Why is water needed for the rusting reaction shown for iron in damp air?

Water is essential for the rusting of iron because it acts as a necessary medium for electrochemical reactions to occur.12

When iron is exposed to moisture and oxygen, a thin surface film forms that serves as an electrolytic medium. This medium facilitates the flow of ions and the transfer of electrons between different regions of the metal, which function as anodes and cathodes. Without this moisture, the oxidation process—where iron atoms lose electrons to oxygen—would be significantly slower or might not occur at all.231

Essentially, the water enables the redox reactions that convert iron into iron oxide, commonly known as rust. The presence of additional substances in the water, such as salts or acids, further increases electrical conductivity and accelerates this corrosive process.412