Chemistry – Metals - Corrosion of metals | e-Consult

The rusting of the steel bolt occurs due to electrochemical corrosion. Iron is a more reactive metal than chromium, which is the main component of steel. When the iron and chromium are in contact with an electrolyte (like water containing salts or pollutants), a galvanic cell is formed. The iron acts as the anode (where oxidation occurs), losing electrons and becoming iron ions (Fe²⁺). These ions dissolve into the electrolyte. The chromium acts as the cathode (where reduction occurs), gaining electrons. Oxygen in the water acts as the oxidizing agent, accepting electrons and forming hydroxide ions (OH^-). The iron ions then react with the hydroxide ions and oxygen to form hydrated iron(II) oxide (rust, Fe₂O₃.nH₂O). The difference in electrochemical potential between the iron and chromium drives the flow of electrons, causing the corrosion process.

One modification to the bolt that would help prevent corrosion is to use a galvanised bolt. This involves coating the steel bolt with a layer of zinc. Zinc is less reactive than iron, so it acts as a sacrificial anode. The zinc corrodes preferentially to the iron, protecting the iron from corrosion. The zinc corrodes to form zinc ions (Zn²⁺), which are less harmful than iron ions. This is a form of galvanic protection.