High-temperature Oxidation And Hot Corrosion Resistance Of K452Alloy And Al-Si Coatings

For the gas turbine, high temperature corrosion is the most frequent adverse events, andthe increasing of the using temperature put forward highly demands to the materials, the hightemperature alloy can′t fully meet this requirements at the moment, so preparate protectivecoating in alloy surface to improve the resistance to high temperature oxidation and thermalcorrosion. The aluminide coating is an ideal protective coating, but there are still somedeficiencies. The addition of silicon can significantly improve the performance of the coating.We preparated Al-Si diffusion layer in the surface of superalloy K452by slurry method.Experimentally, two groups of the superalloy K452with and without Al-Si coating weretreated in stationary air at1000oC for500h and treated in Na₂SO₄+25%NaCl at900oC for150h. The microstructure, surface topography and corrosion products of the layer weredetermined using scanning electron microscope （SEM）, X-ray diffractometer （XRD）, andenergy dispersive spectroscopy （EDS）. And the high temperature corrosion mechanism of Al-Si layer was analyzed.The Al-Si layer by vacuum diffusion deposited about210μm on the surface of alloyK452, consisted of three layers–external, transient and diffusion layers. The refractory alloyand Ni-Si、Cr-Si compounds compound diffuse distributed on the external layer majorconsist of Ni-Si and β-NiAl, both of transient and diffusion layers rich of Cr formed withCr23C6and the major phase in diffusion layer was γ ’, Si increased the thickness of the layerby improving the chemical potential of Al.Both of the oxidation of alloy K452and Al-Si coatings followed parabollic rate under1000℃for500hours.The high-temperature oxidation property of Al-Si coatings was muchsuperior than alloy K452, TiO₂、Cr₂O₃and Al₂O₃was formed in the early and then Al and Crwas consumed, the oxide film can not pretect alloy effectively. The alloy with Al-Si coatingsretained excellent oxidation resistant performance. θ-Al₂O₃was formed in the early and thenreduced with transforming into uninterrupted α-Al₂O₃membrane. In the later stage of high-temperature oxidation Cr₂O₃was formed with Cr diffused from diffusion layers to surface. Itshowed that the oxidation mechanism changed, but the Al-Si coating still had a very effectiveantioxidant.K452alloy formed Cr₂O₃、Al₂O₃and TiO₂in molten Na₂SO₄+NaCl under900℃, sulfide–oxidation occurred with S diffusion into the substrate. Acidity dissolved and volatileformed in the process of hot corrosion maked the law complicated. The product of Al-Sicoatings in hot corrosion was Al₂O₃and SiO₂, Si reacted with Mo、W, fixing them in thecoatings and avoiding them diffusion outward. So the Al-Si coatings enhanced the ability ofresist hot-corrosion of K452alloy.