How do the following factors affect the rate of corrosion ? (i) Retative areas of anodic and cathodic parts. (ii)Passive character of metal. (iii) pH of medium.
1 Answer

i) Relative areas of anodic and cathodic parts

• The rate of the corrosion is greatly influenced by the relative sizes of cathodic and anodic areas.

• If the metal has smaller the anodic area and larger the cathodic area exposed to corrosive atmosphere, more intense and faster is the corrosion occurring at anodic area because at anode oxidation takes place and electrons are liberated.

• At the cathode these electrons are consumed. When anode is smaller and cathode region is larger all the liberated electrons at anode are rapidly consumed.

• This process makes the anodic reaction to takes place at its maximum rate thus increasing the corrosion rate.

• If the cathode is smaller and reverse process takes place decrease rate of corrosion.

• For e.g.: If tin (Sn) coated on iron (Fe) and in that some area are not covered or some pin holes are left, there forms smaller anodic area and larger cathodic area because tin is cathodic with respect to iron so intense localized corrosion takes place. On the other hand if Zn coated to Fe then if there are some pin holes are there creates larger anodic area and smaller cathodic area because Fe is cathodic with respect to zinc so that rate of corrosion is very less.

ii) Passive character of metal.

• The tendency of a metal to undergo corrosion is dependent on the nature of the metal. Metals with lower reduction potential undergo corrosion easily whereas metals with higher reduction potential do not undergo corrosion easily.

• The reactive metals like Na, K, Mg, and Zn are more susceptible for corrosion. The noble metals like Ag, Au, Pt, Pd are less susceptible for corrosion.

iii) pH of the medium:

• In general, lower the pH higher is the rate of corrosion.

• If the pH is greater than 10, corrosion of iron is very less due to the formation of protective coating of hydrous oxides of iron.

• If pH is between 10 and 3, then presence of oxygen is essential for corrosion of iron.

• If the pH is 3 or lower than 3 severe corrosion occurs in the absence of air due to the continuous evolution of H2 at cathode. • However metals like Al, Zn etc. undergo fast corrosion in highly alkaline medium.

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