Corrosion Control by Electrochemical Protection or Cathodic Protection

CORROSION CONTROL BY ELECTROCHEMICAL PROTECTION OR CATHODIC PROTECTION

The principle involved in the electrochemical protection (or) cathodic protection is to force the metal to behave like a cathode. The important cathodic protections are

(i) Sacrificial anodic protection

(ii) Impressed current cathodic protection

1. Sacrificial anodic protection method

In this method, the metallic structure to be protected is made cathode by connecting it with more active metal (anodic metal). So that all the corrosion will concentrate only on the active metal. The artificially made anode thus gradually gets corroded protecting the original metallic structure.

Hence this process is otherwise known as sacrificial anodic protection.

Aluminium, Zinc, Magnesium are used as sacrificial anodes.

Applications of sacrificial anodic protection

(a) This method is used for the protection of ships and boats. Sheets of Mg or Zn are hung around the hull of the ship (Fig. 4.12). Zn or Mg will act as anode compared to iron (ship or boat is made of iron), so corrosion concentrates on Zn or Mg. Since they are sacrificed in the process of saving iron, they are called sacrificial anodes.

(b) Protection of underground pipelines, cables from soil corrosion (Fig. 4.13(a)).

(c) Insertion of Mg sheets into the domestic water boilers to prevent the formation of rust (Fig. 4.13(b)).

(d) Calcium metal is employed to minimize engine corrosion.

2. Impressed current cathodic protection method

In this method, an impressed current is applied in the opposite direction of the corrosion current to nullify it, and the corroding metal is converted from anode to cathode.

This can be done by connecting negative terminal of the battery to the metallic structure, to be protected, and positive terminal of the battery is connected to an inert anode. Inert anodes used for this purpose are graphite, platinised titanium. The anode is buried in a “back fill" (containing mixture of gypsum, coke, breeze, sodium sulphate). The "back fill" provides good electrical contact to anode (Fig. 4.14).

Applications of impressed current protection

Structures like tanks, pipelines, transmission line towers, underground water pipe lines, oil pipe lines, ships, etc., can be protected by this method.

Table 4.3 Comparison of Galvanic method (Sacrificial anode) and Impressed current Cathodic method

Sacrificial anodic method

1. No external power supply is necessary.

2. This method requires periodical replacement of sacrificial anode.

3. Investment is low.

4. Soil and microbiological corrosion effects are not taken into account.

5. This is most economical method especially when short‒term protection is required.

6. This method is suitable when the current requirement and the resistivity of the electrolytes are relatively low.

Impressed current method

1. External power supply must be present.

2. Here anodes are stable and do not disintegrate.

3. Investment is more.

4. Soil and microbiological corrosion effects are taken into account.

5. This method is well suited for large structures and long term operations.

6. But this method can be practiced even the current requirement and the resistivity of the electrolytes are high.