Na2SO4 + NaCl molten salts corrosion mechanism of thermal barrier coatings used in ships

6-8 mass% Y2O3 stabilized ZrO2 (6-8YSZ) thermal barrier coatings (TBCs) are widely applied to protect the hot ends of gas turbines in large navy ships. In this work, the 8YSZ TBCs were prepared by air plasma spraying technique, and their microstructure and phase composition were investigated. The hot corrosion mechanism of YSZ TBCs in molten salts consisting of 80% Na2SO4 + 20% NaCl at 900 degrees C was comprehensively analyzed. The results showed that the corrosion product Y-2(SO4)(3) was formed due to the reaction between Na2SO4 media and the stabilizer Y2O3. As the result of the depletion of Y2O3 phase, the transformation from the tetragonal phase to monoclinic phase of ZrO2 could not been totally inhibited, which consequently induced the 4-6 vol.% expansion and more cracks of YSZ TBCs. Meanwhile, the cracks could work as transfer paths for oxygen and molten salts. The kinetic analysis on hot corrosion also showed that more reaction products (from 2 to 8.1 mg cm(-1)) were generated from 20 to 60 h due to more cracks generated by molten salts and oxygen infiltrating. More thermal grown oxides generated between ceramic layer, bonding layer and substrate, and the volume expansion caused by phase transition, increased the stresses in the coatings. Consequently, the peeling-off failure of 8YSZ TBCs could happen.