Because of its good corrosion resistance, mechanical properties and low price, stainless steel has become the most commonly used metal material after carbon steel, and has a good use value in the strong corrosion environment. But it has a strong susceptibility to intergranular corrosion, so it is necessary to test the intergranular corrosion of stainless steel in material selection, heat treatment and processing. There are many reasons for intergranular corrosion: the dilution theory (precipitation of Cr23C6 at grain boundary, the formation of poor chromium in the grain boundary), the second phase precipitation theory (high chromium molybdenum containing ultra low carbon stainless steel in 650~850 C heat treatment, precipitation of sigma) and grain boundary adsorption theory (adsorption of impurity P on grain boundary). The phenomenon of sensitized intergranular corrosion of Cr Ni austenitic stainless steel and identification of sensitized intergranular corrosion in the weld heat affected zone or component heated by 450~850 C in the welded component, resulting in the leakage or breakage of these parts under the action of medium; Intercrystalline corrosion equipment, components, etc. whose size and shape are almost unchanged and without any plastic deformation; there are no signs of corrosion in the other parts of the corroded area, and there are still obvious metallic luster; local sampling inspection, the strength of the corroded parts, the plasticity has been seriously lost, and the cold bending not only appears. Cracks, when serious, often appear brittle fracture and grain shedding and fall to the ground without metallic sound. Under the metallographic microscope and scanning electron microscope, it is obvious that the grain boundary of steel is widened because of corrosion, and it is mostly reticulate, and the grain fall off in serious case. Two) the non sensitized (solid solution) intergranular corrosion phenomenon of chromium nickel austenitic stainless steel and the identification of non sensitized (solid solution) intergranular corrosion system refers to the solid solution state of Cr-Ni austenitic stainless steel after heating at high temperature (1000~1150 C) and cooling after heat preservation, and no further sensitization (welding or 450~850 degree C) is needed. Intergranular corrosion also occurs in some corrosive media after sensitized temperature heating. Cr-Ni austenitic stainless steel, which produces non sensitized intergranular corrosion, includes both ordinary stainless steel, ultra low carbon stainless steel with sensitized intergranular corrosion and Ti containing stabilizing elements, and stainless steel of Nb. Three) the intergranular corrosion of ferritic stainless steel and the identification of the intergranular corrosion of ferritic stainless steel is different from the previous Cr-Ni austenite. It usually occurs after heating (or after welding) higher than 900~950 [C], even under water and so on. After a short time heating of 750~850 [C], the ferrite is not rusted. The intergranular corrosion sensitivity of steel can be reduced and even eliminated; the intergranular corrosion of ferritic stainless steel is produced near the weld line near the weld line, not in the heat affected zone of Cr-Ni austenitic stainless steel. The recognition of intergranular corrosion of ferritic stainless steel is basically the same as that of Cr-Ni austenitic stainless steel except for the differences in location. Intergranular corrosion of ferritic stainless steel occurs not only in strong corrosive medium, but also in weak medium, such as tap water. Test method and standard method for stainless steel intercrystalline corrosion test method GB/T 4334 metal and alloy corrosion stainless steel intercrystalline corrosion test method: oxalic acid sulphuric acid ferric sulphuric acid ferric sulphate 65% nitric acid method nitric acid - hydrofluoric acid method 16% sulfuric acid copper sulphate 35% sulfuric acid - copper sulphate JIS G0571 stainless steel 10% oxalic acid corrosion test method ASTM A 262 Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels stainless steel sulphuric acid iron corrosion test method Of resistance to intergranular corrosion of stainless steels JIS G0573 stainless steel 65% nitric acid corrosion test methods JIS G0575 stainless steel sulphuric acid copper corrosion test method for intergranular corrosion test for stainless steel welded joint of 3949 ship, stainless steel intercrystalline corrosion test sample for stainless steel 10% oxalic acid Corrosion test, stainless steel sulfuric acid - iron sulphate corrosion test, stainless steel 65% nitric acid corrosion test, stainless steel nitric acid - hydrofluoric acid corrosion test: 30mm*20mm* (3-5) mm, take 2 or 4 samples by product; stainless steel 16% sulphuric acid copper sulphate corrosion test: (80-100) mm*20mm* (3-5) mm, take 2 or 4 samples according to the product; stainless steel 35% Sulphuric acid - cupric sulfate corrosion test: the surface area is 15-35 square centimeters, plate thickness is more than 6mm, other best thickness should be less than 6mm, 1 groups are 2 samples, stainless steel 5% sulfuric acid corrosion test method: sample total area 10-30cm2, stainless steel pressure vessel intergranular corrosion sensitivity test: reference intercrystal corrosion national standard ABCDE