Elevated temperature, immersion in a humid and oxygen rich environment, contact with a metal that is lower in the electrochemical series, and mechanical agitation are conditions that cause an iron nail to rust faster. Rusting is a decomposition reaction where iron reacts with oxygen and water to form hydrated ferrous oxide and hydroxide.
Iron rust is flaky, porous and friable, providing no protection to subsurface layers against rusting. Rusting can occur on pure iron and iron alloys, such as steel. Steel rust can be dangerous because it may eat away at specific regions of the exposed bulk that are electrochemically favorable for rusting. This can lead to unanticipated failure of steel parts when the localized rusting is not counteracted in time. Pure iron immersed in a corrosive environment rusts uniformly all over its surface, making visual inspection and corrosion rate calculations easier. Anaerobic rusting can occur in the presence of other oxidative compounds, such as chlorine salts. Iron reacts with chlorine forming iron (II) and iron (III) chloride, which possesses a characteristic green color. Rusting can be prevented by coating an iron surface with an impermeable coating of paint, oil or metal. Protective metals can be deposited on iron through an electroplating process or by dipping the iron in a hot alloy having a relatively low melting point.