Recent advances in steel surface treatment via novel/green conversion coatings for anti-corrosion applications: a review study

Replacing the chromate and phosphate conversion coatings (CCs) with nontoxic ones has played a key role in CC research in recent years. Although considerable efforts have been devoted to discovering and modifying the toxic CC alternatives, much of the research has been restricted to a few numbers of green CCs. The present study has aimed to review the recent attempts that have been made in this area of research to address the toxicity issue of CCs, with a focus on the steel substrate. It includes investigations on different CCs: cerium (CeCC), neodymium (NdCC), lanthanum (LaCC), zirconium (ZrCC), titanium (TiCC), trivalent chromium (TCC), molybdate (MoCC), silicate (SiCC), and silane (SilCC). Surface studies, electrochemical measurements, CCs’ comparison, and film formation mechanisms were summarized. Imparting adhesion to the steel/organic coating interface or the delamination of the organic coating was discussed. Moreover, the influence of several steel substrates (coated and uncoated) and solution parameters (pH, temperature, concentration, stirring, and immersion time) on the CCs properties were outlined. Various methods (such as additives, heat treatment, and subsequent film treatment) that have been employed to improve the green CCs properties and make them comparable to their harmful counterparts were explored.