The small pits that you see are salt corrosion of the stainless steel. Stainless steel can corrode when salt is present, especially when heated. Unfortunately, these cannot be removed. It is possible that these pits can become deep enough that they penetrate the stainless steel outer layer. When this happens, iron based rust can begin to develop. To avoid getting salt pits, do not add salt directly on the surface of the pan. Any added salt should be dissolved in added water or the foods natural juices that form when cooking.