The corrosion resistance of austenitic stainless steel comes from the protective layer of chromium oxide formed on the metal surface. If this material is heated to a temperature of 450°C to 900°C, its structure changes and chromium carbides form along the crystal edges. This prevents the formation of a protective chromium oxide layer at the crystal edges, resulting in reduced corrosion resistance. This corrosion is called intergranular corrosion.
As a result, stainless steel 304L and 316L were developed to resist this corrosion. Both stainless steel 304L and stainless steel 316L have lower carbon content. Because the carbon content is reduced, there will be no chromium carbide and intergranular corrosion. It should be noted that greater susceptibility to intergranular corrosion does not imply that non low carbon is more susceptible to corrosion. This sensitivity is also greater in an environment containing high chloride.
It should be noted that this situation is caused by high temperatures between 450℃ to 900℃. Usually, welding is the direct cause of reaching this temperature. For soft seat conventional butterfly valves, since we do not weld on the valve plate, it doesn't make much sense to use stainless steel containing low carbon. However, stainless steel 304L or 316L is required.
Why does stainless steel rust?
Why does stainless steel also rust? When rust spots appear on the surface of stainless steel pipes, people are amazed. They think stainless steel does not rust; if it rusts, it is not stainless steel. There may be a problem with the steel. In fact, this is a one-sided misconception of a lack of understanding of stainless steel. Stainless steel will also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation, that is, rust resistance, and it also has the ability to resist corrosion in media containing acids, alkalis, and salts, that is, corrosion resistance. However, its anti-corrosion ability changes with the chemical composition of the steel itself, the protection state, the use conditions and the type of environmental media. For example, SS304 steel pipes have absolutely excellent anti-corrosion ability in a dry and clean atmosphere. However, if it is used in the seaside area, it will rust soon in the sea fog containing a lot of salt. The SS316 pipe performs well. Therefore, not all stainless steel can resist corrosion and rust in any environment.
As a result, stainless steel 304L and 316L were developed to resist this corrosion. Both stainless steel 304L and stainless steel 316L have lower carbon content. Because the carbon content is reduced, there will be no chromium carbide and intergranular corrosion. It should be noted that greater susceptibility to intergranular corrosion does not imply that non low carbon is more susceptible to corrosion. This sensitivity is also greater in an environment containing high chloride.
It should be noted that this situation is caused by high temperatures between 450℃ to 900℃. Usually, welding is the direct cause of reaching this temperature. For soft seat conventional butterfly valves, since we do not weld on the valve plate, it doesn't make much sense to use stainless steel containing low carbon. However, stainless steel 304L or 316L is required.
Why does stainless steel rust?
Why does stainless steel also rust? When rust spots appear on the surface of stainless steel pipes, people are amazed. They think stainless steel does not rust; if it rusts, it is not stainless steel. There may be a problem with the steel. In fact, this is a one-sided misconception of a lack of understanding of stainless steel. Stainless steel will also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation, that is, rust resistance, and it also has the ability to resist corrosion in media containing acids, alkalis, and salts, that is, corrosion resistance. However, its anti-corrosion ability changes with the chemical composition of the steel itself, the protection state, the use conditions and the type of environmental media. For example, SS304 steel pipes have absolutely excellent anti-corrosion ability in a dry and clean atmosphere. However, if it is used in the seaside area, it will rust soon in the sea fog containing a lot of salt. The SS316 pipe performs well. Therefore, not all stainless steel can resist corrosion and rust in any environment.