A study of corrosion processes in a demineralized aqueous medium in the presence of bacterial cells

Abstract

Our earlier results indicate the feasibility of considering the microbiological contamination of closed and reversible cooling and water supply systems, given the effect of the microbial cells on the corrosion processes taking place there. The results show a possibility of transition metal ions acting as regulators and modifiers of microorganisms’ growth processes under conditions of an osmotic stress and starvation.The aim of the present work is to study the effect of transition d-metal ions (Zn2+ and Cr3+), both separately and in the presence of known antiskalants, oxyethylenediphosphonic acid (OEDFh) and nitrilotrimethylphosphonic acid (NTMFh), on the effectiveness of the corrosion protection and the course of the biological corrosion in a demineralized environment.Microorganisms belonging to different systematic affiliations were selected for the study of the biocorrosion processes: Bacillus sp. and Pseudomonas sp. These microorganisms belonged to different physiological groups of bacteria and had opposite types of life strategies. They differred significantly in their ability to survive under the influence of various factors. They could exist in closed systems of water consumption (with sharp temperature changes, oxygen starvation, a presence of toxic substances used for water conditioning). Bacillius and Pseudomonas strains were cultured on commonly used agar media, after which a cell suspension was prepared. For all bacterial strains used, the seeding dose was 108 cells‧cm-3. It was determined using turbidometric control methods and bacterial density standards. The effect of the microorganisms on the corrosion processes was followed in stationary bioreactors of a volume of 125 cm3. A solution of 0.1 % NaCl was added as a model medium to which corrosion inhibitors (Zn2+, Cr3+, OEDPh and NTMPh) were added at a concentration of 5 mg‧dm-3. At each stage of preparation, the suspension was thoroughly mixed, after which the metal samples were introduced to the reactor in the form of electrodes so that the entire surface of the electrode was completely immersed in the water. The metal samples were made of St 20 carbon steel. Each study of model water with metal plates was of 3 weeks duration. The intensity of the corrosion process was evaluated once a week. The rate of the biological corrosion was determined by the gravimetric method. The inhibition factor (j) and the degree of the corrosion protection (Z, %) of the samples in presence of corrosion inhibitors and bacteria were calculated. The negative value of this indicator indicated a change of the direction of the process, i.e. an activation of the corrosion process.The obtained data show that the presence of microorganisms significantly modifies the anti-corrosion effect of the heavy metal ions. The range offluctuations of the degree of the corrosion protection in presence of bacteria of the genus Pseudomonas is greater than that of the genus Bacillus. The dynamics of the protective effect in presence of both strains is characterized by significant changes (a decrease and a subsequent gradual increase of the degree of the corrosion protection). The highest intensity of the corrosion process is observed during the first week of the interaction in a model environment containing bacterial cells and anticorrosive agents. Then, the direction of the process changed and there was shown a slight protective effect at the 5-15 up to 36 % level. It is also demonstrated that the anti-corrosion compositions containing NTMPh provide a greater increase of the corrosion process rate in presence of microorganisms.The study of the presence of microorganisms on the corrosion proceeding demonstrates their ability to provide a significant effect. The bacteria can activate the corrosion process, which complicates the inhibitors behavior.