The Study On Microbiologically Influenced Corrosion Behavior And Mechanism Of Hull Structure Material 907 Steel In Seawater

Warships and civilian ships carry out protection tasks and transport operations in the real sea.A variety of microorganisms in the seawater environment adhere to their surface to metabolize and proliferate to form biofilms.The synergistic action of various microorganisms affects the corrosion process of hull structural materials,which leads to corrosion aggravation or inhibition.It is of great theoretical and practical significance to study the corrosion process of hull structural materials under the synergistic action of various microorganisms.In this thesis,the hull structure material 907 steel was taken as the research object,and the corrosion experiments were carried out successively,including sea hanging test,laboratory simulated real sea test,single microorganism and the synergistic action of aerobic and anaerobic microorganisms.Corrosive microorganisms were isolated,purified,cultured and identified by coating culture method and high-throughput sequencing technology,and their community richness,diversity and structure information were studied.The electrochemical behavior of corrosion was studied by wire beam electrodes and electrochemical impedance spectroscopy.The corrosion morphology was observed by scanning electron microscopy,X-ray photoelectron spectroscopy,X-ray diffraction,and the composition of corrosion products were analyzed.The formation and distribution of biofilm were observed by confocal laser microscopy and fluorescence microscopy,and the changes of environmental parameters such as DO and pH were monitored.Based on the above results,the corrosion characteristics and mechanism are summarized and analyzed.The main research contents are as follows:?1?The effects of sea area,corrosion zone and exposure time on the corrosion of907 steel were discussed.The results showed that the corrosion rate of samples immersed in the same corrosive zone for 6 months was higher than that of samples immersed in 3 months in the same sea area.The corrosion rate of the tidal zone in the same immersion time was higher than that of the whole immersion area.The corrosion rate of Sanya sea area in the same corrosive zone was higher than that of Qingdao sea area in the same immersion time.The composition of corrosion products of 907 steel seamount had little change,mainly consisting of Fe₃O₄,?-FeOOH,?-FeOOH and so on.In addition,CaCO₃ mixed with corrosion products could accumulate continuously.Many kinds of microorganisms coexisted in the real sea.The number of culturable microorganisms isolated from Sanya sea area was higher than those from Qingdao sea area.SRB was ubiquitous in the corrosion products of 907 steel in the full immersion zone of different sea areas.Under the influence of space-time conditions,the results of microbial isolation were different and the distribution changed with the change of conditions.?2?The effects of real sea environment on corrosion behavior and microbial community diversity of 907 steel were discussed.The results showed that the corrosion rate of 907 steel hanging in the full immersion zone of Qingdao sea area for three months was about 0.23 mm/a.The corrosion products mainly consisted of?-FeOOH,?-FeOOH,?-Fe₂O₃ and Fe₃O₄.The corrosion forms were uniform corrosion.The richness and diversity of microbial communities in the inner layer of corrosion products were higher than those in the outer layer.The number of OTUs of inner and outer corrosion products was 1763 and 1691,respectively,and the common number of OTUs was 941.The number of OTUs of inner and outer SRB was 908 and 798,respectively,and the common number of OTUs was 438.Common communities of inner and outer samples at generic level were mainly Cupriavidus,Sphingomonas,Lactobacillus and Ochrobactrum.There were SRB bacteria in the inner and outer layers at genus level,Desulfovibrio,Desulfobacter and Allochromatium were the main gena.?3?The effect of microorganisms on corrosion behavior of 907 steel was studied by building a real sea simulation system.The corrosion rate of 907 steel was slightly reduced by microorganisms,from about 0.0727 mm/a to about 0.0615 mm/a.Microorganisms changed the corrosion morphology and composition of corrosion products.The size of needle-like corrosion products in the aseptic system increased gradually and covered the surface of samples in clusters.The corrosion products were Fe₃O₄,FeCO₃ and FeOOH.The microorganisms and metabolites in bacterial system were dispersed unevenly on the surface of samples to form biofilms,and the corrosion products were FeS and Fe₃O₄.The types of microbial metabolism,the formation and development of biofilm and the properties of metabolites affected the corrosion process comprehensively.Aerobic metabolism of microorganisms inhibited the depolarization reaction of oxygen in the cathode and reduced the reaction rate.The formation of biofilm in the early stage of the experiment hindered charge transfer,and the later stage of the experiment led to the weakening of corrosion inhibition.Organic acids accumulated by microbial life activities could reduce the pH of solution and promote corrosion.The growth and metabolism of SRB could also promote corrosion in the later stage of the experiment.The inhibition of corrosion was stronger than the promotion of corrosion.?4?The effect and mechanism of Desulfovibrio sp.on corrosion process of 907steel were revealed.It was found that the corrosion rate of 907 steel in anaerobic seawater medium was consistent with the growth curve of Desulfovibrio sp..Desulfovibrio sp.metabolized to form an inhomogeneous biofilm.After removing the corrosion products,local pitting occurred.FeS was the main corrosion product.The effect of Desulfovibrio sp.on corrosion mechanism of 907 steel might be due to the change of local micro-electrochemical properties caused by the cathodic depolarization of the metabolite H₂S.In aseptic media,the continuous decrease of i_max value had not been further developed.The location of active anode sites was not fixed,resulting in the formation of a large area of the anode and uniform corrosion.In Desulfovibrio sp.medium,the characteristic current distribution of large cathode and the small anode was stable after the formation of electrochemically active sites,which promoted the formation and development of local pitting.?5?An experimental system under the combined action of Desulfovibrio sp.and Vibrio alginolyticus was constructed,and the joint effect of the two microorganisms on the corrosion behavior and corrosion mechanism of 907 steel were investigated.It was found that the corrosion rate,corrosion type,product composition and corrosion degree of 907 steel were changed by the interaction of Desulfovibrio sp.and Vibrio alginolyticus under seawater conditions.The corrosion rate of aseptic system was about0.2 mg·cm^-2·d^-1,and the corrosion products were mainly FeOOH,Fe₃O₄ and FeCO₃.The corrosion rate of mixed bacteria system was about 0.07 mg·cm^-2·d^-1.The local corrosion was mild,and FeS appeared in the corrosion products.Vibrio alginolyticus consumed dissolved oxygen in seawater to produce biofilm to inhibit corrosion and created anaerobic microenvironment for Desulfovibrio sp.to survive.Desulfovibrio sp.had changed the structure and morphology of biofilm,but its promotion to corrosion was not dominant.