Scanning electron micrograph of a piece of rusted bodywork from a car. The picture shows three layers of paint above a mass of rust (red) at the base. The uppermost paint layer (mauve) is from a respray, and is just over 1mm thick; the middle and lower layers are the manufacturer's coatings. Rust is the result of oxidation of iron by the oxygen in air, in the presence of moisture. To bottom left, the rust appears as a coarse meshwork of fibres. This reflects the crystal structure of hydrated ferric oxide. Air and moisture cannot pass through intact layers of paint; rusting follows initial damage to the coating. Once rusting begins, the coating may be lifted due to the greater bulk of the rust, causing it to crack and spall (seen here), allowing in more moisture. This specimen was removed from the edge of a hole in a car door; there is no uncorroded metal visible in the picture