Effects Of Two Typical Fungi On Corrosion Behavior Of AZ31B Magnesium Alloy

The presence of microorganism can cause microbiologically influenced corrosion (MIC). The economic loss by MIC is about twenty percent. Because of the complexity of the microorganism, corrosion mechanism is still unclear and does not have a consistent viewpoint at present. Researches of MIC mainly have been focused on bacteria corrosion, little is known about fungus corrosion. In fact, fungus occupies an important position in the microorganism; it widely exists in various environments. Therefore, it is necessary to study the effects of fungus on metal corrosion, laying the foundation for the further study of MIC.Magnisium alloys are wildly used in auto industry, military industry,3C industry and medical meterials due to their light weight, high strength to weight ratio, good cast ability and recycling ability, etc. The poor corrosion resistance is still the main factor limiting the development and application of magnesium alloys. The application of magnesium alloys often contact with microorganism; but very few stydies have been focused on MIC of magnesium alloys, and fungus corrosion on magnesium alloys has not been reported yet.Two typical fungi, Aspergillus niger(A. niger) and Trichoderma harzianum (T. harzianum), were chosen to be cultivated respectively, dry weight method was used to measure the growth curve of the two fungi in artificial seawater, selecting reasonable experimental point according to the growth curve. The electrochemical methods (open circuit potential, potentiodynamic polarization curve, cyclic polarization curve and electrochemical impedance spectroscopy) and surface analysis techniques (fluorescence microscope and scanning electron microscope) have been used to study the the corrosion behaviour of AZ31B magnesium alloy in sterile, in the presence of A. niger and in the presence of T. harzianum in artificial seawater. The corrosion behaviour and mechanism of three kinds of cases have been discussed. The differences and similarties of the two fungi were compared. Specific results are as follows:(1) The pH values of sterile artifical seawater increased with incrasing the immersion time, indicating that the dissolution of AZ31B magnesium alloy along with hydrogen evolution occurred. The polarization curves showed passive behaviour at initial immersion time, suggesting the presence of oxide film on surface. But with the extension of time passivation phenomenon disappeared, it gradually transformed into active corrosion, indicating that passivation film was destroyed and did not have passivation repair ability. The polarization curves also showed that the icorr first increased and then decreased, and the correspondingorr first decresed and then increased, indicating that corrosion rate increased first and then had a tendency to decrease. The corrosion products covering on the surface of AZ31B magnesium retarded the further corrosion. The scanning electron microscope showed corrosion occurrence and corrosion products formed on the surface of the sample.(2) The results showed that A. niger adsorbed on the surface of samples by fluorescence microscope and scanning electron microscope, and the hyphaes were near the corrosion pits. The pH values of sterile artificial seawater were observed higher than artificial seawater containing A. niger, indicating that the adsorption of A. niger produced acidic metabolites and accelerated the corrosion rate. The electrochemical testing results showed that, compared with samples in sterile artificial seawater,icorr increased and Ecorr decreased of samples containing A. niger, indicating that the addition of A. niger further accelerated the corrosion of AZ31B magnesium alloy. The corrosion rate of AZ31B magnesium alloy co-affected by artificial seawater and adsorption of A. niger.(3) The pH values of artifical seawater containing T. harzianum increased with increasing the immersion time, indicating the dissolution of AZ31B magnesium alloy balanced by hydrogen evolution. The electrochemical testing results showed that icorr increase and Ecorr decrease of samples containing T. harzianum compared with those in sterile artificial seawater, suggesting the addition of T. harzianum further accelerated the corrosion of AZ31B magnesium alloy. It can be seen that T. harzianum adsorbed on the surface of samples by fluorescence microscope and scanning electron microscope, presenting a mesh structure. The adsorption of T. harzianum accelerated the corrosion rate of AZ31B magnesium alloy. (4) The results showed that pH values of medium in the presence of T. harzianum was lower than that in the presence of A. niger, indicating acidic metabolites produced by T. harzianum was more than A. niger, resulting in a greater effect on corrosion. Although the two fungi accelerated the corrosion rate, their adsorptions were different. The adsoption of A. niger was near corrosion pits, but T. harzianum was on the surface of samples, which showed the complexity of the MIC. The two fungi just changed the corrosion rate of AZ31B magnesium, but did not change the corrosion mechanism.