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Die Instandsetzung von Stahlbetonbauteilen bzw.-werken, welche durch chloridinduzierte Korrosion der Bewehrung gefährdet oder bereits geschädigt sind, hat in den letzten Jahrzehnten weltweit an Bedeutung gewonnen. Der kathodische Korrosionsschutz, als elektrochemisches Instandsetzungsverfahren, bildet in diesem Fall eine zuverlässige und oftmals kostengünstige Methode zur Instandsetzung. Am Beispiel des Parkhauses Am Gericht in Frankfurt am Main werden die Durchführung solch einer Maßnahme sowie die Besonderheiten des Verfahrens beschrieben. Es werden die theoretischen Hintergründe erläutert und die praktische Umsetzung sowie zu klärende Detailfragen beschrieben. Die Daten der ersten Depolarisationsmessungen zeigen den aktuellen Zustand der Anlage sowie des Schutzobjektes.
Corrosion of steel reinforcement in concrete exposed to chloride containing environments is a serious problem in civil engineering practice. The standards and guidelines contain only minimum requirements regarding concrete quality and concrete cover. In very critical cases, for example the ingress of chlorides, the application of different coatings is currently the only possibility to protect the reinforced concrete structures. In this case the modelling of reinforcement corrosion would be more practical. One sub-project of a DFG-research project (FOR 537) is currently performed at the Federal Institute for Materials Research and Testing (BAM). Aim of the sub-project is the description of corrosion propagation and the characterization of the corrosion on specimens with small and large cathodes, by electrochemical parameters. The electrochemical parameters are determined by suitable electrochemical measurements, considering concrete parameters as well as different environmental conditions.
Pitting corrosion of carbon steel and iron in chloride containing alkaline environment is already well investigated, although a couple of questions are not clarified satisfactorily. The critical chloride content for steel in concrete and the critical chloride to hydroxide ion ratio for steel in alkaline solutions is still a major concern, considering chloride induced corrosion problems of steel in concrete. Another not satisfactorily clarified point is the repassivation of carbon steel in consequence of cathodic polarization. In the recent literature some indications are shown that repassivation occurs after a certain time of polarization. GLASS [4-9] and PEDEFERRI [1-3] draw the conclusion that the ennoblement of open circuit potentials (OCP) indicates re-passivation effects due to cathodic polarization. SAGÜÉS [10] carried out potentiodynamic repassivation tests and draws the conclusion that the repassivation potential Er is nearly independent of the solution composition, but influenced by severity of corrosion attack and is in the potential range from - 0.8 V to - 0.4 V vs. Saturated Calomel Electrode (SCE). The transferability of SAGÜÉS results to questions concerning cathodic protection (cp) is questionable, as potentiodynamic polarization tests (PPT) give no information on changes induced by long time polarization.
Therefore, the investigations discussed in this paper aim to clarify, to what extend the re-passivation of carbon steel due to cathodic polarization occurs, and if the ennoblement of OCP is a sufficient indication for repassivation. In a first step the corrosion state of five nominal equal test specimens was determined by electrochemical techniques, i.e. electrochemical noise measurements (ENM) and electrochemical impedance spectroscopy (EIS). After determining the initial corrosion state by evaluating the charge transfer resistance and the polarization resistance respectively as well as evaluating potential and current noise signals, the specimens were polarized cathodically. Impedance data were recorded previous to, during and after polarization. The impedance data were evaluated by equivalent circuit fitting with special attention to charge transfer resistances and diffusion properties. The results show that reduction of oxides and oxygen diffusion during cathodic polarization has strong impact on the systems behavior.