the corrosion resistance of these alloys. The die-cast skin of AM60B exhibited better corrosion resistance than the interior at potentials below the breakdown potential due to the higher fraction of Al-rich β-phase, which improved the protective ability of the surface film. The semi-continuous β-phase network did not provide a strong micro-galvanic activity to drive anodic dissolution of the α-phase, therefore, did not significantly affect the corrosion resistance above the breakdown potential. The crystallographic texture exhibited by the extruded AM30 did not strongly affect the short-term and long-term corrosion resistance. AM30 showed a relatively steady passive state during long time exposures in a room temperature 0.01 M NaCl solution.</p>