Question

Discuss the factors which govern the rate of corrosion of a metal.

Answer 1

Corrosion of a metal depends upon several factors. Some of the important ones are as follows.

1. Presence of reactive gases in air: High reactive gases, including CO₂, SO₂, NO, and others, dissolve with water to form acids. The resulting acids function as electrolytes and quicken the corrosion process.
2. Presence of electrolytes in water: The rate of corrosion is accelerated when electrolytes are present in water. The corrosion of iron, for instance, occurs more frequently in seawater than in purified water. This is due to the presence of several electrolytes, including KCl and NaCl, in saltwater.
3. Reactivity of metal: Compared to a less reactive metal, a reactive metal is susceptible to corrosion. For example, because iron is more reactive than silver, it corrodes more quickly. A metal’s electropositive nature determines its reactivity. A metal that is more electropositive, that is, has a lower standard reduction potential, is also more reactive. As a result, metals positioned higher in the electrochemical series, such as Na, Ca, Mg, Al, Zn, Fe, etc., corrode more quickly, whereas metals positioned lower in the series, such as Ag, Hg, etc., corrode less readily.
4. Presence of impurities in metals: Pure metals usually do not corrode much. It is the presence of other metals (impurities) which compels the host metals to undergo the corrosion process. This is because the less electropositive impurity metals set up local galvanic cells with the host metal and oxidise the more electropositive host metal, which results in the corrosion of the host metal.