Microbially induced corrosion of zinc based coatings by A. niger

摘要

Deterioration of metal due to the microbial activity is called microbially influenced corrosion (MIC). MIC is a highly complicated process and the electrochemical concept of corrosion was taken as a basis for its explanation. Aspergillus niger (A. niger) belongs to a broad spectrum of fungi species. In this contribution, the effect of A. niger on the corrosion behaviour of electrodeposited zinc based coatings has been studied. In this contribution, the term zinc based coatings is assigned to the zinc and zinc alloy coatings (ZnNi, ZnFe). These coatings are used especially in automotive industry. The zinc based coatings were prepared by the electrodeposition. Corrosion behaviour was investigated in artificial nutrient and in glucose enriched solutions (with and without inoculation) over a period of 192 hours. The corrosion attack/activity of the specimens was evaluated at selected time intervals during this period. The evaluation of corrosion involved measuring changes in mass and the measurement of the electrochemical characteristics using potentiodynamic polarization method. Study of the quality of zinc based coatings after corrosion tests involved the use of energy-dispersive X-ray spectroscopy (EDX), X-ray (XRD) and scanning electron microscopy (SEM) methods. Formation of a barrier layer on all investigated coatings in all investigated media was found. The barrier layer consists of a phosphate mixture which is formed by the reaction of the hydrogen phosphates present in the solution with the coating. Zinc oxalate was identified by XRD under the studied conditions in the inoculated glucose solution. Oxalate is a product of A. niger metabolite - the oxalic acid. On the basis of obtained results, we came to the conclusion that the synergic effect of environment and the tested microorganisms is decisive for corrosion rate. Firstly, phosphating occurred and afterwards started the reaction with metabolites produced by microorganisms. This phenomenon was observed for zinc ad both types of zinc alloy coatings (ZnFe, ZnNi).