Question

What is cathode reaction and anode reaction in cathodic protection ?

Answer 1

Sacrificial anode cathodic protection

For cathodic protection by sacrificial anodes made of alloys of zinc, aluminum or magnesium are used to form a galvanic cell. Owing to the potential differences existing between sacrificial anode alloys (less noble) and the cathodic area (steel), positively charged metal ions leave the anode surface while electrons leave the surface at the cathode.

In the case of aluminium alloy anodes, the reaction at the anode surface is:

4Al → 4Al^{+ + +} + 12e^-

Impressed current cathodic protection

Impressed current systems use anodes of a type that are not easily dissolved into metallic ions, but rather sustain alternative anodic reactions. In good sea water environmental conditions oxidation of the dissolved chloride ions will be the predominant anodic reaction resulting chlorine gas developed at the anode surface.

2Cl^- => Cl₂ + 2e^-

In low salinity waters the predominant anodic reaction will be decomposition of water.

2H₂O => O₂ + 4H⁺ + 4e^-