For a wide variety of applications, stainless steel competes with carbon steels supplied with protective coatings, as well as other metals such as aluminum, brass and bronze. The success of stainless steel is based on the fact that it has one unique advantage. The chromium in the stainless steel has a great affinity for oxygen, and will form on the surface of the steel at a molecular level a film of chromium oxide. The film itself is about 130 Angstroms in thickness, one Angstrom being one millionth of one centimeter. This is like a tall building being protected from the rain with a roof the thickness of one sheet of ordinary copy paper. This layer is described as passive, tenacious and self renewing. Passive means that it does not react or influence other materials; tenacious means that it clings to the layer of steel and is not transferred elsewhere; self renewing means that if damaged or forcibly removed more chromium from the steel will be exposed to the air and form more chromium oxide. This means that over a period of years a stainless steel knife can literally be worn away by daily use and by being resharpened on a sharpening stone and will still remain stainless. Silver plated cutlery will eventually wear through to the base alloy, but stainless steel cutlery cannot wear through. Manhole and access covers in the water treatment and chemical industry are widely made out of both galvanized steel and stainless steel. In normal use galvanized steel can last many years without corrosion occurring and in these cases there would be little advantage apart from aesthetic reasons to switch to stainless steel. Where stainless comes into its own is where the galvanized coating is constantly being worn away, for example by chains being dragged over it, or constantly being walked over, or where very corrosive chemicals are being randomly splashed onto it.
This leads on to the important point that the initial investment cost of producing a component or fabrication in stainless steel will always be more expensive that using ordinary steel, not just because of the higher cost of stainless steel, but also because it is more difficult to machine. However it is the better life cycle costs of stainless steel that make it attractive, both in terms of much longer service life, less maintenance costs, and high scrap value on de-commissioning.