| It is believed that corrosion loss accounts for about 3~4% of GDP, and microbially-influenced corrosion (MIC) accounts for about 20% of metals and building materials. The losses caused by MIC are huge. The direct economy losses of worldwide per annum caused by MIC are about 300~500 billion us dollars, and about 100~200 billion RMB in China. The leakages of natural gas, coal gas, oil pipeline induced by MIC could cause not only great personal injuries, but also severe damages of ecology and environment. MIC of metals exists widely in various industrial sectors. These sectors mainly include nuclear energy, power generation, petroleum and marine industries. For a long time, the reseaches of MIC of metals are mainly in stainless steels, carbon steels, aluminum alloys, copper alloys and nickel alloys etc. The reseaches of MIC of magnesium alloys are still not arouse people's enough attention. So far the reseaches of MIC of magnesium alloys in oversea have not been reported, and few studies had involved ZM-5 cast magnesium alloy in domestic. At present, the researches of MIC of magnesium alloys are still in starting stage, and the rearches are still not systemic and deeply. However, with the incresase applications of magnesium alloy in the civil fields of automobile and IT, and continuously development of modern new magnesium alloys, and constant progress of surface treatment processings of magnesium alloys, these all provide unprecedented opportunities for the the wide applications of magnesium alloys. With the wide popularization of magnesium alloy materials, the problems of MIC of magnesium alloys will be inevitable. Therefore, the present research is of great importance.At present, there are some defects in the study of MIC of magnesium alloy: 1) the deeply researches of MIC of AZ91 are still not carried; 2) the influences of environmental medium for MIC of AZ91and 3) the reseaches of MIC associate with magnesium alloys under heat treatments and surface modifications are still not studied. Therefore, the dissertation aimed at the above-mentioned problems to carry deeply and systemtic researches.Firstly, through the experiments of enrichment, separation and purification for microorganism, attained a pure culture with single bacterium. Through qualitative detection for bacterium metabolites of H2S, method of gram stain, observation of TEM, identificated the species and genera for the pure culture. Through detection technique of amount of bacteria, studied the growth characteristics of the bacterium, made good preparation for the step experiments of MIC.The experimental results show that, the fabricated bacterium culture was SRB pure culture. The SRB pure culture had higher biological activity and can be used as experimental bacterium. The separated SRB cells were rod shaped and short rod shaped. Bacterial colonies diameters were about 0.1~1mm. The sizes of the SRB cells were 0.5~0.6μm×1.3~2.0μm. The SRB cells without flagella and gemmae moved with torsing and rolling way. According to berger bacteria identification manal, the SRB does not belong to the common Desulfovivrio and Desulfotoma-culum. The study of the growth characteristic of the SRB indicates that, this SRB has a longer arrearage stage for about 4.5d, a very short logarithm growth stage for about 0.5d, a growth stabilization stage for about 1d and a contabescence stage after 6th day.Secondly, through immersion experiment, combining measure of corrosion rate, analyses of XRD, EDS, SEM and electrochemistry, studied the influence of SRB on the corrosion behaviors of cast AZ91. The experiment firstly detected that the main corrosion product of AZ91 in API culture medium was NH4MgPO4?6H2O, and the other corrosion products with lower content were Al2O3,Al(OH)3,Al4Ca,AlPO4. The experiment indicates that, SRB can adhere and grow on the surface of AZ91, and form a biofilm on AZ91 surface. The biofilm delayed the corrosion of cast AZ91. The structure of biofilm had consanguineous relationship with medium ingredient. When the culture medium contained (NH4)2Fe(SO4)·6H2O and C6H8O6, amount of SRB in the biofilm was higher, and SRB cells distributed equably. However corrosion products were lower and the structure of the biofilm loosened. The biofilm has weak protection function. When the culture medium did not contained (NH4)2Fe(SO4)·6H2O and C6H8O6, amount of SRB in the biofilm was lower, and SRB cells distributed unequably. However corrosion products were higher and the structure of the biofilm compacted. The biofilm has perfect protection function. (NH4)2Fe(SO4)·6H2O and C6H8O6 significant accelerated the growth and metabolism of SRB, but formed FeS deposit on magnesium alloy surface, accelerated the corrosion of cast AZ91.Thirdly, the influence of Cl-, Fe2+, Cu2+ and their concentrations on the SRB corrosion behaviors of cast AZ91.1) The corrosion characteristic of AZ91 in SRB medium containing Cl- was pitting corrosion. When Cl- concentration was not more than 1.5g/L, SRB