Evaluation of the Potential Range for Sacrificial Mg Anodes for the Cathodic Protection of Mg Alloy AZ31B-H24

Corrosion protection of Mg and its alloys remains a challenge hindering widespread application. The propensity for Mg alloy AZ31B-H24 (Mg-3Al-1Zn) to be protected via sacrificial anodic based cathodic protection was evaluated. Potentiostatic polarization of AZ31B-H24 in 0.6 M NaCl and 0.1 M MgCl₂ revealed that a wide range of potentials for practical or kinetic-based sacrificial cathodic protection is possible for Mg alloys but a practical range of potentials for substantial cathodic protection in 0.1 M TRIS was not observed. In addition, residual corrosion rates at potentials cathodic to their open-circuit potential were greater than expectations from electrochemical mixed potential corrosion theory interpreted to be a result of the negative difference effect. Zero resistance ammeter (ZRA) testing was performed on couples of AZ31B-H24 to commercially pure (CP) Mg and Mg alloy WE43B-T5 (Mg-4Y-3RE) to validate the range of protection for AZ31B-H24 and to evaluate their suitability for the protection of AZ31B-H24. Gravimetric mass loss of AZ31B-H24 from ZRA coupled samples revealed only a 2× to 12× decrease in anodic charge density compared to that at open-circuit potential. Coupled WE43B possessed a lower self-corrosion fraction than coupled CP Mg because it possesses a larger potential difference between its open-circuit potential and galvanic couple potential. Anode-cathode polarity reversal was not observed for couples of AZ31B-H24 to CP Mg and WE43B, but further ZRA testing did reveal polarity reversal of laser surface modified AZ31B-H24 coupled to base AZ31B-H24.