1 - Hot Dip Galvanized Coating

A galvanized cell is formed. The zinc around the point of damage corrodes. Corrosion products precipitate on the steel surface and protect it. The steel is also protected because it is cathodic in relation to the zinc coating.

2 - Paint Coating

The steel rusts where the paint film has been damaged. Rust creeps under the paint film, which is lifted up from the steel surface. Corrosion continues until the damage has been repaired.

3 - Coating of more electro positive metals than steel

Nickel, chromium, and copper - give rise to more rapid corrosion at the point of damage than if the steel had been uncoated. The corrosion often takes the form of pitting, which can even go through steel.