Gradual deterioration of a solid (especially metals) due to chemical processes such as oxidation or the action of a chemical agent. Some corrosion products, like metal patinas, can be protective; others, like rust, can be harmful. Corrosion is often accelerated by contact with materials that generate acidic vapours – e.g. wood, plastics.  

Green corrosion product on a bronze bell. Photo courtesy of Alice Cannon.
White corrosion product is visible on the metal components of this pocket knife handle. Photo courtesy of Christine Ianna. 

Corrosion product can be seen around the metal clasp on this leather briefcase. Photo courtesy of RM Cannon.