Abstract
The role of microbes in the corrosion of metals is due to the chemical activities (metabolism) associated with the microbial growth and reproduction. A lot of researches have shown that enormous loss of resources and even lives has been experienced in many countries of the world due to corrosion. There is therefore need to device a means of combating this menace so as to save different countries of the world including Nigeria the cost encured due to corrosion. This research has helped to determine the role of bacteria in the corrosion of high carbon steel under aerobic and anaerobic conditions was investigated using sulphate reducing bacteria and other aerobes. A sample of high carbon steel grade of chemical composition: 96% iron (Fe), 1.08% carbon (C), 0.349% silicon (Si), 0.841% manganese (Mn), 0.005% phosphorus (P) and 0.005% sulphur (S), was obtained in the form of steel plate. Eighteen (18) samples of the high carbon steel were prepared. Standard microbiological methods were used to isolate aerobic and anaerobic bacteria isolates. The microbes isolated from the soil included; aerobes (Bacillus species, Pseudomonas species) and anaerobes (Desulfovibrio species, Thiobacillus species). The role of bacteria in the corrosion of high carbon steel was estimated using the weight loss technique and the electrochemical potential measurement technique. Eighteen samples of a high carbon steel of known compositions were exposed to sulphate reducing bacteria under different environments (aerobic, anaerobic and control). The physical and chemical changes were examined and monitored on weekly basis for six weeks. It was observed that the coupons immersed in the media with sulphate reducing bacterium underwent fast activation and numerous corrosion sites were formed on the surfaces. The average corrosion rate for six weeks (42 days) period of time as determined by the weight loss method and electrochemical testing were found to be 0.0004595 mm/year, -0.712 mV in aerobic environment, 0.0005646 mm/year, -0728 mV in anaerobic environment and 0.0004458 mm/year, -0702 mV in the control or reference environment (distilled water environment). High carbon steel was found to corrode more only in the anaerobic environment. In view of this, high carbon steels were found to be relatively more corrosion resistant than mild and medium carbon steels and may be suitable for application in industries such as the petrochemical industry. Therefore, treating the environment with chlorine but with caution as solutions of chlorine gas in water is corrosive to steels while the use of bactericides or biocides to arrest the action of microbes is important.
Highlights
Corrosion may be defined as the deterioration of a material resulting from chemical attack by its environment [1]
This mineral film acts as a cathode for hydrogen evolution, increasing the corrosion rate [9], but can have a protective passivation effect depending on the crystalline mineral composition [9] [10]
This is in agreement with Carpen et al [24] who demonstrated that corrosion rate might be as high as 10 - 63 micrometer per annum in anaerobic environment
Summary
Corrosion may be defined as the deterioration of a material resulting from chemical attack by its environment [1]. Metals corrode because they succumb to environmental interaction The environment in this regard may refer to the service conditions where the metal may find application. Most studies on anaerobic—micro aerobic MIC use sulphate reducing bacteria (SRBs) because they are sulphide producers and promoters of the cathodic depolarization process in steel [6] [7]. Sulphide produced by SRBs combines with and Fe2+ to form ferrous sulphide and generate an adhesion film. This mineral film acts as a cathode for hydrogen evolution, increasing the corrosion rate [9], but can have a protective passivation effect depending on the crystalline mineral composition [9] [10]. A natural environment, the reaction leading to steel corrosion intensify with temperature [11], and high-temperature environment is characterized by diverse bacteria communities [12], many of which may have corrosive effect [13] [14]
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