Research On The Corrosion Behaviors Of Steels Under The Sulfate-Reducing Bacteria Biofilms

With increasingly intensive exploitation and effective utilization of marine resources, problems due to microbiologically influenced corrosion (MIC) of metal constructions/components had aroused people's high concern and extensive study, and among them sulfate-reducing bacteria (SRB) are of the most dangerous culprits.In this thesis, SRB strain was isolated from seamud in Qingdao nearby, incubated in the modified Postgate's C culture medium which was also prepared for the corrosion medium. Changes of seawater-based medium state due to bacterial activities were detected. As the main objectives, effects of SRB on the corrosion behaviors of carbon steel, low-nickel alloyed high-strength steel and 18-8 stainless steel (18-8SS) in seawater-based media were investigated by electrochemical methods (Ecorr^t curve, electrochemical impedance spectroscopy and polarization curve) and surface analyses techniques (scanning electronic microscopy, atomic force microscopy and X-ray photoelectron spectroscopy). Mechanisms related to both anodic active dissolution of iron and pit-initiation/pit-growth of stainless steel were discussed in detail.Specifically, when carbon steel is immersed in the cultures of SRB, a protective surface ferrous sulfide (FeS) film would form during the early days of immersion; but some time later the protective FeS film would suffer breakdown locally, and then the localized corrosion rate would be very high in a mode of active dissolution. In the case of high-strength steel immersed in the culture of SRB, the electrochemical nature of corrosion is almost the same as carbon steel although the corrosion morphology is a little different. With further investigation, it was found that sulfur-containing metabolites by SRB are the main factor enhancing the anodic active dissolution of carbon steel. Thus, such a corrosion mechanism was proposed that sulfur-containing bacterial metabolites in the form of adsorbed sulfur on iron enhance anodic active dissolution of carbon steel or high-strength steel. Thereticaly, judged from the E—pH diagrams for adsorbed sulfur on iron, adsorbed sulfur on iron remains very stable in a wide range of pH values as well as potentials values.As for 18-8SS, pitting corrosion of 18-8SS in the cultures of SRB is very complex process, in which many factors affect pit-initiation/pit-growth. Pit-initiation of 18-8SS can occur in a very short term, and the localized free oxygen resulted from sulfidation of passive film should be the main factor that cause the breakdown of passive film because Epit of 18-8SS in the cultures of SRB is more positive than Eh of the media; Meanwhile, the depth of micro-pits increases approximately linearly with the immersion time. From standing point of cathodic depolarization of corrosion, there are many factors that affect the pit-growth of 18-8SS in the cultures of SRB; However, controlled by the repassivation behaviors of 18-8SS (Erep is more positive than the equilibrium potential of hydrogen electrode), sulfur element or polysulfide is the main factor that sustains the pit-growth instead. Additionally, due to the changes of the medium state parameters under influences of SRB, bacterial activities not only instantaneously affect the pit-initiation and the pit-growth, but also induce the modification of passive film in structure/properties and in turn affect the pitting corrosion as well.In this thesis, sulfidation of passive film of 18-8SS in the cultures of SRB and its effects on the pitting corrosion were thorougly investigated. Results shows that sulfidation of passive film in the cultures of SRB causes a loss of passivity of 18-8SS, but sulfidation happened in the short term is more detrimental to the passive film than that in the long term. However, sulfidation polarizes the cathodic reactions to a greater extent, indicating that sulfidation can impede the pit-growth. Surface properties analyses of 18-8SS show that almost only iron oxides in the passive film were converted into sulfides in forms of FeS or FeS₂, while element Cr and Ni still remained essentially as oxides/hydroxides, which can explain definitely why sulfidation happened in the short term is more detrimental to the passive film than that in the long term, because the free oxygen resulted from sulfidation of iron oxides in the passive film probably can later recombine with other alloy elements to form other forms of unsulfidable oxides/hydroxides. On the other hand, it was also found that protective SRB biofilms can be formed in the cultures of SRB, which can impede pit-initiation /pit-growth of 18-8SS to some extent.