What does a passive metal mean in terms of corrosion?

A passive metal refers to a type of metal that develops a protective oxide layer on its surface, which helps to prevent further corrosion. This passive film acts as a barrier that inhibits the electrochemical reactions necessary for corrosion to occur. Many metals, such as aluminum, stainless steel, and titanium, exhibit this behavior when they are exposed to an oxidizing environment.

The protective layer that forms is often stable and adheres tightly to the metal, which is crucial because it reduces the rate at which the underlying metal can be oxidized. The passive state can be enhanced through processes like anodization, which increases the thickness of the oxide layer, thereby enhancing corrosion resistance.

The other options address different aspects of corrosion and metal behavior. Rapid corrosion typically describes active metals, high temperatures relate to thermodynamic factors influencing corrosion rates, and the idea that a metal cannot be oxidized contradicts the natural oxidation processes that all metals undergo under certain conditions.