Why stainless steel valves will rust?

In layman’s terms, stainless steel is steel that is not easy to rust.
Let me talk about the principle of stainless steel first.
The stainless and corrosion resistance of stainless steel is due to the formation of a chromium-rich oxide film (passive film) on its surface. This rust and corrosion resistance are relative. The test shows that the corrosion resistance of steel increases with the increase of chromium content in steel in weak medium such as air and water and in oxidizing medium such as nitric acid. When the chromium content reaches a certain percentage, the corrosion resistance of steel changes abruptly. , that is, from easy to rust to not easy to rust, from non-corrosion resistant to corrosion resistant.
Why does stainless steel rust?
Anyone who works in the stainless steel industry knows that unless you get kicked in the head by a donkey, no one will guarantee you that it will not rust. Some people think that stainless steel will never rust. In fact, this is a one-sided wrong view of lack of understanding of stainless steel.
Stainless steel is not necessarily rust-free, but its corrosion rate is much lower than that of other steels in the same environment, sometimes even negligible. But not any kind of stainless steel can resist corrosion and rust in any environment.
Stainless steel relies on a layer of extremely thin, firm, dense and stable chromium-rich oxide film (protective film) formed on its surface to prevent oxygen atoms from continuing to infiltrate and oxidize, thereby obtaining the ability to resist corrosion. Once for some reason, the film is continuously damaged, the oxygen atoms in the air or liquid will continuously infiltrate or the iron atoms in the metal will continuously separate out to form loose iron oxide, and the metal surface will be continuously rusted.