| Iron and aluminum are the most widely-accepted materials for the construction of offshore structures and marine hulls. The investigation and design on iron and aluminum may probably be crucial to achieve the combination of industrial applications and functionalizing of ocean materials, and are also theoretically and experimentally of great significance. However, single iron and aluminum alloys are far from meeting the requirement of the rapid development of ocean technology. For example, iron alloys exhibit good mechanical properties but with bad corrosion resistance and high density. Relatively, aluminum alloys exhibit lower density, and improved corrosion resistance due to the passive films, but possess lower mechanical strength, making them unsuitable candidates for marine engineering structures.Simultaneously, the present ongoing researches about the corrosion of marine materials are limited to the characterization of corrosion morphology or mechanisms. Little effort has been done on the formation mechanisms of the representative aerobic biofilm and the interface dynamics between biofilms and substrate. In addition, examinations about anti-MIC (microbiologically influenced corrosion) performance of new kinds of marine materials are still inadequate and need further explored.Based on the above discussions, Fe3Al intermetallic compounds and its composites were brought into marine area in this dissertation. The species of dominantly-adhered bacteria on the surface of Fe3Al, their growth characteristics, the adhesion mechanism and models of biofilms and the interface features between biofilms and Fe3Al were investigated. Colloid chemistry theory, fluid mechanics theory, first density functional theory (DFT) and molecular simulation methods were alos synthetically used to characterize the interface dynamics, which provide valuable theoretical basis for the potential applications of Fe3Al metallic compounds and its composites in the ocean.The dominantly-adhered bacteria on Fe3Al surfaces were obtained by hanging samples in seawater. Through a variety of analysis methods such as 16 S rDNA, the three dominantly-adhered bacteria were confirmed to be Bacillus baekyungensis, Arthrobacter chlorophenolicus and Streptomyces flavus, respectively. These bacteria are all aerobic gram-positive bacteria. Bacillus ba. could consume oxygen rapidly and reduce the pH value of the environmental through metabolization; with the presence of Fe3+, complex would be formed by the reaction between the metabolic products of A. chlorophenolicus and Fe3+; Streptomyces flavus, which is a kind of nitrate-reducing bacteria, could easily adhere to the material surface because of its hyphae.The general formation mechanisms of biofilms associated with the three dominant bacteria were revealed by investigating the morphology and composition of different biofilms on the Fe3Al surfaces. The attached biofilms formed by Bacillus ba. were loose and porous at first, then became thick and dense, and finally fractured and peeled off from Fe3Al. Serious pitting could be found on the Fe3Al surface after the samples have been immersed in seawater for about 10 days. In the case of A. chlorophenolicus, the characteristic nanostructured Al2O3 films were firstly formed due to the attached biofilms; then, metabolic products of A. chlorophenolicus, Oxine (HQ), reacted with Fe3+ to form complex, which could effectively help repairing the fracture of Al2O3 films. Biofilms caused by Streptomyces flavus underwent morphological evolutions from the roselike oxide film to homogeneous honeycomb-like structure, which led to the adsorption of NO2- and correspondingly mitigated to some extent the acid-induced corrosion of Fe3Al.According to a set of measurements of physical and electrochemical properties of Fe3Al, the formation mechanisms of biofilms corresponding to different dominant bacteria and their effects on the surface characteristics of Fe3Al were verified. On this basis, interface dynamic models of biofilms caused by the three dominant bacteria were further established through colloid chemistry theory and Newtonian fluid dynamics methods. Referring to the reversible adhesion theory and the exponential growth model of biofilms, the maximum adhesion amount and the total adhesion coefficient during the reversible adhesion stage were confirmed by analyzing the variation of the adhesion amount of the three dominant bacteria on the Fe3Al surfaces as a function of time and their growth index during the initial growth stage. Finally, dynamic models of the three dominant bacteria on the Fe3Al surfaces during both reversible adhesion stage and developing stage were established. The variation of critical bonding strength between the three dominantly-adhered biofilms and Fe3Al as a function of immersion time was also fixed. This part provides a solid foundation for further exploring the unique physical and chemical properties of Fe3Al-based materials and their potential applications.The lowest energy state of a single teichoic acid molecule was determined through in-depth understanding the bonding pattern, adhesion parameters and electronic structure features of mutual interaction between dominantly-adhered microbial cells. According to the speculation of atomic charge density, it can be identified that the phosphatide contributed largely to the adsorption of teichoic acid molecule on the surfaces. It can also be proved that the binding of single teichoic acid molecule to the Al2O3 substrate depend mainly on the adhesion ability of oxygen atoms on the surface, strong binding energy between single teichoic acid molecule and the Al2O3 substrate occurred, evidencing that gram positive bacteria is the source of the dominantly-adhered microbe on the Fe3Al surfaces.At the end of this dissertation, we devoted our attention to the preparation of Fe3Al/ZrO2 composites with different Fe3Al-to-ZrO2 ratio and their anti-corrosion performance under both sterile environment and Bacillus ba. existing environment. The results showed that Fe3Al/ZrO2 composites with 80 vol.% ZrO2 exhibited superior anti-corrosion behavior under the Bacillus ba.-adhered condition, which may be due to the mutual cooperation of high content of ZrO2 and the passivation films formed by a small amount of Fe3Al. |
PDF Full Text Request