The cause of the 304 stainless steel rust

Stainless steel has a strong ability to resist corrosion. But this does not mean that it never rust. Some stainless steel rusts in practice. Taking SUS304 as an example, the material appears rusty phenomenon, led by some reasons, such as service environment process mode and material defects.

Stainless steel is very easy to rust in an environment with chloride ion which is widespread in salt, sweat spot, soil, water, wind, etc. In this environment, stainless steel can be corroded easily, the corrosion speed is even greater than tha of ordinary low carbon steel. So, there is a requirement in the use of stainless steel environment. Besides, the steel should be often wiped for removing dirt and keeping clean and dry. Secondly, the steel without solid solution treatment is also easy to rust. Alloying elements do not dissolve into the matrix, leading to low ally content of matrix organization and poor corrosion resistance. Another reason of 304 stainless steel rust is that the steel has a natural tendency to intergranular corrosion, because it does not contain the titanium and niobium. With stable processing, adding titanium and niobium can reduce the intergranular corrosion.

According to metallography, there is thin chromium membrane on the surface of stainless steel due to the chromic in the steel. The membrane has an effect on corrosion resistance by isolateding from oxygen. In order to maintain inherent corrosion resistance of stainless steel, the chromium content should be more than 12%. Furthermore, when stainless steel in some certain environments, precipitation of carbides may result in intercrystal corrosion (welding erosion). Low carbon content can make the precipitation of carbon compound to a minimum in heat affected zone near the weld. For the reason, 304L stainless steel, which is a lower carbon variations of 304 stainless, is applied to welding of the occasion.

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