Half Cell Corrosion Potentiometer

About Half cell potential test:

The half cell potential test is a widely utilised, non-destructive electrochemical technique used primarily for assessing the likelihood of the corrosion activity in steel reinforcement. It is especially designed to evaluate the electrochemical potential of steel reinforcement embedded in concrete by comparing it to the reference electrode placed on the surface. The test serves as an indirect method of estimating the corrosion activity, without physically damaging the structure or extracting reinforcing bar (rebar).

When the steel corrodes in concrete, it undergoes oxidation reaction, releasing electrons. These electrochemical processes generate a measurable potential difference between the embedded steel and the reference electrode. In a half cell test, a high impedance voltmeter is used to measure this potential difference , which reflects the electrochemical state of the steel. The reference electrode, typically a copper/copper sulfate (Cu/CuSO4) or a silver/silver chloride (Ag/AgCl), provides a stable known potential against which the steels potential can be compared. The steel reinforcement, if corroding, will show negative potential due to the anodic reactions taking place on its surface.

For analysis of the obtained value it becomes necessary to understand the significance of the measured potential. According to the standards like ASTM C876, a potential value measurement that is more negative than -350 mV generally indicates high probability of corrosion activity occurring at the time of measurement. It is important to note that the test only reflects the potential of corrosion at the time of testing and does not quantify the rate and the extent of corrosion damage. Several factors can influence the test accuracy and its interpretation. Moisture content plays an important role, as higher moisture content naturally increases the ionic conductivity of the concrete. Surface conditions, such as coatings or contaminants can affect electrical conductance or measurement accuracy.

Purpose of half cell potential test:

• To detect whether corrosion activity of the steel reinforcement is actively occurring.

• To assess the probability of corrosion in the steel reinforcement.

• To locate corrosion prone areas across the concrete surface

• Monitor the effectiveness of protection measures.

Principle of half cell potential test:

The half-cell potential test is based on the principle that the steel reinforcement embedded within the concrete matrix behaves as an electrochemical phase capable of participating in redox processes at the steel-concrete-pore solution interface. When a metallic electrode such as reinforced steel is immersed in a conductive medium an equilibrium is established between the metallic iron phase and its ionic species (Fe2+, Fe3+) present in the adjacent pore solution. This equilibrium gives rise to measurable potential difference, referred as half-cell potential, which reflects the thermodynamic tendency of the embedded steel to undergo oxidation or reduction.

The underlying principle lies in the distribution of electrochemical potential at the interface between the embedded steel and the electrolyte contained within the concrete's pore structure. Concrete contains a microscopic network of interconnected pores filled with an aqueous ionic solution consisting of hydroxyl ions, alkali metals cations (Na+, K+), and dissolved oxygen and carbonates. The equilibrium potential near the steel depends on the redox state of the steel surface, ionic composition of the pore solution, and physicochemical properties of the surrounding medium.

When the steel reinforcement surface is under a high alkaline environment with pH values above 12.5, the surface of the steel is covered by a thin, adherent, and protective oxide film, primarily composed of Fe2O3 or Fe3O4. This protective layer drastically reduces the rate of anodic dissolution of the iron , and the steel's potential stabilises at a relatively nominal value (I.e., less negative value). Conversely when aggressive species such as chloride ions penetrate the concrete cover or when carbonation lowers the local pH below the threshold of passivity, this protective film becomes thermodynamically unstable. The destruction of the protective film exposes the metallic surface of the steel to direct electrochemical interaction with the pore solution, initiating the active corrosion processes characterised by the anodic dissolution of the iron into Fe2+ and Fe3+ ions. In the context of reinforced concrete, the steel does not exist as an isolated bar; rather, it constitutes a distributed electrochemical network within heterogeneous electrolytes in the concrete. The electrical continuity of the rebar and ionic conductivity of the pore solution enables establishment of galvanic cells across the concrete structure. Within such systems, spatial variations in moisture content, oxygen availability, chloride concentration and pH give rise to localised anodic and cathodic regions. Hence the half-cell potential represents the mixed potential resulting from these competing electrochemical reactions, primarily the oxidation of iron at the anodic sites and reduction of water or oxygen at cathodic sites.

