Explain the effect of following factors on the rate of corrosion : (1) Nature of corrosion product. (2) pH (3) Anodic and cathodic areas (4) purity of metal

(1) Nature of the corrosion product (oxide film): in dry corrosion the oxide layer formed on the surface decides further attack.

• Stable, non-porous, adherent film (Al, Cr, Ti, Sn, Pb): it covers the metal completely and stops O₂ diffusing in, so corrosion stops (passivation).
• Porous film (alkali and alkaline earth metals; Pilling–Bedworth ratio below 1): O₂ reaches the metal through the pores, so corrosion continues.
• Unstable film (Ag, Au, Pt): the oxide decomposes back to metal, so no lasting corrosion occurs.
• Volatile film (MoO₃): it evaporates and exposes fresh metal, so corrosion is rapid and continuous.

(2)

pH of the medium: the rate of corrosion generally rises as pH falls, because acidic media supply H⁺ for the cathodic reaction (2H⁺ + 2e⁻ → H₂).

• For iron, corrosion is fast below pH about 4. Between pH 4 and 10 the rate is roughly constant and set by how fast O₂ diffuses. Above pH 10, a protective hydroxide/oxide film makes corrosion very slow.
• Amphoteric metals such as Al, Zn and Pb corrode in both strongly acidic and strongly alkaline media and are most stable near neutral pH.

(3)

Relative areas of anode and cathode: rate of corrosion \( \propto \dfrac{\text{area of cathode}}{\text{area of anode}} \).

With a small anode and a large cathode, all the electrons demanded by the large cathode must come from the small anodic area. The anode then corrodes very fast and becomes pitted. Example: a steel rivet in a copper plate corrodes rapidly. A small break in a tin coating on iron leads to severe pitting of the iron.

With a large anode and a small cathode, corrosion is spread over a large area and is slow. Example: copper rivets in a steel plate.

(4)

Purity of the metal: impurities in a metal set up tiny galvanic cells (local anode–cathode pairs), which speed up corrosion. The more impure the metal, the faster it corrodes. Example: 99.999% pure Zn corrodes much more slowly than 99.5% Zn containing Fe or Pb impurities. Very pure metals such as high-purity Al resist corrosion well.