Let’s talk about intergranular corrosion and its prevention measures
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Corrosion progresses from the surface along the grain boundaries inward; there are no signs of corrosion on the outer surface, but loose corrosion products accumulate at the grain boundaries. Under a metallographic microscope, network-like corrosion can be observed at the grain boundaries. Severe intergranular corrosion can cause the metal to lose its strength and ductility, leading to fracture under normal loads. Intergranular corrosion occurs when chemical and compositional changes take place at the grain boundaries under certain conditions, resulting in a decrease in corrosion resistance; such changes are usually caused by heat treatment or cold working. Taking austenitic stainless steel as an example, a chromium content of over 11% is required to achieve good corrosion resistance. During welding, the areas 2–3 mm away from both sides of the weld can be heated to 400–910°C. At this temperature (the sensitization temperature), chromium and carbon at the grain boundaries tend to combine to form Cr3C6; chromium precipitates out of the solid solution. The spread of chromium within the grains to the grain boundaries is slow, resulting in chromium-deficient regions at those boundaries. The chromium content there can drop well below the 11% threshold. In suitable corrosive solutions, this creates a \"Cr3C6 grains (cathode) – chromium-deficient regions (anode)\\" cell, leading to the corrosion of those chromium-deficient regions. Intergranular corrosion in austenitic stainless steels is relatively common in industry and poses the greatest hazards. Prevention methods include: ① \"solution treatment quenching,\" which involves heating the steel in which chromium-depleted zones have formed to around 1100°C to dissolve the chromium carbides, followed by water quenching to rapidly pass through the sensitization temperature range, thereby keeping the alloy in a homogeneous state with respect to chromium content. ②A small amount of elements such as titanium or niobium, which are more prone to forming carbides, is added to the steel. ③When the carbon content is reduced to below 0.03%, very little chromium precipitates at the grain boundaries.
Intergranular corrosion is a type of corrosion that occurs at the grain boundaries of metal materials, and it is particularly common in austenitic stainless steels. The main cause of intergranular corrosion is the chemical and compositional changes that occur at the grain boundaries of the material during heat treatment or cold working; in particular, chromium and carbon combine to form chromium carbide at certain temperatures, resulting in a decrease in the chromium content at those grain boundaries and thereby reducing the corrosion resistance of that area. Measures to prevent intergranular corrosion include: 1. Performing solution quenching, which involves heating the steel to about 1100°C to redissolve chromium carbides, followed by rapid cooling in water to avoid staying at the sensitization temperature. 2. Adding elements such as titanium or niobium to the steel, which form carbides more easily, can effectively reduce chromium consumption. 3. Reduce the carbon content in steel to below 0.03%, thereby minimizing the deposition of chromium carbide at grain boundaries and allowing for a higher chromium content, which enhances corrosion resistance. .