Theory for the Enrichment Limit Associated with Noble Metal Impurities on Corroding Mg Anodes

Abstract

Magnesium (Mg) alloys are becoming increasingly popular for weight reduction in the automotive industry due to their low density and high strength to weight ratio [1, 2]. However, the deleterious intrinsic corrosion rates of Mg alloys have hindered their widespread application. The low corrosion potentials (< 1.4 VSCE [3]) of Mg alloys result in rapid dissolution when galvanically coupled to more noble structural alloys such as aluminum or steel. Mg when anodically polarized exhibits increasing rates of the hydrogen evolution reaction (HER) contrary to the notions of conventional mixed potential theory. Several theories for this anomalous behavior exist, including noble metal enrichment, however, no theory has reached a consensus [4, 5]. Recently we have shown that Fe enriches on the metal surface of a high purity Mg (80 ppmw Fe) anode after a period of anodic polarization [6]. In addition, cathodic potentiodynamic polarization scans revealed increased cathodic kinetics on samples which had undergone different doses of prior anodic dissolution when compared to samples which had undergone no prior dissolution thus demonstrating that areas of Fe enrichment can support increased cathodic kinetics. However, the cathodic reaction rates reached an approximate limiting current density and didn’t increase further with anodic dissolution suggesting a limit to enrichment. Recently a simple model for the enrichment of Fe was proposed by Lysne et al. [7] assuming 100% Fe enrichment efficiency on a corroding Mg anode. In terms of this model, Fe enrichment on the surface of an Mg anode is not 100% efficient and indeed far below theoretical estimations. The work presented discusses several concepts that could limit enrichment including the idea that there is an equilibrium concentration of each impurity element on corroding Mg surfaces. Various enrichment models will be explored and considered given the inefficient enrichment of other transition metal impurities such Cu and Mn [8]. From this analysis, a mechanistic model for noble metal enrichment will be given.