ABSTRACT The dynamics of corrosion losses caused by biofilms after changing culture conditions was investigated. Mild steel coupons were kept in the presence of biofilm generated by natural microbial micro-environment isolated from oil-processing waters. In addition to oil-oxidizing aerobes, the biofilm comprised sulfate-reducing bacteria. During formation of biofilm in glucose-mineral medium with peptone (GMP), corrosion losses increased vs control. The decrease of losses vs control was observed after transferring coupons carrying this biofilm into enriched with peptone Luria-Bertani medium (LB). During formation of biofilm in LB medium, corrosion losses decreased vs control. The increase of losses vs control was observed after transferring coupons with this biofilm into glucose-mineral medium.
The high level of corrosion losses under intensive aeration conditions was registered. As soon as coupons with the biofilms were transferred into microaerophilic conditions, corrosion losses reduced sharply. Thus, it was confirmed that the subdivision of microorganisms into destructors and protectors is arbitrary. Results conclude that due to their combined metabolic activity, biofilms of natural microbial assemblages promptly respond to environmental factors, thereby either stimulating or inhibiting corrosion processes. This provides new opportunities for developing a promising approach to control MIC.
INTRODUCTION Microbiologically influenced corrosion (MIC) destroys steel as well as electrochemical and chemical corrosion1,2. Biocides are currently the main tool for controlling MIC. However, it is known that sometimes microorganisms reduce corrosion losses3-5. For this reason microorganisms usually are divided into those capable of accelerating corrosion, those providing no effect and those that minimize corrosion. Possible reversibility in acceleration of corrosion had been attributed only to extracellular polymer production during biofilm formation6-8.
Investigations of corrosion processes in the presence of microorganisms are usually carried out by using media specially selected for specific microorganisms. However, the influence of environmental composition on the direction of corrosion processes induced by microorganisms needed additional study.
Previously, we studied the corrosion activity of 10 taxonomically different aerobic heterotrophs in microaerophilic conditions9. It was found that the direction of the biological effect on corrosion depended on the composition of the nutrient medium. If nutrient substrate favored the production of acidic metabolites all 10 heterotrophs accelerated corrosion. If substrate favored the production of alkaline metabolites the same microorganisms mitigated corrosion. A statistically reliable correlation between corrosion losses and pH of the culture liquid at the end of the incubation was obtained.
Our results prove that the subdivision of microorganisms into biodegraders and protectors is merely conventional. We can only state that microorganisms differ in the ability to induce break down or protection, depending on the type of the nutrient substrate.