Huzhou Nanxun Yintuo Special Material Technology Co., Ltd. specializes in the development and production of high-quality austenitic stainless steel seamless pipe, nickel-nickel alloy and seamless pipe.
Since the industry adopted austenitic stainless steel in the 1920s, it has been discovered that after welding of this type of steel, the heat-affected zone at a temperature of 450°C to 800°C produces intergranular corrosion in many media. These media are mainly nitric acid with a heat concentration of 50% to 65%, sulfuric acid solutions containing copper salts and iron oxides, and hot organic acids. Later, it was found that this type of steel works at 450℃～800℃, or when aging treatment (or heat preservation or slow cooling) is carried out at this temperature, the same effect due to welding heating will be obtained. This kind of aging treatment will cause the sensitivity of stainless steel to intergranular corrosion, so it is also called sensitization treatment. The temperature range of 450℃～800℃ that easily causes intergranular corrosion is called the sensitization temperature.
Recent studies have proved that this form of corrosion not only exists in chromium steel and chromium-nickel steel, but also exists in nickel, copper, and aluminum-based alloys. The cause of intergranular corrosion is the inhomogeneity of chemical composition in the grain boundaries and within the grains.
In stainless steel and nickel-based alloys, the mechanism of intergranular corrosion can be divided into three basic types: one is that corrosion is related to the depletion of elements along the grain boundary to ensure that the material is resistant to corrosion in the medium; the other is that corrosion is related to the depletion of elements along the grain boundary. The chemical stability of the precipitate is related; the third is corrosion caused by the segregation of surface active elements along the grain boundary that reduce the corrosion resistance of the matrix.
The intergranular corrosion of austenitic stainless steel is mainly caused by the precipitation of continuous chromium-rich (Cr, Fe) 23C6 along the grain boundary in the sensitizing temperature range. As a result, a chromium-depleted area is generated in the matrix around the grain boundary, and the width of the chromium-depleted area is about 10-5 cm. In the time when the precipitation of (Cr, Fe)23C6 is not too long, the chromium-depleted area cannot be recovered due to the slow diffusion of chromium. The generation of the chromium-depleted zone reduces the chromium content near the grain boundary to below the n/8 limit, so the chromium-depleted zone becomes a micro anode and corrodes. If it is heated for a long time within the sensitization temperature range, the chromium-depleted area can be eliminated through the diffusion of chromium, and the tendency of intergranular corrosion can be eliminated.
Because the corrosion resistance of steel is related to the carbide reaction, it is obvious that the carbon content in austenitic steel and its thermodynamic activity determine the tendency of steel to intergranular corrosion. Regarding the carbon content, since the austenite in Cr18Ni9 steel has a solubility of 0.02% below 600°C, there is almost no (Cr,Fe)23C6 precipitation at this time. In fact, when C≤0.03%, no intergranular corrosion occurs.
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