Galvanic vs Impressed Current Cathodic Protection for Reinforced Concrete

Once chlorides from deicing salts or seawater reach the reinforcement and break down its passive film, rust takes up more volume than the bars it replaces and the cover cracks and spalls, so the repair has to stop the corrosion itself instead of patching its damage. A galvanic design wins on small, wet or hard-to-power elements because its zinc drives the current with no electricity, while impressed current takes the large, dry or heavily contaminated structures, where a rectifier can raise the driving voltage and adjust the output for decades.

Cathodic protection is then designed around the element, its exposure, access to power and the monitoring its owner can keep up. Both methods share the same electrochemistry, so the comparison starts with the cell they create in the concrete.

What Is Cathodic Protection in Reinforced Concrete?

Cathodic protection turns the reinforcing steel into the cathode of an electrochemical cell: a small protective current flows to the bars, and they stop corroding while an anode takes the corrosion instead. In that cell, the moist concrete serves as the electrolyte, and the process runs as long as the current keeps flowing.

Corrosion is itself an electrochemical process: cells form on their own along a bar, where one area sits at a different potential from the next and starts to corrode. The protective current shifts the potential of the whole reinforcement network in the negative direction, and 100 mV of polarization is the most common criterion for negligible corrosion, which makes that shift the first figure to check in any commissioning or monitoring report.

Cathodic protection takes its current from one of 2 sources: galvanic systems draw it from the electrochemical difference between steel and metals that corrode more readily, while impressed current systems get it from a DC power source. Freyssinet USA designs and installs both types through its cathodic protection services.

How Does Galvanic Cathodic Protection Work?

In a galvanic system, cathodic protection comes from metals less noble than steel, each given a metallic connection to the bars and left to corrode in their place, so the protective current flows with no outside power. In seawater, zinc sits at about -1200 mV and steel at about -500 mV against a copper-copper sulfate reference electrode, so electrons flow from the zinc to the bars.

Zinc is the sacrificial metal most systems use, and magnesium and aluminum alloys are the other metals used for galvanic anodes. Zinc takes 4 forms, each matched to the element and its exposure:

• A thermally sprayed coating on the element’s surface, applied once with no added weight
• A mesh, sheet or ribbon fixed to the element, often inside a pile jacket
• Bulk anodes, such as 50 lb zinc blocks, for submerged piles
• Discrete units, each a zinc core in a mortar shell, embedded in patch repairs and in congested reinforcement

The bars stay protected while the zinc is consumed, so its life is limited, and its performance drops in dry, high-resistance concrete, where humectants can help hold moisture around it. Its low driving voltage keeps the risk of hydrogen embrittlement near zero, making galvanic cathodic protection suitable for prestressed and high-strength steel.

How Does Impressed Current Cathodic Protection Work?

With an impressed current, cathodic protection draws its power from a rectifier: the reinforcement, as the cathode, is connected to the negative terminal of that external power source and inert anodes to the positive terminal. A permanent direct current then flows from the anodes through the concrete to the bars.

The anodes are usually titanium coated with mixed metal oxide, installed in 3 forms:

• Mesh fixed to the surface under a cementitious overlay or shotcrete
• Ribbons grouted into slots or bonded to the concrete face
• Discrete anodes grouted into drilled holes, for deep or dense reinforcement

Because these anodes barely change in volume, an impressed current system can run for decades on the same anodes, provided their maximum current capacity is not exceeded; the rectifier and wiring usually need maintenance first to keep the system reliable.

The rectifier sets the output, so the system can apply the higher voltage that heavily contaminated concrete, larger elements and harsh environments require; in normal service, it runs at a low voltage. Pushed too far, that voltage generates hydrogen at the bars, which raises the risk of embrittlement in prestressed or high-strength steel, so designers cap the steel potential on those structures, a limit to look for in any design that includes prestressing strand.

Galvanic vs Impressed Current Cathodic Protection Compared Side by Side

In cathodic protection, the galvanic method trades control for simplicity, while the impressed current one gives up simplicity for control and range.

Installation differs less than operation: for either design, cathodic protection usually goes in during reinforced concrete repair, so the right system has to be chosen before the repair is designed. On the Florida pile jackets, both methods followed the same 5-step process and differed only at step 4:

1. Unsound concrete is located by sounding and removed; corrosion keeps spreading between design and construction, so the repair area usually grows.
2. Electrical continuity between the bars is verified or established, since only bars in metallic contact receive the current.
3. The old surface and the exposed bars are blast cleaned, with power tools below the water line.
4. The anodes go in: bulk zinc anodes before a galvanic jacket is placed, titanium mesh or ribbon in the annulus of an impressed current jacket.
5. Grout is pumped into the jacket from the bottom within 72 hours of cleaning, without fly ash, slag or silica fume, which would raise its electrical resistance.

Which Cathodic Protection System Fits a Given Structure?

The right cathodic protection system for a given structure follows 6 factors, each read from a condition assessment, and the same data also shows when other infrastructure corrosion protection solutions, such as electrochemical chloride extraction, fit better.

