How do following factors affects the rate of corrosion i) PH medium ii) Purity of metal iii) Area of anode and cathode.

i)

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.

ii)

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.

iii)

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.