An investigation on the inhibitory effect of Allopurinol on mild steel st 37 corrosion in hydrochloric acid solution

Steel corrosion is a costly problem and is one of the most important issues in materials science. The corrosion of mild steel is the most common form of corrosion, especially in acid solution. Inhibitors are commonly used to reduce the corrosion attack on metallic materials. Most of the commercial inhibitors are toxic in nature and therefore it is necessary to replace them by environmentally benign inhibitors. The use of pharmaceutical compounds offers interesting possibilities for corrosion inhibition due to the presence of hetero atoms like nitrogen, sulphur and oxygen in their structure, and they are of particular interest because of their safe use, high solubility in water and high molecular size [1]. In the present work, the inhibition effect of allopurinol drug on the mild steel in 1 M hydrochloric acid solution is studied using the potentiodynamic polarization, the electrochemical impedance spectroscopy (EIS), the infrared spectroscopy (FT-IR) and the scanning electron microscopy (SEM) methods [2]. The experiments have been performed for various inhibitor concentrations using these methods and the optimum concentration of the inhibitor has been determined. Furthermore, the influence of the temperature on the corrosion inhibition of the allopurinol at the optimum concentration has been investigated. Results show that the addition of the allopurinol to the hydrochloric acid solution reduces the corrosion of the mild steel. It has also been shown that the corrosion inhibition efficiency increases by increasing the concentration of allopurinol to a certain value and increasing temperature. Potentiodynamic polarization measurements show that allopurinol acts as mixed-type inhibitor. Electrochemical impedance measurements show that increasing the inhibitor concentration to a certain value increases the charge transfer resistance and decreases the capacity of the double layer ( 𝐶𝑑𝑙 ). The results obtained using the potentiodynamic polarization and electrochemical impedance techniques are in good agreement. 293 Furthermore, the adsorption of the allopurinol inhibitor on the alloy surface obeys the Langmuir adsorption isotherm model. The absorption process is a combination of the physical and the chemical adsorption and it is spontaneous. In the hydrochloric acid solution, the adsorption process is endothermic with an increase in the entropy. The results obtained from infrared spectra confirmed the formation of a protective layer on the alloy surface after immersion in hydrochloric acid solution containing allopurinol. SEM investigations also validate the adsorption of inhibitor.