One question per week (09.09.21) – Measures to prevent and control intergranular corrosion in austenitic stainless steels

Causes and prevention measures of intergranular corrosion in austenitic stainless steels. Austenitic stainless steels suffer from intergranular corrosion when held at temperatures between 450–850°C or when cooled slowly. The higher the carbon content, the greater the tendency to intergranular corrosion. Furthermore, intergranular corrosion can also occur in the heat-affected zone of welded parts. This is due to the precipitation of Cr-rich Cr23C6 at the grain boundaries. This results in a chromium-depleted zone forming in the surrounding matrix, thereby creating a corrosion galvanic cell. In engineering, the following methods are commonly used to prevent intergranular corrosion in austenite: (1) Reducing the carbon content in the steel so that it is below the saturated solubility of carbon in austenite under equilibrium conditions; this effectively solves the problem of chromium carbides (Cr23C6) precipitating at the grain boundaries. Generally, a carbon content in steel of 0.03% or less is sufficient to meet the requirements for intergranular corrosion resistance. (2) By adding elements such as Ti and Nb, which can form stable carbides (TiC or NbC), the precipitation of Cr23C6 at grain boundaries can be prevented, thereby avoiding intergranular corrosion in austenitic stainless steels. (3) By adjusting the ratio of austenite-forming elements to ferrite-forming elements in the steel, a dual-phase structure of austenite + ferrite is obtained, with ferrite accounting for 5% to 12%. This dual-phase structure is less prone to intergranular corrosion. (4) By adopting appropriate heat treatment processes, intergranular corrosion can be prevented, thereby achieving optimal corrosion resistance. Heating the steel to 1050–1150°C and then quenching it in water serves primarily to dissolve the carbides in austenite and maintain this state at room temperature, thereby significantly improving the steel’s corrosion resistance. As mentioned above, to prevent intergranular corrosion, solution treatment is typically used to dissolve Cr23C6 in austenite, followed by rapid cooling. Air cooling can be used for thin-walled components, while water cooling is generally employed.

Methods to prevent intergranular corrosion in austenitic stainless steels: ① \"Solution quenching\" treatment, in which the steel containing chromium-depleted regions is heated to around 1100°C to dissolve chromium carbides, followed by water quenching; this rapid passage through the sensitization temperature range helps maintain a uniform chromium content in the alloy. ②A small amount of titanium or niobium, elements that more readily form carbides, is added to the steel. ③When the carbon content is reduced to below 0.03%, very little chromium precipitates from the grain boundaries.

①Low-carbon grades such as 00Cr19Ni10 (304L) or 00Cr17Ni14Mo2 (316L), or stable grades like 0Cr18Ni11Ti (321, more common in Europe) or 0Cr18Ni11Nb (347, more common in the United States) can be used. The use of these stainless steel grades helps to prevent the precipitation of carbides during welding, which could have harmful effects. ②If the structural component is small and can be heat-treated in a furnace, it can be heat-treated at 1040–1150°C to dissolve chromium carbide, and then rapidly cooled in the range of 425–815°C to prevent carbon precipitation. Welded ferritic stainless steels can also experience intergranular corrosion in certain media. This is caused by the precipitation of carbides or oxides and strain in the metal lattice when steel is cooled rapidly from above 925°C; stress relief heat treatment after welding can eliminate this stress and restore corrosion resistance. Adding titanium in an amount more than 8 times the carbon content to 1Cr17 stainless steel can generally reduce intergranular corrosion of welded steel structures in certain media. However, adding titanium in concentrated nitric acid is not effective.