Corrosion of reinforcing steel and other embedded metals is a leading cause of deterioration in concrete. Highly alkaline concrete normally protects the steel around it. However, corrosion can start when carbonation reduces that alkalinity or chlorides disrupt the protective film, provided moisture and oxygen are available. The corrosion products occupy more volume than the original steel. As a result, pressure builds around the bar and may cause cracking, delamination, and spalling.
This article is the broad CEW guide to corrosion mechanisms, diagnosis, prevention, and repair decisions for embedded metals. For the narrower carbonation pathway and its relationship to reinforcement corrosion, see carbonation and corrosion in concrete.
How corrosion of embedded metals occurs
Steel corrodes because it is not a naturally occurring material. Producers smelt and refine iron ore to make steel, and those production steps add energy to the metal. Consequently, steel tends to release that energy and return to a more stable oxide or hydroxide state. This process is called corrosion.
In reinforced concrete, corrosion is an electrochemical process involving the flow of electrons and ions. Four parts are needed:
- An anodic area, where iron dissolves.
- A cathodic area, where a reduction reaction occurs.
- An ionic path through moist concrete pore solution.
- An electronic path through the reinforcing bar or other connected metal.
Importantly, the anode and cathode can be different locations on the same reinforcing bar. They do not have to be two separate pieces of metal.
Reaction at the anode
At active sites on the bar, called anodes, iron atoms lose electrons and enter the surrounding pore solution as ferrous ions. This is the anodic half-cell reaction:
Reaction at the cathode
The electrons remain in the bar and flow to cathodic sites. In aerated alkaline concrete, a common cathodic reaction combines oxygen, water, and electrons to form hydroxyl ions:
The ferrous and hydroxyl ions can combine to form ferrous hydroxide:
Further reactions with oxygen and water form a mixture of iron oxides and hydroxides commonly called rust. These products occupy more volume than the steel consumed. Because concrete confines the bar, the expansion creates tensile stress in the cover. Initially, cracks may follow the line of the reinforcement; delamination and spalling can follow. Continuing corrosion can also reduce bar area and bond.
Concrete and the passivating layer
Although steel has a natural tendency to corrode, the alkaline environment of sound concrete normally protects it. A thin oxide film forms on the steel at high pH and greatly reduces the rate at which iron dissolves. This passive film does not make the steel permanently corrosion-proof. It remains effective while the surrounding concrete provides suitable chemical and physical protection.
Carbonation and chloride ingress are the two common ways the steel loses protection. In addition, cracks, low-quality permeable concrete, inadequate cover, and wet exposure can shorten the path for aggressive substances and increase the risk. Moisture also lowers the electrical resistivity of the concrete and supports ionic current.
Carbonation corrosion vs chloride corrosion
| Mechanism | What changes at the steel | Typical exposure or trigger | Useful evidence |
|---|---|---|---|
| Carbonation | Carbon dioxide reacts with the cement paste and reduces alkalinity. When the carbonation front reaches the bar, the passive film may no longer remain stable. | Atmospheric exposure, especially with permeable concrete, cracks, or insufficient cover. | Carbonation depth compared with measured cover, moisture history, cover survey, and corrosion mapping. |
| Chloride ingress | Chloride ions can locally disrupt the passive film even while the concrete remains alkaline. Attack is often localized and can produce pitting. | Marine spray, seawater, deicing salts, contaminated materials, or chloride-bearing admixtures. | Chloride profile at the depth of steel, exposure history, half-cell mapping, and corrosion-rate measurements. |
| Stray-current corrosion | External direct current leaving the steel drives metal loss at the discharge area. | Nearby DC traction, welding, or impressed-current systems with unintended current paths. | Electrical continuity, potential/current monitoring, and specialist investigation of the external system. |
| Galvanic or dissimilar-metal action | Electrically connected metals or steel regions with different electrochemical conditions form a cell. | Dissimilar embedded metals, connections, or strong differences between repaired and unrepaired areas. | Metal identification, continuity testing, potential mapping, and review of repair boundaries. |
For more detail on the two main initiators, see CEW’s pages on carbonation and corrosion, testing concrete for carbonation, and chloride ingress into concrete.
