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.
Highlights
The corrosion behaviour of the reinforcement in concrete can be assessed by enhanced electrochemical methods: AC impedance, galvanostatic pulse, linear polarization or Tafel polarization
With the help of a potentiostat and a measurement cell containing a reference- and a counter electrode, these methods force the rebar – acting as a working electrode – to change its potential from the open circuit value according to the method specifications and to get additional corrosion relevant data: electrolyte resistance, polarization resistance and corrosion current
Regardless of all uncertainties and imperfections, the Tafel polarization seems feasible for the practical assessment of corrosion current densities on reinforced concrete structures, which includes a “safe” approach to do no IR correction to the measurement curve and to relate the Icorr to a rebar area not greater than the counter electrode
Summary
The corrosion behaviour of the reinforcement in concrete can be assessed by enhanced electrochemical methods: AC impedance, galvanostatic pulse, linear polarization or Tafel polarization. With the help of a potentiostat and a measurement cell containing a reference- and a counter electrode (setup according to fig 1), these methods force the rebar – acting as a working electrode – to change its potential from the open circuit value according to the method specifications and to get additional corrosion relevant data: electrolyte resistance, polarization resistance and corrosion current. These data provide enhanced qualitative and quantitative information which are of better accuracy than those of a potential survey only. Considering the much larger uncertainties in selecting an appropriate electrode surface, no IR correction has been made in the following assessment, because the uncorrected result is on the “safe side”
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