Half-Cell Potential Mapping: Predicting Corrosion Before the Rust Shows

Reinforced concrete corrodes from the inside. By the time rust staining, delamination, or spalling appears on the surface, the electrochemical process has typically been underway for years. Half-cell potential testing gives engineers a way to assess corrosion activity before visible damage develops, allowing asset managers to intervene at a point where repair is still targeted and cost-effective.

The Electrochemical Principle

Steel embedded in concrete exists in a passive state under normal conditions. The alkaline pore solution surrounding the reinforcement, with a pH typically above 12.5, maintains a thin oxide layer on the steel surface that inhibits corrosion. When that passivity breaks down, through carbonation reducing the pH or chloride ions penetrating to the steel depth, an electrochemical cell forms.

In this cell, anodic zones on the steel surface oxidise, releasing electrons and iron ions. Cathodic zones consume those electrons in the presence of oxygen and moisture. The potential difference between these zones drives corrosion current through the concrete, which acts as an electrolyte. Half-cell potential testing measures the electrochemical potential at the concrete surface relative to a stable reference electrode, giving a spatial picture of where anodic activity is occurring.

The reference electrode used in practice is typically a copper/copper sulphate electrode (CSE), though silver/silver chloride electrodes are also used in some applications. The electrode is placed on the wetted concrete surface and connected via a high-impedance voltmeter to a wire attached to the reinforcement. Readings are taken on a grid, typically at 200 mm to 500 mm centres depending on the required resolution, and the results are plotted as an equipotential contour map.

ASTM C876 Probability Thresholds

The interpretation framework most widely applied is ASTM C876, *Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete*. The standard defines probability thresholds based on potential readings measured against a CSE:

• More positive than -200 mV CSE: Greater than 90% probability that no active corrosion is occurring at the time of measurement
• Between -200 mV and -350 mV CSE: Corrosion activity is uncertain; further investigation is warranted
• More negative than -350 mV CSE: Greater than 90% probability that active corrosion is occurring at the time of measurement

These thresholds are probabilistic, not deterministic. A reading of -380 mV does not confirm that section loss has occurred; it indicates that the electrochemical conditions for active corrosion are present. Conversely, a reading of -150 mV does not mean the steel is in perfect condition, only that active corrosion is unlikely at that location on that day.

Moisture content affects readings. Dry concrete produces artificially positive potentials, which can mask active corrosion. Pre-wetting the surface before testing is standard practice to ensure adequate electrical contact and representative readings. Temperature also influences results, and ASTM C876 includes guidance on correction factors where measurements are taken outside the standard temperature range.

Australian practice generally follows ASTM C876 in the absence of a direct AS equivalent for this test method, though engineers should document the reference electrode type and moisture conditioning procedure in any test report to allow meaningful comparison across surveys.

What a Corrosion Map Shows

Plotting potential readings as a colour-graded contour map transforms a grid of numbers into spatial intelligence. Zones of highly negative potential cluster around areas of active anodic activity. The shape and extent of those zones tells the engineer several things at once.

A localised cluster of readings below -350 mV, surrounded by readings in the passive range, suggests a discrete corrosion cell, possibly associated with a crack, a construction joint, or a localised chloride contamination event. A broad zone of negative readings across an entire bay suggests a more systemic problem, such as carbonation front reaching the steel depth or widespread chloride ingress from years of de-icing salt exposure.

The gradient between zones also carries information. A steep gradient, where potentials shift from -150 mV to -450 mV over a short distance, indicates a well-defined active cell with a clear anode/cathode boundary. A shallow gradient across a large area may indicate a more diffuse corrosion mechanism.

Half-cell mapping is rarely used in isolation. Pairing it with carbonation depth testing, chloride profiling, and cover depth measurements from GPR scanning or a cover meter gives a much clearer picture of the corrosion mechanism and likely progression rate.

Car Park Investigation: Hidden Active Corrosion

A multi-level precast car park constructed in the late 1990s was flagged for investigation following routine maintenance observations. Minor rust staining was visible at a small number of column connections on the lower levels, but the slab soffits and beam faces appeared largely intact. The asset manager needed to understand whether the staining was isolated or indicative of a broader corrosion front before committing to a repair programme.

