What impact does intergranular corrosion have on steel?
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Intergranular corrosion is one of the localized corrosions in boilers. It occurs along the boundaries of rigid grains. It is generally believed that the difference in composition and structure between the grain boundaries and the grains causes potential difference to cause corrosion, and develops rapidly along the grain boundaries. For example, caustic brittleness occurs in the steam drum. The cracks after sensitization of stainless steel and stainless steel superheater tube weld seams are all along grain boundaries. After intergranular corrosion occurs in the steel, although the shape does not change, the connection between the grains is destroyed, making the steel brittle, the strength drops sharply, and sometimes it breaks suddenly, causing serious accidents.
Intergranular corrosion English name: intergranular corrosion; intercrystalline corrosion Description: A type of localized corrosion. Corrosion that extends inward along the interface between metal grains. Mainly due to the difference in chemical composition between the surface and interior of the grain and the existence of grain boundary impurities or internal stress. Intergranular corrosion destroys the bond between grains, * * Reduce the mechanical strength of metal. Moreover, after corrosion occurs, the surfaces of metals and alloys still maintain a certain metallic luster, and no signs of damage can be seen. However, the bonding force between grains is significantly weakened, the mechanical properties deteriorate, and cannot withstand knocking, so it is a very dangerous corrosion. Usually found in brass, duralumin alloys and some stainless steels and nickel-based alloys. Intergranular corrosion of stainless steel welds is a significant problem in chemical plants. Intergranular corrosion of stainless steel: A corrosion phenomenon that occurs between the grains of stainless steel under the action of corrosive media is called intergranular corrosion. When stainless steel suffers from intergranular corrosion, it will fracture along the grain boundaries and its strength will almost completely disappear when it is subjected to stress. This is one of the most dangerous forms of damage to stainless steel. Intergranular corrosion can occur in the heat-affected zone (HAZ), weld or fusion line of the welded joint. The intergranular corrosion produced on the fusion line is also called knife line corrosion (KLA). The necessary condition for stainless steel to have corrosion resistance is that the mass fraction of chromium must be greater than 10~12%. When the temperature increases, the diffusion rate of carbon inside the stainless steel grains is greater than the diffusion rate of chromium. Because the solubility of carbon in austenite at room temperature is very small, about 0.02% to 0.03%, and the carbon content in general austenitic stainless steel exceeds this value, so the excess carbon continues to diffuse to the austenite grain boundaries and combines with chromium to form chromium carbide compounds between the grains, such as (CrFe) 23C6, etc. The data shows that the activation energy of chromium diffusion along the grain boundary is 162~252KJ/mol, while the activation energy of chromium diffusion within the grain is about 540KJ/mol, that is: the diffusion speed of chromium within the grain is smaller than the diffusion speed of chromium along the grain boundary, and the internal chromium does not have time to diffuse to the grain boundary, so the chromium carbide formed between the grains requires Chromium mainly does not come from inside the austenite grains, but from near the grain boundaries. As a result, the chromium content near the grain boundaries is greatly reduced. When the mass fraction of chromium at the grain boundaries is as low as less than 12%, a so-called "chromium-depleted zone" is formed. Under the action of corrosive media, the chromium-depleted zone will lose its corrosion resistance and produce intergranular corrosion. Sensitization of stainless steel and preventive measures Unstable austenitic stainless steel with a carbon content exceeding 0.03% (i.e. 0Cr18Ni9 stainless steel without titanium or niobium) is prone to intergranular corrosion in certain environments if heat treatment is not performed properly. When these steels are heated between 425-815°C, or slowly cooled through this temperature range, intergranular corrosion will occur. Such heat treatment causes carbides to precipitate at grain boundaries (sensitization) and causes chromium depletion in the immediate areas making these areas susceptible to corrosion. Sensitization can also occur during welding, causing subsequent localized corrosion in the welding heat-affected zone. The most common method for checking the sensitivity of stainless steel is the 65% nitric acid corrosion test method. During the test, the steel sample was placed in a boiling 65% nitric acid solution for a period of 48 hours, a total of 5 cycles, and the weight loss was measured in each cycle. Generally, the average corrosion rate of 5 test cycles should not be greater than 0.05mm/month. Intergranular corrosion of austenitic stainless steel welded structures can be prevented by the following methods: ①Use low-carbon grades 00Cr19Ni10 (304L) or 00Cr17Ni14Mo2 (316L), or stable grades 0Cr18Ni11Ti (321, more common in Europe) or 0Cr18Ni11Nb (347, more common in the United States). Using these grades of stainless steel can prevent the amount of carbide precipitation that would cause harmful effects during welding. ②If the structural part is small and can be heat treated in a furnace, it can be heat treated at 1040-1150°C to dissolve chromium carbide, and rapidly cooled in the range of 425-815°C to prevent carbon precipitation. Welded ferritic stainless steel may also suffer from intergranular corrosion in certain media. This is caused by the precipitation of carbides or oxides and the strain of the metal lattice when the steel is rapidly cooled from above 925°C. Stress-relieving heat treatment after welding can eliminate stress and restore corrosion resistance. Adding more than 8 times the carbon content of titanium to 1Cr17 stainless steel can usually reduce intergranular corrosion of welded steel structures in some media. However, the addition of titanium is not effective in concentrated nitric acid. Related standards: Our country already has GB/T 4334. (1~5)-2000 Standard for test methods of intergranular corrosion susceptibility of stainless steel (select corresponding standards according to different material sensitivities) GB/T 15260-1994 "Standard for test methods of intergranular corrosion susceptibility of nickel alloys" GB/T 21433-2008 "Inspection of intergranular corrosion susceptibility of stainless steel pressure vessels"
Intergranular corrosion is mainly for stainless steel. Now that the carbon content of stainless steel is relatively low, this kind of corrosion can basically be avoided.