Half Cell Potential Testing

Scribe Half Cell Potential Equipment (Courtesy Hammond Concrete Testing)

Steel embedded in good quality concrete is protected by the high alkalinity pore water which, in the presence of oxygen, passivates the steel. The loss of alkalinity due to carbonation of the concrete or the penetration of chloride ions (arising from either marine or de-icing salts, or in some cases present insitu from the use of a calcium chloride additive) can destroy the passive film. In the presence of oxygen and humidity in the concrete, corrosion of the steel starts. A characteristic feature for the corrosion of steel in concrete is the development of macrocells, that is the co-existence of passive and corroding areas on the same reinforcement bar forming a short-circuited galvanic cell, with the corroding area as the anode and the passive surface as the cathode. The voltage of such a cell can reach as high as 0.5V or more, especially where chloride ions are present. The resulting current flow (which is directly proportional to the mass lost by the steel) is determined by the electrical resistance of the concrete and the anodic and cathodic reaction resistance.

The current flow in the concrete is accompanied by an electrical field which can be measured at the concrete surface, resulting in equipotential lines that allow the location of the most corroding zones at the most negative values. This is the basis of potential mapping, the principal electrochemical technique applied to the routine inspection of reinforced concrete structures^,.

The use of the technique is described in an American Standard, ASTM C876-80, Standard Test Method for Half Cell Potentials of Reinforcing Steel in Concrete.

Factors affecting the potential field

When surface potentials are taken, they are measured remote from the reinforcement due to the concrete cover. The potentials measured are therefore affected by the ohmic drop potential drop in the concrete. Several factors have a significant effect on the potentials measured

1. Concrete Cover Depth

With increasing concrete cover, the potential values at the concrete surface over actively corroding and passive steel become similar. Thus the location of small corroding areas becomes increasingly difficult.

2. Concrete Resistivity

The concrete humidity and the presence of ions in the pore solution affect the electrical resistivity of the concrete. The resistivity may change both across the structure and with time as the local moisture and salt content vary. This may create an error of plus or minus 50 mV in the measured potentials

3. High Resistive Surface Layers

The macrocell currents tend to avoid highly resistive concrete. The measured potentials at the surface become more positive and corroding areas may be undetected.

4. Polarisation Effects

Steel in concrete structures immersed in water or in the earth often have a very negative potential due to restricted oxygen access. In the transition region of the structure (splash zone or above ground), negative potentials can be measured due to galvanic coupling with immersed rebars. These negative potentials are not related to corrosion of the reinforcement.

Procedure for Measurement

To measure half cell potentials, an electrical connection is made to the steel reinforcement in part of the member you wish to assess. This is connected to a high impedance digital millivoltmeter, often backed up with a datalogging device. The other connection to the millivoltmeter is taken to a copper/copper sulfate or silver/silver chloride half cell, which has a porous connection at one end which can be touched to the concrete surface. This will then register the corrosion potential of the steel reinforcement nearest to the point of contact. By measuring results on a regular grid and plotting results as an equipotential contour map, areas of corroding steel may readily be seen. Using 3D mapping techniques, a more graphical representation of the corrosion can be shown.

Results and Interpretation

According to the ASTM method, corrosion can only be identified with 95% certainty at potentials more negative than -350 mV. Experience has shown, however, that passive structures tend to show values more positive than -200 mV and often positive potentials. Potentials more negative than -200 mV may be an indicator of the onset of corrosion. The patterns formed by the contours can often be a better guide in these cases.

In any case, the technique should never be used in isolation, but should be coupled with measurement of the chloride content of the concrete and its variation with depth and also the cover to the steel and the depth of carbonation.