Technical Information

Cathodic protection (CP) is one of the most effective methods for mitigating corrosion of steel structures in soil and water. The theory and application of CP is well established, widely used, and has been in existence for many years. The science of cathodic protection (CP) was born in 1824, when Sir Humphrey Davy made a presentation to the Royal Society of London: "The rapid decay of the copper sheeting on His Majesty's ships of war, and the uncertainty of the time of its duration, have long attracted the attention of those persons/bodies most concerned in the oil & Gas Industries.

The pipeline Safety Act of 1972 made the application of CP on pipelines transporting hazardous material mandatory for safety concerns, however natural gas and oil companies had already been using CP for economic, as well as safety reasons since the 1930's.
The EPA has enacted rules for mandatory cathodic protection of all new steel underground fuel storage tanks, and deadlines for protection of existing tanks. While these regulations are intended to protect the environment from spillages, an effective CP system will also protect your investment. Repair, clean-up and replacement costs can be averted with cathodic protection.

 

Corrosion of ferrous metal is a natural process in which the metal returns to its native ore. Changing anodic and cathodic areas develop on the metal for a variety of environmental or metallurgical reasons; Corrosion occurs at the anodic areas. Cathodic protection mitigates corrosion by polarizing the structure to a potential equal to or more negative than its most active anode with the application of a low voltage DC current.
There are basically two types of cathodic protection (methods of applying DC current). One method utilizes galvanic anodes such as magnesium; This method is also known as sacrificial cathodic protection; It is the method of choice for low current requirement, low resistivity parameters, no close source of external power, however it is limited by the driving potential of the anode material.
When confronted with a structure or environment that has high resistivity parameters, high current requirement etc. impressed current cathodic protection becomes imperative. This method utilizes an insoluble anode such as graphite or high silicon cast iron. The anodes are energized with a DC power supply from a rectifier or battery, to apply current to the structure.

The basic principle of CP is simple. A metal dissolution is reduced through the application of a cathodic current. Cathodic protection is often applied to coated structures, with the coating providing the primary form of corrosion protection. The CP current requirements tend to be excessive for uncoated systems.



Particle size and shape are also important when specifying a backfill. Both parameters determine the contact area between anode and earth whilst influencing the porosity of the column which is important for gas ventilation. A general purpose coke breeze is for use in shallow horizontal and vertical groundbeds. It has a resistivity of 35 W cm. For deep well applications a special calcined petroleum coke breeze is available. It has a resistivity of 15 W cm and can be pumped.