Investigation of the inhibition effect of indole-3-carboxylic acid on the copper corrosion in 0.5 M H2SO4
Introduction
Copper is widely used in equipment and plants of many industrial fields because of its valuable physical and mechanical properties and good resistance to corrosion in several media. The chemical cleaning treatments of process equipments are usually carried out to remove mill scale, to dissolve incrustations, corrosion products. The deposits can be dissolved or removed by using treatments with solutions of inorganic or organic acids for a proper time and temperature range. Among the acid solutions the most frequently used are the sulphuric acid ones. During the chemical cleaning treatment there is the danger that copper corrosion upon the already cleaned metal surface will occur after the removal of the scale or oxides. This involves a loss of dissolved copper as well as acid consumption [1]. In several circumstances organic molecules containing heteroatoms with lone-pair electrons, multiple bond or aromatic rings were used to inhibit copper corrosion and to prolong the life of the equipment and plants [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. This minimizes the use of natural resources including ore, petroleum, water, etc. The first step in the action mechanism of an inhibitor in acid solutions is adsorption of the organic molecule (adsorbate) onto metal (adsorbent). Adsorption mainly depends on the nature and surface charge of the adsorbent, the chemical structure of the adsorbate, the type of aggressive electrolyte, the temperature of the corrosion reaction [13]. Physisorption and chemisorption are the principal types of interaction between adsorbate and adsorbent [14]. The first one is weak undirected interaction and it is the result of electrostatic attractive forces between inhibiting organic ions or dipoles and electrically charged metal surface. Physisorption involves rapid interaction between adsorbent and adsorbate but it is also easily removed from surface with the temperature increase. In the physisorption the potential of zero charge (Epzc) plays an important role. The charge on metal surface can be expressed in terms of potential difference (Φ) between the corrosion potential (Ecorr) and the potential of zero charge of the metal (Φ = Ecorr − Epzc). If Φ is negative, adsorption of cations is favourite. On the country, the adsorption of anions is favourite if Φ is positive. In the chemisorption, strong, directed forces govern the interaction between adsorbate and adsorbent and involve charge sharing or charge transfer from the adsorbate to the adsorbent in order to form a coordinate type of bond. With regard to electron transfer, it may take place in presence of transition metals having vacant or low-energy electron orbitals and of an adsorbate with multiple bonds, aromatic rings or heteroatoms (S, N, O) with lone-pair electrons. This process takes place more slowly than physisorption and it increases with the temperature. Chemisorption has free energy of adsorption and activation energy higher than physisorption and, hence, normally it is irreversible [15]. The adsorption of an adsorbate on the metal-solution interface is considered formally equivalent to a chemical reaction between the adsorbate in the aqueous solution, Org(sol), and the water molecules adsorbed on the metallic surface, H2O(ads)where x is the size ratio representing the number of water molecules replaced by one molecule of adsorbate. The size ratio can take on values of 1, 2, 3, 4, 5 [16], [17]. Adsorption process can be described by several adsorption isotherms such as: Langmuir, Frumkin, Hill de Boer, Parsons, Temkin, Flory–Huggins, Dhar–Flory–Huggins, and Bockris–Swinkels [18].
The inhibiting properties of indole on copper corrosion in 0.5 M H2SO4 solutions were shown previously [19]. The aim of this work was to study the inhibition of copper corrosion by ICA in 0.5 M H2SO4 solutions in the temperature range of 25–55 °C. The adsorption behaviour of ICA was also analysed in order to choose the appropriate adsorption isotherm.
Section snippets
Experimental
Measurements were performed on purity 99.994% copper disk electrodes with 1 cm2 surface area by potentiodynamic polarization and electrochemical spectroscopy impedance (EIS) techniques. Each disk was soldered first to a copper wire and then it was embedded in a cold metallographic resin. The surface of the specimens was polished mechanically using silicon carbide (SiC) paper up to grade 1200, degreased with acetone, rinsed with distilled water, dried with a stream of air and finally placed in
Polarization curves
Fig. 1, Fig. 2 present the results of the effect of ICA concentration on cathodic and anodic polarization curves of copper in 0.5 M H2SO4 solutions at 25 °C and 35 °C. Polarization curves similar to those of Fig. 1, Fig. 2 were obtained at 45 °C and 55 °C.Electrochemical parameters obtained from the polarization curves are shown in Table 2. These include corrosion potential (Ecorr), corrosion current density (icorr) determined by extrapolation of the anodic Tafel line to the corrosion potential,
Conclusion
- (1)
ICA displayed inhibitive properties towards the corrosion of copper in 0.5 M H2SO4 solutions in the temperature range of 25–55 °C.
- (2)
The inhibitive effect increased by increasing the concentration of the investigated compound. At 55 °C the best inhibitive effect was obtained.
- (3)
Equivalent circuit model for copper/0.5 M H2SO4 interface was proposed.
- (4)
Both physisorption and chemisorption seemed to contribute to the adsorptive behaviour of ICA.
- (5)
The adsorption behaviour of ICA followed the Langmuir’s isotherm.
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