How it works
All cathodic protection shifts the potential of a metal structure in the negative direction until corrosion is suppressed. In an impressed current system, the driving force comes from a power supply instead of a galvanic couple. The rectifier's negative terminal is connected to the structure and its positive terminal to an anode bed buried in the soil, suspended in water or installed inside a vessel. Current leaves the anodes, travels through the electrolyte and enters the structure, which becomes the cathode of the circuit. Anode materials are chosen to be consumed very slowly: mixed metal oxide on titanium, high-silicon cast iron, graphite and platinized titanium or niobium are common choices.
ICCP versus sacrificial anodes
| Aspect | Impressed current | Sacrificial anodes |
|---|---|---|
| Power source | External rectifier | Galvanic potential of zinc, aluminum or magnesium |
| Output | Adjustable | Fixed by anode material and electrolyte |
| Best fit | Large, bare or poorly coated structures, high-resistivity soils | Smaller or well-coated structures, local protection |
| Operation | Needs power, monitoring and maintenance | No power, but anodes must be replaced when consumed |
| Interference risk | Can cause stray current corrosion on nearby foreign structures | Low |
A reversed connection is a known hazard: if the leads are swapped, the structure becomes the anode and corrodes rapidly.
Overprotection and coatings
More current is not better. At excessively negative potentials, water is reduced at the steel surface, producing hydroxyl ions and hydrogen. The alkali attacks the coating bond and drives cathodic disbondment and blistering, while atomic hydrogen can embrittle high-strength steels. Coating selection and CP operation are therefore linked: coatings for protected structures are chosen for proven resistance to cathodic disbondment, and potentials are kept inside the window set by the protection criteria. Criteria and survey methods are defined in standards such as NACE SP0169 for buried and submerged piping and ISO 15589-1 for onshore pipelines.
Who designs it
ICCP design — current demand, anode bed layout, rectifier sizing, interference testing and commissioning surveys — is the work of a qualified cathodic protection specialist. A coatings applicator contributes the other half of the system: a sound, compatible coating that reduces current demand and repairs to coating damage, coordinated with the CP specialist so that the coating and the protection levels stay compatible.