4.C] Explain the factors affecting the rate of corrosion
(i) Nature of corrosion product,
(ii) Ratio of anodic to cathodic areas
(iii) pH
Answer:-
(i) Nature of corrosion product
The corrosion product formed like metal oxide may act as a protective film if it is stable, insoluble, stoichiometric, and non-porous. Thus acts as a protective film and prevents further corrosion by acting as a barrier between the metal surface and corrosion medium. On the other hand, if the corrosion product is unstable, porous, non-stoichiometric, and soluble, it further enhances corrosion.
For E.g. in oxidizing environments, metals like aluminum, chromium, titanium, etc., are highly passive as their oxides as corrosion products form protective films on the metal surface, preventing further corrosion. Metals such as iron, zinc, magnesium etc., do not
form any protective film and are highly corrosive.
(ii) Ratio of anodic to cathodic areas
The rate of corrosion is influenced by the size of the cathodic to the anodic area. If the metal has a small anode and large cathodic region, the corrosion rate is very high. This is because at the anode electrons are liberated. Which are consumed in the cathodic region, if a cathodic region is larger the liberated electrons are rapidly consumed at the cathode. This further enhances the anodic reaction leading to an increase in the overall rate of corrosion.
(iii) pH
The general rate of corrosion is higher in acidic pH than in neutral and alkaline pH. In acidic solutions, they react quickly and initiate the reactions. In the case of iron, at a very high pH protective coating of iron oxide is formed which prevents corrosion. whereas at low pH severe corrosion takes place. But for metals like Aluminum, the corrosion rate is high even at high pH.