The investigation scope included half-cell potential mapping across approximately 1,800 square metres of suspended slab, combined with carbonation depth testing on cores and chloride profiling at selected locations. Cover depths had been recorded during an earlier Ferroscan survey.

Pre-wetting was applied to all survey areas the evening before testing to ensure consistent moisture conditions. Readings were taken on a 300 mm grid using a CSE reference electrode, with the voltmeter connected to exposed reinforcement at column locations.

The resulting map showed a pattern that was not visible from the surface. Two full bays on level two returned readings predominantly below -350 mV, with a significant proportion below -400 mV. Adjacent bays on the same level returned readings largely in the -150 mV to -250 mV range. On level three, directly above, readings were predominantly passive across all bays.

The two active bays on level two corresponded to the areas directly below vehicle entry ramps on the level above. Chloride profiling on cores extracted from those bays confirmed elevated chloride concentrations at the steel depth, consistent with years of drainage from the ramp surfaces carrying chloride-laden water from vehicle tyres during winter months. The ramp drainage design directed runoff toward the slab edges, where it tracked along construction joints and penetrated to the reinforcement.

Carbonation depth on the same cores was less than 5 mm, well short of the cover depth of 30 mm to 40 mm recorded in the Ferroscan data. Carbonation was not the driver. The mechanism was chloride-induced pitting corrosion in an otherwise sound concrete matrix.

The passive zones on level three, and in the bays away from the ramp drainage path on level two, confirmed that the problem was not structural age or concrete quality but a specific water management failure. That distinction changed the remediation strategy entirely.

Rather than a broad concrete repair programme across all levels, the engineer specified targeted patch repairs in the two active bays, replacement of the ramp drainage system, and application of a penetrating silane sealer to the ramp surfaces and adjacent slab areas. A follow-up half-cell survey was scheduled for 18 months post-remediation to confirm that the corrosion front had stabilised.

Without the mapping data, the visible staining at column connections would likely have driven a repair scope based on what could be seen. The half-cell survey redirected that scope to where the electrochemical activity was actually occurring, and away from areas that did not warrant intervention.

Limitations to Understand

Half-cell potential measures corrosion probability, not corrosion rate. Two areas with identical potential readings may have very different rates of section loss depending on concrete resistivity, oxygen availability, and moisture content. Where corrosion rate data is needed, linear polarisation resistance testing can be used alongside half-cell mapping to quantify the corrosion current density.

Epoxy-coated or galvanised reinforcement, and post-tensioned systems with grouted ducts, require modified interpretation. The standard ASTM C876 thresholds apply to uncoated black steel. Testing on structures with alternative reinforcement types should be interpreted with that qualification clearly stated.

Concrete overlays, waterproof membranes, and thick toppings can attenuate the surface potential reading and reduce the reliability of the data. Where these conditions exist, the investigation scope should account for them, either by testing through penetrations or by combining methods.

Integrating Half-Cell Data into Asset Management

For asset managers, a half-cell survey provides a baseline condition map that can be repeated at intervals to track corrosion progression. The change in potential distribution between surveys, rather than any single reading, often provides the most actionable information. An area that shifts from the uncertain range to the active range between surveys indicates accelerating corrosion activity that warrants investigation before the next scheduled survey cycle.

Paired with cover data and chloride profiling results, a half-cell map supports a risk-ranked repair schedule. Sections with active corrosion, low cover, and high chloride concentrations sit at the top of the priority list. Sections with passive readings and adequate cover can be monitored rather than repaired immediately, preserving budget for where it is genuinely needed.

This kind of evidence-based prioritisation is the practical value of the method. It replaces area-based repair estimates with location-specific data, and it gives engineers a defensible basis for the repair scope they recommend.

SiteOps carries out half-cell potential surveys as standalone investigations and as part of broader NDT programmes combining GPR scanning, cover meter surveys, carbonation testing, and chloride profiling. More information on investigation packages is available at siteops.au.