| There are a wide variety of microorganisms in the tropical marine environment. The presence of microorganisms has significantly impacted the average corrosion rate of metals and their corrosion behavior, which is also an important reason for the localized corrosion and the decrease of mechanical properties of metals. The effect of microorganisms on the corrosion behavior of metals is closely related to their species and contents. In the initial stage of seawater immersion, Pseudomonas and Vibrio are in the highest amount in the surface corrosion products of carbon steel. Thus, studying the effect of Pseudomonas and Vibrio on corrosion behavior and electrochemical behavior together with their mechanisms of S45C steel, which is widely used in the construction of marine facilities, is of great significance. At the present, the most common immersion condition used in the study of microbiological corrosion at home and abroad is medium-immersion, while seawater-immersion may be closer to the natural state. Therefore, comparatively studying the difference of corrosive behavior of S45C steel exposed to seawater and medium separately also has important significance.In this thesis, after the separation and purification of Vibrio and Pseudomonas, the effects of single Pseudomonas and Vibrio on the corrosive behavior and electrochemical behavior of the carbon steel were explored by comparing the corrosive behavior and electrochemical behavior of S45C steel in sterile seawater, in sterile medium, in seawater containing Pseudomonas, in seawater containing Vibrio, in medium containing Pseudomonas and in medium containing Vibrio. The cross-synergy effect of the mixed bacteria of Pseudomonas and Vibrio on the corrosive behavior and electrochemical behavior of carbon steel was also studied by comparing the corrosive behavior and electrochemical behavior of S45C steel in sterile seawater, in seawater containing Pseudomonas, in seawater containing vibrio and in seawater containing the mixed bacteria of Vibrio and Pseudomonas.In the initial stage of corrosion, Pseudomonas could inhibit the corrosion of carbon steel. However, with the extending of immersion time, the effect was weakened:in seawater, the corrosion rate of the steel immersed in an environment inoculated with Pseudomonas for7days and15days were39.2%and76.9%of that in a sterile environment. Pseudomonas could reduce the concentration of dissolved oxygen in the environment and form a layer of biofilm with compact structure on the surface of carbon steel, which may slow down the mass transfer process. By prolonging the corrosion time, however, due to the oxygen concentration cell corrosion on the surface of carbon steel and the accumulation of alkaline substances and amino siderophores, the inhibition was weakened and the localized corrosion of carbon steel was promoted.The life activities of Pseudomonas could reduce the corrosion potential of carbon steel, then increase its surface impedance, reduce the corrosion current densities of the cathode and anode, change the electrochemical process of carbon steel corrosion.2216E medium could inhibit the corrosion of carbon steel. After immersion in sterile medium for7days and15days, the corrosion rates of carbon steel were27.8%and19%of that in marine environment. The reason was that organic compounds in the medium could be absorbed into steel surface, which might slow down the mass transfer process and change the electrochemical process, thus inhibit the corrosion of carbon steel. The impact of2216E medium on the corrosion rate of carbon steel was highly stronger than that of Pseudomonas.In the initial stage of corrosion, Vibrio could inhibit the corrosion of carbon steel. After immersion in seawater containing Vibrio for7days and15days, the corrosive rates of carbon steel were72.3%and77.5%of that in a sterile environment. The corrosive rates of carbon steel were50%and82.6%of that in a sterile environment after immersion in medium containing Vibrio for7days and15days. In the initial stage of corrosion, Vibrio could reduce the concentration of dissolved oxygen in the environment and form a layer of biofilm with compact structure on the surface of carbon steel, thus inhibited the corrosion of carbon steel. With the extending of immersion time, however, due to the oxygen concentration cell corrosion on the surface of carbon steel, the localized corrosion of carbon steel was promoted.The life activities of Vibrio could improve the corrosion potential of carbon steel, then increase its surface impedance, reduce the corrosion current density, change the electrochemical process of the corrosion of carbon steel.At the initial stage of corrosion, the mixed bacteria of Pseudomonas and Vibrio could reduce the concentration of dissolved oxygen in the environment and form a biofilm on the surface of carbon steel which hindered the transmission of oxygen, thus inhibited the corrosion of carbon steel with a corrosion rate of81.6%of that in a sterile environment. However, with the extending of immersion time, the oxygen concentration corrosion caused by the uneven distribution of Pseudomonas and Vibrio and pH shift of the microbial membrane caused by its metabolites in the distribution, finally increased the corrosion rate of carbon steel and promoted the localized corrosion. After30days, the average corrosion rate of carbon steel under mixed bacterial environment was121.5%of that in a sterile environment.After7-day corrosion, the cross-synergy effect of the mixed bacteria of Pseudomonas and Vibrio could improve the corrosion potential of carbon steel, increase its surface impedance, and reduce the current density. But with the extending of immersion time, the corrosion potential was dropped, the impedance was reduced and the current density was increased significantly.Vibrio, Pseudomonas and the mixed bacteria of them could be vaccinated into seawater and be propagated largely by culturing to make the system more closer to the natural state in which the effect of corrosion inhibition caused by medium composition could be avoid. |
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