Induction of Pitting on Stainless Steel 304 and 316 by Bacillus sporothermodurans

Abstract Text: Stainless Steel (SS) undergoes corrosion in both polished and unpolished regions of dairy evaporators and dryers leading to product quality and economic concerns. The objective of this research was to investigate Microbially Induced Corrosion (MIC) or pitting on SS 304 and 316 by a common milk contaminant; Bacillus sporothermodurans. These bacteria and their spores can persist on the SS milk contact surfaces in evaporators and dryers in the form of biofilms and may induce corrosion due microbial metabolic products such as enzymes, exopolymers, organic and inorganic acids and hydrogen sulfide. Many Sulfate reducing bacteria (SRB) are active under oxygen stress and contribute to oxygen reduction reactions, thus influencing corrosion. In this study, polished and unpolished SS coupons (1’x1’) of grades 304 and 306 were used to form biofilms of a dairy origin strain 10599 of B. sporothermodurans (Deutsche Sammlung von Mikroorganismen und Zellkulturen). This strain was found to exhibit Sulfate reduction, as well as, proteolytic activities. About 10⁷ cfu/mL of the overnight grown culture were inoculated in sterile 11 % reconstituted Non Fat Dry Milk (NFDM) in a Petri dish, in which sterile SS coupons were immersed. The incubation was carried out at 30^oC, being the optimum growth temperature of the culture. The suspension medium (NFDM) was replaced at regular intervals based on its pH drop to 5.0. Bacterial counts from the spent media were taken at weekly intervals, while the biofilm counts were taken after every two weeks to monitor the culture viability. The data were statistically analyzed to compare means. The counts in spent media and biofilms ranged from 10⁵-10⁶ cfu/mL and 10²-10⁴ cfu/cm², respectively. The coupons were examined using Scanning Electron Microscope (SEM) for any corrosion or pit formation. Energy Dispersive X-ray Spectroscopy (EDS) was used to find the elemental composition of the control SS surface and was compared with that of the pits. From SEM observations and EDS data, it was established that pitting of surfaces for both grades of SS with biofilms got induced from 4^th week onwards.  These pits were distinct for the two grades; some pits were deep, while others were on the surface with corrosion products deposited over them. Elemental increase in Sulphur and Oxygen on the pit surfaces further confirmed the induction of corrosion. In conclusion, the biofilms of Bacillus sporothermodurans induced corrosion on the surface of stainless steel of both grades. 

Keywords: Corrosion, Bacillus, Stainless Steel, Pitting