STAIN AND STAINLESS STEEL
Alloy steel can be used, say P9 up to 750 degree C maximum. But engineering material are required for operating at higher temperature. Corrosion rate are generally increasing with increasing in temperature and hence higher alloys are required for reliable performance.
Hence the need for better and better alloys that led to invention of class of steel called stainless steel. As the term specified the steel does not stain and hence called stainless steel.
What is stain?
Take a piece of carbon and through it in atmosphere for 3 to 4 days. Then take that piece and see, there would be brown spots appearing on the steel surface exposed to atmosphere. Leave the piece for few more days and you would find the number of spot have increased covering the complete surface area.
After a few days more there would be a thin brownish layer that can be easily removed. The brownish spots that initially occurred are called stain and final brownish layer is called rust scale. In chemical term the stain and brownish scale is nothing but called iron oxides. So when steel exposed to atmosphere gets stained.
What is called stainless steel?
Steel that does not stain when exposed to atmosphere is called stainless steel. For steel to be converted to stainless steel it requires minimum 11% chromium to be added. So stainless steel with 11% chromium and more are term as stainless steel.
This mean if we cut a piece of stainless steel and allowed it to expose in atmosphere then it never form iron oxide but it forms chromium oxide and this is an adherent micron thick film. This prevents further attack of the steel by the environment. So there is no staining hence called stainless steel.