Rapid Corrosion Performance Evaluation of Metallic Alloys by Electrochemical Polarisation Technique

A rapid corrosion performance evaluation technique would be of considerable use to automotive sector especially during the alloy development phase. The repeatability of electrochemical polarisation tests for magnesium alloys in aqueous solutions has been investigated in this study. The investigation includes pure Mg and two magnesium alloys (AM60 and AZ91D), various pH's (both buffered and unbuffered) and chloride concentrations. In unbuffered solutions AM60 showed extreme scatter in corrosion rate relative to AZ91D, especially at pH 4 and pH 10. In general, as the salt concentration increased the relative corrosion rate of AM60 with respect to AZ91D dropped, though at all times it was greater than AZ91D. At pH 4, the greatest spread in corrosion rates of AM60 was observed at all chloride concentrations. In buffered solutions the extreme scatter found in the case of unbuffered solutions was absent, thus suggesting that use of buffered solutions offered a more reproducible analysis by reducing the local pH change that occurs near the working electrode during polarisation. In a buffered solution of pH 6 and 0.1% NaCl concentration, pure Mg, AM60 and AZ91D showed the greatest spread in relative corrosion rates, suggesting this regime to be optimal for further evaluations.