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  • Shaanxi Tonghui Steel Co., Ltd.

  • E-mail: sxthsteel@sxth-group.com

  • Tel: +86-29-89636557

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  • What is the intergranular corrosion of austenitic stainless steel? (2)
    Aug 03, 2018

    (following from previous page)

    Intergranular corrosion of austenitic stainless steel is mainly in the sensitized temperature range, which easily leads to the precipitation of continuous reticulate chromium-rich (Cr, Fe) 23C6 along grain boundaries. Thus, the chromium deficient area around the grain boundary is formed, and the width of the chromium depleted zone is about 10-5cm. When the precipitation time (Cr, Fe) 23C6 is not too long, the chromium deficient zone is not recovered due to the slow diffusion rate of chromium. The chromium depletion zone makes the chromium content near the grain boundary lower than the n/8 limit, so the chromium depletion zone becomes a micro-anode and corrodes. If heated for a long time in the sensitized temperature range, the chromium-poor zone can be eliminated by the diffusion of chromium, and the tendency of intergranular corrosion can be eliminated.

    Because the corrosion resistance of steel is related to carbide reaction, it is obvious that the content of carbon in austenitic steel and its thermodynamic activity determine the tendency of intergranular corrosion of steel. For carbon content, the solubility of austenite in Cr18Ni9 steel is 0.02% below 600 C, at which time almost no (Cr, Fe) 23C6 precipitates. When C is less than 0.03%, there is no intergranular corrosion. Therefore, the most effective way to solve the intergranular corrosion tendency of stainless steel is to produce ultra-low carbon stainless steel, so that the steel C < 0.03%, such as 00Cr18Ni10 steel. For the elements that affect the thermodynamic activity of C, all elements that increase the activity of C (such as nickel, cobalt and silicon) promote intergranular corrosion; all elements that reduce the activity of C (manganese, molybdenum, tungsten, vanadium, niobium, titanium) are all impeding the formation of intergranular corrosion. For this reason, a strong carbide is often added to austenitic stainless steel to form titanium or niobium, forming a stable TiC or NbC and fixing C in steel. Such as 1Cr18Ni9Ti, 1Cr18Ni11Nb and so on.

    ( to be continued...)


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