This paper discusses the capability of Electrochemical Noise Measurements (ENM) in terms of applications on steel in concrete with respect to pitting corrosion. Passive and active steel specimens in solution as well as in concrete are investigated and the results particularly with regard to the conductivity of the electrolyte and the influence of porosity respectively are evaluated. Two different types of concrete, with fly ash and without, were used to obtain noticeable differences in conductivity. The results clearly show the general influence of resistance on the corrosion behavior of steel in concrete and give some indication that ENM is a capable method also for systems with comparatively high electrolyte resistances.
Generally, steel in high alkaline environment is well protected against corrosion by formation of a dense passive layer consisting of various iron oxides/hydroxides [1]. That applies also for steel in concrete, where the alkalinity of the pore water provides conditions for the formation of passive layers. However, steel in high alkaline environment can lose passivity i.e. in consequence of chloride ingress, when a critical cCl-/cOH- ratio at the steel surface is exceeded [2, 13]. Normally, the corrosion of steel in concrete is investigated using conventional methods like linear polarization resistance technique (LPR), galvanostatic pulse technique or electrochemical impedance spectroscopy (EIS) [14-18]. Considering the critical corrosion inducing chloride content for steel in concrete [13], it becomes obvious that further research work is necessary to fully understand the chloride-induced corrosion of steel in concrete. Electrochemical noise measurements are well known to be very useful to investigate the early state of pitting corrosion. The capability of ENM to investigate pitting corrosion with respect to scientific questions is well acknowledged and has been shown by several researchers [3-9, 22- 25]. The most of these publications deal with investigations on metals and alloys in different solutions of more or less high conductivity. In the literature, only a few papers can be found, that give attention to pitting corrosion of steel in concrete [10-12] and those mostly regard questions concerning the determination of corrosion rates. Thus it appears that ENM is undoubted applicable on steel in concrete. However, the question to what extend ENM can contribute to a better understanding of corrosion of steel in concrete is not answered so far [26]. Therefore, the investigations presented in this paper shall contribute to clarify questions on limits concerning applications of ENM on steel in concrete.
When investigating steel in concrete choosing a suitable reference electrode is all times a major problem. In terms of electrochemical noise measurements, it is additionally a very special problem. In consequence of the comparatively high electrolyte resistance, it would be beneficial having a low impedance reference electrode very close to the working electrode in order to reduce the potential drop to a minimum. This would mean a laboratory reference electrode has to be cast-in into the concrete and would produce quite expensive specimens. To simplify the specimen setup and in order to find a practical solution, three different electrodes (saturated calomel laboratory reference (SCE), activated titanium mesh (TiMO) and platinum wire (Pt)) for potential noise measurements were compared and evaluated with regard to the set noise.
