Review on the phosphate-based conversion coatings of magnesium and its alloys

Magnesium (Mg) and its alloys are lightweight as well as biocompatible and possess a high strength-to-weight ratio, making them suitable for many industries, including aerospace, automobile, and medical. The major challenge is their high susceptibility to corrosion, thereby limiting their usability. The considerably lower reduction potential of Mg compared to other metals makes it vulnerable to galvanic coupling. The oxide layer on Mg offers little corrosion resistance because of its high porosity, inhomogeneity, and fragility. Chemical conversion coatings (CCs) belong to a distinct class because of underlying chemical reactions, which are fundamentally different from other types of coating. Typically, a CC acts as an intermediate sandwich layer between the base metal and an aesthetic paint. Although chromate CCs offer superior performance compared to phosphate CCs, yet still they release carcinogenic hexavalent chromium ions (Cr 6+ ); therefore, their use is prohibited in most European nations under the Registration, Evaluation, Authorization and Restriction of Chemicals legislation framework. Phosphate-based CCs are a cost-effective and environment-friendly alternative. Accordingly, this review primarily focuses on different types of phosphate-based CCs, such as zinc, calcium, Mg, vanadium, manganese, and permanganate. It discusses their mechanisms, current status, pre-treatment practices, and the influence of various parameters—such as pH, temperature, immersion time, and bath composition—on the coating performance. Some challenges associated with phosphate CCs and future research directions are also elaborated.