State necessary condition of iron corrosion.

Explanation

**Step-by-Step Solution:**

1. **Understanding Iron Corrosion:**
- Iron corrosion, commonly known as rusting, is a chemical process where iron reacts with oxygen and moisture in the environment.

2. **Necessary Conditions for Iron Corrosion:**
- **Exposed Metal Surface:** The iron must have an exposed surface that is not protected by paint, oil, or any other coating. This allows the iron to come into contact with the environment.

...