Research On Metal Corrosion By Microorganisms In Circulating Cooling Water

As is known to us, it is an effective solution to replace surface water withreclaimed water as the make-up water in the industrial circulating cooling watersystem, since the water consumption of those systems is enormous. However theproblem of Microbial Infuenced Corrosion (MIC) and microbial slime would bemuch more serious, considering the influence of the water quality of reclaimed water.It is widely convinced that the nature of metal corrosion is electrochemical reaction,so some related experimental methods were adopted to achieve these researchpurposes: obtaining the way how sulfate-reducing bacteria(SRB) and iron-oxidizingbacteria(IOB) influence the electrochemical behavior， surface morphology andsurface chemical element of Q235carbon steel, supplying data support for explainingthe mechanism of MIC.SRB and IOB were achieved by enriching, separating, purifying from circulatingcooling water generated by operating the dynamic analog device. After that thegrowth characteristics of those bacteria were researched. Then the influence of someelectrochemical indexes such as open circuit potential (OCP), polarization resistanceand corrosion rate were researched with the application of CS2350electrochemicalworkstation. Besides, the changes of surface morphology of carbon steel couponsaffected by microbe were researched by observing with the Environmental ScanningElectron Microscope (ESEM) and the surface chemical element changes of thosecarbon steel coupons were analyzed by the method of X-ray Energy DiffractionSpectrum (EDS).The results of electrochemical experiments showed that SRB and IOB couldconsiderably decrease the OCP so as to increase the corrosion reaction kinetics. SRBcould increase the corrosion rate of the medium up to3.17times, and IOB couldincrease the corrosion rate of the medium up to1.53times compared with the mediumwithout inoculating microorganism. After the polarization resistance and corrosionrate reached the stable stage, the corrosion rate could be separately increased byapproximately62%and29%at the effect of SRB and IOB. The polarization curvemanifested that SRB and IOB may change the electrochemical kinetics parameterssuch as βa、βc，especially affect the cathodic reaction. The phenomenon reflected by Electrochemical Impedance Spectroscopy (EIS) coincided well with that bypolarization resistance.The direct observation of carbon steel coupons indicated that, SRB could turn thesurface color into black and generate some protuberances while IOB could causecorrosion pit. The observation with ESEM demonstrated that both the two kinds ofbacteria could remarkably change the surface morphology of carbon steel and formspecial structures. The analysis of chemical element showed that SRB could increasethe percentage of oxygen and phosphorus remarkably, and decrease the percentage ofiron and sulphur slimly. At the same time, IOB could also increase the percentage ofoxygen, phosphorus, and potassium considerably, and decrease the percentage ofcarbon and iron at certain degree. In one word, SRB and IOB could apparently affectthe electrochemical behavior, surface morphology and chemical element proportion ofQ235carbon steel so as to promote its corrosion.