Chemical resistance and corrosion mechanism of aluminophosphate-based low temperature enamel (LTE) in H2SO4 and NaOH solutions

Abstract

Enamel coatings, showing advantages in material durability, aesthetics, hardness, corrosion resistance, and high-temperature tolerance, are widely used for corrosion protection of metallic substrate. Recently, a new type of aluminophosphate-based low temperature enamel (LTE) coating was developed to avoid the strength degradation of steel substrate, which was also applicable to relatively low-melting metal and was more environmentally friendly with reduced energy consumption. LTE coating might suffer the corrosion of acids or alkalis in some special cases like kitchen utensils, petroleum pipelines, and reinforcing bars. In this study, the chemical corrosion behavior of aluminophosphate-based LTE was investigated in H2SO4 and NaOH solutions with varying pHs of 1∼13. Results indicated that after 30 days of immersion, LTE survived at pHs 3–10, but severely deteriorated at pHs 1, 2, 12, and 13, with the corroded layer presenting a loose structure, completely losing mechanical strength, and even peeling off. For pH 11, the color changed slightly whereas the mass loss was negligible. Specially, LTE suffered selective corrosion in strong acid and alkali conditions, that is, H2SO4 corroded the Ca-compounds (Ca5(PO4)3F and CaF2) phases to generate CaSO4·2H2O and H3PO4, whereas NaOH corroded the LTE matrix (AlPO4) to generate soluble Na3PO4 and NaAlO2. The findings of this study confirmed the acid and alkali tolerance range of LTE and clarified the corrosion mechanism of LTE under both acid and alkaline environments, which guides the further composition design and application of LTE coatings.