Stainless steel is so called because it resists rusting. Imparting this quality to steel is accomplished by an alloy addition of at least 10% chromium. The chromium unites with oxygen to form a very tight transparent layer over the steel surface that prevents rusting by precluding further oxidation. This transparent layer is self-healing when damaged by scratches, wear or denting. Stainless steels also withstand the corrosive attack of many acids. They possess strength and toughness at both extremes of the temperature scale, yet can be fabricated into intricate shapes for many uses. There are more than 250 different stainless steel grades which are divided into five major families or classes. The general classes have been developed to consolidate the chemistries and mechanical properties required to meet specific applications