It is believed that "stainless steel is not rusted, and rusty steel is not stainless steel".In fact, this is a lack of understanding of stainless steel, stainless steel under certain conditions will rust. If we can understand the various corrosion types of stainless steel, we can have corresponding measures to reduce the loss when facing stainless steel corrosion.
A number of different mechanism of corrosion of stainless steel are recognised. Corrosion damage of stainless steel is mostly partial corrosion, the most common corrosion are intergranular corrosion (9%), pitting corrosion (23%) and Stress Corrosion Cracking (49%). Today we will combine practical cases to analyze the most common corrosion types of stainless steel.
Stress Corrosion Cracking (SCC)
316L pipe fitting with stress corrosion cracking
Stress corrosion cracking is a localised mechanism of corrosion that requires a tensile stress, a susceptible material and a specific environment. This is a relatively rare form of corrosion which requires a very specific combination of tensile stress, temperature and corrosive species, often the chloride ion, for it to occur. SCC in Austenitic stainless steels normally only occurs above about 50°C. Duplex stainless steels can also suffer from SCC in extreme conditions, such as very high chloride levels or temperatures in excess of 100°C.
Stainless steel sink with pitting corrosion
A form of extremely localised corrosion that leads to the creation of small holes in the metal. In stainless steels, pitting corrosion is generally caused by halide ions (which are usually chlorine ions, as these are found in sea water, soils and many raw materials). Pitting corrosion is avoided by making sure that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel which is more resistant to attack.
Intergranular corrosion Under a microscope and on 304 steel tubing
This corrosion is also called sensitisation and typically occurs during welding. Intergranular corrosion occurs where carbon content in the stainless steel is high, or where cooling rates in welding or heat treatment are slow. If the Carbon level in the steel is too high, Chromium can combine with Carbon to form Chromium Carbide. This occurs at temperatures between about 450-850 deg C. The Chromium available to form the passive layer is effectively reduced and corrosion can occur.
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