Half Cell Potential Meaning and Usage

Half cell potential in general and technical terms, corresponds to a steady state of electrical current between metal dissolution and the equivalent reduction reaction, leading to the formation of OH- or ions hydroxyls from oxygen gas. In concrete, the dissolution zones are usually rather large (often a centimeter or more) and fixed. A current line or electric field connects the zones to areas where oxygen is reduced. For a given value of the potential, these electric fields are normal on equipotential surfaces, and can then be intersected by the concrete surface. The potential value can then be measured using a reference electrode on the surface of the concrete. Half cell potential is the parameter used or evaluating the steel reinforcement condition of the concrete.

The corrosion of reinforcing steel is primarily an electro chemical process and the resulting behavior of the metal can be characterized and analyzed by determining and measuring its half cell potential. The greater this measurement, the higher the risk that corrosion has taken place or is occuring. An electrode accounts for one half of the cell while the reinforcing steel in the concrete forms the other. Silver or silver chloride in potassium chloride solution is the preferred reference electrode for on-site testing, although many devices still use copper or copper sulphate.

Locating the steel and determining their bar spacing by means of a cover meter is the first step of a half cell potential survey procedure. Cover concrete is locally removed over a suitable bar so that electrical connection can be made to the steel. It is important to make sure that the steel is electrically continuous. This can be done by measuring the resistance between two widely separate points. The reinforcing bar is then connected to the half cell using a digital voltmeter and readings of half-cell potential are taken by means of a regular grid of measurement points.