Study Of Hot Corrosion Properties Of Low Expansion Thermo-span Superalloys

Low expansion Thermo-Span superalloys is mainly used as thin stator structural components in modern engines, which could reduce the gap, weight of the engine but also could improve the efficiency of the fuel. Considering the condition which the egine undertook, such as high temperature(650-700℃) with corrosive gases, it will be corroded by industrial pollution, ocean atmosphere, and the sulfates, halides generated by fuel, so all of these will lead a premature failure to an engine. Modified Thermo-Span alloy has a high temperature oxidation resistance and good mechanical properties by decreasing Cr, increasing Al, and adding a small amount of B. In this paper, by comparing modified Thermo-Span alloy with standard Thermo-Span alloy, the project mainly focused on electrochemical corrosion at different temperature in different solustions, and aslo the molten salt corrosion at 650℃.From the electrochemical corrosion experiments which were conducted in different temperature(25℃, 50℃, 80℃ and 100℃) also different concentration solutions(NaCl or Na2SO4), the polarization and AC impedance curves were got. All these curves show the results: first of all, when at the same temperature, the corrosion resistance will decrease with increasing the concentration of the solutions. On the other hand, when choose the same concentration of the solution, the property for resistant corrosion will be inversely with the temperature. When the corrosive environment is the same, the general standard alloy appears better.The molten salt corrosion experiments will be conducted at 650℃ and 700℃ with the suface of each specimen brushed with mixed solution(75%Na2SO4+25%NaCl). From the experiments, following conclusion could be got. First of all, with the same corrosion medium, the corrosion resistant will decrease when increase the temperature. Moreover, at the same temperature, the corrosion products need to be analyzed. For the standard alloy, the corrosive products will form a thin film which is composed by Chromium, Alumina and some Sulfide. The chromium film is thick, and the alimina is thin, so the sulfur will diffuse into the bulk along with the grain boundaries. Compared with the standard alloy, the film formed on the modified alloy is composed by thin Chromium, thick Alumina and just small amount of Sulfide, which is very close to the surface, so the diffusion rate of sulfur could be slow down in the modified one. The alumina thin film for the modified alloy is also denser than the regular one, so it appears better corrosion resistant property by the combined effects of Al and Cr.The experiments of electrochemical corrosion of the two low expansion superalloys at 75%Na2SO4 mixed with 25%NaCl molten salt in 650℃ show that: the standard alloy corrosion current is bigger, the passivation interval is shorter, and the capacitive arc of AC impedance is smaller than the modified alloy. Therefore, the modified alloy has a better corrosion resistance than the standard one.