Corrosion mechanism of potassium silicate coating on Ti-6Al-4V alloys under solid NaCl deposit at 700 oC

A potassium silicate coating cured by calcium hydrogen phosphate was prepared on Ti-6Al-4V, and its protection behavior against NaCl-induced corrosion was investigated at 700 oC for 500 h by the means of thermogravimetry, SEM, EMPA, XRD and TEM. The bare Ti-6Al-4V alloy suffered severe corrosion as the thickness of corrosion production layer exceeded 2 mm after 500 h. The major corrosion production was TiO2, and sodium titanates was observed within 100 h while disappeared after long-term tests. Accordingly, a novel model for NaCl-induced corrosion of Ti alloys at high temperatures was proposed. That is, the porosity of the corrosion product is strongly correlated with the reaction between water vapor in air and sodium titanates at high temperatures. The coating exhibited excellent protection as the corroded substrate layer, which is Ti5Si3 resulted from coating-substrate reaction, is ∼ 6.6 μm only, and no oxide scale was formed. Dissolution of Na in the coating was identified and is believed to be associated with the protective mechanism of the potassium silicate coating, i.e. this dissolution avoids chlorination-oxidation of Ti alloy.