Signs and tests for reinforcement corrosion
Rust staining, cracks along the bar, hollow-sounding concrete, delamination, and spalling are useful warning signs. However, no single sign proves the cause or shows how much steel the bar has lost. Therefore, the investigation should combine observations, exposure history, and complementary tests.
| Question | Suitable check | What it can tell you | Important limitation |
|---|---|---|---|
| Where is concrete already detached? | Visual survey and delamination sounding, such as hammer or chain drag where appropriate | Maps cracks, spalls, rust staining, and hollow areas. | Finds damage, not the corrosion cause or remaining bar area. |
| Is the cover shallow or variable? | Cover meter survey | Locates reinforcement and estimates cover depth. | Instrument results need calibration and interpretation around congested steel. |
| Has carbonation reached the steel? | Freshly exposed surface with a suitable pH Indicator, measured against cover depth | Estimates the depth of the low-alkalinity carbonation front. | The color boundary is not a direct corrosion-rate measurement. |
| Are chlorides present at bar depth? | Depth-resolved powder or core samples analyzed for chloride content | Shows the chloride profile and concentration near the reinforcement. | Sampling location, depth, test basis, binder content, and exposure must be recorded; one universal threshold is not suitable for every structure. |
| Where is corrosion likely active? | Half-cell potential mapping to ASTM C876-22, where applicable | Maps relative corrosion-potential patterns and probability of activity for uncoated reinforcement. | Does not measure corrosion rate or section loss. Moisture, cover, coatings, resistivity, and temperature affect readings. |
| How fast is corrosion progressing? | Specialist corrosion-rate testing, often combined with concrete resistivity | Estimates current corrosion activity and helps compare areas. | Results depend on test method, geometry, moisture, and interpretation. |
| Has structural capacity been affected? | Local exposure and bar measurement, bond/anchorage review, drawings, and structural assessment | Checks section loss, bond, anchorage, and load-path consequences. | May require intrusive work, temporary support, and a licensed design professional. |
How to reduce corrosion risk in new concrete
Well-consolidated, properly cured concrete with low permeability and sufficient cover greatly reduces corrosion of embedded metals. Although the detailed requirements depend on the structural code and exposure class, the main controls are consistent:
- Use a mixture with a water-cementitious materials ratio and durability provisions suitable for the exposure.
- Provide and maintain the specified concrete cover; use stable chairs and good placement control.
- Consolidate and cure the concrete so that the cover zone is dense and continuous.
- Control cracking and detail drainage so that salt-bearing water does not remain on the surface.
- Control chloride contributed by concrete ingredients and avoid unapproved chloride-bearing admixtures around reinforcement.
- For severe exposure, evaluate corrosion-resistant reinforcement, membranes, sealers, inhibitors, or cathodic protection as part of the service-life design rather than as isolated products.
When corrosion damage needs engineered repair
A surface patch may hide the visible damage without stopping corrosion in adjacent contaminated concrete. Therefore, repair planning should first determine the mechanism, the area affected, the condition of the reinforcement, and the load-path consequences. The repair team must then select a system that addresses both the damaged concrete and the cause of depassivation.
Arrange a qualified structural and corrosion assessment when any of the following is present:
- loose or falling concrete over occupied areas;
- exposed reinforcement, measurable section loss, or loss of bond;
- damage at columns, beams, connections, prestressed members, or anchorage zones;
- widespread chloride contamination or recurring failure beside previous patches;
- uncertain structural drawings, bar layout, or extent of concealed deterioration.
Current ACI repair practice begins with condition assessment and cause identification before the team selects a repair method or material. Depending on the findings, the solution may include removal and replacement of deteriorated concrete, reinforcement treatment or replacement, protective systems, electrochemical methods, or strengthening. Finally, CEW’s overview of cathodic protection of steel in concrete explains one specialist option.
References
- American Concrete Institute, ACI 222R-19: Guide to Protection of Reinforcing Steel in Concrete against Corrosion.
- ASTM International, ASTM C876-22: Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete.
- Federal Highway Administration, Guidelines for Detection, Analysis, and Treatment of Materials-Related Distress in Concrete Pavements.
- U.S. Bureau of Reclamation, Guide to Concrete Repair, Second Edition, 2015.
- American Concrete Institute, ACI CODE-562-25: Assessment, Repair, and Rehabilitation of Existing Concrete Structures.