Green corrosion inhibition for carbon steel reinforcement in chloride-polluted simulated concrete pore solution using Urtica Dioica extract

In this work, Urtica Dioica (nettles) extract, which contains a variety of organic molecules with N, O and S heteroatoms, was added to provide green corrosion inhibition for carbon steel reinforcements in chloride-polluted (1% NaCl) simulated concrete pore solution. This specific extract had not been previously tested in high alkaline (pH 13.6) solutions. A global analysis was carried out, including electrochemical testing (to assess the inhibition ability of the extract), surface analysis (to check for the presence of organic molecules on the steel surface) and densityfunctional theory (DFT) quantum chemical calculations (to compute the absorption of inhibitor components on steel exposed to pH 13.6). After 24 h and determined with electrochemical spectroscopy test (EIS), the most efficient dosage (0.075%) of nettles showed that charge transfer resistance (Rct) values for ribbed steel were 4.3 times higher than in non-inhibited solutions, providing an inhibition efficiency of 77%. A similar reduction was determined for current density values from polarization tests. Analysis of the specimen after dipping it in the inhibited alkaline electrolyte via Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) revealed the surface adsorption of organic molecules in the nettles extract and their contribution to reinforcing the oxide/hydroxide layer. Moreover, the semi-quantitative carbon amount inside the pits is 9 times higher than on the steel surface. DFT ab-initio calculations indicate that the Urtica Dioica extract can be adsorbed at the Fe2O3 surface with a charge transfer of about 0.28-0.23 e due to binding of N- and O- containing adsorbates. All results suggest the presence of organic molecules related to the nettles extract on steel surfaces under those tested solutions. The nettles extract is able to precipitate on the pits, hindering the development of the anodic reactions. The inhibition efficiency of nettles, their worldwide availability and their environmentally-friendly nature, combined with the readily manufactured extract all point to promising application in this field.This research was funded by the European Union Horizon 2020 Research and Innovation MSCA-IF-2019 Programme under grant agreement No. 892074 (NATCON project). Support from the Ministerio de Ciencia, Innovación y Universidades of Spain (RTI2018-096428-B-I00) is also acknowledged