Galvanic corrosion takes place when dissimilar metals make physical or electrical contact. Corrosion which results from electrical contact arises when through the effects of electrolytes which makes the anode to deplete at a faster rate than the cathode. Using electromotive force estimation, it is possible to predict which metal is prone to faster corrosion when contact is made with another metal. This is the anchor upon which cathodic protection is predicated.
Cathodic Protection was developed as a means of electrochemical corrosion control such that the oxidation reaction within the galvanic cell is concentrated concertedly at the anode in order to suppress the cathode corrosion. For steel pipelines, cathodic protection is made possible by the use of a connection to a sacrificial magnesium anode that is buried in a common soil electrolyte.
The discovery of cathodic protection is attributed to Sir Humphrey Davy, who unveiled the technique in 1824 to combat the onslaught of corrosion in the British naval fleet. In modern usage, pipelines are protected by an organic protective coating which is aided by sized cathodic protection systems which come alive when the protective coating fails. This procedure is used for carbon steel pipes that are within subsea levels or buried underground apart from oil rig platforms and associated concrete structures.
When cathodic protection is ensured, there might still be cases of iron corrosion, but this will be dependent on the dimensions of the iron and zinc electrodes. Sometimes, hydrogen reduction could occur on the zinc anode as it is the site for net oxidation reaction while net reduction reactions occur at the cathodes.
For a cathodic protection system, there must be a cathode, anode, an electric circuit linking the cathode and the anode as well as an electrolyte. For cathodic rust & protection to take effect, the underlying structures must not be exposed to air environments. Air works as a poor conductor, and it will inhibit the flow of current to the cathode from the anode like in other electronic rust protection measures.
Cathodic Protection is deployed by two major means:
- by the coupling of a given structure like Iron with any other active metal like magnesium or zinc with higher activity ratings. This will lead to the production of a galvanic cell with an anode represented by the more active metal which leads to electrons flux within the structure to serve as the cathode. As the anode gets destroyed progressively, it acts out its role as the sacrificial anode while the cathode gets protected.
- The alternative measure is to channel direct current between the structure and an inert anode to provide the required protection. With the flow of electrons to the structure, it secures protection and is prevented from becoming the anode or the source of electrons.