Microbe-induced pitting of Fe-based amorphous coating

Abstract

It is well known that Fe-based amorphous coatings demonstrate excellent performance against marine corrosion, but the corrosion resistance is significantly reduced in the presence of ocean-microbes. However, the mechanism of microbe-induced corrosion, named as microbiologically influenced corrosion (MIC), has yet to be understood. In this paper, the MIC mechanism of an Fe-based amorphous coating (with the composition of Fe43.7Co7.3Cr14.7Mo12.6C15.5B4.3Y1.9) in the presence of Pseudomonas aeruginosa (one of the typical ocean-microbes) was elaborately investigated by electrochemical impedance spectroscopy (EIS), electrochemical noise analysis (EN), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The electrochemical results show that P. aeruginosa reduced the corrosion resistance of the amorphous coating, manifested by the lower impedance values and higher metastable pitting initiation rate compared to those in a sterile medium. XPS analysis indicates the enrichment of soluble high-valent metal oxides in the passive film formed in the P. aeruginosa medium. In addition, TEM observation reveals pitting initiated exactly beneath P. aeruginosa cells, rather than defect regions (i.e., intersplat) as expected. In the pitting region, a thick biofilm was formed, which resulted in thinning of the underlying passive film. Based on these results, an MIC mechanism based on the inward extracellular electron transfer (EET) effect was proposed, and a potential strategy to relieve MIC was examined.