Formation mechanisms of environmentally acceptable chemical conversion coatings for zinc: a review

Eco-friendly conversion treatments have attracted a great interest for corrosion protection of Zn-based materials. Considerable work has been undertaken on the development of advanced conversion coatings. However, the complete range of formation mechanisms of these coatings is not fully understood. Comprehensive understanding of the mechanisms of coating formation, from coating methodologies to fundamentals, is lacking. This review covers recent research that has led to advances in formation mechanisms of environmentally acceptable conversion coatings for zinc, including the thermodynamic stability of chemical systems, the illumination of intermediate reactions, the characterization of the element compositions of coatings, and the deduction for the formation mechanisms of coatings. Representative surface treatment techniques, including phosphate coating, molybdate coating, rare earth coating, vanadate coating, IV(B) metal coating, and silane coating, are discussed in detail. Finally, focused on the strategies used to develop new technologies of conversion treatments, discussions on future trends and perspectives of clever design solutions and advanced analytical methods for formation processes will be given finally.