Environmental effects on selective dissolution from ZnAlMg alloy under low frequency alternating current perturbations

Abstract

Real time measurement of elemental dissolution rates by atomic emission spectroelectrochemistry under alternating current (AC) perturbations of ZnAlMg alloy was used to precise mechanisms controlling selective dissolution as a function of pH in chloride containing solutions in the presence of NH4+. At pH values of 8.5 and 10.5 Zn and Mg dissolution currents (jZn, jMg) followed the total current je* respecting the stoichiometry of Zn2Mg at the maximum of anodic current in solution at pH 8.5. Al dissolution current (jAl) followed je* at pH 10.5 and 8.5 with a phase shift of 180° further called cathodic Al dissolution. No correlation between AC perturbations and Al dissolution was seen at pH 8.5. At pH 13 anodic Zn dissolution, cathodic Al dissolution and no Mg dissolution were observed. The elemental dissolution rates of Al at pH 8.5 and Mg at pH 13 did not vary with AC perturbations. The detected concentrations of Al at pH 8.5 and Mg at pH 13 correlated well with the saturation concentrations of Al3+ and Mg2+ in studied electrolytes expected from solubility constants. Thus, their dissolution was controlled by the solubility of the surface films observed by infrared and Raman spectroscopy. The finding implies the separation of cathodic Al dissolution, and anodic Zn and Mg dissolution in the appropriate equivalent circuit for an adequate analysis of conventional EIS data.