What element is responsible for providing the tough surface oxide on stainless steel?

The element that provides the tough surface oxide on stainless steel is chromium. When stainless steel is exposed to oxygen, chromium reacts with the oxygen to form a thin, protective layer of chromium oxide. This oxide layer is crucial as it is stable and adheres tightly to the underlying steel, effectively protecting it from corrosion and oxidation. The presence of at least 10.5% chromium in stainless steel is what classifies it as stainless, as this content is necessary to form the protective chromium oxide layer. This property is fundamental to the corrosion resistance that stainless steel is known for, making it a preferred material in various applications, particularly in environments where moisture and corrosive substances are present.