Searching for Evidence of Hydride Formation on Corroding Magnesium

This article examines the hypothesis that magnesium hydride (MgH₂) forms during the corrosion of magnesium and its alloys. A brief literature review presents the existing analytic evidence of MgH₂ in the corrosion products of magnesium, along with pertinent information related to the chemical characteristics of MgH₂.. Anodic polarization tests were conducted on high-purity magnesium in unbuffered 0.1 M sodium chloride solutions. A substantial amount of H₂ gas evolved from the corrosion products when dissolved in chromic acid solution, indicating the presence of a reducing substance in the corrosion products. Optical microscopy showed the release of thin, plate-shaped metal particles. A numerical estimate indicated that the mass of the disintegrated metal particles was negligible in comparison to the overall mass loss of the electrode. The corrosion was localized, and the attacks displayed a layered appearance with a preferential orientation, most likely parallel to the basal (0001) plane of Mg. The complete charge balance for galvanostatic anodic polarization tests at 10 mA/cm² was established by quantifying 1) the external current, 2) the volume of H₂ gas being evolved during the polarization test, 3) the volume of H₂ gas being evolved during the dissolution of the corrosion products in chromic acid, 4) the quantity of Cr(III) ions formed through the reduction of chromic acid during the dissolution, and 5) the metal mass loss. The charge balance showed that on average 27% of the cathodic reaction products, quantified as charge, remained as a reducing residue in the corrosion products after the tests. The composition of the reducing substance was primarily examined within the framework of a hypothesis regarding MgH₂, which recent research has indicated is partially protected from hydrolysis by a layer of reaction products. Other plausible metastable substances were also discussed.