What element contributes to the formation of a tough surface oxide in stainless steel?

The formation of a tough surface oxide in stainless steel is primarily due to the presence of chromium. When stainless steel is exposed to oxygen, chromium reacts with the oxygen to form a thin, passive film of chromium oxide on the surface of the steel. This oxide layer is highly protective and helps to prevent further corrosion by blocking the diffusion of oxygen and other corrosive elements into the underlying steel.

The effectiveness of chromium in enhancing corrosion resistance is one of the key reasons why stainless steel is valued in various applications where exposure to moisture and corrosive environments are common. Additionally, this oxide layer is self-repairing; if it is damaged, chromium will react with oxygen in the air to restore the protective layer, maintaining the integrity and durability of the material over time.

While nitrogen can also contribute to enhancing the properties of some types of stainless steel, it does not play as central a role in oxide formation as chromium does. Zinc and copper do not contribute to the formation of the protective chromic oxide layer crucial in stainless steel's corrosion resistance.