Factors Influencing the rate of corrosion

FACTORS INFLUENCING THE RATE OF CORROSION

The rate and extent of corrosion mainly depends on the following two factors.

1. Nature of the metal

2. Nature of the environment

1. Nature of the metal

1. Position in emf series

Extent of corrosion depends on the position of the metal in the emf series. Metals above the hydrogen in emf series get corroded vigorously. Lower the reduction potential, greater is the rate of corrosion. When two metals are in electrical contact, the more active metal (or the metal having high negative reduction potential) undergoes corrosion.

The rate and severity of corrosion depends on the difference in their positions in the emf series. Greater the difference faster is the corrosion rate.

2. Relative areas of the anode and cathode

Rate of corrosion will be more, when the cathodic area is larger. When the cathodic area is larger, the demand for electrons will be more and this results in an increased rate of corrosion (dissolution) of metals at anodic area.

3. Purity of the metal

100% pure metal will not undergo any type of corrosion. But, the presence of impurities in a metal create heterogeneity and thus galvanic cells are set up with distinct anodic and cathodic area in the metal. Higher the percentage of impurity, faster is the rate of corrosion of the anodic metal.

The effect of impurities on the rate of corrosion of zinc is given below.

4. Over voltage or over potential

Over voltage of a metal in the corrosive environment is inversely proportional to corrosion rate.

Example: The normal hydrogen over voltage of zinc metal, when it is dipped in 1 M H₂SO₄, is 0.7 volt. Here the rate of corrosion is low. By adding small amount of impurity like CuSO₄ to H₂SO₄, the hydrogen over voltage is reduced to 0.33 V. This results in increased rate of corrosion of zinc metal.

5. Nature of the surface film

Nature of the oxide film, formed on the metal surface, decides the extent of corrosion which can be decided by Pilling‒Bedworth rule

(i) In case of alkali and alkaline earth metals such as Mg, Ca, etc. form oxide, whose volume is lesser than the volume of metal. Hence the oxide film will be porous and non‒protective and bring about further corrosion.

(ii) But in heavy metals like Al, Cr, etc. form oxide, whose volume is greater than that of the metal. Hence the oxide film will be non‒porous and protective and prevents further corrosion.

6. Nature of the corrosion product

If the corrosion product is soluble in the corroding medium, the corrosion rate will be faster. Similarly, if the corrosion product is volatile (like MoO₃ on Mo surface), the corrosion rate will be faster.

2. Nature of the environment

1. Temperature

Rate of corrosion is directly proportional to temperature. This is because, the rate of chemical reaction and the rate of diffusion of the ions increases with rise in temperature. Hence the rate of corrosion increases with temperature.

2. Humidity

Rate of corrosion will be more, when the humidity in the environment is high. The moisture acts as a solvent for the oxygen in the air to produce the electrolyte, which is essential for setting up a corrosion cell.

3. Presence of corrosive gases

Acidic gases like, CO₂, SO₂, H₂S and fumes of HCI, H₂SO₄, etc., produce electrolytes, which are acidic and increases the electrochemical corrosion.

4. Presence of suspended particles

Particles like, NaCl, (NH₄)₂SO₄ along with moisture act as powerful electrolytes and thus accelerate the electrochemical corrosion.

5. Effect of pH

Possibility of corrosion with respect to pH of the electrolytic solution and the electrode potential of the metal is correlated with the help of a pourbaix diagram.

Pourbaix diagram

The pourbaix diagram for iron in water is shown in fig. 4.10. It shows clearly the zones of corrosion, immunity and passivity. In the diagram, 'Z' is the point where pH = 7 and the electrode potential = ‒0.4 V. It is present in the corrosion zone. This clearly shows that iron rusts in water under those conditions. In actual practice, it is observed to be true.

From the diagram (Fig. 4.10) it is clear that the rate of corrosion can be altered by shifting the point 'Z' into immunity or passivity regions. The iron will be immune to corrosion, if the potential is changed to about ‒0.8 V by applying external current. On the other hand, the rate of corrosion of iron can also be reduced by moving into the passivity region by applying positive potential.

The diagram clearly indicates that the rate of corrosion can also be reduced by increasing the pH of the solution by adding alkali.

Thus, the rate of corrosion will be maximum when the corrosive environment is acidic. i.e., pH is less than 7.