High Temperature Oxidation and Corrosion of Ni-Based Superalloys for Industrial Gas Turbines
Doctoral thesis, 2014
High temperature oxidation and corrosion of Ni-based superalloys used for blades in industrial gas turbines was studied using advanced high-resolution electron microscopy methods. Both coated and non-coated alloys were tested.
In the case of alloys covered by protective coatings, the influence of Pt on the material corrosion properties at 900°C was investigated. Due to an extensive research it is known that Pt improves oxidation resistance of coatings above 1000°C. However, the effect of Pt on the oxidation and corrosion resistance at lower temperatures and in the presence of sulphates has not been studied to the same extent. To simulate hot corrosion occurring in gas turbines, a novel method, based on the evaporation of Na2SO4 in the furnace was developed. The investigation showed that the presence of Pt improves the protective properties of the coating against oxidation and hot corrosion. The propagation stage of the corrosion on the platinum-free coating was reached already after 100 h of exposure while the scale formed on the platinum-rich coating was thinner and denser, and still appeared to be protective even after 500 h. It was proposed that Pt suppresses the formation of interfacial cavities, degrading the scale adherence by the faster diffusion of Al ions, which can fill the voids.
The performed work on uncoated superalloys includes studies of effects of a) the surface treatment; b) the aggressive environments and c) sulfur in the material on the corrosion properties of the newly developed alloy, SCA425+ and the extensively used alloy IN792. Cyclic exposures up to 260 hours were performed in different atmospheres such as: air, SO2+O2+Ar, SO2+H2O+Ar and H2O+O2. The study showed that due to the relatively high Al content, 10 at.%, SCA425+ might form a protective alumina layer, provided that it receives a surface treatment that creates a zone of defects allowing for a fast enough supply of Al to the metal surface. On the other hand, IN792 does not form a protective oxide scale in any of the investigated conditions and the structure and morphology of the oxide scale are not very sensitive to the surface treatment.
Furthermore, it is shown that the most pronounced effect on the corrosion behavior of the investigated material has the presence of moisture in the atmosphere while the effect of SO2 up to 260 h exposure seems to be less important. In the presence of water vapor Cr is depleted from SCA425+ due to the evaporation of chromia. The loss of Cr2O3 was estimated using a comparison of the Cr depletion in the metal with the amount of Cr2O3 in the oxide scale. The results also indicate that the volatilization of chromia slows down with increased exposure. The experiment concerning the influence of sulfur in the material, desulfurization of SCA425+ in hydrogen indicates that S degrades the adherence of the oxide scale.