Abriox

PRINCIPLES

Corrosion is the action of a metal that has been extracted from ore reverting to its primary state when exposed to oxygen and water. The most common example is the rusting of steel. Corrosion is an electrochemical process, normally occurring at the anode but not the cathode.

The principle of cathodic protection is to connect an external anode to the metal to be protected and to pass a DC current between them so that the metal becomes cathodic and does not corrode. In a pipeline system there are two ways of doing this:

• Using an external galvanic anode, where the DC current arises from the natural difference in potential between the metals of the anode (eg Zn, Al or Mg) and the pipe (eg carbon steel). The anode is electrically connected to the pipeline, causing a positive current to flow from the anode to the pipe so that the whole surface of the steel becomes more negatively charged, ie the cathode.

• Using an external DC power source (rectified AC) to impress a current through an external anode (usually inert) onto the surface of the pipe, which becomes the cathode.

Galvanic systems are easy to install, have low operating costs and minimal maintenance requirements, do not need an external power supply and rarely interfere with foreign structures. However, they offer limited protection of large structures and are therefore used for quite localised CP applications.

Impressed current systems are more frequently used to protect pipelines and underground storage tanks. Their high current output is capable of protecting large underground metal structures economically, is flexible to deal with varying conditions and less susceptible to soil resistivity. However, they rely on the continuity of their AC power source and can interfere with other nearby buried structures.

The level of CP current that is applied from impressed current systems is important. Too little current will lead to corrosion damage; excessive current can lead to disbanding of the coating and hydrogen embrittlement.

For these reasons impressed current systems require regular monitoring.

Corrosion effects at (a) pipeline coating; (b) pipeline weld