(346j) Effect of Cations on the Adsorption of NO2?/NO3? Corrosion Inhibitors for STEEL Reinforced Concrete

To enhance the tensile strength and extend the lifespan of concrete structures, carbon steel rebars are embedded in concrete. In this environment, rebars are naturally protected from corrosion, due to the high alkalinity of the concrete environment (pH ~12.6) creating a protective passive layer. Nonetheless, external environmental factors such Cl^‒ attacks caused from marine proximity or deicing salts can promote the passivity breakdown. Cl^‒ ions ingress from the surface of the concrete to the rebar and once it exceeds a certain concentration threshold, then an autocatalytic acid hydrolysis reaction will occur lowering the pH locally and causing the breakdown of the passive ‒ initiating corrosion. Accordingly, expansive corrosion products will accumulate, increasing the pressure in the system and compromising the integrity of the structure. The most common proactive corrosion prevention method used in concrete are corrosion inhibitors as they are economic, effective, and easy to apply. Nitrites (NO₂^‒) and nitrates (NO₃^‒) are among the most commonly used and effective corrosion inhibitors for this specific environment. There are many studies found in literature associated with NO₂^‒/NO₃^‒ based corrosion inhibitors in different media. However, all these studies focused on one salt form of these inhibitors, either Ca²⁺ or Na⁺ at a constant temperature.

To this end, the effect of the cationic species ionically bounded to NO₂^‒/NO₃^‒ corrosion inhibitors was studied for carbon steel rebars in 0.6 M Cl^‒ simulated concrete pore solution. A 1:1 molar ratio of inhibitor to Cl^‒ was applied, and the corrosion inhibition efficiency of NaNO₂, Ca(NO₂)₂, and Ca(NO₃)₂, and NaNO3 were evaluated using potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). Different temperatures were used (25, 35, 45, and 55 ºC) to study the effect of temperature on inhibition and find the activation energy of the corrosion process in the absence and presence of the different inhibitors tested.

According to electrochemical testing NO₂^‒/NO₃^‒ were able to impart protection by forming a stable passive oxide layer achieving an inhibition efficiency (IE) up to 85%. The inhibition mechanism of NO₂^‒ corrosion inhibitors arise from its ability to compete with Cl^‒ and adsorb though the nitrogen lone pair on the surface of the rebar, oxidizing ferrous ions to form a stable protective layer made of maghemite (Fe₂O₃) and lepidocrocite (γ–FeOOH). The IE decreased with increasing temperature, which can be attributed to increased corrosion kinetics or desorption of the corrosion inhibitor. According to the EIS and PDP the best corrosion inhibition at every temperature was achieved by NaNO₂ followed by Ca(NO₂)₂, NaNO₃, Ca(NO₃)₂, respectively. The lower IE achieved by NO₃^‒ can be attributed to its inhibition mechanism, since NO₃^‒ gets reduced to NO₂^‒.

The activation thermodynamics of the corrosion process was studied for each inhibitor through an Arrhenius relationship. The activation energy of the corrosion process was 41.15, 35.75, 31.46, 28.28, and 25.35 kJ/mol for NaNO₂, Ca(NO₂)₂, and Ca(NO₃)₂, NaNO₃, and blank (i.e. no inhibitor), respectively. This illustrates that the presence of NO₂^‒/NO₃^‒ corrosion inhibitors were able to hinder the initiation of the corrosion reaction. This trend agrees with the electrochemical testing concluding that the best corrosion inhibition is achieved by NaNO₂ followed by Ca(NO₂)₂, and Ca(NO₃)₂, NaNO₃. Accordingly, the thermodynamic equilibrium states of the electrolyte was studied since the Na⁺ salts of NO₂^‒/NO₃^‒ are performing better than the Ca²⁺ salts in inhibiting the surface of the rebar, although the concentration of the active part (NO₂^‒/NO₃^‒) of the corrosion inhibitor is constant.

For the purpose of this work, the Pitzer model was utilized to study the equilibrium state of the solutions, as it can accurately model systems with ionic strength under 6 M, making it fit for this study. The activity coefficients were found using PHREEQC – a geochemical modeling open-source code used to calculate different geochemical properties for highly concentrated electrolytes. It was found that Ca²⁺ ions increased the ionic strength, increasing the system's stability, and inducing more ionic interactions compared to Na⁺, because of its multivalent charge nature. The increased ionic interactions hinder the mobility of the NO₂^‒/NO₃^‒ ion to inhibit the carbon steel rebar. The presence of Ca²⁺ ions relative to Na⁺ decreased the activity coefficient of NO₃^‒ from 0.46 to 0.42, indicating that there is more active NO₃^‒ present in Na⁺ form than Ca²⁺, explaining the difference in performance.