Corrosion Behaviour of Ni-Based Alloys 230, 617 and 601 in CO2 Gas at 750 and 850°C

Abstract

This paper investigated corrosion behaviors of three nickel-based chromia-forming commercial alloys (230, 617 and 601) at 750°C and 850°C in a CO2 environment for up to 500 h. All three alloys showed good oxidation resistance by forming mainly a protective chromia layer with low weight changes. Internal Al2O3 was precipitated beneath a thin protective chromia layer for all cases. For 230 and 617 alloys, NiO and Cr-rich spinel outer layers were formed, but for 601 less iron and nickel outward diffusion was observed at both temperatures. Furthermore, some minor alloy elements (Mn, Ti, and Co) were observed to diffuse into the chromia layers.Wagner’s theory predicted that alloy concentrations were marginal for chromia formation at both temperatures for the test alloys. The observation of protective chromia formation can be attributed to the effect of other alloying elements, e.g. Al, Mn, Ti, Si etc. Wagner’s theory also predicted that this critical chromium concentration decreased with increasing the oxidation temperature, which is consistent with the experimental observation.The Al and Si oxides formed an additional protective barrier between the matrix and the chromia layer, preventing the diffusion of iron and nickel outwards and therefore increasing oxidation resistance. Manganese effectively prevented carburization by combining with chromia to form the outermost spinel oxide layer, increasing oxidation/carburisation resistance. Titanium diffused through the chromia layer, accelerating chromium diffusion and leading to higher kinetic rates. Tungsten reduced oxidation resistance by increasing the metal vacancies and therefore outward diffusion of Ni, Fe and Mn through the chromia scale.