When measuring the electrochemical potential difference between the rebar and reference electrode which is typically as copper/copper sulfate or silver/silver chloride electrode, the concrete acts as an ionic conductor that facilitates the charge transport between the steel and the reference electrode. The measured potential is therefore an indirect reflection of the thermodynamic force for corrosion reaction occurring at the steel surface. A more negative potential corresponds to a greater tendency of anodic dissolution (i.e., active corrosion).

The heterogeneous nature of concrete adds further electrochemical intricacy. Variations in pore structure, degree of saturation, and electrical resistivity across the concrete matrix causes the spatial potential gradients that are not solely attributed to corrosion activity but also the transport properties of the medium. The resistivity of the concrete governs the internal potential drop between the steel and the surface, the moisture and temperature affects the mobility of the ionic species. Hence the half-cell potential shows an integrated electrochemical response encompassing thermodynamic equilibrium, inter-facial kinetics, and ion transport across the phase.

Measurement of Half-cell potential test:

Measurements are generally taken on a grid pattern with spacing between 0.25 m and 1.0 m, depending on the required mapping resolution. The concrete surface should be moist to ensure proper electric contact, which is maintained using a wet sponge or conductive gel under the electrode.

The value of potential difference (E) measured is expressed in Volts (V), more commonly recorded in milli volt with respect to reference electrodes.

Components of Half-cell potential test:

1. Reference Electrode: Provides a stable, known potential for comparison. Common types include Cu/CuSO for soil and Ag/AgCl for concrete. It ensures accurate and consistent readings of corrosion activity.

2. Connecting Leads: Insulated wires that connect the reference electrode and voltmeter to the metal structure. They must be low-resistance and corrosion-resistant for reliable measurements.

3. Voltmeter : Measures the potential difference between the reference electrode and the metal. A high-impedance voltmeter (over 10 M) prevents current flow that could affect the true potential.

4. Testing connection on rebar: The location on the metal (e.g., rebar or pipeline) where the measurement is taken. A clean, firm electrical connection ensures accuracy.

5. Contact Solution or Surface Preparation: A wet sponge or conductive gel is used to improve contact between the electrode and the surface, ensuring stable and accurate potential measurements.

Standard Procedure: overview (As per ASTM C876, IS 516)

The Half-Cell Potential Test is a non-destructive method used to assess the likelihood of corrosion activity in reinforced concrete structures. The following procedure outlines the standard method for performing the test in the field.

Step 1: Surface Preparation and Grid Marking

Clean the concrete surface thoroughly to remove dust, coatings, grease, or any contaminants that could hinder electrical contact. Establish a testing grid with points typically spaced between 0.5 m and 1.0 m, and clearly mark each location to ensure systematic data collection.

Step 2: Exposure and Connection to Reinforcement

Expose a small section of reinforcement at an appropriate location to serve as the electrical connection point. Clean the steel surface using a wire brush or sandpaper to achieve a sound metallic contact. Connect the negative terminal of a high-impedance voltmeter to the reinforcement and the positive terminal to the half-cell electrode.

Step 3: Conditioning of the Test Surface

If the concrete surface is dry, lightly moisten it with a damp sponge or cloth to improve electrical conductivity between the half-cell electrode and the concrete surface. Avoid excess water accumulation that could affect readings.

Step 4: Placement of the Half-Cell Electrode

Position the half-cell electrode (commonly coppercopper sulfate or silversilver chloride) firmly on the first grid point, ensuring good contact with the moistened concrete surface. Maintain steady placement during the reading to ensure accuracy.

Step 5: Measurement and Data Collection

Record the potential difference displayed on the voltmeter once the reading stabilises. Continue moving the electrode across all marked grid points, repeating the measurement process to obtain a complete set of potential readings across the test area.

Step 6: Data Interpretation and Reporting

Interpret the data in accordance with ASTM C876 or other applicable standards. Areas showing more negative potentials indicate a higher probability of active corrosion, assisting in identifying zones requiring further investigation or remediation.

