Study on Biocorrosion Induced by Sulfate-Reducing Bacteria on Heat Exchanger Material in Cooling Water

Abstract

Corrosion associated with microorganisms has been recognized for over 50 years and yet the study of microbiologically influenced corrosion (MIC) is relatively new. MIC can occur in diverse environments. Industrial cooling water from rivers, lakes and sea water contain lots of microorganisms which are able to grow and multiply under certain conditions when pH, water temperature and sunlight etc are suitable. MIC is one of key cause of heat exchanger faults. MIC of heat exchanger materials in cooling waters has caused expensive unplanned outages, the need for local repairs and, in some cases, completes system replacement. Sulfate-reducing bacteria (SRB) are the main harmful bacteria in circulating cooling water. Under anaerobic conditions, SRB reproduce a lot to produce mucus, which speed up the formation of corrosion, erode the metal equipment, plug the pipeline, affect the efficiency of heat transfer, and bring a lot of inconvenience to the production. The corrosion behaviors of 304 stainless steel induced by SRB were studied by measuring the polarization curves, electrochemical impedance Spectrum, weight loss measurements of fore-and-aft biocorrosion, and electrochemical noise method. The electrochemical noise signal of 304 stainless steel corrosion were de-noised by using a wavelet threshold de-noising method, which made the quadratic biorthogonal spline wavelet as the mother wavelet and adopted an soft threshold processing function. The result showed that the slope of cathodic polarization curves measured included with SRB is lower than the one obtained without SRB, while the slope of anodic polarization curves is higher than it. It is concluded that the process of anode polarization was repressed at the presence of SRB. With the growth of the culture time, the value of electrochemical impedance without bacteria reduced at first, then rose, while with bacteria fell at all times. It indicated that SRB accelerated the corrosion of stainless steel. With the dipping time, a biofilm, under which corrosion products congregate to form local battery corrosion, was formed on the surface of stainless steel, so that the serious pitting corrosion is induced. The results from electrochemical noise method showed that the quadratic biorthogonal spline wavelet much smoother and it can remove the noise from the electrochemical noise effectively, and can effectively identify the location of the sudden changes in the signal and accurately reflect the useful information of the signal. The more useful information and data about biocorrosion induced by SRB are also gotten.