COMPARISON, BY ELECTROCHEMICAL MEASUREMENTS, OF THE MICROBIOLOGICALLY INDUCED CORROSION BEHAVIOR IN STAINLESS STEELS J. I. Pastor Manso; F. Montero Ortego and J. Izquierdo Gómez IBERDROLA GENERACIÓN – SETEG - Tecnología de Materiales, Pº Virgen del Puerto, 53-57 28005 Madrid Spain jipastor.manso@iberdrola.es ABSTRACT Many components of floodgates are built in martensitic stainless steels. Different behaviour of these components has been found depending on the river. We have carried out some investigations in order to find the root cause of this phenomenon. Samples of the usual material AISI 420, together with other possible candidates, were introduced in the typical environment of the components. Simultaneously metallurgical and electrochemical analyses were made, comparing different material heat treatments. Electrochemical techniques are a very useful tool for determining and compare the corrosion behaviour of metals in a limited experimental period, giving at the same time a very high accuracy. The results were compared and the material with the best behaviour selected. INTRODUCTION The influence of the micro-organisms over the corrosion process in metals was suggested at the end of the last century telling about the corrosive action of the NH3, NO3 -, NO2 - produced by the bacteria over the lead. In 1910 it was suggested that ferrous bacteria and sulphate-reducing bacteria were in part responsible of the corrosion in buried metals. Other authors tried to explain the buried corrosion in electrochemical terms by the possibility of the sulphate-reducing bacteria to take H2 when the sulphates were reduced following the reaction: SO4 = + 4H2 Æ S = + 4H2O They proposed that this bacterium could use atomic hydrogen from polarised cathode to reduce sulphated compounds. Sulphured ions will react in the anode with part of the ferrous ions in the solution: S= + Fe++ Æ FeS The other part of ferrous ions will react with hydroxyl ions: 3 Fe++ + 6 OH- Æ 3 Fe (OH)2 The global reaction is:
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