Microbially Induced Pitting Corrosion on an Austenitic Stainless Steel Pipe

Abstract After only a few weeks of use, leaks caused by pitting corrosion appeared in the weld-affected area of a fire main made of austenitic stainless steel (material number 1.4307). The quality of the used steel was checked and potential reasons for an increased susceptibility of the pipe material to pitting corrosion were searched for by examining samples of pipe sections. For this purpose, metallographic sections were prepared, etched with three different etching solutions (V2A, Beraha 2, Lichtenegger and Blöch), and examined in the light optical microscope. The examinations revealed a considerable proportion of deformation martensite and a δ-ferrite fraction of about 5 % (mainly in the cross section center) and slip lines in the edge zone. A sensitization of the material in the heat affected zone of a weld could not be observed. Deformation structures and the chromium enrichment in the δ-ferrite promote the propagation of the corrosion processes within the material. However, they cannot be related to the initiation of the pitting corrosion. Heat tinting could be observed in the weld-affected zone, and the chloride content in the water of only about 10 mg/l does not adequately explain the initiation and maintenance of pitting corrosion processes (additionally confirmed by an electrochemical examination). However, during the pipe surface examination, small black spots could also be observed on the unaffected surface. EDS analysis in the SEM and wet chemical methods identified them as microscopic manganese dioxide nodules. These deposits are an indication of microbially induced corrosion during which the microbially formed manganese dioxide, being a strong oxidant, significantly increases the stainless steel sensitivity to chloride. In the present case, this finding could be confirmed by measuring the free corrosion potential. In addition to that, indications were found suggesting that the pipe was temporarily charged with water having a higher chloride content, plausibly explaining the initiation of the pitting corrosion. The operating conditions of this pipe are characterized by stagnating water which strongly promotes the stabilization of pitting corrosion and thus represents an additional aggravating factor.