(Invited) Evaluating Corrosion Processes in an Aluminum Alloy, AA2024, Assisted By a Consortium of Microorganisms

Abstract

The influence of microorganisms in the deterioration of metal surfaces is an important issue in relation to the life of metal structures, equipment, and in terrestrial, marine, and aerospace transportation. In addition, changes in processes and formulations in surface treatments, in order to respond to environmental requirements, have reduced the protective quality of anti-corrosion coatings. A good example of this is the restriction in the use of Cr (VI) in surface treatments of aluminum alloys for aeronautical applications. To this we add the fact of climate change on a global level which can make the situation moredramatic. Therefore, there is growing interest in the phenomenon of microbiologically influenced corrosion in order to design intelligent solutions and long service life. In this we have studied the electrochemical behavior of an aluminum alloy exposed to a bacterial consortium. We have also considered the electrochemical responses of aluminum alloys exposed to some of the bacteria that make up the consortium. Thus, potentiodynamic polarization measurements and electrochemical impedance spectroscopy were performed. Moreover, the morphological changes associated to the corrosion phenomena were followed by optical microscopy and scanning electron microscopy. Furthermore, we check indirectly the enzymatic biocorrosion process, using macromolecules of a molecular catalyst adsorbed on the surface of a carbon electrode. The identification of microorganisms was performed by sequencing of the 16S gene of ribosomal DNA (rDNA), which allowed to distinguish a number of bacteria belonging to the genera Staphylococcus, Kocuria, Nosocomiicoccus and Tersicoccus. For all microorganisms, the catalase test was positive. Electrochemical measurements and morphological characterization (SEM and AFM) of the surfaces were performed before, and after, different immersion times (1, 2, 3, 7 and 14 days) in minimum culture media, Twin (MMT), sterile and inoculated with the consortium and also, with each isolated microorganism. Impedance diagrams for all immersion times showed a capacitive behavior characterized by a single time constant. Although for short exposure times no corrosion process was evident, for longer times the resistance of aluminum oxide (Rox) decreased significantly. The enzymatic approach of corrosion was studied using pyrolytic carbon electrodes modified with cobalt phthalocyanine. These results show that the presence of microorganisms significantly alter the mechanism of oxygen reduction, particularly the reduction of oxygen to hydrogen peroxide, confirming the enzymatic approach mechanism in MIC. Ackowledgements: The authors are grateful to CONICYT (Grant ACT1412) for financial support.