The structure, corrosion resistance, and formation mechanism of a composite conversion film based on oxalate on steel substrates

Abstract

To prepare an environmentally friendly and corrosion-resistant chemical conversion film on a steel substrate, cerium salt, titanium salt, and the organic additives 2-mercaptobenzimidazole (2-MBI) and polyethylene glycol (PEG) were introduced into oxalic acid (OA) to form an oxalate composite chemical conversion film. The composite chemical conversion film is composed of large-sized FeC₂O₄·2H₂O rectangular crystal grains and long aspect ratio FeC₂O₄·2H₂O nanorods interspersed among the grains. A large number of nanoscales FeC₂O₄·2H₂O grains help to disperse the corrosion current, preventing localized corrosion, and the fitted polarization resistance reaches 7203.7ohm cm². 2-MBI complexes with metal ions, participating in the formation of the double electric layer. Titanium salt participates in film formation in the form of titanium oxide, while PEG and OA enhance the bonding strength and density of the film-forming materials in the layer by coating titanium oxide, acting as dispersants and wetting agents. As Fe²⁺ generated from corrosion at the interface between the steel substrate and the working liquid combines with oxalic acid to form insoluble ferrous oxalate, the carbon chain migrates outward to the surface of the conversion film. The stronger bond energies of C = O, CO-Fe, and Fe-O facilitate the inward movement of self-assembly behaviour, which significantly increases the carbon content on the film surface and reducing the surface energy in contact with air. After soaking in a 5 wt.% sodium chloride solution, sodium-containing compounds formed on the surface of the conversion film, hindering chloride corrosion. The large number of nanoscale ultrafine dihydrate ferrous oxalate grains in the film effectively suppresses localized corrosion. During the corrosion process, 2-MBI moves to the film surface through complexes formed with titanium oxide via Ti-N bonds, thereby obstructing the intrusion of corrosion liquid ions and enhancing the corrosion resistance of the film layer, endowing it with a certain degree of self-healing capability. Cerium salt participates in film formation as a non-stoichiometric compound with an ambiguous valence state, and the transformation of the cerium element's valence state is another reason for the self-healing property of the conversion film.