Why do brown rust spots appear on the surface of 304 stainless steel pipe?

Why do brown rust spots appear on the surface of 304 stainless steel pipe? 2018-05-18 When brown rust spots appeared on the surface of 304 stainless steel pipes, people were surprised: they thought that 304 stainless steel pipes did not rust, and rust is not stainless steel. In fact, this is a one-sided misconception about the lack of understanding of 304 stainless steel pipes. 304 stainless steel pipe will rust under certain conditions. 304 stainless steel pipe has the ability to resist atmospheric oxidation---that is, non-rust, but also has the ability to corrode in media containing acid, alkali, and salt---that is, corrosion resistance. However, the size of its anti-corrosion ability varies with the chemical composition of the steel itself, the state of mutual addition, the conditions of use and the type of environmental media. 304 steel pipe has excellent anti-corrosion ability in a dry and clean atmosphere, but when it is moved to a coastal area, it will soon rust in sea fog containing a lot of salt; while 316 steel pipe performs well. Therefore, it is not any kind of stainless steel, which is corrosion-resistant and does not rust in any environment.