INHIBITION OF CORROSION

As Evan's diagram give the rate controlling step for corrosion process, methods of inhibiting corrosion are understood by using, diagrams. The corrosion current is given by

| I | corr = [ | I |so × ∣ I ∣ m]¹/₂ 

eF [ E₀ so − E₀ M ] /4RT

According to the above equation, | I | corr can be lowered by two methods or these are two ways inhibiting corrosion.

(i) It's possible to lower exchange current densities for electronation and deelectronation. These are known are inhibiting methods.

(ii) By bringing the corrosion potential closer to reversible potential. This is known as cathodic protection.

1) INHIBITION OF CORROSION BY LOWERING EXCHANGE CURRENT DENSITIES :-

(A) Electronation process is inhibited

(i) Hydrogen evolution (cathodic)

The metal is coated with elements like P, As, or Sb    These elements occupy the electron source area on the metal and hence prevent H⁺ reaching the electron source area or electronation is prevented.

The addition of P, As or Sb lower the corrosion current and ∆φ corr is also lowered as shown in the diagram. Hence the corrosion process is inhibited.

ii) OXYGENATION CORROSION :-

O₂ + 4e⁻ 2O ⁻ ⁻

2 O ⁻ ⁻ + 2 H₂O →+ OH ⁻

To inhibit oxygenation corrosion, the metal surface is coated with reducing agents like SO₃⁻ ⁻  N₂H₄. This inhibitors occupy electron source area and remove O₂ gas. Evan's diagram for this inhibitions process is ∣∣∣ ʳ to the one drawn before for H₂ evolution process. 

B) DEELECTRONATION PROCESS IS INHIBITED :-

Organic amines > Aldehyde > thiourea are good e⁻ donors. When these compounds are attached to the surface of corroding metal, they will occupy electron rich area. Hence the deelectronation process is inhibited. 

The same is explained by the following Evan's diagram.

The added inhibitor lowers the corrosion current by, occupying e⁻ sink area or by inhibiting electronation process. But the corrosion potential process up. 

                            From the change in direction of corrosion potential, it's possible to show that the inhibitor blocks or occupies either electron source area inhibits electronation or e⁻ sink area inhibit deelectronation. 

EXAMPLE :1

At a particular alkaline PH, when metal dissolves to from metal ion, metal hydroxide will precipitate and gets coated on the metal. This ppt occupies electron source area and inhibits electronation process. 

EXAMPLE : 2

                 The addition of PO₄ ion precipitate Fe³⁺ as ferric phosphate. This ppt. is coated on iron as the corrosion potential goes up after coating with ppt, the ppt occupies electron sink area and, hence inhibits deelectronation process.