How do humidity and temperature affect corrosion of metals in air?

• A. They decrease corrosion rates by drying the surface.
• B. Higher humidity and temperature generally increase corrosion rates by promoting moisture films and faster chemical reactions.
• C. Only humidity, not temperature, affects corrosion.
• D. Only temperature, not humidity, affects corrosion.

Answer: (B)

The key idea is that corrosion in air is an electrochemical process that needs a thin electrolyte and enough energy to drive reactions. When humidity is high, a film of water forms on the metal surface, and this moisture acts as the electrolyte that lets ions move between anodic and cathodic sites. Oxygen, water, and metal ions react together, producing corrosion products like rust for iron. Temperature matters too because warmer conditions speed up chemical reactions and increase the conductivity of the thin moisture film, so ions move faster and the overall corrosion rate rises. So, higher humidity supplies the water layer that enables the electrochemical process, and higher temperature speeds that process—together they generally increase corrosion rates. The other ideas fall short because corrosion in air doesn’t occur as effectively on a dry surface, and it isn’t driven by temperature alone without the presence of a moisture film to carry the electrochemical reactions.