Novel perspectives on the mechanism of phosphate conversion coating on magnesium alloys

Abstract

Chemical conversion coating (CCC) is a cost-effective and industrially scalable technique to tackle the issue of corrosion of Magnesium and its alloys, crucial for reducing global greenhouse gas emissions by the road transportation sector. Amongst all the possible CCCs, the phosphate conversion coating (PCC) has one of the highest industrial application potential. This article demonstrates limitations in the traditional understanding of Mg-PCC mechanism, primarily dictated by increased local pH near the Mg-substrate leading to double deprotonation of phosphoric acid (H3PO4 → H2PO4− + H+; H2PO4− → HPO42− + H+). The HPO42− further reacts to Mg2+ and subsequent supersaturation of precipitate, MgHPO4.3H2O results in the coating formation. The new hypothesis proposed for Mg-PCC is based on the argument that the second deprotonation reaction is energetically unfavorable, instead, the reaction is mostly limited to the first deprotonation of the phosphoric acid resulting in H2PO4−. This H2PO4− reacts to the Mg2+ and forms an intermediate complex ion, (Mg2+-H2PO4−)+. The Mg substrate, when exposed to an aqueous environment, releases Mg2+, resulting in a negatively charged substrate. The complex (Mg2+-H2PO4−)+ ions are electrostatically attracted to the negatively charged substrate resulting in the reaction responsible for MHPT (MgHPO4.3H2O) coating. This new hypothesis is supported by thermodynamic calculations through density functional theory (DFT) and experiments.