has little effect on the corrosion of magnesium alloy. When Cl- concentration was more than 1.5g/L, the area of pittings increased and developed ulcer corrosion. Corrosion rate increased with the increase of Cl- concentration, and corrosion potential decrease with the increase of Cl- concentration. The mature biofilm acted as physics barrier, decreased the corrosion sensitivity for Cl-, and delayed the corrosion of AZ91.2) The main corrosion products of AZ91 in SRB medium containing Cu2+ were not NH4MgPO4?6H2O. The corrosion prodcts are Mg(OH)2 and Cu. Cu2+ had significant sterilization function for SRB cells. SRB trended to adhere and grow on metal surface. The lower Cu2+ concentration (~50mg/L) was contributed to inhibit the growth of sessile bacterium. When Cu2+ concentration was at the range of 50~200mg/L, with the increase of Cu2+ concentration, conglomeration of sessile bacterium increased, and EPS and corrosion products increased. This increased the aggrieve resistance of SRB for Cu2+, so the account of SRB in biofilm increased. When Cu2+ concentration was 250mg/L, the diffusion of Cu2+ increased, and the account of sessile bacterium decreased. When Cu2+ concentration was at the range of 50~250mg/L, Cu2+ could not sterile SRB cells in solution and metal surface completely. In the SRB medium containing Cu2+, EPS in biofilm can bind Cu2+, decreased the corrosion function of the solution. Cu2+ is a deleterious ion for the growth of SRB. It can form Cu-Mg galvanic corrosion electrocircuits on metal surface leading to the severe corrosion of magnesium alloy. Hence, Cu2+ can not act as sterilization agent or a composition of sterilization agent for the MIC of magnesium alloy.3) With the variety of Fe2+ concentration in solution, SRB can cause the change of corrosion products on AZ91 surface. Although Fe2+ can accelerate the growth and metabolism of SRB, Fe2+ can form Fe-Mg galvanic corrosion electrocircuits on magnesium alloy surface leading to the severe corrosion of magnesium alloy. When Fe2+ concentration was not more than 40mg/L, SRB biofilm delayed the corrosion of magnesium alloy. When Fe2+ concentration was not less than 80mg/L, SRB biofilm accelerated the corrosion of magnesium alloy.Fourthly, the influence of SRB on the corrosion behavior of heat treatment AZ91 was studied. The experiments indicate that T4, T6 and SRB did not change the composition of the corrosion products of AZ91 magnesium alloy. The corrosion products were still NH4MgPO4·6H2O,Al2O3,Al(OH)3,Al4Ca,AlPO4. AZ91-T4 had good corrosion resistence in sterile medium and its corrosion characteristic was pitting corrosion. AZ91-T4 had bad corrosion resistence in SRB medium and its corrosion characteristic was forming large area of local corrosion regions. In sterile medium, corrosion rate increased with the increase ofβphase content. In SRB medium, corrosion rate decreased with the increase ofβphase content. In sterile medium,βphase acted as cathode accelerating the corrosion of magnesium alloy. Moreover, with the increase ofβphase content, the area of cathode increased, and hydrogen evolution reaction was easier, and the dissolution of anthode was accelerated. In SRB medium,βphase acted as inhibition phase and delayed the corrosion of magnesium alloy. In SRB medium, corrosion was induced by the activation of H2S forαphase, and the increase ofβphase decreased the effective activation area, and the dissolution of anthode was inhibited. The presence of SRB did not cause the significant varitation of the pH values of solutions, and corrosion resistance had not relationship with the pH values of solutions. As to AZ91-T6, SRB metabolites of H2S had cathode depolarization effect, and H2S can accelerate the reaction of cathode leading the dissolution of anthode.Finally, the influence of SRB on the corrosion behavior of MAO treatment AZ91 was investigated. The experiment indicates that, MAO film and SRB did not change the corrosion products. MAO films were destroyed either in sterile medium or in SRB medium, and the corrosion characteristics were all pitting corrosion. The activiation of H2S and the erosion effects of organic acids and hydrosulphuric acid accelerated the pitting corrosion of MAO treatment AZ91. The Ecorr of MAO samples in sterile medium shifted positively, and the Ecorr of MAO samples in SRB medium shifted negatively.To sum up, the dissertation combined biotechnology, corrosive measurement technology, surface analyses technology and electrochemistry technology. In lab environment, through the experiments of simulation acceleration, studied the influence of SRB on the corrosion behaviors of cast AZ91, heat treatment AZ91, MAO treatment AZ91, and the influence of environmental medium on the corrosion behavior of cast AZ91, and discovered the basic rules of the corrosion of AZ91. This provided reliable experimental and theoretical bases for the further corrosion protection of MIC of magnesium alloys.
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