Estudo da corrosão microbiológica em tubulações de sistemas de proteção contra incêndio (sprinkler) na presença de bactérias redutoras de sulfato

Abstract

Sprinklers are fire protection equipment that discharge water in order to extinguish a fire quickly and automatically, preventing the spread of fire. Most of pipes used in these systems are constructed of carbon steel. Sprinklers systems provide an ideal environment for the growth of microorganisms that promote internal oxidation of the pipes and, consequently, corrosion. Sulphate-reducing bacteria (SRB) are the most important microorganisms involved in microbiologically induced corrosion (MIC) that generally occurs localized. One way to control this type of corrosion is the addition of biocides. However, most of the commercialized biocides are toxic and have difficult degradation. Therefore, the objective of this work was to evaluate the MIC behavior of carbon steel AISI 1020 in industrial sprinkler water in the absence and presence of mixed culture of microorganisms containing sulfate-reducing bacteria (SRB). The tests were carried out at 3, 7, 14, 21 and 28 days. In addition, a natural biocide, neem oil, was evaluated as an alternative in the treatment of water used in sprinkler systems and compared with glutaraldehyde, which is a biocide used commercially. Microbial growth on the steel surface was measured by quantifying the sessile SRB using the most probable number (MPN). The corrosion evaluation was performed by electrochemical techniques (open circuit potential (OCP), potentiodynamic polarization curves (PP) and electrochemical impedance spectroscopy (EIS)). The biofilm formation and the corrosion products on the steel surface were observed by scanning electron microscopy (SEM). The carbon steel surface characterization was observed by energy dispersive spectroscopy (EDS). Was observed a low growth of SRB adhered to the material due to the little concentration of sulfate in the medium. A maximum value was verified in 14 days of incubation. The OCP evaluation showed ennoblement of the material, due to the formation of the biofilm on surface. EIE indicated an acceleration of the corrosive process in the presence of SRB, evidenced by the displacement of the curves to lower frequency values in the Bode (Module and Phase diagrams. SEM analysis, performed in 14 days, was observed the formation of a nonuniform biofilm on the surface of the material. After the biofilm removal, the presence of pits was verified. The addition of the biocides was carried out for the time of 14 days and prevented the growth of SRB on the surface of the carbon steel. Electrochemical tests in the presence of biocides showed a decrease in the corrosive process compared with the tests without biocide performed on the same time period.