Cathodic and Anodic Protection

Cathodic and Anodic protection are applied to any metallic structure in contact with a bulk electrolyte. In practice its main use is to protect steel structures buried in soil or immersed in water. It cannot be used to prevent atmospheric corrosion.

Cathodic Protection (CP)

The reduction or Prevention of corrosion by making metallic structure as cathode in the electrolytic cell is called cathodic protection. Since there will not be any anodic area on the metal, corrosion does not occur. There are two methods of applying cathodic protection to metallic structure as following :

Impressed current cathodic protection

Corrosion in aqueous solutions proceeds by an electrochemical process, and anodic and cathodic electrochemical reactions must occur simultaneously. Anodic reactions involve oxidation of metal to its ions, e.g. for steel the following reaction occurs.

Fe = Fe++ + 2e (1)

The cathodic process involves reduction and several reactions are possible. In acidic water, where hydrogen ions (H+) are plentiful, the following reaction occurs.

2H+ + 2e = H2 (2)

In alkaline solutions, where hydrogen ions are rare, the reduction of water will occur to yield alkali and hydrogen.

2H2O + 2e = H2 + 2OH- (3)

However, unless the water is deaerated reduction of oxygen is the most likely process, again producing alkali at the surface of the metal.

O2 + 2H2O + 4e = 4OH- (4)

We can change the rate of these two reactions (1and2) by withdrawing electrons or supplying if we withdraw electrons from the piece of metal the rate of reaction (1) will increase to attempt to offset our action and the dissolution of iron will increase, whereas reaction (2) will decrease. Conversely, if we supply additional electrons from an external source to the piece of metal, reaction (1) will decrease to give reduced corrosion and reaction (2) will increase. The latter case will apply to cathodic protection. additional electrons to the piece of metal.

Sacrificial Anodes Protection

To understand the action of sacrificial anodes for cathodic protection it is necessary to have in mind the galvanic series of metals. The galvanic series for a few selected metals in sea,When the tendency for metal to go into solution as metal ions increases (leaving an excess of electrons on the metal surface), i.e.

Me = Me+ + e (5)

the metal becomes more electronegative. Thus, since zinc, aluminium and magnesium are more electronegative than steel they are increasingly able to supply electrons to the more electropositive steel when in electrical contact in water, and will effect cathodic protection of the steel surface.

Anodic Protection (AP)

AP is a technique to control the corrosion of a metal surface by making it the anode of an electrochemical cell and controlling the electrode potential in a zone where the metal is passive. The application of anodic current to the structure decreases the rate of hydrogen evolution. However can be applied only to metals and alloys that exhibit active-passive behaviour, if an active-passive alloy such as stainless steel is maintained in the passive region through an applied potential (or current) from a potentiostat, its initial corrosion rate can be shifted to a low value.