We are all familiar with its effects, from rusting of airplanes and automobiles to bridges and razor blades. Direct costs include replacement due to failure or finite life cycle.
Scientists recognize two broad categories of corrosion, general corrosion and localized corrosion.
The former involves uniform surface oxidation penetrating into the bulk of the material. This process is now well-understood, and corrosion scientists can reliably predict and control general corrosion rates. In fact, corrosion can be nearly halted by nanometers-thick passive films, which form either naturally as part of the general corrosion process or by design as in anodic protection. Just such a passive layer, with a mass of only a few tens of micrograms, protects a stainless-steel kitchen knife. In localized corrosion, the passivating layer is locally disrupted, causing pits and cracks and leading to mechanical failure.