Study On Corrosion Resistance Of AZ91 Magnesium Alloy In Sulfate-Reducing Bacteria Solution

Magnesium alloys is the lightest quality and the highest strength of metallic materials in engineering applications. At the same time, magnesium has high specific stiffness and good thermal conductivity, good shock absorber damping and electromagnetic shielding properties, processing of easy, good recyclable recycling advantages. As a result, magnesium alloys is considered as "Green Project materials" that has the most potential applications and the future development in the 21st century and have become promising materials for manufacturing light structures and have a bright future in aviation, aerospace, walking machine and 3C etc. However, the poor corrosion resistance of magnesium alloys has been greatly restricting their further application. So, the key of development and application magnesium alloy lies in the solution of magnesium alloy corrosion resistance problems. Microbiologically influenced corrosion (MIC) is the forefront of corrosion one of the fields of scientific research, particularly in the aerospace and automotive fields. At present, magnesium alloy is gradually applied to civilian areas and large magnesium alloy components will be used in lightweight vehicles. People will gradually concern magnesium alloy MIC.In recent years, people have done a lot of research on corrosion resistance of magnesium alloy. People have found that heat treatment can not only improve the mechanical properties of magnesium alloys, but also can significantly improve the corrosion resistance of magnesium alloy. However, concerning on the effects of microbes on as-cast and heat-treated magnesium alloy, people know very few and needs to be done in-depth study and discussion.The magnesium alloys used in this paper was AZ91 and microbe used in this paper was SRB. We have inspected the law of microstructure changed that solution heat treatment (T4) and aged heat treatment (T6) impact on AZ91 magnesium alloys; have researched the average rates of corrosion and corrosion morphologies and products that pure magnesium(Mg), pure aluminum(Al), AZ91-F(as-cast), AZ91-T4(solution) and AZ91-T6(solution and aged) produced in SRB solution; have discussed the corrosion characteristics, corrosion, electrochemical behavior which Mg, Al, as-cast and heat treatment AZ91 immersion in the SRB and sterile solution and the effects on SRB on their corrosion performance. Our researching work was concluded as followings:(1) Pure culture bacilli are got by using the API culture medium and ferrous sulfate ammonium reaction sediments black. Gram stain results showed that the bacteria identified as gram-negative bacteria, therefore conclude that this culture is pure sulfate-reducing bacteria strains. TEM is observed that SRB was short rod-shaped bacterial cells, no flagella, no bacillus, the size of cell body is (0.5-0.6)μm×(1.3-2.0)μm.(2) Corrosion characteristics are comprehensive corrosion which Mg immersed in the SRB and sterile solution, larger continuous corrosion regions were formed, and also a significant localized corrosion was shown in the sample with SRB. It is little that the difference of Al corrosion morphology between innocuous and sterilized solution, corrosion characteristics are comprehensive corrosion. There were no obvious signs of corrosion. Corrosion characteristics are pitting corrosion which AZ91-F,AZ91-T4 and AZ91-T6 immersed in the SRB and sterile solution. It is small that the number of innocuous AZ91-F producing corrosion pits that are rather scattered. Sterile sample corrosion pits more than innocuous samples significantly, and there corrosion pits are continuous distribution. It is large that the number of AZ91-T4 producing corrosion pits in SRB solution, but it is small in sterile solution; Corrosion resistance of AZ91-T6 in the SRB solution less than sterile solution. There are some corrosion pits on innocuous samples and sterile sample surface. With the increase of aging time, the numbers of corrosion pits are reducing.(3)XRD analysis showed that main corrosion product of Mg and Al is Mg(OH)2 in the innocuous and sterile medium; Corrosion products of AZ91-F,AZ91-T6 and innocuous AZ91-T4 are NH4MgPO4·6H2O,Al2O3,Al(OH)3,Al4Ca,AlPO4; main corrosion product of sterile AZ91-T4 is NH4MgPO4·6H2O.(4) Corrosion potential (Ecorr) of Mg and Al in the SRB solution less than in the sterile solution. Compared with sterile solution, corrosion current density (icorr) of Mg and Al is larger than in the SRB solution. SRB promoted corrosion of Mg and Al. Compared with sterile solution, Ecorr of AZ91-F is larger than in the SRB solution. But icorr of AZ91-F in the SRB solution less than in the sterile solution, SRB restrained corrosion of AZ91.For heat-treated AZ91, SRB promote the cathode depolarization at the same time but did not affect the anode process. Ecorr of AZ91-T4 and AZ91-T6 in the SRB solution are less than Ecorr in the sterile solution. Compared with sterile solution, icorr of AZ91-T4 and AZ91-T6 is larger in the SRB solution. SRB promoted corrosion of heat-treated AZ91.In a word, the research of the present study lies not only in the further investigation and broadening on Magnesium alloys MIC, but also in the more embedded comprehension of the traditional MIC technique and Magnesium alloys surface technology as well as the correlation theories. At the same time, it provides the referencable ideas for the studies of in Magnesium alloys MIC which are more prevailed internationally.