The acid resistance and corrosion stability of polyester coatings on phosphatized steel

Protective polyester coatings on phosphatized steel were obtained with different combinations of diols (2-butyl-2-ethyl-1,3-propanediol or 2,2-dimethyl-1,3-propanediol), organic acids (1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, hexahydrophthalic acid anhydride or adipic acid) and hexamethoxymethylmelamine as a crosslinking agent. The acid resistance and corrosion stability of these coatings were investigated during exposure to different corrosive agents (30% H2SO4, 3% NaCl) using the acid resistance test, electrochemical impedance spectroscopy (EIS) and measurement of the film thickness, gloss and hardness. It was shown that coatings based on polyesters with more branched structures of the diol (2-butyl-2-ethyl-1,3-propanediol) and of the organic acid (hexahydrophthalic acid anhydride), which enable better crosslinking, exhibit better acid resistance and corrosion stability. Coatings with the optimal polyester/melamine ratio, which is close to the theoretical ratio for complete crosslinking, also show better protective properties.