| Microorganisms widely exist in the environment.The presence of microorganisms can affect the corrosion of metals.In 1820s,people began to study microbiologically influenced corrosion(MIC),MIC has get more and more attention.According to the survey,the metal corrosion caused by microorganism accounts for about twenty percent of the total metal corrosion.Because MIC involves many subjects,so the mechanism of corrosion is not clear at present.Due to its many excellent properties,magnesium alloy is widely used in automobile manufacturing,military,aerospace,chemical and rocket and other important industries.However,Magnesium alloy's corrosion resistance ability is very poor,which greatly hinders its application.There are many microorganisms in the environments which can easily adsorp onto the surface of magnesium alloy and affect the corrosion of magnesium alloy,but the investigation involved in the MIC of magnesium alloy is vary scare,the mechanism is still unclear.To study the corrosion mechanism of magnesium alloy in the presence of microorganisms,two strains of bacillus species(Bacillus subtilis and Bacillus cereus)which are widespread in the environments were used in this paper.Frist,spectrophotometry was used to measure growth curves of the two bacillus.Second,the reasonable experimental points in subsequent experiments were selected base on the growth curves.Third,electrochemical methods(such as open circuit potential,potentiodynamic polarization and electrochemical impedance spectroscopy)were used to study the corrosion behavior of AZ31B magnesium alloy in the sterile,in presence of Bacillus subtilis and in presence of Bacillus cereus in artifical seawater.pH mater was also used to measure the pH values of each group's experiments.The formation of bacillus biofilm on the surface of magnesium alloys were study by SEM/EDS,IR and UV.The results are as follows:(1)The corrosion rate of magnesium alloy increased first and then decreased in the sterile artificial seawater.The pH of sterile seawater was slightly decreased due to the trace dissolution of CO2.The addition of magnesium alloy increased the pH of solution clearly,which indicated that the corrosion of magnesium alloy was partly hydrogen evolution.Due to the existence of oxide film on magnesium alloy surface,the corrosion rate was low at first,but the corrosion rate increased with the disappearance of oxide film.The corrosion rate in the late slowed down due to the accumulation of corrosion products on the surface,the change of the solution composition and the increase of pH.The results of scanning electron microscope showed that the surface of magnesium alloy had a large crack,indicating the magnesium alloy cracking.(2)Two strains of bacillus formed biofilm on the surface of AZ31B magnesium alloy,the biofilm was much dense that hampered the transmission of small molecules and ions to the surface of AZ31B magnesium alloy.Compared with the protective effect of biofilm,the effects of concentration polarization was too small that could be ignored.Seen as a whole,biofilm formed by the two strains reduced the corrosion rate of AZ31B magnesium alloy.(3)Bacillus subtilis and Bacillus cereus had similar effects on the corrosion of magnesium alloy in many respects.Two strains had an increased corrosion rate in early stage.With the biofilm growed well on the surface of AZ31B magnesium alloy during the mid stage,the corrosion rate of magnesium alloy decreased because of the formation of more compact biofilm.In the last stage,the the protection of magnesium alloy by biological film was weakened for the biofilm was wearing off.Compared with Bacillus subtilis,Bacillus cereus increased the pH more.Electrochemical test results showed that Bacillus cereus reduced the corrosion rate of magnesium alloy more than Bacillus subtilis. |
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