Result interpretation of half-cell potential test:

ASTM C876 and IS 516 provide guidance on conducting half-cell potential measurements and on correlating the measured potentials with the likelihood of reinforcement corrosion. The results are interpreted qualitatively using a copper sulfate electrode (CSE).

Factors influencing half cell potential test:

a) Concrete Moisture Content: The amount of moisture in concrete significantly affects half-cell potential readings. Dry concrete can produce less negative (more positive) potentials, giving the false impression of low corrosion risk, whereas properly moist concrete provides more accurate readings.

b) Type, Condition, and Coverage of Steel Reinforcement: The nature of the steel, its coating (if any), and the thickness of the concrete cover influence potential measurements. Well-protected or deeply embedded steel may show less negative potential even when corrosion is present.

c) Temperature and Environmental Conditions: Temperature variations and environmental factors such as humidity can alter the electrochemical behavior of steel and concrete, affecting potential readings.

d) Surface Preparation and Contaminants: Proper surface cleaning is necessary for good electrical contact. Dust, chlorides, or other surface contaminants can interfere with electrode connection and distort results.

Source of errors in half cell potential test:

a) Electrode Placement and Movement: Incorrect positioning or accidental movement of the reference electrode during measurement may result in inconsistent or misleading potentials.

b) Surface Conditions: Rough, cracked, or contaminated concrete surfaces can prevent proper electrode contact, causing measurement errors.

c) Instrument and Reference Electrode Issues: Faulty, uncalibrated, or degraded reference electrodes and instruments can distort potential readings.

d) Environmental Influences: Stray electrical currents, uneven moisture distribution, or temperature fluctuations can affect the measured potentials and give misleading indications of corrosion risk.

e) Misinterpretation of Localised Readings: Relying on isolated measurements without mapping the overall potential distribution may lead to false conclusions about the likelihood of corrosion.

Assessing the likelihood of corrosion in reinforced concrete is crucial for long-term durability. The half cell potentiometer by Vedantrik technologies is a specialised equipment designed to evaluate the likelihood of the corrosion of the steel reinforcement embedded in the concrete structures, using a non-destructive electrochemical method. The device works on half-cell potential principle, where the difference in the potential at steel reinforcement and the reference electrode indicates the probability of corrosion. The main unit includes copper/copper sulfate (Cu/CuSO) electrode as the reference which is placed on the exposed surface of the concrete, and connected to the multi-meter, which is in turn connected to the rebar. The meter measures the voltage generated due to the natural electrochemical process occurring at the steel surface. More negative potential measured generally indicates higher risk of corrosion, while positive potential suggests that the steel is majorly passive and protected.

The device is fully compliant with ASTM C876 and IS 516, and allows systematic mapping of corrosion-prone areas, providing data for employing preventive measures, maintenance, and structural durability assessment.

The Half Cell Potentiometer test measures the electrical potential difference to indicate whether steel reinforcement is at risk of corrosion. In Mumbai, where marine exposure and humidity are common, this test is particularly valuable. Vedantrik Technologies provides advanced half-cell potentiometer systems that deliver precise, reliable, and easy-to-interpret results. Widely used in research, maintenance, and quality control, these devices help engineers make informed decisions on repair and rehabilitation. By identifying corrosion activity at an early stage, the half-cell test prevents costly repairs and ensures structural longevity. It is a preferred choice for bridge inspections, marine structures, and high-rise developments. For dependable half-cell potentiometers in Mumbai, contact Vedantrik Technologies and safeguard your structures against premature corrosion.

Specification and Key features:

• Voltage Range: -999mV to +999Mv

• Temperature Measurement range 0-100 deg Celsius (Temperature sensor given as per IS516)

• Accuracy: +/- 1mV

• Power Supply: pencil cell

• Operating Temp: 0 deg Cels to 50 deg Cels

• Auto power cut-off to save battery

• Back light display to use in dark

• Hold function for stable reading

• Removable copper assembly with two side caps to improve Life of copper rod and less consumption of copper sulphate.

• NABL calibration certificate