Corrosion Mechanism And Control Of Stainless Steel Under Biofilms Of Citrobacter Freundii In Circulating Cooling Water System Of Power Plant

As one of the effective water-saving methods,urban reclaimed water was used as a supplementary water source for the circulating cooling water system of power plants.However,due to the problems of high water quality concentration and large number of microbial species,the stainless steel tubes of condenser in the circulating cooling water system was prone to complex biological corrosion and scale formation.In this paper,the biological film characteristics,hazards and control of C.freundii were studied on the surface of stainless steel tubes of condenser.The growth and adhesion characteristics,biofilm structure,metabolism products and extracellular polymeric substances(EPS)components of C.freundii in circulating cooling water environment were studied by using principles,methods and techniques of hydrochemistry,environmental microbiology,surface chemistry and electrochemistry,etc.The corrosion mechanism of S2’ and EPS on stainless steel was studied.The effect of scale and corrosion inhibitors on corrosion inhibition of stainless steel were studied in circulating cooling water.The main conclusions are as follows:(1)The thermodynamic parameters of SS317,SS316L and SS304 were obtained in the experiment.It was found that the surface of stainless steel was hydrophobic and bacterial adhesion to the surface of stainless steel is a spontaneous process.The dynamic adhesion experiment revealed that the adhesion rate of bacteria on SS317 and SS316L was higher than that on SS304.(2)The experiment proved that C.freundii could reduce SO42-to S2-and the sulfate reduction rate increased with the increase of biological activity in circulating water.The main groups of EPS were polysaccharide and protein,and the content of polysaccharide was higher than that of protein.The functional groups of EPS were O-H,N-H,C-O-C,phosphate-containing groups and sulfur-containing groups.(3)In the presence of S2-,S2-reacted with Fe2+ and Ni2+to form FeS and NiS on the surface of stainless steel.The results showed that S2’ changed the composition and structure of the passivation film and inhibited the formation of oxide on the surface of stainless steel.Sulfide degree of passivation film on the surface of stainless steel was obviously different.The order of sulfide content was SS304>SS316L>SS317.(4)LB-EPS or TB-EPS isolated from C.freundii had a certain sustained release effect on stainless steel in circulating cooling water.In the presence of LB-EPS or TB-EPS,metal dissolution on the surface of three kinds of stainless steel was effectively inhibited.(5)Inhibitors of HEDP and PBTCA both have an influence on the initial adhesion of bacteria cell to surfaces of SS317,SS316L and SS304 coupons.The kinetics adhesion constants were reduced in the presence of PBTCA and HEDP.The total of free energy(ΔGTot)values of the SS surfaces were increased(moved towards positive)in the condition of coating inhibitors.The adhesion to SS surfaces coating with PBTCA were less than those with HEDP.(6)The results of electrochemical impedance experiment showed that the surface polarization impedance values of SS304,SS316L and SS317 were increased by 68.8%～93.4%in the presence of PBTCA,and 63.3%～95.3%in the presence of HEDP.The inhibition efficiency of PBTCA was better than that of HEDP.