By making the potential of the metal surface more negative with respect to its environment (i.e. soil, water, reaction solution, etc), the rate of corrosion can be reduced. Thus, by applying additional current to the buried metal surface, the corrosion rate can be controlled – this technique is known as cathodic protection (CP).
Isinyithi has extensive experience in the design of CP systems for the external protection of buried pipelines, flat bottom tanks, buried tanks, offshore gas and oil platforms, inside of water tanks, pipelines and condensers, settling tank rake arms, reinforcing steel in concrete structures, ships and many other novel applications.
CP is a proven, successful and widely used electrochemical method of protecting buried or submerged steel as well as the reinforcing steel in concrete against corrosion. Cathodic protection was first described in 1824 by Sir Humphry Davy after he noted a significant decrease in the corrosion of copper hulls of ships to which iron ingots had been attached. However, it took more than 100 years before cathodic protection was used to any great extent on pipelines.
By applying a D.C. voltage between a specially provided electrode (the anode) and the steel structure (the cathode) the resultant current flow prevents the loss of metal from the steel surface and protection against corrosion is achieved.
Cathodic protection lends itself to protection in all soils and natural waters, including brackish and sea water. It can also be applied in a variety of chemicals and solutions, although consultation with a CP Design Specialist is recommended.
In general, the combination of coating + cathodic protection is more economical than either alone. In such instances, the coating protects the majority of the pipeline and the cathodic protection current only flows to and protects the exposed steel at any defects in the coating.
Cathodic Protection is not generally used in the following instances…
For any CP system to operate successfully, the following are essential:
Consultation with a CP Design Engineer will also ensure optimal construction practice of peripherals including valve chambers and actuators, flow meters, earthing systems, reservoirs, pump stations, etc.
In practice, Cathodic Protection is applied to a structure by one of two different methods.
- The power required is obtained from the main AC supply, transformed to a lower voltage and rectified to DC. The DC output is connected between the structure and an external anode. The anode is constructed of a material made to last as long as possible under anodic conditions. This is known as an `Impressed Current Cathodic Protection System’ (ICCP).
- The current is obtained by creating a cell or ‘battery’ between the steel structure (cathode) and a more active metal – such as magnesium (the anode). In this cell, the anodic metal is corroded away to provide the power. This is known as ‘Sacrificial Anode’ Cathodic Protection (SACP).