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The influence of manganese on the time course and on the final value of the open-circuit potential of microbially influenced stainless steel (SS) has been examined by variation of the concentration of manganese in the aqueous phase and on the surface of the metal sample. Under the influence of bacteria stable final values of the open-circuit potential are attained at 540 mV or 620 mV depending on the manganese concentration in the aqueous phase, the duration of the experiment, and the supply of oxygen. These potential values can also be fixed electrochemically by anodic deposition of MnO2 on the metal surface. According to the results obtained with different techniques, the electron flow from the metal towards the oxygen is a combination of biotic and abiotic processes. Besides the microbial oxidation of manganese the electron flow via a Mn-based redox enzyme system has to be discussed.

Ausgangspunkt fur die vorliegende Untersuchung waren Korrosionsschaden an Kuhlwasserrohrleitungen aus nichtrostenden Stahlen, die unterhalb von 50°C durch mikrobielle Belage verursacht wurden. Diese Biofilme bewirkten eine Potentialverschiebung der Werkstoffe in den Bereich der labilen Lochfrasbestandigkeit. Die mikrobiologische und elektrochemische Untersuchung ergab, das die bakteriellen Zelldichten in den Belagen ebenso temperaturabhangig sind wie die im Laborexperiment erreichten Endwerte des Freien Korrosionspotentials. Weiterhin wird das Freie Korrosionspotential des Werkstoffes mit der Redoxsituation der umgebenden wasrigen Phase verglichen und davon ausgehend eine direkte Kopplung zwischen der Potentialverschiebung und dem bakteriellen Wachstum postuliert. Gemas dieser energetischen Betrachtung ist eine Zunahme der Zelldichte nur bis zum Erreichen einer minimalen Potentialdifferenz moglich, die den Grenzwert fur den Erhalt der Biomasse darstellt. An examination of the interrelationship between bacterial growth and microbially induced potential shift of stainless steels Corrosion damage was found in an earlier examination of stainless steel piping used for cooling circuits at temperatures below 50°C. This attack was caused by a microbial slime layer which was responsible for the shifting of the open circuit potential to a region of instable pitting resistance. Both the bacterial cell densities of the biofilm and the final values of the open circuit potential in laboratory experiments showed temperature dependence. Furthermore, a comparison of the open circuit potential with the redox situation of the surrounding aqueous phase leads us to postulate a direct coupling between the potential shift and the growth of the bacterial population. According to these observations the cell density can only increase until a minimal potential difference is reached. This value is the limit necessary for maintaining the biomass.

A new type of MIC testing facility has been developed based on the data gained through MIC-experiments, and the analysis of the potential shift caused by MIC. A parallel trial experiment was also conducted lasting more than two months. The results demonstrated that the facility enables one to undergo a detailed investigation of responses to MIC behavior. This allows for the collection of a substantial amount of data required for the investigation of MIC. The experiment also demonstrated that the improved facility offers more flexibility to MIC experiments. The facility can maintain predefined conditions allowing experiments to be conducted without outside environmental influence. Verbesserung eines Versuchsaufbaus fur Untersuchungen der „Mikrobiell induzierten Korrosion” an rostfreien Stahlen Aufgrund von Ergebnissen einiger bisher durchgefuhrten Untersuchungen wurde ein neuer Prufstand fur Untersuchungen zur MIC entwickelt. Eine erste Versuchsserie wurde uber einen Zeitraum von mehr als 2 Monaten durchgefuhrt. Die Ergebnisse zeigen, das der Versuchsaufbau geeignet ist unter frei wahlbaren definierten Bedingungen ohne ausere Einflusse Versuche durchzufuhren.