Review of rare-earth-based conversion coatings for magnesium and its alloys

Rare-earth elements-based conversion coatings are actively researched as an environmentally benign alternative to conventionally used chromate-based conversion coatings for magnesium and its alloys. Here, we reviewed the topic comprehensively. The review starts with a short introduction on corrosion of magnesium alloys and chemical conversion treatments, and that is followed by a detailed description of cerium-based conversion coatings. Morphology, structure and composition of the cerium conversion coatings, their mechanism of formation and competence for corrosion control explained. Influence of various inorganic (hydrogen peroxide, phosphates, permanganates, vanadates, molybdates, etc.) and organic (silane, gelatin, phytic acid, etc.) co-additives discussed. Effect of various pre- and post-treatments described. Reported works on rare-earths other than cerium, viz. lanthanum, yttrium, samarium, neodymium, gadolinium and praseodymium are also reviewed. Future potentials of R & D presented.