Corrosion

Corrosion is the spontaneous oxidation of metal in the presence of an oxidizing agent, usually oxygen. It is the same phenomenon behind the blue-green color of the Statue of Liberty or a rusty anchor.

Some corrosion is protective. For example, copper oxidizes in the presence of oxygen, carbon dioxide, and water, to form basic copper carbonate, a blue-green and structurally stable protective layer.

Other corrosion is detrimental. For example, iron oxidizes in the presence of oxygen, water, and hydrogen ions, to structurally unstable metal-oxides called rust, which continuously flakes off, exposing the underlying metal to further oxidation.

Rusting is a spontaneous electrochemical reaction. Defects on the surface serve as the anodic regions where iron, with an electrode potential of −0.44 volts, readily oxidizes to iron(II).

The electrons migrate towards the cathodic region, where oxygen, with an electrode potential of +1.23 volts, is reduced to water in an acidic environment. The hydrogen ions come from the atmospheric reaction between carbon dioxide and water, forming carbonic acid.

Next, the iron(II) ions migrate to the cathodic region via the surface moisture and are further oxidized to iron(III) oxide hydrate, commonly known as ‘rust.’ The hydrate’s stoichiometry varies with the amount of water to which iron(II) is exposed.

Rusting is accelerated with additional moisture, acids, and electrolytes, which enhance the flow of charges, and the concentration of H⁺ ions.

Methods to prevent corrosion include painting the metal surface to prevent contact with water and oxygen or usage of alloys, a combination of multiple metals. Stainless steel is an alloy of iron containing a small amount of chromium, which corrodes to form an unreactive oxide layer on the surface protecting the iron.

Alternatively, galvanization is used, too. Here, an iron screw is coated with a layer of more easily oxidized metal, like zinc, which forms a protective layer of zinc oxide protecting the iron.

Lastly, cathodic protection controls corrosion by converting the metal being protected into the cathode. Underground iron pipes are often connected to a more easily oxidized metal, such as zinc, where the zinc acts as a sacrificial anode being oxidized, and the iron is the cathode protected from corrosion.