The alloy nature of Stainless Steel and its thin, transparent layer on the surface provides a self healing feature. When activated by oxygen within the air or water then it is this layer which repairs lightly scratched or any damage to the surface. Stainless steels contain a significant amount of chromium, which forms Chromium Oxide when it oxidizes (rusts). In contrast to a simple carbon steel, which forms iron oxide when it rusts, (the flaky reddish brown oxide we typically associate with rust). Iron oxide is brittle and permeable, doing little to prevent further oxidation of the steel surface. In contrast, chromium oxide does not have an objectionable appearance, being the same effective color as the base metal, just a bit duller, and is hard and resilient. When you scratch or mar stainless, the exposed chromium just forms a new oxide layer, protecting the underlying metal.