What is necessary for rusting?

• A. Water (humidity) and oxygen (air).
• B. Heat and sunlight.
• C. Only water.
• D. Only oxygen.

Answer: (A)

Rusting happens when iron reacts with oxygen in the presence of water. Water provides the medium that allows ions to move, so it acts as an electrolyte, letting the iron lose electrons and oxygen to gain them. Without water, the reaction still needs a way for ions to move, so rust doesn’t form readily; without oxygen there’s nothing to oxidize. That’s why both water (humidity) and oxygen are required for rust to occur. Heat and sunlight aren’t necessary for rust, though they can speed it up; in dry conditions rust forms far more slowly.