In this study, the inhibition efficiency of four inhibitors (methylenephosphonic acid, benzotriazole, 2-mercaptobenzimidazole and 2-ethylbenzimidazole) for corrosion prevention of carbon steel samples in one molar HCl solution was evaluated by weight loss method. Experimental design and modeling of the corrosion control process for two inhibitors with the highest effectiveness was carried out using a combined mixture-process design method at different temperatures, concentrations, and mixing ratios. 2-Mercaptobenzimidazole and 2-ethylbenzimidazole were selected for investigation as a mixture. Through analysis of variance of experimental data, a new model was developed to predict the inhibition efficiency of the mixture of these two inhibitors. The inhibition efficiency predicted by the model was in good agreement with the laboratory data, and the P value of the model was less than 0.0001. In addition, the coefficient of determination, adjusted coefficient of determination, and predicted coefficient of determination were 0.9989, 0.9986, and 0.9982, respectively. The results showed that the inhibition efficiency increased with increasing the mixture concentration. Also, temperature had a negative effect on the inhibition efficiency. As the temperature increased, the inhibition efficiency of the mixture decreased.
Through modeling, the optimal values for temperature, mixture concentration and mixing ratio were determined to be 25°C, 130 ppm and 75:25 of 2-mercaptobenzimidazole to 2-mercaptobenzimidazole, respectively. Under these conditions, the inhibition efficiency of the mixture was more than 93%. Moreover, synergistic effect between 2-mercaptobenzimidazole and 2-ethylbenzimidazole inhibitors was determined. The highest synergistic effect was more than 6.5%, which increased the corrosion inhibition efficiency by more than 6.5%.