The interaction of phosphate coatings on a carbon steel surface with a sodium nitrite and silicate solution

Mono-cation PZn, bi-cation PZnCa, PZnNi and three-cation PZnNiMn crystalline phosphate coatings were modified with an inhibitor mixture: a sodium nitrite and sodium silicate solution with the aim to establish the reasons of protective ability enhancement of passive films on a carbon steel surface in an alkaline media. The SEM, EDS, XRD and XPS techniques were applied for the structural, phase and composition characterization of the phosphate coatings, voltammetric measurements were carried out to determine the passive layer protective ability, while EIS studies yielded information on the coatings porosity. Compact films of Si compounds were formed on the surface of the phosphate coatings during their modification procedure, which was accompanied by an increase in the protective ability of phosphate layer. A higher porosity and regularly shaped crystallites of the phosphate layer were favourable for accumulation of a greater amount of Si in the modified coatings. The protective ability of the modified coatings remains fairly pronounced, which testifies that the phosphate layer porosity is not the only factor influencing the corrosion behaviour of the coating. The difference in the nature of Si compounds comprising modified phosphate coatings leads to the differences in their protective ability.