¹ Department of Chemistry, Ahmadu Bello University, Zaria, Nigeria

² Department of Chemistry, Akwa Ibom State University, Ikot Akpaeden, P. M. B. 1167, Uyo, Akwa Ibom State, Nigeria

Abstract

In an attempt to improve the corrosion inhibition potential of Anogessius leocarpus gum exudates for mild steel in solutions of HCl, corrosion inhibition efficiencies of the gum, KI, KCl, and KBr were determined experimentally using weight loss and gasometric methods. 0.05 M KI, 0.05 M KBr and 0.05 M KCl were combined with various concentrations of Anogessius leocarpus gum and their combined inhibition efficiencies were used in computing synergistic parameters for the respective combinations. The results obtained indicated that combination of 0.4 and 0.5 g/L of AL gum with 0.05 M solutions of KI and KBr, respectively, enhanced inhibition efficiency of the gum. However, for all concentrations of AL gum, combinations with 0.05 M KCl recorded a remarkable increase in inhibition efficiency, but calculated values of the synergistic parameter indicated that the adsorption of AL gum on mild steel surface is antagonized by the presence of Cl^-. Theory of competitive and co-operative adsorption has been used to explain the co-adsorption of the gum and the halides. From calculated values of activation energy (< 80 kJ/mol), free energy of adsorption (< -40 kJ/mol) and the variation of inhibition efficiency with temperature, the adsorption characteristics of the gum, halides and that of gum-halide mixtures were found to be consistent with the mechanism of physical adsorption. The adsorption was also found to be endothermic, spontaneous and favored the Langmuir adsorption model.

Introduction

Corrosion is the deterioration of materials by chemical interaction with their environment. The consequences of corrosion are many and varied and its effects on the safe, reliable and efficient operation of equipment or structures are severe.

Most corrosion inhibitors are synthetic chemicals that are expensive and hazardous to the environment [1]. The use of natural products (green corrosion inhibitors) has been given extensive research interest because most of them are easily available, less expensive, less toxic and biodegradable [2-14]. In our research group, we have found that some natural polymers are effective corrosion inhibitors and can compete with green corrosion inhibitors [15]. According to Eddy et al. [15], polymers are effective corrosion inhibitors because through their functional groups they form complexes with metal ions and on the metal surface. These complexes occupy a large surface area that can blanket the metal surface and protect it against corrosion [16-17].

In spite of their numerous advantages as corrosion inhibitors, some natural polymers are found to be characterized with low values of inhibition efficiency. Therefore, it could be beneficial to work out efforts that can be adopted to improve upon their inhibition potentials. One of the ways of achieving this is through synergism [18]. Synergism has become one of the most important effects in inhibition processes and serves as the basis for all modern corrosion inhibitor formulations. It is an effective method to improve the inhibitive performance, to decrease the amount of usage, and to diversify the application of the inhibitor in acidic media [19]. Eddy et al. [3,20] stated that addition of Br^- and I^- ions to Lasianthera Africana and Gnetum African leaves extract antagonized their inhibition potentials for mild steel in solutions of H₂SO₄, while Cl^- exerted synergistic effect in the same media. Synergistic inhibition effect between iodide ion and natural exudate gums of Raphia hookeri [20] and Pachylobus edulis [21] for mild steel in acidic media has been reported. In view of the high favourable composition of Anogessius leocarpus gum exudates and their low corrosion inhibition potential [15], the objective of the present study is to investigate the effect of halide ions (KI, KCl and KBr) on the enhancement of the corrosion inhibition potential of Anogessius leocarpus gum exudates.

