Performance of Inconel alloys in Supercritical water oxidation process
The supercritical water oxidation is a high temperature and pressure oxidizing reaction, usually the feeds require to be preheated to supercritical conditions. Therefore, it is of great significance to understand the performance of eligible materials in supercritical water conditions without additional oxidants, corresponding to preheating stage of supercritical water oxidation. With higher mechanical strength and higher corrosion resistance as compared to stainless steel at the elevated temperatures, nickel based alloys are usually used for producing components of supercritical water reactors and pipes, and equipment functioning in supercritical condition. At present, various nickel based alloys like Inconel 625, Inconel 600, Inconel 718 and Hastelloy C276 are evaluated for performing in pure supercritical water.
The oxide layers produced on nickel based alloy surface subjected to pure supercritical water usually show a duplex layer structure with nickel and iron rich in external layer and chromium enriched inner layer which suitably prevents the external diffusion of alloying elements and the internal diffusion of oxygen ions, hence decreasing the corrosion rate of the substrate. Different from the service fluids used in supercritical power plants and conceptual supercritical water cooled reactors, the feed streams used in supercritical heater for supercritical water oxidation and gasification usually contain large magnitudes of organic compounds and inorganic salts. Vigorous anions derived from the inorganic salts like chloride, bromide, nitrate, sulfate etc cause accelerate or decelerate the corrosion process in supercritical water conditions, based on how interact with the security oxide layers. The localized corrosion is usually used for chloride and bromide. Although, many anions like phosphate can be attributed in the oxide layer of nickel based alloys and result in their increased stability due to secondary passivation.
Although, there has been little research on resistance of nickel based alloys in super critical water containing sulfides. Corrosion resistance of Inconel wire in supercritical water oxidation process is excellent. The corrosion properties of Inconel 600 and Incoloy 825 in these conditions have attracted widespread attention because of their comparatively significant economic advantages. Therefore he study on corrosion resistance of alloy 600 and alloy 825 in SCW comprising sulfides provide basic understanding of the effects of sulfides on the performance of these alloys.
The function of these alloys was evaluated in SCW in a sulfide concentration of 12800 ppm at 520oC and pressure 25 Mpa. The weight gain of alloy 825 was lower than Inconel 600. Corrosion resistance of Incoloy 825 was better than alloy 600. The change in weight gain noticed from low to high rate, which referred the occurrence of breakway corrosion, means fast weight gain.
The surface morphology of alloys evaluated after 3 and 60 hours of test. After test of 3 hours, thin layers are produced on samples of these alloys. Further it may be noted that dark phases developed on Inconel 600, its particle size was less than that of bright phases. Scaled developed on Incoloy 825 showed good adhesion to the matrix.
The function of alloy 600 included the external diffusion of nickel through inner layer and inward flow of water through the external layer.