Cathodic protection theory - chemical reactions

Cathodic protection

The corrosion of steel in aerated sea water can be represented by the following reaction:

2Fe       +      O2        +      2H20      →          2Fe+ +      +      4OH-
steel     +   dissolved  +      water      →        ferrous      +    hydroxyl
                   oxygen                                      ions

In practice, the ferrous ion (Fe+ + ) is likely to oxidize further to ferric ion (Fe+ + +) then to react with the hydroxyl ion (OH-) to produce insoluble ferric hydroxide (Fe(OH)3) which may loosely be called rust.

In order to stop corrosion, the steel must be made cathodic so that the corrosion reaction Fe → Fe2+ + 2e- does not occur.

Any process reaction releasing electrons at the surface would speed up the anodic reaction, and any process consuming electrons at the surface would slow down anodic reaction and accelerate the cathodic reaction.

The predominant cathodic protection reaction in aerobic environmental conditions is:

½ O2 + H20 + 2e- → 2OH-

The cathodic protection reaction alters the ionic concentration at the interface, increasing the hydroxyl ion concentration. The consequent pH increase reduces the solubility of calcium, magnesium and bicarbonate ions at the steel structure interface resulting precipitation of a calcareous scale according to the following reactions:

HCO3- - + OH- → H2O + CO3- - 

Ca+ + + CO3- - → CaCO3

Mg+ + + 2OH- → Mg(OH)2

The precipitation of calcareous deposits (CaCO3 and Mg(OH)2) interfere with oxygen mass transfer to the cathodic protected surface. They are therefore protective and, rather like a paint film, they reduce the current required for cathodic protection.

Correspondingly to the cathodic reactions, an anodic reaction must occur on the surface of the anode provided for the cathodic protection (CP) system. There  are two methods whereby electrons ban be supplied for cathodic protection of a steel structure:

  • Galvanic or sacrificial anode cathodic protection
  • Impressed current cathodic protection

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-