Why does intergranular corrosion occur in stainless steel plates?

Not only stainless steel plates, but many other metals have a tendency to intergranular corrosion. However, intergranular corrosion is more prominent in stainless steel materials, molybdenum-containing nickel-based alloys and aluminum alloys. If stress exists, intergranular corrosion will turn into stress corrosion damage along the intergranular.

From the perspective of depletion theory, intergranular corrosion is caused by the precipitation of new phases at the grain boundaries, resulting in a lack of certain components near the grain boundaries. For example, during the tempering process of austenitic stainless steel, supersaturated carbon is partially or completely precipitated at the grain boundaries in the form of Cr23C6, which will quickly reduce the carbon and chromium concentrations near the carbides, resulting in chromium-poor areas on the grain boundaries, and the chromium-poor areas will be corroded as anodes.

For low-carbon and ultra-low-carbon stainless steels, there is no condition for carbides to precipitate at the grain boundaries to cause chromium depletion. However, practice has shown that low-carbon and ultra-low-carbon stainless steel plates, especially high-chromium and high-molybdenum stainless steel plates, will also produce intergranular corrosion when heated at 650-850℃, in strong oxidizing media, or when their potential is in the passivation zone. When the temperature of ferritic stainless steel plate is above 900℃, rapid cooling in the high temperature zone is prone to intergranular corrosion. Even superferritic stainless steel with extremely low carbon and nitrogen content will also experience intergranular corrosion. However, reheating at 700-800℃ can remove intergranular corrosion.

After welding, ferritic stainless steel plate is prone to intergranular corrosion at the weld metal and fusion line. 18Cr-9Ni stainless steel will not form intergranular corrosion when the temperature is greater than 750℃, but in the range of 600-700℃, the tendency of intergranular corrosion is also very serious. When the temperature is less than 600℃, it will take a long time to form intergranular corrosion tendency, and when the temperature is less than 450℃, there will be basically no intergranular corrosion tendency.

The method to detect whether stainless steel materials have intergranular corrosion tendency is usually to use sensitization processing technology. Stainless steel materials are sensitive to intergranular corrosion temperature when heated to intergranular corrosion temperature. After constant temperature treatment for a corresponding time, this treatment process is called sensitization treatment, and the temperature at which intergranular corrosion sensitivity is formed is called sensitization temperature. The sensitive temperature of 18Cr-8Ni stainless steel is 650-700 ℃, and the short time required to form intergranular corrosion tendency is 1-2 hours.
There are four ways to avoid intergranular corrosion of stainless steel. The first is to reduce the carbon content. When the carbon mass fraction in stainless steel is less than 0.03%, intergranular corrosion will not occur even if it is tempered at 700℃ for a long time. The second way is to add alloy elements that can fix carbon. The third way is to perform solid solution treatment on stainless steel materials. The fourth way is to use duplex steel. The use of ferrite and austenite duplex stainless steel can effectively resist intergranular corrosion.