Alternative conversion coatings to chromate for the protection of magnesium alloys

• We discuss the chemical mechanism of Chromium Conversion Coatings (CCC).
• We discuss the chemical mechanism of alternative coatings.
• We compare the properties of coatings obtained by CCC and alternative coatings.
• We compare the morphologies of all studied coatings.
• Two alternatives coatings present some equivalent properties to CCC.

Chromium Conversion Coatings (CCCs) are widely used for the protection of magnesium alloys against the corrosion but this approach is limited due to the toxicity of hexavalent chromium. Alternative conversion coatings are based on solutions containing chromium (III), permanganate, phosphates, Rare Earth Elements (REEs) or vanadium. This review details the deposition and protection mechanisms of CCC technology and other promising processes. Permanganate/phosphate-based coating presents a corrosion resistance equivalent to CCC and REEs have self-healing properties but no coating present all the properties of CCCs. The development of new non-toxic conversion coatings remains a priority.