Assessment of the MICP potential and corrosion inhibition of steel bars by biofilm forming bacteria in corrosive environment

Abstract

The porous network and micro-cracks of concrete facilitate the movement of deleterious ions inside the concrete cover which could corrode the embedded steel. The calcifying bacteria fill up the pore spaces and cracks through a self-healing mechanism, however their capability to precipitate CaCO3 in chlorides rich environment and impact on the corrosion of rebars was unknown. Therefore, the influence of 0%–5% NaCl contents was investigated on the CaCO3 producing capability of four Bacillus strains in this study. In addition, corrosion attributes of steel bars were explored upon interacting with bacteria in the simulated cracked concrete exposed to the 3.5% NaCl solution. The results indicated that on average 1.5% of NaCl content positively affects the bacterial CaCO3 production capability and B. subtilis and B. pumilus produced more CaCO3 at 7 days and 15 days, respectively. Besides, increase in NaCl content significantly altered the type of produced CaCO3 polymorph, microstructure, and crystallinity. The bacterial biofilm forming potential and effect of biofilm on corrosion of steel bars was evaluated through Tafel polarization and electrochemical impedance spectroscopy. The B. subtilis and B. safensis were the optimum strains in developing strong and sticky biofilms during both in-vitro and in-vivo studies and appeared as excellent corrosion inhibitors of reinforcing steel bars compared to the control specimen under an exposure of 3.5% NaCl. Increase in the concentration of NaCl aggregated more bacterial cells together and resulted in the secretion of maximum amount of extracellular polymeric substances. Besides, the biofilm formed by B. safensis was comparatively more uniform and thicker without any blemishes thus considered optimum strain to prevent the corrosion of rebars.