Method of preventing intergranular corrosion of seamless steel pipe: 1) Reduce the amount of carbon in the steel so that the amount of combined carbon in the steel is lower than the saturated solubility in the austenite in the equilibrium state, which fundamentally solves the problem of precipitation of chromium carbides (Cr23C6) on the grain boundaries. Generally, the amount of combined carbon in steel can be reduced to less than 0.03% to meet the requirements of intergranular corrosion resistance. Resistance to intergranular corrosion: 304L>304>316
2) Adding Ti, Nb and other elements that can form stable carbides (TiC or NbC) to avoid precipitation of Cr23C6 on the grain boundaries, seamless steel pipes can prevent intergranular corrosion of austenitic stainless steel pipes. Therefore, the resistance to intergranular corrosion of 321 is better than that of 304.
3) The seamless steel pipe has a dual-phase structure of austenite + ferrite by adjusting the ratio of austenite forming elements to ferrite forming elements in the steel, in which ferrite accounts for 5% to 12%. This two-phase structure is not easy to produce intergranular corrosion. This is why 2205 has better resistance to intergranular corrosion than ordinary austenitic stainless steel pipes.
4) The seamless steel pipe adopts proper heat treatment process to prevent intergranular corrosion and obtain the best corrosion resistance.
Tips: In case ASTM A53 Grade B in ERW (electric resistance welded) pipe, the weld seam shall be done the heat treatment with a minimum 1000°F [540°C]. In this way the no untempered martensite remains. In case ASTM A53 B erw pipe in cold expanded, then expansion should not exceed 1.5% of the required OD.
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