Rusting

Corrosion of iron and steel due to rusting is responsible for millions of pounds of damage each year. Rusting, is oxidation of the metal to form a metal oxide. Rust does not firmly adhere to the surface of the metal allowing it oxide further. The oxide causes damage to the surface of the metal known as pitting which, over time, reduces thes structural integrity of the metal.

What has this got to do with batteries? Rusting is a chemical process the occurs when the iron or steel is exposed to moist air, it reacts with the oxygen in the air to create Iron (III) oxide. We saw earlier how electricity is generated by the process of oxidization and reduction. The formation of rust can occur at some distance away from theactual pitting or erosion of iron as illustrated below. This is possible because the electrons produced via the initial oxidation of iron can be conducted through the metal and the iron ions can diffuse through the water layer to another point on the metal surface where oxygen is available. This process results in an electrochemical cell in which iron serves as the anode, oxygen gas as the cathode, and the aqueous solution of ions serving as a “salt bridge” as shown below.

Rusting of Iron by water droplet.

Fe → Fe⁺² + 2e^- and Fe → Fe⁺³ + e^- in the anode

The amount of water complexed with the iron (III) oxide (ferric oxide) varies as indicated by the letter “X”. The amount of water present also determines the color of rust, which may vary from black to yellow to orange brown. The formation of rust is a very complex process which is thought to begin with the oxidation of iron to ferrous (iron “+2″) ions.

Fe → Fe⁺² + 2 e^-

Both water and oxygen are required for the next sequence of reactions. The iron (+2) ions are further oxidized to form ferric ions (iron “+3″) ions.

Fe⁺² → Fe⁺³ + 1 e^-

Tthe electrons provided from both oxidation steps are used toreduce oxygen as shown.

O₂ (g) + 2 H₂O + 4e^- → 4 OH^-

The ferric ions then combine with oxygen to form ferric oxide [iron (III) oxide] which is then hydrated with varying amounts of water. Other metals, such as Aluminium, form an oxide layer when they come into contact with oxygen from the air but the layer of oxide bonds very strongly to the surface of the Aluminium preventing further oxidation from occurring. However, Aluminium can rust in a very short time if a thin layer of mercury is applied to the surface. Mercury readily combines with aluminium to form a mercury-aluminum amalgam when the two pure metals come into contact. When the amalgam is exposed to air, the aluminium oxidizes, leaving behind mercury. The oxide flakes away, exposing more mercury amalgam, which repeats the process thus a small amount of mercury can rust a large amount of aluminium over time, by progressively forming amalgam and relinquishing the aluminium as oxide. For this reason mercury is prohibited on aircraft.