tends to rust in humid air and thus needs protection by coating / galvanising etc. SS stands for Stainless Steel - an alloy of iron and carbon (steel) with minimum of 11% Chromium (that makes the steel stainless)…can be soft or hard depending on the carbon content and heat treatment.

Stainless steel fasteners retain good resistance to corrosion over long useful lifetimes, and as such, they retain a shiny appearance for much longer than a traditional low alloy steel fastener. The chemistry of exactly how stainless steel fasteners are able to resist corrosion is crucial to understand.

1 Stainless Steel Surface Quality. It means that the hot rolled stainless steel surface is annealed. It is widely used in the production of cold drawn materials, industrial tanks and industrial-chemical applications.

Even with these impressive features, stainless steel can and does rust after all, it's 'stainless' not 'stainfree'. Some types of stainless steel are more prone to corrosion than others, depending on the chromium content. The higher the chromium content, the less likely the metal will rust.