Effect of Bacillus and Pseudomonas biofilms on the corrosion behavior of AISI 304 stainless steel

In this research work, the corrosion tendency of stainless steel (SS 304) caused by the Pseudomonas aeruginosa ZK (PA-ZK) and Bacillus subtilis S1X (BS-S1X) bacterial strains is investigated. The topographical features of the biofilms and SS304 substrate achieved after 14 days of incubation at 37 °C were examined by scanning electron microscopy (SEM). Fourier Transform Infrared Spectroscopic (FTIR) analysis of the extracellular polymer substance (EPS) was also carried out to estimate the chemical composition of the biofilm. Electrochemical Impedance Spectroscopy (EIS) and Tafel Polarization test methods were applied to understand the in-situ corrosion tendency of the SS304 in the presence of PA-ZK and BS-S1X strains. Compared to the biofilm produced by the PA-ZK, the EPS in the BS-S1X containing bacteria was porous and non-uniform as revealed in the SEM analysis. The improved hydrophobicity and uniformity of the PA-ZK containing biofilm retarded the corrosion of the underlying SS304 sample. Appreciably large resistance of the PA-ZK biofilm (~ 6.04 kΩ-cm2) and hindered charge transport (11.12 kΩ-cm2) were evident from the EIS analysis. In support of these results, a large cathodic Tafel slope (0.2 V/decade) and low corrosion rate (1.69 µA/cm2) were corroborated by the inhibitive properties of the PA-ZK containing biofilm. However, the formation of porous biofilm and non-homogeneity of the EPS layer produced by the BS-S1X bacteria facilitated localized corrosion. Also, low charge transfer resistance, a high corrosion rate and pitting of the surface under BS-S1X biofilm were comparable to the surface features of SS304 obtained after exposure to a controlled medium. These results highlighted the poor corrosion inhibitive properties of the BS-S1X biofilm compared to the PK-ZK bacterial strain.