Many people simply think-stainless steel will not rust, and rusty thing is not stainless steel.
Ordinary carbon steel reacts with oxygen in the atmosphere to form an oxide film on the surface of the metal, and then continues to oxidize, so that the corrosion keeps expanding, forming a “thousand cake” type of corrosive material until it breaks down. Of course, we can use paint or oxidation-resistant metals (such as zinc, nickel and chromium) for electroplating to protect the surface of carbon steel. However, the protective layer is only a thin film. If the protective layer is damaged, the V profile press fittings under it begin to rust again.
However, the processing of stainless steel press fitting is different. There will form a passivation film (Cr2O3) on the surface of the steel when its chromium content reaches about 12%. It is a dense chromium-rich oxide, to protects the surface of stainless steel press fittings, especially to prevent from further oxidation. This kind of oxide film is extremely thin, and the natural luster of the steel surface can be seen through it, which makes stainless steel have unique surface characteristics.
If the surface oxide film is destroyed, the exposed steel surface will react with oxygen in the atmosphere to form a passivation film again. Continue to play a protective role.
Therefore, under the guidance of this theory, many people believe that stainless steel press fittings will not rust. But in fact, it is normal for different levels of rust to be caused by different dry and humid environments in many sites.