Corrosion rate of steel in concrete – measurements beyond the Tafel law

The rapid galvanostatic pulse technique was applied on site on a large number of measuring points with reinforcement varying from severely corroding to passive state. The measurements provide reliable results on corrosion potential, ohmic resistance and polarization resistance in very short time. The overall scatter of the data is not bigger than with any other instrumentation used for corrosion rate determination in the field. For actively corroding zones the current from the counter electrode is self-confined, no guard-ring is needed and the corrosion rate can be calculated on the basis of the reinforcement area under the counter-electrode. For passive zones the calculated “corrosion rate” is overestimated (on a very low level). Corrosion rate calculated from polarization resistance data are always instantaneous values. For engineering application (residual service life) the daily and seasonal changes in corrosion rate have to be considered. In the frequent case of chloride induced localized corrosion the local penetration rates calculated from Rp data can vary up to a factor of 5–10 and local penetration rates of 1 mm/year may occur. This uncertainty on a very high level of corrosion rate is much more important than variations induced by using devices with or without guard ring.