How it works
Corrosion happens at anodic sites, where metal gives up electrons and dissolves. CP forces current onto the structure through the electrolyte from an external anode, polarizing the structure in the negative direction until its anodic sites stop dissolving. The corrosion moves to the anodes, which are either designed to be consumed or chosen to be nearly inert. CP only reaches surfaces that share an electrolyte with the anode: buried and immersed steel, tank bottoms, water boxes. It does nothing for atmospheric surfaces.
Two ways to supply the current
| Galvanic (sacrificial anode) | Impressed current (ICCP) | |
|---|---|---|
| Current source | Potential difference between the anode metal and the structure | External DC supply, usually a transformer-rectifier |
| Anodes | Zinc, aluminum or magnesium alloys, consumed in service | Low-consumption materials such as mixed metal oxide, high-silicon cast iron or graphite |
| Output | Limited and self-regulating | High and adjustable |
| Typical use | Well-coated or compact structures, water boxes, localized protection | Long pipelines, large tank farms, large bare or poorly coated areas |
| Main risks | Depleted anodes, broken bonds | Reversed polarity, stray-current interference on nearby structures, overprotection |
The sacrificial anode entry covers the galvanic option in more detail.
Criteria and verification
Whether a structure is protected is judged against the criterion in the governing standard, not a rule of thumb. Examples: NACE SP0169, now maintained by AMPP, for external corrosion control of buried or submerged metallic piping; ISO 15589-1 for onshore pipeline CP; and, in Canada, CSA Z662 for oil and gas pipeline systems. The criterion value, the reference electrode and the way voltage drops in the soil or water are accounted for all come from the standard and edition the specification invokes. Verification relies on structure-to-electrolyte potential surveys at test stations, rectifier output records and periodic anode inspection.
Coatings and CP work as a pair
On buried and immersed structures the coating does most of the work and CP protects the defects the coating inevitably has, which keeps the current demand economical. Driving the potential too far negative is not extra safety: it can cause cathodic disbondment of the coating and hydrogen-related damage in susceptible metals such as high-strength steels and titanium. The heat exchanger galvanic corrosion article shows CP applied in water boxes and the potential limits that titanium tubes impose.