Corrosion resistance of 825 alloy

Alloy 825 is a solid solution strengthened nickel-based corrosion-resistant alloy that may precipitate carbides at the grain boundaries during hot processing or during heat treatment. These carbides adversely affect the corrosion resistance of Alloy 825. The SR101 reactor of a domestic perfumery manufacturing company was made of domestically produced 825 alloy. After six months of use, the alloy material was found to be severely corroded. After analysis, it was found that the corrosion crack was intergranular crack, the crack was coarse and the depth was shallow. Metallographic analysis of the material revealed a large amount of carbides in the material. The corrosion product was analyzed and found to be a carbide. Due to the large amount of carbides present in the can material, these carbides are distributed in the material, which reduces the corrosion resistance of the material. The study found that the solution treated tube has better corrosion resistance during use. The recommended solution temperature is 980 oC.
825 alloys are austenitic structures from high temperature to normal temperature. Normally, when the intergranular corrosion of austenitic stainless steel is sensitized, the diffusion of carbon to the grain boundaries is faster than that of chromium, so The chromium in the adjacent region is depleted due to precipitation of the type carbides between the grains. If the chromium content is reduced below the chromium content limit required for passivation, corrosion along the grain boundaries is accelerated due to the microbattery constituting the large cathode-small anode. When the 825 alloy is sensitized at medium temperature, Cr-rich M23C6 precipitates on the grain boundaries, resulting in intergranular corrosion. In addition, the MC phase (TiC) may also cause intergranular corrosion of the 825 alloy. TiC precipitates a high temperature, it begins to form from about 800 ºC, the fastest form about 900 ºC, a large number of fine dispersed TiC precipitates with increasing temperature begins to dissolve TiC and heated from 900 ºC to 1200 ºC, TiC dwindling number, 1150 Above oC has high solubility, when the temperature exceeds 1150 oC, TIC will dissolve a lot. Therefore, in a strongly acidic environment, both M23C6 and TiC have an effect on the corrosion performance of Alloy 825. Accordingly, in order to reduce corrosion tendency, the final hot working temperature should be above 825 alloy 1050 ºC. In order to prevent the tendency of intergranular corrosion of 825 alloy, in addition to the conventional reduction of C content, the proportion of components such as butyl Ti/C should be increased. During the hot working, the deformation of TiC should be repeated in the temperature range where the precipitation of TiC is large, so that the formed TiC precipitates. Distributed in the austenite matrix.