Studies On The Hot Corrosion Resistance Of YSZ Thermal Barrier Coatings

Hot corrosion resistance of thermal barrier coating system was investigated in the high temperature melting salt, including Ni-based high-temperature alloy Hastelloy X, NiCrAl metal coating and 8YSZ ceramic coating. Two kinds of coating , Hastelloy X+NiCrAl and Hastelloy X+NiCrAl+YSZ were prepared through Atmospheric Plasma Spray technique. After oxidation treatment, the three kinds of materials were heated in 25wt. % NaCl-75wt. %Na₂SO₄ molten salt at 750℃. The structural transformation of the three kinds of materials were investigated by scan electron microscopy(SEM), energy dispersive spectroscopy(EDS), X-ray diffraction, electrochemical method. The mechanism of destroy and corrosion resistance behavior were further analyzed.The impacts of oxidation treatment for the corrosion resistance of Ni-based alloy Hastelloy X are investigated. The results showed that the oxide film was shaped at the surface after oxidation treatment which was intact and tight and could prevent the molten salt diffusing into the alloy. The oxide film was shaped from the surface of the alloy without oxidation treatment, and the structure was not complete and loose. The molten salt was able to penetrate into the alloy surface. The oxide layer in the corrosion product migrated to the film surface, whereas the sulfide did not migrate at all. As the thickness of sulfide increased and the film crazed and was destroyed, the corrosion rate speeded up.After the oxidation treatment, a complete and tight film was formed at the top of NiCrAl metal coating, which was able to prevent the molten salt diffusing into the surface of coating. The oxide film was dissolved by the Na₂O in the molten salt and produced Na₂CrO₄,NaAlO₂ et al. The metal elements inside the coating moved to the surface and produced defects inside the coatings, which lead to the increase of lattice index. The defects gathered and yielded holes inside the coatings.The thermally grown oxide (TGO) was also produced between the NiCrAl coating and YSZ film after oxidation treatment. The YSZ ceramic coating also was dissolved by Na₂O in the molten salt, with the corrosion product of Na₂ZrO₃. The dissolve of YSZ produced particles, and the multilayer structure of the coatings dissociated. After the Na₂ZrO₃ reached saturation in the molten salt, Na₂ZrO₃ came out from the cracks and gaps of YSZ coating. The coating was later ruptured and fell off. The multilayer structure of YSZ ceramic coating actually enhanced the resistance against hot corrosion.