Stainless steel is a material with a wide variety of applications–from use in medical devices to cooking utensils. Stainless steel contains iron, carbon, and anywhere from 12-30% chromium. Stainless steel can contain other elements such as nickel and manganese, but it is the chromium that resists corrosion, making the steel “stainless.” In theory, it doesn’t rust.
When the surface of normal steel is exposed to oxygen in the presence of moisture, it usually forms iron oxide or "rust". Corrosion from iron oxide is very common, because iron reacts so easily with oxygen. In fact, it reacts so readily that it is very rare to find pure iron in nature
Chromium has a very strong affinity for oxygen. The chromium in stainless steel resists rust by combining with oxygen to form a thin layer of chromium oxide on the surface of the steel. This is called the “passive layer” and it is a reliable protective coating that is even capable of repairing itself when damaged in many cases. If stainless steel is scratched and the chromium oxide layer is removed, a new chromium oxide layer will form and protect the rest of the stainless steel beneath it. As long as there is sufficient chromium present, the chromium oxide layer will continue to protect the stainless steel and prevent it from rusting.
When rust is visible on the surface of stainless steel, it is often the result of what is called “free iron”. Free iron is iron that is separate from the alloy, and therefore not protected by the chromium oxide. Chromium can protect stainless steel if the localized concentration is in excess of 12%, but if you cover the stainless surface with sufficient steel particles, then the localized concentration of chromium can fall below the 12% threshold and the chromium oxide layer fails to protect the stainless steel from oxygen attack. If this type of corrosion happens to stainless steel, it is fixable by cleaning off all the rust, and then removing the tiny steel particles by thoroughly cleaning the stainless steel part. These two steps should allow the chromium oxide layer to protect the stainless from further oxidation.