Unlocking Durability in Concrete: The Superior Edge of Calcium EDTA in Corrosion Inhibition

Chloride-induced corrosion is a significant cause of deterioration in reinforced concrete structures, affecting the integrity and lifespan of infrastructure such as bridges, highways, marine structures, and buildings. This form of corrosion leads to substantial economic losses due to repair and maintenance costs, as well as safety hazards associated with the compromised structural integrity of affected constructions. Understanding the mechanisms, impacts, and prevention strategies for chloride-induced corrosion is essential for engineers, architects, and maintenance personnel involved in the design and construction. Reinforced concrete is a composite material that combines the high compressive strength of concrete with the high tensile strength of steel reinforcement. While concrete is generally a durable construction material, its porous nature allows for the ingress of harmful substances, such as chlorides, which can initiate and accelerate corrosion processes. Chlorides, primarily from deicing salts or seawater, can penetrate concrete through its porous network and reach the steel reinforcement. This penetration can be facilitated by cracks, voids, or imperfections in the concrete. Addressing chloride-induced corrosion involves a combination of prevention strategies during the design and construction phases, as well as mitigation techniques for existing structures. Traditional methods to counteract this issue include the use of corrosion inhibitors, coatings, alloyed reinforcing steel, and cathodic protection. The shift towards organic inhibitors, particularly poly-carboxylates, has been motivated by their environmental friendliness and efficiency in forming protective films through adsorption at electron-rich centers on the steel surface.

In a new study published in Cement and Concrete Composites by Zhicheng Liu in collaboration with Fengrui Zhang, Xuan Li and led by Danqian Wang conducted an investigative study to assess the effectiveness of calcium disodium EDTA (EDTA-Ca) as a corrosion inhibitor for mild steel within chloride-contaminated concrete pore solutions, contrasting its performance with disodium EDTA (EDTA-Na). They demonstrated superior performance of EDTA-Ca over EDTA-Na due to its unique chelation properties and the formation of a more compact and thinner oxide film on mild steel, thereby enhancing corrosion resistance. The team used in their experiments a suite of advanced electrochemical techniques, including open circuit potential, electrochemical impedance spectroscopy, cyclic potentiodynamic polarization, and Mott-Schottky analysis. These methods provided insights into the thermodynamics and kinetics of corrosion processes, the effectiveness of the inhibitors, and the properties of the oxide films formed on mild steel surfaces. They also used scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy to further understand the interaction between the inhibitors and mild steel, and to visualize the morphology and composition of the protective films formed.

The authors found EDTA-Ca to be more effective than EDTA-Na in inhibiting the corrosion of mild steel. The efficiency of EDTA-Ca improved with increasing concentrations, whereas higher concentrations of EDTA-Na led to decreased efficiency. This inverse relationship highlights the significance of the inhibitor’s concentration in the corrosion process. The adsorption behavior of EDTA-Ca on the steel surface followed the Langmuir isotherm model, indicative of a uniform and monolayer adsorption, suggesting a combination of physical and chemical interactions. In contrast, EDTA-Na adhered to the Freundlich isotherm model, suggesting multilayer adsorption on heterogeneous surfaces, primarily through physical forces. The presence of EDTA-Ca resulted in the formation of a thinner, more compact oxide film with a higher content of Fe²⁺ compared to EDTA-Na. This characteristic is crucial for the protective capability of the film, as a denser film with higher Fe²⁺ content is generally more effective in preventing corrosion. The research highlighted the critical role of the chelating agent’s structure and the availability of coordinate bonds in determining corrosion inhibition efficiency. EDTA-Ca’s preoccupied coordinate bonds by Ca²⁺ limit its ability to form soluble complexes with Fe²⁺ ions, thereby reducing the dissolution of iron and enhancing protective film formation. Conversely, the available coordinate bonds in EDTA-Na facilitate the formation of soluble EDTA-Fe complexes, potentially exacerbating corrosion under high concentrations. When the researchers conducted surface analysis, it revealed that the protective film formed in the presence of EDTA-Ca was not only thinner and more uniform but also exhibited a composition conducive to corrosion protection, including the presence of Fe²⁺ oxide/hydroxide and minimal corrosion products.

In sum, the new detailed investigation conducted by the researchers underscores the superior corrosion inhibiting properties of EDTA-Ca over EDTA-Na in chloride-contaminated concrete pore environments. The findings elucidate the underlying mechanisms of action, including adsorption behaviors, film formation, and the pivotal role of chelation dynamics, providing a robust foundation for future advancements in corrosion inhibition technology for reinforced concrete structures.