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Title: Corrosion Performance of Migratory Inhibitors Under Combined Chloride and Carbonation Ingress
Authors: Tiwari, Ashish Kumar
Supervisor: Goyal, Shweta
Keywords: Reinforced Concrete;Corrosion;EIS;Inhibition mechanism
Issue Date: 30-Aug-2023
Abstract: Deterioration of reinforced concrete (RC) structures, due to exposure of rebar to aggressive environment, is a major cause of concern in terms of durability and overall structural integrity. The two main aggressive environments for rebar corrosion are the ingress of chloride ions and carbonation. While the former cause pitting corrosion and localized breakdown of passive film, the later results in a uniform corrosion of the rebar surface. In some typical environments where the rate of both chloride and carbonation penetration is high (due to simultaneous presence of pollution and de-icing or sea salts), a more severe damage to steel happens than the individual environments. The present study explores the severity level of combined action of carbonation and chloride on corrosion rate of RC. Protection of RC against corrosion can be done by employing various techniques. Amongst all, application of corrosion inhibitors is found to be most effective in terms of cost, application, environment, and ease of handling. Corrosion inhibitors are chemicals that can effectively reduce the corrosion rate even when used in small quantities. The inhibition characteristics depend on their molecular structure, polarity, and presence of heteroatoms. Based on their mode of application, they are generally categorized as admixed inhibitor or migratory inhibitors. The major advantage of using migratory inhibitor is that they do not cause any negative affect on the concrete properties. Another advantage of migratory inhibitors is that they can be used both as preventive measure (i.e., can be applied before corrosion initiation) and as repair strategy (i.e., application after the steel has reached active corrosion state). Present study explores the potential of these inhibitors in migratory form to act as preventive as well as repair measure. The study has been divided into three levels: pore solution test, migration ability test and ultimate application on reinforced concrete surface. The Level 1 of testing (i.e., pore solution test) was done to study the inhibition efficiency of some selected generic compounds with typical functional groups against combined (carbonated + chloride) exposure condition. Four different generic compounds, namely, triethyl phosphate (TEP), salicylaldehyde (SA), 2-Aminopyridine (AP) and 4-aminobenzoic acid (ABA) were selected on the basis of the presence of heteroatoms in their molecular structure and their solubility in high pH of pore solution. The potentiodynamic polarization curve behaviour and surface analysis test results (namely, Scanning Electron Microscopy, Energy Dispersive X-ray analysis and FourierTransform Infrared Spectroscopy) confirmed that the selected generic compounds effectively inhibit corrosion process in combined aggressive environment. From the test data, it was established that the inhibition efficiency of generic compounds depends upon the molecular structure, presence of functional group and number of heteroatoms. Based on pore solution testing, three generic compounds ABA, AP and SA that have the inhibitor effeciency of > 99% were finalized for the Level 2 of study i.e. percolation ability in concrete. In Level 2 of testing, efficient compounds from Level 1 were applied on hardened concrete made by using either Portland cement (OPC) or fly-ash-blended cement (PPC) in order to study their percolation capacity in different concrete systems. The percolation ability was monitored at 15, 30, 45 and 60 days of inhibitor application by thin layer chromatography (TLC) and Ultraviolet visible spectroscopy (UV-Vis). Also, the effect of their application on the penetration of carbonation front and chloride ions was a accessed through carbonation depth and free chloride concentration. For percolation test, five different concentrations of generic compounds were applied (i.e., 0.1M, 0.3M, 0.5M, 0.8M and 1.0M) on 100 mm cubes. It was observed that at higher concentrations, all the compounds were able to penetrate within 15 days of their application. Further, the concentration reaching at different depths was observed to be dependent on the molecular weight, molecular structure of the compound, along with the type of cement. Among all the tested compounds, the concentration of ABA reaching at rebar level of 15 mm was highest, followed by AP and SA. Insignificant variation in carbonation depth and chloride profile of control and inhibitor applied specimens suggested that the tested generic compounds were not blocking the pores in concrete system. They were penetration upon application on concrete surface; and were also allowing the aggressive species to percolate through the concrete cover. Based on the migration studies conducted at this level, a concentration of 1M of the chosen generic compounds was finalized for application on RC specimens. In Level 3 of the study, the selected generic compounds were applied on the surface of RC prism specimens that were prepared with one rebar at 15 mm cover depth. The inhibitors were applied in two formats: preventive and repair. The CoI is said to be applied in preventive format when the rebar in concrete was still in is its passive state (non-corroded); whereas in repair strategy, the inhibitor was introduced on the concrete surface when the rebar had reached the active state of corrosion. The specimens were subjected to a typical exposure cycle of seven days, which consisted of 2 days of chloride ponding (3.5% NaCl solution), followed by 2 days of air drying, 2 days of CO2 exposure in a closed chamber (maintained at a concentrationof 5% by volume at 30 ± 2˚ C temperature and 60-70% relative humidity) and 1 day of air drying. The corrosion activity of specimens was monitored periodically after 10, 20, 30, 40 and 50 exposure cycles by electrochemical techniques namely, Half-cell potential (HCP), Linear Polarization resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS). The test results confirmed the effectiveness of ABA in both the preventive and repair measure. AP was found to inhibit corrosion only in preventive measure, while SA could not perform in either of the two measures in both the concrete systems. Based on the test results, the inhibition mechanism of all the generic compounds was developed. Furthermore, pore solution testing was repeated with already corroded rebars to analyse the reason of performance/non-performance of any generic compound in repair measure. The results also indicated that the employment of inhibitors was highly effective in reducing the vulnerability of PPC concrete in the combined aggressive exposure.
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