When chlorides or carbonation have reached the reinforcement in a concrete structure, conventional repair runs into a problem. Breaking out and patching the visibly damaged areas leaves contaminated concrete around them, and corrosion tends to move to the steel next door. Cathodic protection takes a different approach. Instead of removing all the contaminated concrete, it changes the electrochemistry at the steel so that corrosion is stopped or greatly slowed.
This guide explains the principle, the two main types of system, the incipient anode effect, what monitoring involves and the situations in which engineers consider cathodic protection. It is an introduction for owners, managers and students, not a design guide. Cathodic protection design, installation, commissioning and monitoring are specialist work, and every system needs a qualified corrosion engineer, the manufacturer’s data for the anodes and equipment, and the applicable standards and owner requirements.
Key takeaways
- Corrosion in concrete is electrochemical. Anodic sites on the steel lose metal, cathodic sites consume the current, and the concrete pore solution carries the ions.
- Cathodic protection supplies current from an external anode so that the whole protected steel acts as a cathode.
- Galvanic systems use zinc or other active metal anodes; they are simple but their output is limited and falls with time and with drying of the concrete.
- Impressed current systems can protect large areas of chloride-contaminated concrete, but need power, controls and monitoring for their whole life.
- ISO 12696 covers cathodic protection of steel in concrete, and EN 1504-9 includes cathodic protection among its repair principles.
- Engineers consider cathodic protection where contamination is widespread, where breaking out all contaminated concrete is impractical, or where repeated patching has failed.
How corrosion works in concrete
Steel in sound concrete is protected by a thin passive film formed in the highly alkaline pore solution. Chlorides break that film down locally, and carbonation lowers the pH of the cover concrete until the film is no longer stable. Our guides to chloride-induced corrosion and why concrete deteriorates explain both mechanisms.
Once corrosion starts, the steel divides into anodic areas, where iron dissolves, and cathodic areas, where oxygen and water are reduced. Electrons flow through the steel from anode to cathode, and ions move through the moist concrete to complete the circuit. The rust formed at the anodes takes up more volume than the steel it came from, which cracks and spalls the cover, as described in concrete spalling prevention.
What cathodic protection does
Cathodic protection adds a new anode outside the steel and connects it to the reinforcement. Current flows from this anode, through the concrete, to the steel. The steel’s potential shifts in the negative direction, the anodic reaction on the steel is suppressed, and the reinforcement as a whole behaves as a cathode.
Over time, the current also produces useful chemistry at the steel surface: hydroxide ions are generated, which raise alkalinity, and negatively charged chloride ions are drawn away from the steel toward the anode. These effects help restore passive conditions, which is one reason a well-run system can often operate at lower current once the steel has been polarized.
Protection and prevention
Two terms are used:
- Cathodic protection is applied to steel that is already corroding, to stop or slow active corrosion.
- Cathodic prevention is applied to steel that is still passive but at risk, for example in new structures in severe chloride exposure. It needs much less current because it only keeps passive steel passive.
Galvanic and impressed current systems
Galvanic (sacrificial) anodes
A more active metal, usually zinc or a zinc alloy, is connected to the reinforcement. The zinc corrodes instead of the steel and supplies current by itself, without a power supply.
Common forms:
- Discrete anodes embedded in the edges of patch repairs or installed in drilled holes in sound concrete.
- Thermally sprayed zinc or zinc layers applied to the concrete surface.
- Zinc mesh or sheet in jackets around piles and columns in marine environments.
Galvanic anodes are simple to install and need little maintenance, but their output is limited by the voltage difference between zinc and steel and by the resistivity of the concrete. In dry concrete their output falls sharply. Their life is limited by the amount of metal available, and the zinc’s corrosion products and surrounding activating material affect performance. Expected life and output are product-specific, so check the manufacturer’s data sheet and the engineer’s design.
Impressed current cathodic protection (ICCP)
Inert or slowly consumed anodes are connected to the positive terminal of a DC power supply, and the reinforcement to the negative terminal. The power supply drives the current, so output can be adjusted by zone and over time.
Common anode types include:
- Mixed metal oxide coated titanium mesh or ribbon, set in a cementitious overlay or in slots cut into the concrete.
- Conductive coatings applied to the concrete surface, with embedded primary conductors.
- Discrete anodes installed in drilled holes, useful for thick sections and where an overlay is not acceptable.
- Thermally sprayed zinc or titanium used as an impressed current anode in some designs.
ICCP can protect large areas of heavily contaminated concrete and can be controlled and adjusted for decades. It also brings ongoing obligations: power, transformer-rectifiers or controllers, cabling, reference electrodes, monitoring and periodic inspection.
Hybrid systems
Some systems first apply impressed current for a limited period to polarize the steel and move chlorides, then run as galvanic systems from the same anodes without a permanent power supply. These are specialist designs, and their suitability depends on the structure, the exposure and the manufacturer’s data.
