Mechanisms of Inhibition of Corrosion of Metal Reinforcements for Eco Friendly Concrete Containing a Biological Admixture

International audienceThe corrosion of steel in concrete is an important phenomenon of degradation of reinforced concrete structures, which starts with the breakdown of the passive layer on steel surface. Reinforcing steel in concrete is usually passivated due to the formation of a protective oxide layer that remains stable in the high alkaline surrounding pore solution. However, carbonation and contamination with chlorides induce passive film breakdown and pitting corrosion initiation. Thus, corrosion inhibitors can be added to fresh concrete as admixtures. A new eco-friendly bio-admixture (BA) produced with extra-cellular substances from a Gram-negative bacteria (Pseudomonas) was tested as inhibitor of corrosion. BA was added at concentration of 10 v/v % to various simulated electrolytes representative of concrete pore solutions (CPS) extracted from different concrete mixtures (CEM I, CEM II and CEM III; w/c = 0.45). The objectives were to study the influence of (1) the chemical composition of CPS and (2) the bio-admixture on the electrochemical behaviour and the surface chemical composition of C15 carbon steel, used for reinforcement of building concrete. Electrochemical measurements: corrosion potential Ecorr as a function of time, polarization curves, electrochemical impedance spectroscopy were performed first in 0.1 M NaOH reference solution to understand the electrochemical behaviour of C15 mild steel by and then compared to those obtained in simulated CPS without or with BA. The pH of the synthetic CPS was adjusted to 13. The electrochemical results were combined to surface reflectivity. Experiments were also carried out in chlorides solution in order to study the effect of BA in localized corrosion conditions. The estimation of the oxide layer thickness using the Cole-Cole approach confirmed the presence of an oxide layer in an extended cathodic potential range in presence of BA