Caffeic acid as a green corrosion inhibitor for mild steel

Abstract

The inhibitor effect of the naturally occurring biological molecule caffeic acid on the corrosion of mild steel in 0.1 M H₂SO₄ was investigated by weight loss, potentiodynamic polarization, electrochemical impedance and Raman spectroscopy. The different techniques confirmed the adsorption of caffeic acid onto the mild steel surface and consequently the inhibition of the corrosion process. Caffeic acid acts by decreasing the available cathodic reaction area and modifying the activation energy of the anodic reaction. A mechanism is proposed to explain the inhibitory action of the corrosion inhibitor.

Introduction

The isolation of a metal from corrosive agents is the most effective way to prevent electrochemical corrosion. Among the different methods available [1], [2], the use of corrosion inhibitors is usually the most appropriate way to achieve this objective. Nowadays, naturally occurring molecules exhibiting a strong affinity for metal surfaces are the focus of research oriented toward the development of environmentally friendly corrosion inhibitors; compounds showing good inhibition efficiency and low environmental risk. Among the so-called “green corrosion inhibitors” are organic compounds that act by adsorption on the metal surface, such as ascorbic acid [3], [4], succinic acid [5], [6], tryptamine [7], caffeine [8] and extracts of natural substances [9], [10], [11], [12], [13]. The efficiency of these organic corrosion inhibitors is related to the presence of polar functions with S, O or N atoms in the molecule, heterocyclic compounds and π electrons. The polar function is usually regarded as the reaction centre for the establishment of the adsorption process [14].

Caffeic acid (3-(3,4-dihydroxyphenyl)-2-propenoic acid (Fig. 1), is the phenylpropenoid most commonly found in nature and has been demonstrated to have medicinal properties, especially as an antioxidant agent [15], [16]. Its electrochemical behavior in aqueous solutions of pH 2.0–8.5 has been studied [17]. As can be deduced from its chemical structure, caffeic acid has characteristics suitable for its application as a corrosion inhibitor. Additionally, because it is a highly water-soluble compound, it can be used across the whole pH range. Nevertheless, there are few reports on the corrosion behavior of metals in the presence of caffeic acid. McCafferty and Hansen [18] studied the corrosion inhibition of iron and aluminum by various naturally occurring biological molecules, among them caffeic acid. The abstract accessible on-line did not reveal data on the performance of caffeic acid as a corrosion inhibitor, but parabactin, a catecholate siderophore, has been found to be effective in inhibiting both the corrosion of iron in hydrochloric acid and the pitting of aluminum in sodium chloride. Sekine and co-workers [19] have shown that poly(acrylate-co-caffeic acid) and other acrylate-caffeylamide copolymers can improve the corrosion inhibition of mild steel in cooling water systems, in low inhibitor concentrations, due to polymer adsorption on the steel surface. Environmentally-compatible poly(caffeic acid) and other varieties of polyphenol polymers have been synthesized as corrosion inhibitors in boiler systems under alkaline conditions [20]. The copolymers were able to inhibit the corrosion of mild steel in the same manner as tannic acid, used as a reference. A composition for cleaning semiconductor wafers containing metals, particularly copper, has recently been proposed [21], comprising a cleaning agent, a chelating agent and a corrosion inhibiting compound. The latter can be a mixture of organic compounds, including caffeic acid. However, the performance of these corrosion inhibitors was not reported.

Iron and steel are the metallic materials most used in structures exposed to the atmosphere and to the acidic media in industrial applications [22] due to their availability and low cost. Aqueous solutions of hydrochloric and sulfuric acid are extensively used in acid cleaning, pickling and descaling processes, as well as for drilling operations [10]. The rate of corrosion to which iron and mild steel are submitted in aggressive media is high. Therefore, the development of novel corrosion inhibitors is important for practical applications aiming to decrease the metal corrosion processes.

This study aims to gain some insight into the corrosion of mild steel in H₂SO₄ in the presence of caffeic acid as a corrosion inhibitor. Systematic studies on the inhibitor effect of caffeic acid have not been reported. The inhibitor effect of this naturally occurring biological molecule on the corrosion of mild steel in 0.1 M H₂SO₄ was investigated by open-circuit potential (OCP) measurements, potentiodynamic polarization, weight loss, electrochemical impedance and Raman spectroscopy. Additionally, thermodynamic data were obtained from adsorption isotherms and Arrhenius plots.