Materials and methods

Materials

Materials used for this study were mild steel sheets of composition (wt %, as determined by quantiometric method): Mn (0.6), P (0.36), C (0.15), Si (0.03) and Fe (99.01). The sheets were mechanically cut into different coupons, each of dimensions 5 × 4 × 0.11 cm. Each coupon was degreased by washing with ethanol, cleaned with acetone and allowed to dry in the air before preservation in a desiccator. All reagents used for the study were analar grade and double distilled water was used in preparing the required solutions. The Anogessus leocarpus gum exudates used for the studies were obtained from the Anogessius leocarpus tree, purified and concentrated using the method reported in our last work [15].

where CR is the corrosion rate of mild steel in g/cm²/h, W1 and W2 are the weight losses (g) for mild steel in the absence and presence of the inhibitor, Θ is the degree of surface coverage of the inhibitor, A is the area of the mild steel coupon (in cm²), t is the period of immersion (in hours).

Results and discussion

Effect of AL gum and halides

The figure depicts that weight loss of mild steel in the blank is higher than those measured for solutions of HCl containing various concentrations of AL gum. This indicates that AL gum retarded the rate of corrosion of mild steel in solutions of HCl and that AL gum is an inhibitor for the corrosion of mild steel in solutions of HCl. At higher temperature (333 K), weight loss of mild steel in solution of HCl was found to follow similar pattern. However, at higher temperature (333 K), measured values of weight loss were higher than those obtained at a lower temperature (303 K).

From the figure, it is significant to note that inhibition efficiencies of the respective AL-halide mixture are higher than those of the corresponding halides and that of AL-gum. From the plots, it is evident that the increase in inhibition efficiency of AL gum as due to interaction with potassium halides is consistent with the following trend, KI > KBr > KCl.

In order to ascertain synergistic, antagonistic and non effect of halides on the adsorption of AL gum, synergistic parameters, S, were calculated for various combinations of the inhibitor and fixed concentrations of the halides (KI, KCl and KBr) using the following equation [19],

Consequently, calculated values of the synergistic parameters were greater than unity at AL concentration of 0.5 g/L when the inhibitor was co-employed with 0.005 M KI and KBr, respectively. Also S values calculated for combination of 0.4 and 0.5 g/L of AL gum inhibitor and 0.005 M KI were greater than unity. Therefore, at these concentrations, adsorption of AL gum on mild steel surface is enhanced by synergistic combination of KI and KBr at concentrations ≥ 0.4 g/L and ≥ 0.5 g/L, respectively. Below these critical concentrations and for all concentrations of Al gum in combination with KCl, calculated values of the synergistic parameter were less than unity, indicating that the adsorption of AL gum on mild steel is antagonized by the presence of KCl. The synergistic effect observed for iodide and bromide ions may be due to co-adsorption of iodide or bromide with the inhibitors' cations. It has been found that co-adsorption can be either competitive (in which the anion and the cation are adsorbed at different sites on the metal) or co-operative adsorption, in which the anion is chemisorbed on the surface of the metal and the cation on the layer of the anion [23]. However, it is possible to have both types of adsorption.

Effect of temperature

The effect of temperature on the corrosion of mild steel in solutions of HCl containing various concentrations of AL was studied using the Arrhenius equation, which can be written as follows [24]:

From the results obtained, it can be seen that the activation energies are less than the threshold value required for chemical adsorption (80 kJ/mol), indicating that the adsorption of AL gum and those of AL gum-halides mixtures are consistent with the mechanism of charge transfer from the charged inhibitor to the charged metal surface, which indicates physisorption. The results also revealed that there is an improvement in the adsorption of AL gum due to synergism, which was observed for the following systems: 0.5 g/L AL + 0.05 M KI, 0.5 g/L AL + 0.05 M KI and 0.05 M KBr + 0.5 g/L KBr. This also implies that at higher concentration of AL gum, enhancement of inhibition efficiency through synergistic combinations with KI or KBr can be achieved through improved adsorption.

Heat of adsorption

The heat of adsorption of AL gum and for various combinations of AL gum with halides were estimated using the following equation [15],

where b is the equilibrium constant of adsorption and is related to the standard Gibb free energy of adsorption (ΔG_ads) according to the following equation,