Comparing the families
| Factor | Galvanic anodes | Impressed current |
|---|---|---|
| Power supply | None | Required for system life |
| Current output | Limited; falls as concrete dries and anodes are consumed | Adjustable by zone |
| Typical use | Around patch repairs, piles, smaller areas | Large areas of widespread contamination |
| Monitoring | Often simple or periodic | Regular monitoring and adjustment |
| Design effort | Still needs an engineer | Substantial specialist design |
| Life | Limited by anode mass | Depends on anode type and maintenance |
| Risks | Under-protection in dry or high-resistivity concrete | Overprotection risks to sensitive steel if poorly controlled |
The incipient anode effect
When a corroding area of steel is active, it acts as an anode and, in effect, provides some protection to the surrounding steel, which acts as a cathode. Patch repair removes the active area and replaces it with fresh, alkaline mortar. The steel in the patch becomes passive, and the adjacent steel, still in chloride-contaminated concrete, loses the protection it had. New corrosion often starts around the edges of the patch.
This is called the incipient anode or ring anode effect. It is one of the main reasons patch repairs on chloride-contaminated structures fail within a few years. Discrete galvanic anodes tied to the reinforcement at the edges of patches are a common way to reduce it, and for widespread contamination, a full cathodic protection system may be more effective than repeated patching. The choice of repair mortar also matters, because the anode and the patch must be compatible, with suitable resistivity.
What a design involves
A cathodic protection design is not a product choice. It typically covers:
- Condition assessment: chloride profiles, carbonation depth, cover, half-cell potentials, resistivity and breakouts, as described in concrete condition surveys.
- Electrical continuity: the reinforcement in each zone must be electrically continuous, which is tested and corrected before anodes are installed. Isolated bars or epoxy-coated bars complicate this.
- Zoning: dividing the structure into zones with similar steel density, exposure and moisture so current can be distributed evenly.
- Anode selection and layout: for current density, life, geometry, weight, appearance and access.
- Concrete repair: delaminated and spalled areas are repaired with materials compatible with cathodic protection before or during installation.
- Reference electrodes and monitoring: embedded sensors to measure steel potential and confirm performance.
- Sensitive steel: prestressing and post-tensioning steel can be susceptible to hydrogen embrittlement if polarized too far, so designs for these structures need particular care.
- Commissioning: energizing the system, adjusting output and verifying performance against criteria in the design and the applicable standards.
Monitoring and performance
The performance of ICCP systems is normally checked by measuring steel potentials with embedded reference electrodes, often by switching the current off briefly and recording how the potential decays over a set period. Standards including ISO 12696 set out performance criteria based on such measurements. The exact criteria, test intervals and actions belong to the design and the standard, not to rules of thumb.
Many systems now use remote monitoring, which reports potentials, currents and faults without site visits. Even so, cabling, junction boxes, power supplies and anodes need periodic inspection. A system that is switched off or left unmonitored for years protects nothing.
| Monitoring item | Why it matters |
|---|---|
| Current and voltage by zone | Shows the system is running and stable |
| Steel potentials and decay tests | Shows whether the steel is adequately polarized |
| Reference electrode health | Faulty electrodes give misleading data |
| Visual inspection of anodes and overlays | Finds debonding, damage or acid attack at the anode interface |
| Records over time | Show trends and support adjustment |
When engineers consider cathodic protection
- Chloride contamination is widespread and removing all contaminated concrete would be very costly or structurally difficult.
- Repeated patch repairs have failed around their edges.
- Long remaining service life is required from a structure in chloride exposure, such as a bridge substructure, marine pier, parking structure or tunnel.
- Cast-in chlorides are present throughout the concrete, so surface barriers cannot help.
- The owner can commit to monitoring and maintenance for the system’s life.
Cathodic protection is less suitable where the owner cannot maintain an impressed current system, where electrical continuity cannot be achieved economically, or where the structure’s remaining life is short. Other options, such as barrier protection with penetrating sealers or membranes, chloride extraction or targeted removal, may be better fits, and are often combined with cathodic protection.
Frequently asked questions
Does cathodic protection remove chlorides? It draws chloride ions away from the steel over time and raises alkalinity there, but it is not designed as a chloride removal treatment. Electrochemical chloride extraction is a separate, temporary treatment.
Can it be added to an existing structure? Yes, most systems on concrete are retrofitted to existing structures, after concrete repair and continuity testing.
Is cathodic protection a substitute for repair? No. Delaminated and spalled concrete is still repaired. Cathodic protection controls corrosion so that sound but contaminated concrete can stay.
Can a contractor size anodes from a catalog? Anode selection and spacing should come from a design by a qualified corrosion engineer, using the manufacturer’s data sheet for output and life.
Bottom line
Cathodic protection stops or slows reinforcement corrosion by making the steel the cathode of an electrochemical cell. Galvanic anodes need no power and work well around patches and in moist concrete, while impressed current systems can protect large contaminated areas but must be powered and monitored for life. The incipient anode effect explains why patch repairs alone often fail on chloride-contaminated structures. Design, installation, commissioning and monitoring are specialist work for qualified corrosion engineers, working from condition data, the manufacturer’s information and standards such as ISO 12696 and the EN 1504 series.