Corrosion inhibition of steel reinforcement in solutions simulating carbonated and chloride-polluted concrete

Premature degradation of concrete structures is one of the most important causes contributing to the wastage of non-renewable resources in the construction industry and in most cases it is due to corrosion of steel reinforcement. The aim of this research is to identify new inhibiting formulations, useful to hinder the corrosion of steel reinforcement, in the concomitant presence of chlorides and carbonation and to investigate their mechanism of inhibition. With this purpose, exposures of steel samples to solutions simulating the composition of the pore electrolyte in carbonated concrete containing chlorides (pH 7, 0.1 M NaCl) have been carried out. The inhibitors studied are sodium glycerophosphate (GPH), sodium salts of benzoic acid derivatives, that is 2-aminobenzoate (2AMB), 3-aminobenzoate (3AMB), 4-aminobenzoate (4AMB) and N-phenyl-2-amino-benzoate (PhAMB) sodium salts. Both the single additives and GPH / amino-benzoates mixtures have been tested, at the additive concentration of 0.05M (with the exception of PhAMB used under saturated condition, that is about 0.025M). The evolution of the corrosion process has been monitored during three days exposures by electrochemical impedance spectroscopy and polarization curve recording. It has been shown that, among the substances used as single additives, only 3AMB has a good inhibiting efficiency, while GPH induces synergistic corrosion inhibition when mixed with 2AMB, 4AMB and PhAMB, but not 3AMB. At the end of the exposure period, surface analysis techniques (Field Emission Scanning Electron Microscopy - FESEM, Energy Dispersion Spectroscopy – EDS, X-Ray Photoelectron Spectroscopy - XPS, Fourier Transform Infrared Spectroscopy - FTIR) have been applied to investigate the inhibition mechanism of the different formulations and in particular of the mixture 0.05M GPH + 0.05M 2AMB. In this case, the FESEM observations have shown the growth of a rather thick protective film onto the steel surface. XPS analysis performed during continuous sputtering of the sample surface by means of an in situ 3 KeV Ar-ion beam (depth profile mode) reveals that this film consists of an organic salt of Fe(II), containing a low percentage of phosphorous (0.8-1.7 wt%), mainly present as P(V). FTIR clearly evidences it is a 2-amino-benzoate salt with a bidentate coordination of the ferrous cation, via carboxylate and amino groups