Structures Where Galvanic Anodes Fit Best

Galvanic anodes suit elements where the area to protect is localized, the concrete remains wet and no power reaches the site, the ideal case for a system with no electronics:

• Patch repairs: discrete zinc anodes placed in the repair stop the new corrosion that tends to start in the old concrete around it.
• Marine piles: a bulk zinc sacrificial anode below the water and zinc mesh pile jackets in the tidal zone, the usual options in those 2 marine environments.
• Prestressed elements: the low driving voltage avoids hydrogen embrittlement of high-strength strand.
• Limited access sites: no rectifier, power line or communications link has to reach the element.

Above mean high tide, galvanic options narrow to sprayed zinc and embedded anodes, and they become less effective as the concrete dries out.

Structures Where Impressed Current Systems Fit Best

Impressed current systems suit large areas, heavily chloride-contaminated concrete and drier elements, with pier caps and pile bents as the typical case. Dry concrete has a much higher electrical resistance than a wet one, and galvanic current may be too weak to polarize the steel at higher elevations, so the Florida bridges of the FHWA case study use a powered system there.

Bridge decks, long substructures and elements with uneven chloride levels also favor the rectifier wherever a higher current is needed, and so do sites with many pile jackets, where impressed current jackets typically cost less than galvanic ones. Its output stays adjustable for decades, which suits owners who need long-term control, have an external power source nearby and can keep monitoring in place.

Structures That Combine Galvanic and Impressed Current Systems

The 2 methods can share one structure, chosen element by element: galvanic anodes on piles and patch repair areas, impressed current on larger or drier zones. Florida combined them on the Howard Frankland and Crescent Beach bridges, where cathodic protection cost less than $20 million and added more than 40 years of service life to both structures, according to the FHWA case study of 2 Florida bridges. At Crescent Beach alone, a 2017 to 2019 project installed 45 galvanic pile jackets alongside titanium mesh anodes on 14 bent caps, 2 pier caps, 4 struts and 8 columns.

Freyssinet USA used the same split at the Dominion Terminal Associates piers in Newport News, Virginia, where a rectifier-fed system protects the pier structure and a bulk zinc anode network protects the 106 piles.

Cathodic protection can also be hybrid, sequencing the 2 methods in time rather than splitting them by zone: impressed current runs for a few weeks to restore the passive layer of the steel, then sacrificial anodes hold it. Hybrid designs have a shorter track record than either method alone, so each installation needs case-by-case review by its designer.

What Does Long-Term Cathodic Protection Monitoring Involve?

Cathodic protection needs monitoring for its whole service life, because the protective current has to keep meeting the polarization criterion as moisture, chlorides and temperature change. Permanent reference electrodes embedded in the concrete give the potential of the bars, and readings taken with the current switched off for an instant show how far they are polarized.

An impressed current system requires the closest attention because its current demand shifts with seasons and weather, and only the rectifier can follow it. Its checks cover electrical shorts, meter accuracy, rectifier efficiency and circuit resistance. Galvanic output cannot be adjusted, so its readings mainly confirm that the anodes remain effective and still deliver enough current, and they are taken far less often.

Without monitoring, a loss of performance goes unseen: a tripped rectifier or a cut cable can leave cathodic protection off for years before an inspection finds it. Remote data loggers now record the readings automatically, and that monitoring belongs in a structural integrity management program, alongside periodic inspections such as the annual visits to the Dominion Terminal Associates piers over a 14-year period.

Choose the Right Cathodic Protection System With Freyssinet USA

Galvanic or impressed current comes down to the structure’s condition, its exposure and how long the protection must last. At Freyssinet USA, we start with a condition assessment that measures the cause, extent and rate of corrosion, then design, install and monitor the system that fits. 

Request a condition assessment for your bridge or marine structure.

Frequently Asked Questions About Galvanic vs Impressed Current Cathodic Protection

How Is Cathodic Protection Different From Galvanization?

Galvanization is a corrosion prevention step for new steel, coating it with zinc before it is built into a structure, while cathodic protection with galvanic anodes wires a separate zinc anode to reinforcement already cast in place. It can be added decades after construction, which suits existing bridges and piers, and its impressed current form uses a rectifier and an external power source in place of zinc.

What Are the Disadvantages of Cathodic Protection?

Cathodic protection stops corrosion but cannot restore concrete or steel section already lost, so spalled areas and badly corroded bars still need repair or structural strengthening first. A poorly designed impressed current system can also push stray current into nearby buried metallic structures, such as a utility pipe or a storage tank, which then corrode where the current leaves them. Both methods add a lifetime cost of monitoring, and galvanic anodes must be replaced once their zinc is consumed.

How Much Does Cathodic Protection Cost for Reinforced Concrete?

Cost is set project by project by the area to protect, the anode system, access to each element and the monitoring that comes with it, so a price per square foot from another bridge is only a first order of magnitude. Galvanic anodes keep a small patch repair inexpensive to protect, with no rectifier, supply line or adjustment visits to pay for, while the power supply, wiring and remote monitoring weigh on the price of an impressed current system.

When Should Impressed Current Cathodic Protection Be Inspected?

The rectifier output is checked at short intervals, often through remote monitoring, and a full survey confirms every year that the potential of the bars still meets the polarization criterion. Rectifiers damaged by lightning strikes or vandalism add unplanned repair visits beyond that schedule.