Question

Why Mg corrodes in both acidic and alkaline oxygen deficient conditions, whereas Fe does not corrode in alkaline oxygen deficient condition?

Answer 1

The corrosion behavior of a metal depends on several factors, including the nature of the metal itself, the environmental conditions in which it is placed, and the electrochemical reactions that occur at the metal surface. In the case of magnesium (Mg) and iron (Fe), their corrosion behavior in oxygen-deficient acidic and alkaline conditions can be explained as follows:

1. Mg corrosion in acidic and alkaline oxygen-deficient conditions: Mg is a highly reactive metal, and it readily reacts with water to form magnesium hydroxide and hydrogen gas. In an oxygen-deficient environment, this reaction can continue even in the absence of oxygen. In acidic conditions, the hydrogen ions present in the solution can accelerate this reaction, leading to faster corrosion of the metal. In alkaline conditions, the hydroxide ions present in the solution can also accelerate the reaction. This is why Mg corrodes in both acidic and alkaline oxygen-deficient conditions.

2. Fe corrosion in alkaline oxygen-deficient conditions: Iron, on the other hand, is a less reactive metal than magnesium. In alkaline oxygen-deficient conditions, the corrosion of iron is inhibited because the formation of iron oxide (Fe2O3) is favored over the formation of iron hydroxide (Fe(OH)2). This is because the solubility of Fe2O3 is lower than that of Fe(OH)2 in alkaline conditions. As a result, the surface of the iron metal is covered with a layer of Fe2O3, which protects the underlying metal from further corrosion.

Answer 2

As we know,

Mg⁺² + 2e^- → Mg, E° = -2.36 V

Fe⁺² + 2e^- → Fe, E° = -0.44 V

H⁺ + e^- → \(\frac{1}{2}H_2\) , E° = 0

2H₂O(l) + 2e^- → H₂(g) + 2OH^-(aq), E° = -0.83

∵ reduction potential of Magnesium (Mg) is less than the reduction potential of H⁺ , H₂O therefore. It can easily reduce H⁺ and H₂O because of that Mg corrodes in acidic and as well as in basic medium.