What is Steel?

Steel is an alloy primarily containing iron with other alloying elements in minor percentage. Pure iron is very soft and brittle hence cannot be used for making usable parts. So carbon is added which gives the strength. Carbon atoms get embedded in the tiny gaps of iron atoms making them less likely to slip one on another under a force or impulse. This is how steel gets its strength more than pure iron. Typically carbon percentage in steels is 2% and alloy beyond 2% carbon is called as cast iron. The maximum carbon that can stay in the lattice structure is 6.67% by weight after which it forms cementite (Fe3C). 

What is Rust?

Iron gets oxidized to iron oxide in the presence of oxygen and this is what is commonly known as rust. Rust not only deteriorates the surface appearance but also lowers the strength and mechanical properties of steel. Plain carbon steel is susceptible to forming rust if not coated with anti-corrosion paints. 

What is Stainless Steel?

The way to prevent steel from reacting with oxygen is by adding any other element to steel so that it can more quickly react with oxygen thus leaving the iron in the steel as is. Chromium is more reactive than iron hence 11 to 18% chromium is added to steel that makes stainless steel stainless. The more chromium in the composition the more resistance it offers against corrosion. When there is oxygen in the environment of the part, the chromium atoms on the surface react with oxygen to form chromium oxide which further prevents it from reacting with oxygen. The process of exposing the surface to oxygen so that a quick layer of chromium oxide layer is formed is called passivation process.