Synergistic Effect of Iodide Ion and 4-Hydroxy Coumarin on the Corrosion Inhibition of Mild Steel in Hydrochloric Acid

No discussion of the use of inhibitors in acid solution is complete without mentioning the phenomenon of synergism. Synergism also operates in corrosion protection where an enhanced inhibition may be related to interaction between inhibitor compounds. This effect has been observed since the earlier days of inhibitor technology and continues to be a potent tool in the development of acid inhibitors for specialized uses. Hence, in this paper, the corrosion inhibition behavior of mild steel (MS) in 1？M hydrochloric acid in the presence of 4-hydroxy coumarin (4HC) and potassium iodide (KI) has been investigated using the mass loss method and electrochemical techniques. The inhibitive performance of 4HC is considerably enhanced by the addition of KI. The addition of KI to different concentrations of 4HC has intensified its efficiency through considerable reduction in the mass loss, corrosion current density , double layer capacitance , and increase in charge transfer resistance . The calculated synergism parameter “ ” is greater than unity, thereby proving the fact that the improvement in inhibition efficiency of 4HC, generated by the addition of KI, is due to synergism. 1. Introduction Synergism literally means joint action of different processes in producing an effect greater than the sum of their individual effects. This effect has been observed since the earlier days of inhibitor technology and continues to be a potent tool in the development of acid inhibitors for specialized uses. In fact, the inhibitive nature of most of the extracts of common plants used to combat corrosion of metals is known to be fortified through synergism of many organic constituents present. In modern practice, inhibitors are rarely used in the form of single compounds. A formulation of two or more organic compounds and/or inorganic compounds is usually employed. The synergistic action of anions on the inhibition of mild steel corrosion in acid solution containing organic compound has been reported by several authors [1–4]. In view of this, the enhancement of protection efficiency of the chosen coumarin inhibitor by iodide ions against mild steel corrosion in 1？M hydrochloric acid has been studied. 2. Experimental Parts 2.1. Synthesis of 4-Hydroxy Coumarin The microwave assisted synthesis of 4-hydroxy coumarin (4HC) was carried out in an ordinary domestic microwave oven by the von Pechmann reaction of phenol, malonic acid fused ZnCl2, and a few drops of POCl3 for 60–40？seconds at 450？W. The mechanism of the reaction is shown in Scheme 1 [5]. Scheme 1: Synthesis of 4-hydroxy