The formation of phosphate conversion coatings on St20 low-carbon steel was studied in order to evaluate the effect of organic accelerators on coating formation and protective performance. The influence of pH, temperature, treatment time, and stirring rate on coating formation was studied by gravimetry and SEM-EDS, the influence of accelerators was studied by Raman spectroscopy, salt spray testing, polarization measurements, EIS, and dynamic LPR monitoring. Efficient coating formation was achieved at an initial pH of 3.0, elevated temperature, and intensive stirring; the process showed diffusion-controlled behavior with an apparent activation energy of 28 kJ/mol. Sodium nitrite accelerated coating formation but did not increase the limiting coating mass. Among the organic additives, catechol provided the most pronounced improvement in corrosion resistance, which was attributed to lower effective porosity and possible incorporation of catechol-derived oxidized fragments into the phosphate layer. The catechol-modified coating also improved electrochemical resistance and did not impair the adhesion of subsequent acrylic paint. These results indicate that catechol is a promising organic modifier for zinc phosphate intermediate pretreatment layers.
Sass et al. (Thu,) studied this question.