Overview
Cathodic protection (CP) is an electrochemical technique for controlling the corrosion of a metal surface — most commonly steel in seawater, soil or another electrolyte. Rather than trying to keep the environment away from the steel (the job of a coating), CP changes the electrical condition of the steel itself so that the reaction which dissolves it effectively stops. It is one of the few corrosion-control methods that can protect surfaces a coating can never fully reach: the back of a pile, a damaged holiday in a coating, the internals of a tank.
CP is almost always used together with coatings rather than instead of them. The coating does most of the work by reducing the exposed steel area; CP protects what the coating cannot — defects, edges, and areas of breakdown that grow over the asset's life.
The engineering principle
Corrosion of steel in an electrolyte is an electrochemical process. On the same surface, some areas behave as anodes (where iron dissolves and metal is lost) and others as cathodes (where a balancing reaction, typically oxygen reduction, consumes the electrons released). Current flows through the metal and the electrolyte between them — this is the corrosion cell, and the anodic sites are where damage occurs.
Cathodic protection works by supplying electrons to the whole structure from an external source, pushing the entire surface in the negative (more cathodic) direction. When the structure is polarised far enough, the areas that were acting as anodes are suppressed and metal loss is arrested. In effect the structure is made the cathode of a larger cell, and the anodic reaction is transferred to a component you have chosen to sacrifice or power.
How far you have to polarise the steel is judged against a protection potential criterion — a measured potential, relative to a reference electrode, at which corrosion is considered controlled. The exact criterion depends on the metal, the environment and the standard being worked to, which is why it is always quoted with its reference electrode and conditions rather than as a single universal number.
Two ways to deliver it
There are two mechanisms for supplying that protective current. In a sacrificial (galvanic) anode system, a more electrochemically active metal — aluminium, zinc or magnesium alloy — is connected to the steel. Because it is more active, it becomes the anode and corrodes preferentially, driving current to the steel with no external power. In an impressed current (ICCP) system, a DC power supply drives current from inert or semi-inert anodes to the structure, and the driving voltage is set by the equipment rather than by metallurgy.
Both achieve the same electrochemical end — the steel is polarised to a protective potential — but they suit different problems. The comparison of the two is a subject in its own right and is covered separately.
Design and selection considerations
A CP design begins with the current the structure will demand, which depends on the exposed steel area, the environment (seawater, sediment, soil), temperature, flow and the condition and expected breakdown of any coating. Because coatings degrade, current demand is assessed over the whole design life, not just the as-new state. The design then provides enough anode mass (galvanic) or rectifier capacity and anode life (ICCP) to meet that demand for the required period while keeping the structure polarised to the chosen criterion.
Getting the current-demand assumptions right matters more than almost anything else: an under-designed system will not hold protection late in life, and an over-designed one wastes anode mass or over-polarises sensitive materials.
Common mistakes
The recurring errors are conceptual as much as numerical: treating CP as a substitute for coating rather than a partner to it; designing for the as-new coating condition and ignoring breakdown over life; ignoring electrical continuity so that parts of the structure are not actually connected to the CP system; and interpreting a single potential reading without accounting for the reference electrode used or for voltage drop in the electrolyte.
Inspection and verification
CP is verified by measurement, not assumption. Structure-to-electrolyte potentials are measured against a reference electrode to confirm the structure sits at or beyond the protection criterion, and anodes (galvanic) or rectifier output and anode condition (ICCP) are inspected over time. Because protection can be lost gradually — as anodes consume or as coatings break down — periodic survey at a realistic interval is part of the design, not an afterthought.
How CCME-MCPS can help
CCME-MCPS designs, supplies and supports both sacrificial and impressed current cathodic protection for marine and industrial assets. If you are choosing between approaches for a specific structure, an engineering-led enquiry is usually faster than a general guide.
Important note
This article explains general cathodic-protection engineering principles for education. It is not a design, and it is not guidance for any specific structure — every real system must be engineered to the applicable standards and the specific conditions of the asset. Standards are referred to by name only.