Why does corrosion occur?
Corrosion is material deterioration through chemical or electrochemical interaction with the environment. Iron rusting is a familiar example, but not every metal corrosion product is rust. Moisture, dissolved oxygen, and ions can strongly influence the process.
Electrochemical regions
Different regions of a metal surface can support oxidation and reduction. Metal dissolves where electrons are lost, while another region supports processes such as oxygen reduction. Salts can increase conductivity and damage protective films. Electrically connected dissimilar metals in a suitable electrolyte may undergo galvanic corrosion. Surface oxides vary: some form compact passive layers, while others offer weak protection.
How can it be slowed?
Coatings, suitable alloys, drainage-conscious design, and cathodic protection can reduce corrosion. Zinc coatings can protect iron both as a barrier and through sacrificial action under appropriate conditions. A scratch may accelerate local damage in some coating systems, so all coatings cannot be treated alike. Corrosion rate cannot be judged from color alone; thickness measurements, environmental analysis, and maintenance are used together.