Why not stainless?

The corrosion-resistant mechanism of stainless steel is due to the presence of the Cr element. Oxidation occurs on the surface of stainless steel, forming a dense oxide film that hinders further oxidation. This oxide film, primarily composed of Cr2O3, is generally only a few nanometers thick. The existence of this film prevents corrosion of the stainless steel matrix in various media, a phenomenon known as passivation. In fact, stainless steel is not necessarily immune to rust; it just has a much lower corrosion rate than other steels in the same environment, sometimes even negligible.