Insights from corrosion current measurements on corrosion mechanisms in reinforced concrete and on the evaluation of other corrosion data

Abstract

During the past years Tafel polarization measurements have been implemented into the scope of measurements of CITec corrosion diagnosis projects. This has created a vast database of different and corresponding corrosion parameters, such as chloride and water content in the rebar vicinity, open circuit potential, electrolyte resistance, polarization resistance (from galvanostatic pulse and linear polarization) and corrosion current from Tafel polarization measurements. Although general limitations in using these methods on macro cell systems such as reinforced concrete are known, the comparative assessment of these data has led to a better understanding of the corrosion behaviour and of specific circumstances of the structures which deviated partly from usual expectations. For instance, a low polarization resistance at high chloride content will not result necessarily in a high corrosion current, if the reinforcement in the wider vicinity of the test location is similar active, and cathodic rebar areas are either very distant or retarded by very wet concrete. So the extended range of corrosion testing gives a more precise evaluation of the corrosion situation and permits a tailored repair and maintenance concept to be found. It has also been found that the Stern-Geary equation which is often used to calculate corrosion current densities and material loss of the reinforcement from linear polarization (LPR) measurements, doesn’t seem very feasible if used on reinforced concrete structures, as there appears to be a dominant influence of macro cell corrosion over the corrosion model of a homogenous mixed electrode (for which the Stern-Geary equation applies), and the true corrosion current densities may be either larger or (very often) much smaller than those calculated from Stern-Geary. This is not a new observation, and the findings will be discussed for several project cases.