General Background To ICCP Control Systems

Structures requiring impressed current cathodic protection systems (ICCP’s) are split into zones so that each zone has a separately controlled power supply. Zones are used to limit the total power in any one area and to enable different current densities to be used in areas of different steel densities and/or resistivities. Because the power supply required is not constant (typically the power required diminishes over time) half cells are strategically located within each zone to monitor the electrical potential of the reinforcement. The corrosion protection performance is measured by switching off the power supply and measuring the difference between the electrical potential immediately after switching off the power and after a set period. A common criteria is the difference as measured at 24hrs being more than 100mV.

The control system for a CP system contains transformers to drop the voltage down from mains supply to a safe operating voltage for the CP system, a control unit which varies the power to each zone as required, and commonly a computer and software used for monitoring inputs and outputs. This computer adjusts the outputs as a function of the inputs, communications devices, mechanical devices for adjustments, ancillary devices such as fuses, switches, sockets and heaters. This is all housed in a box. On large projects there may be a major control station and substations at various points.

The current density requirements determines the power required to achieve protection. Chess [1] recommends a trial is undertaken to establish these but very often a conservative estimate is made based on experience as there is no time or budget for the trials. Chess [1] suggests the following:

Exp. class	Environment	Examples	Current density (mAm² of rebar)
1	Alkaline, no corrosion occurring, low oxygen supply.	Buried elements such as footings and piles.	0.1
2	Alkaline, no corrosion occurring, exposed structure.	Sheltered structures prior to corrosion activation.	1-3
3	Alkaline, chloride present, dry good quality concrete, high cover, light corrosion observed on rebar.	Sheltered structures with some activation but little corrosion damage.	3-7
4	Chloride present, wet, poor quality concrete, medium-low cover, widespread pitting and general corrosion on steel.	Marine structures where there is general active corrosion and significant corrosion damage.	8-20
5	High chloride levels, wet fluctuating environment, high oxygen levels, hot, severe corrosion on steel, low cover. This is an exceptional circumstance.	Seawater or brine tanks and channels. Severe splash zones. Tropical environments. Low cover and advanced corrosion.	30-50

CP systems are most commonly used in Australia for chloride contaminated structures (e.g. marine exposure) in exposed environments after they evidence some corrosion. Hence a common conservative value used for current density is 20mA/m2 of rebar.