Corrosion resistance of 825 alloy

Alloy 825 is a solid solution-strengthened nickel-based corrosion-resistant alloy. During hot working or heat treatment, carbides may be precipitated on the grain boundaries. These carbides adversely affect the corrosion resistance of the alloy 825. The SR101 reactor of a domestic spice manufacturing company was made of 825 alloy produced in China. After six months of use, it was found that the alloy material was severely corroded. After analysis, it was found that the corrosion crack was an intergranular crack, the crack was thick and the depth was shallow. The metallographic analysis of the material shows that a large number of carbides are distributed in the material. The corrosion products were analyzed and found to be carbides. Due to the large amount of carbides in the can material, these carbides are distributed in the material ridges, which reduces the corrosion resistance of the material. The study found that the tube after solution treatment has good corrosion resistance in use. The recommended solution treatment temperature is 980 ゜ C.

Alloy 825 has an austenite structure from high temperature to normal temperature. Generally, when the intergranular corrosion of austenitic stainless steel is sensitized, the diffusion of carbon to the grain boundary is faster than that of chromium. The chromium in the adjacent area is depleted due to the precipitation of M23C6 carbides between the grains. If the chromium content is reduced below the chromium content limit required for passivation, the corrosion along the grain boundaries is accelerated due to the micro-batteries constituting the large cathode-small anode. During the sensitization treatment of alloy 825, 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 is a high-temperature precipitation phase, which starts to form from about 800 ゜ C and forms fastest around 900 ゜ C. A large amount of fine TiC disperses and precipitates. As the temperature rises, TiC begins to dissolve again, heating from 900 ゜ C to 1200 ゜ C It keeps decreasing. Above 1150 ゜ C has high solubility. When the temperature exceeds 1150 ゜ C, TiC will be dissolved in a large amount. Therefore, in a strongly acidic environment, both M23C6 and TiC will affect the corrosion performance of 825 alloy. Therefore, in order to reduce the corrosion tendency, the final hot working temperature of alloy 825 should be above 1050 ゜ C. In order to prevent the intercrystalline corrosion of alloy 825, in addition to the conventional reduction of C content, the ratio of Ti / C and other components should be increased. During the hot working process, the temperature should be repeatedly deformed in the temperature range where TiC is precipitated, so that the TiC The precipitation is distributed in the austenite matrix.