ENHANCING OXIDATION RESISTANCE OF Ni-BASED ALLOYS FOR HIGH TEMPERATURE APPLICATIONS

Abstract

The demand for energy significantly increases with increasing of world population and industrial activity in developing countries. Nuclear energy can be considered to make an important contribution to future demands for electricity and thermal energy. In order to continue providing inexpensive and safe energy, the operation temperatures of the next generation nuclear reactors should be remarkably increased. Consequently, the alloys used in the high temperature environment will face oxidation degradation problems. Therefore, it is critical to find ways to improve the oxidation resistance of the alloys to be used for high temperature applications. In this thesis, two potential solutions are proposed to enhance the oxidation resistance of three Ni-based alloys. The effect of crystallographic orientation on the oxidation behavior of Hastelloy 230 was investigated to evaluate if texture would affect the oxidation behavior of Ni-based alloys. The obtained results clearly demonstrate that the oxidation rate highly depends on the grain orientation. The oxidation resistance of grains are found to increase in the order of (100) < (110) < (111). The substrates with different grain sizes and similar textures were prepared to inspect the influence of grain sizes on the oxidation behavior of Hastelloy 230 and Hastelloy N. It is demonstrated that the grain size only has a small influence on the oxidation behavior of the high-Cr Hastelloy 230 while it significantly changes the oxidation behavior of the low-Cr Hastelloy N. The oxidation mechanisms of high and low-Cr alloys with coarse and fine grain sizes are discussed. Increasing of the grain boundary density of the low-Cr alloy is believed to promote the diffusivities of Cr ions which results in the formation of a uniform Cr-rich protective oxide layer. The grain refinement process can be an efficient way to enhance the oxidation resistance of investigated low-Cr alloy. The effect of reactive element coating on oxidation behavior of chromium oxide forming alloy was examined. The parameters of electro-deposition process like the current density, deposition time and temperature were optimized to synthesize ceria coating. The coatings were applied to Hastelloy 230 and 625 to investigate the influence of this reactive element on the oxidation behavior. For both alloys, the coating significantly enhanced the oxidation resistance and adherence of the oxide to the substrate. The segregation of reactive element onto grain boundaries is linked to change of the oxidation mechanism from outward diffusion of cations to inward diffusion of anions. Specifically for Hastelloy 230, the addition of ceria coating changed the chromium oxide morphology from columnar to equaxied structure and that is believed to enhance the spallation resistance. In Hastelloy 625, a porous and cracked oxide layer is observed on the uncoated sample, while a layer that is protective against oxidation is formed on the coated sample. The prepared coating suppressed the formation of iron oxide. The oxidation mechanism is suggested and discussed.