| Metal corrosion is a global problem,restricting the development of many industries such as construction,water treatment,shipping,petrochemical,marine,and so on.Microbiologically influenced corrosion inhibition(MICI)is the corrosion inhibition caused by microorganisms’ direct or indirect action.Compared with traditional anticorrosion methods,MICI is a new method to prevent and treat corrosion.However,due to the diversity of microorganisms and the complexity of their metabolic processes influenced by environmental factors,MICI is still facing the challenge of practical application.The research on MICI in China and abroad is still in the initial stage.Sulfate-reducing bacteria(SRB)is a kind of anaerobic bacteria widely existing in soil,seawater,underground pipeline,oil and gas wells and other environments.The presence of SRB accelerates the corrosion of steel.In this paper,the distribution of SRB in soil was investigated,and a strain of thiobacillus denitrificans(TDN)with inhibition SRB corrosion was isolated from the soil through enrichment culture.The corrosion and corrosion inhibition behavior of X70 steel in water and soil in the presence of TDN and SRB were studied by electrochemical testing,weight loss experiment,morphology,and composition analysis techniques,and the corrosion inhibition mechanism was preliminatively studied by using metabolites and enzymology.The main conclusions are as follows:(1)The abundance of SRB in soil was determined by the MPN method,and the significant correlation between SRB abundance and season,soil depth,and the correlation between SRB abundance and environmental factors was studied.It was found that SRB abundance was significantly correlated with the season and soil depth(p<0.05).Soil moisture content and pH in topsoil and subsoil were significantly correlated with SRB abundance(Spearman,p<0.05).There was a significant correlation between organic matter content and total nitrogen content in topsoil and SRB abundance but no significant correlation in subsoil(p>0.05).By ANOVA and Spearman analysis of topsoil and subsoil,it was found that the anion K+,Ca2+,Mg2+、SO42-、and NO3-in topsoil and subsoil were significantly correlated with the abundance of SRB and the salinity.There was no significant correlation between carbonate and SRB abundance.Redundancy analysis(RDA)was used to analyze the relationship between SRB and the main physicochemical factors of soil,and the result was consistent with that of Spearman’s calculation.(2)An SRB and TDN strain were isolated and purified from Beijing soil,the growth characteristics were studied.Both bacteria are bacilli,gram-negative bacteria.TDN was a short rod-shaped,white and transparent colony,which could discolor Giltay medium and generate gas,indicating that the bacterium had denitrification ability.16SrDNA sequencing results verified the properties of TDN strains.The growth conditions of TDN bacteria were similar to that of SRB.The optimal culture temperature was about 30℃,and the growth of the bacteria reached a stable stage in about seven days,and the bacteria could coexist in the same growth environment.The co-culture results suggested that TDN could effectively inhibit the growth of SRB.An orthogonal experiment was used to optimize the medium formula of TDN,and the optimal combination was 2.0 g·L-1 potassium nitrate,0.5 g·L-1 sodium bicarbonate,10 g·L-1 Seignette salts,and the initial pH was 7.0.Among these four factors,potassium nitrate had the greatest effect on TDN growth.(3)Through the weight loss method,electrochemical experiments,morphology and composition analysis techniques,it is found that TDN can inhibit the corrosion of SRB on X70 steel under relatively wide conditions.Through the condition exploration,the weight loss results show that TDN could inhibit the corrosion of X70 steel caused by SRB in the environment of 20～40℃,pH 6～8,and the bacteria injection amount is more than 1:1.The electrochemical results showed that SRB promoted the corrosion of X70 steel,The corrosion of X70 steel was inhibited in the TDN+SRB mixed bacteria environment.By exploring the water and soil environment of 3,6 and 9 months,TDN has an excellent inhibition effect on the corrosion of X70 steel caused by SRB.In the SRB inoculating environment,the X70 steel surface can produce relatively thick corrosion products,mainly iron oxides and sulfides.The steel sheet surface inoculated with mixed bacteria was covered with a layer of biofilm containing more organic matter,such as C and O,and the sulfur content was low.As a result,the surface corrosion of X70 steel was decreased,and no pitting occurred.It indicated that TDN could also inhibit the corrosion of X70 steel caused by SRB for a long time.(4)The corrosion process and corrosion products were observed and analyzed by SEM,EDS,XPS and ion chromatography.The results of SEM and EDS analysis of corrosion products showed that the addition of TDN weakened the cathode depolarization reaction of SRB,and TDN transformed the corrosive sulfide generated by the metabolism of SRB into SO42-which reduced the corrosion effect of sulfide on steel sheets.By SEM and EDS analysis of the corrosion surface and corrosion products,it was found that the contents of S element and trace element in the corrosion products of X70 steel decreased with the extension of soaking time in the environment of TDN+SRB compared with the environment of SRB alone.The mixed bacteria formed a uniform biofilm without S element on the sample surface,which had a better protective effect on X70 steel.The variation trend of pH,H2S,SO42-and NO3-concentrations during the culture cycle reflects that under anaerobic conditions,TDN uses NO3-as the electron acceptor,which can transform the corrosive sulfide generated by SRB metabolism into SO42-.The XPS|analysis of the corrosion products shows that the corrosion products of X70 steel are mainly oxides and hydroxides of iron in the solution of TDN+SRB,while the corrosion products inoculated with SRB are mainly sulfides,oxides and hydroxides of iron.(5)The corrosion rate,electrochemical Tufel polarization curve,and impedance spectroscopy were used to determine the inhibitory effect of TDN intracellular enzyme on the corrosion of X70 steel by SRB,and the presence of active enzymes phosphate-binding protein and glyceraldehyde-3-phosphate dehydrogenase in TDN intracellular enzyme was detected.Extracellular and extracellular TDN crude enzyme solution with enzyme activity was enriched and collected.The Bradford and Folin-phenol reagent methods were used to determine the crude enzyme solution’s protease concentration and enzyme activity.The corrosion rate was 0.0257 mm·a-1 in the SRB system without TDN enzyme,0.0098mm·a-1 in the SRB system with TDN intracellular enzyme,and 0.0283mm·a1 in the SRB system with TDN extracellular enzyme.The inhibition of TDN extracellular enzymes on SRB was measured by electrochemical Tufel polarization curve and AC impedance spectroscopy.It was found that TDN extracellular enzymes did not inhibit the corrosion of SRB on X70 steel,but TDN intracellular enzymes inhibited the corrosion of SRB on X70 steel.LCMS detection revealed that active enzymes were inhibiting SRB in TDN intracellular enzymes,and 51 proteins of TDN intracellular enzymes were detected,among which phosphatebinding proteins and glyceraldehyde-3-phosphate dehydrogenase could inhibit the active expression of SRB protein(enzyme). |
