Corrosion product that may form due to oxidation with oxygen und/or in contact with water (figure 1).
Alloy elements may suppress or even avoid corrosion (stainless steel).
Corrosion (iron(III) oxide-hydroxide) has a dark brown to black color and is a mixture of various oxides, hydroxides and oxide hydrates based on iron. In contrast to other oxide layers of many metals (chromium, zinc, aluminum), corrosion does not protect from further oxidation.
Corrosion is accelerated by salts as they increase the conductivity of water.
Corrosion that can be easily removed is also referred to as flash rust. It occurs at iron and steel components that are exposed to the open environment and can be removed with diluted phosphoric acid, citric acid or ascorbic acid, for example.