An exploratory study of the corrosion of Mg alloys during interrupted salt spray testing

A first systematic investigation was carried out to understand the corrosion of common Mg alloys (Pure Mg, AZ31, AZ91, AM30, AM60, ZE41) exposed to interrupted salt spray. The corrosion rates were also evaluated for these alloys immersed in 3 wt.% NaCl by measuring hydrogen evolution and an attempt was made to estimate the corrosion rate using Tafel extrapolation of the cathodic branch of the polarisation curve. The corrosion of these alloys immersed in the 3 wt.% NaCl solution was controlled by the following factors: (i) the composition of the alpha-Mg matrix, (ii) the volume fraction of second phase and (iii) the electrochemical properties of the second phase. The Mg(OH)₂ surface film on Mg alloys is probably formed by a precipitation reaction when the Mg²⁺ ion concentration at the corroding surface exceeds the solubility limit. Improvements are suggested to the interrupted salt spray testing; the ideal test cycle would be a salt spray of duration X min followed by a drying period of (120-X) min. Appropriate apparatus changes are suggested to achieve 20% RH rapidly within several minutes after the end of the salt spray and to maintain the RH at this level during the non-spray part of the cycle. The electrochemical measurements of the corrosion rate, based on the “corrosion current” at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.