Temporary Inhibition of the Corrosion of AZ31B Magnesium Alloy by Formation of Bacillus subtilis Biofilm in Artificial Seawater

Marine bacterial community analysis on 316L stainless steel coupons by Illumina MiSeq sequencing

In order to evaluate the corrosive action of microorganisms on 316L metal exposed directly to a marine environment, a system was designed to immerse coupons in seawater. After periods of 30, 60 and 90 days, the coupons were recovered, the corrosion rates evaluated and the biofilm samples on their surface were analyzed by 16S rRNA gene sequencing. The results of the corrosion rate showed an acceleration over the entire experimental period. Alpha diversity measurements showed higher rates after 60 days of the experiment, while abundance measurements showed higher rates after 90 days of exposure to the marine environment. The beta-diversity results showed a clear separation between the three conditions and proximity in the indices between replicates of the same experimental condition. The results of 16S rRNA gene sequencing showed that after 30 days of exposure to seawater, there was massive representativeness of the pioneer bacteria, Gamma and Alphaproteobacteria, with emphasis on the genera Alcanivorax, Oceanospirillum and Shewanella. At the 60-day analysis, the Gammaproteobacteria class remained dominant, followed by Alphaproteobacteria and Flavobacteria, and the main representatives were Flexibacter and Pseudoalteromonas. In the last analysis, after 90 days, a change in the described bacterial community profile was observed. The Gammaproteobacteria class was still the largest in diversity and OTUs. The most predominant genera in number of OTUs were Alteromonas, Bacteriovorax and, Nautella. Our results describe a change in the microbial community over coupons directly exposed to the marine environment, suggesting a redirection to the formation of a mature biofilm. The conditions created by the biofilm structure suggest said condition favor biocorrosion on the analyzed coupons.

Corrosion Behavior of AISI 1045 Steel in Seawater in the Presence of Flavobacterium sp

A systematic comparison study was carried out to investigate the effect of Flavobacterium sp. on AISI 1045 steel corrosion by weight loss, fluorescence microscopy (FM), surface analysis, cell count, pH measure, electrochemical impedance spectroscopy (EIS), and polarization curves. The impedances were considerably increased by Flavobacterium sp. between 1 and 7 day exposure and after 30 day exposure but considerably decreased by Flavobacterium sp. after 15 and 21 day exposure, which were supported by the I_corr results and the weight loss data. Furthermore, the biofilm was formed on the coupons. The pH values were considerably decreased by Flavobacterium sp. after 15 and 21 day exposure. The results proved that Flavobacterium sp. decreased the corrosion rates between 1 and 7 day exposure and after 30 day exposure and increased the corrosion rates between 15 and 21 day exposure, which could be ascribed to the protective biofilm and the secreted corrosive acid, respectively. In addition, Flavobacterium sp. considerably increased the pit numbers, the maximum pit depths, and the corresponding widths and considerably decreased the E_pit values. Importantly, the coverage and the heterogeneity of the biofilm were positively correlated with the increases in the maximum pit depths and the corresponding widths and the decreases in the E_pit values by Flavobacterium sp. The results demonstrated that Flavobacterium sp. increased the pitting corrosion, which could involve the heterogeneous biofilm cover.