Stainless steel pipe fittings are rust-proof because they are made of a type of steel that contains chromium, which forms a protective layer of chromium oxide on the surface of the metal. This layer of oxide is extremely thin, but it is also extremely durable and resistant to corrosion, which is why stainless steel is often used in applications where resistance to rust and corrosion is important.

When stainless steel pipe fittings are exposed to oxygen, the chromium in the steel reacts with the oxygen to form a thin layer of chromium oxide. This layer is self-repairing, which means that if it is scratched or damaged in any way, it will quickly reform to protect the underlying metal from further corrosion.

In addition to chromium, stainless steel may also contain other elements such as nickel and molybdenum, which further enhance its resistance to rust and corrosion. This makes stainless steel pipe fittings ideal for use in harsh environments such as marine or industrial applications, where exposure to moisture and corrosive substances is common.