Sources of variability in cation analysis for a quantitative exfoliation corrosion resistance test

Exfoliation corrosion of some high-strength aluminum alloys results in delamination of the metal surface resulting in a reduction of mechanical properties. The susceptibility is a strong function of the metallurgical condition of the material. Artificial aging treatments leading to higher strengths generally lead to higher susceptibility to exfoliation. The results of a new, quantitative test for exfoliation resistance based upon solution analysis of dissolution products are presented. Capillary electrophoresis (CE) has been used to quantify the Mg2+ and Zn2+ leached by 10 mM HCl from the thermal oxide of aluminum alloy AA7075 artificially aged to different degrees. An empirical relationship was found between the ratio of Mg2+ to Zn2+ dissolved during the test and the susceptibility of material to exfoliation. The development of the test is discussed with a focus on an analysis of its reproducibility and sources of variance. The primary sources of variability were found to be spatial variations in the composition of the thermal oxide and inhomogeneities in the test solution.