Rapid evaluation of corrosion inhibitors in acidic solution using the coulostatic method

Abstract

A monitoring method for the corrosion rate of metal based on the coulostatic principle and a new mathematical model for the evaluation of Tafel coefficients from coulostatic experimental data are presented in this paper. Different kinds of equivalent circuit models involving R(CR(RL)) and R(RC) have been applied to examine the characteristics of the response curves to coulostatic and AC perturbations. In addition, weight loss and electrochemical impedance spectroscopy (EIS) experiments on Q235 mild steel in 0.5 M H₂SO₄ solutions with and without thiourea as inhibitor have been used to verify the results from the coulostatic method. It was found that the coulostatic technique can not only determine values of R_p and C_d quickly, but also the Tafel coefficients in the acidic solution with and without inhibitor. Moreover, since coulostatic measurements can always assume R_p equal to R_t in this system, the corrosion rate calculated by the coulostatic method is free of confusion between R_p and R_t. This facilitates the rapid calculation of corrosion current density based on the Geary–Stern equation, and provides a simple procedure for the evaluation of corrosion and corrosion inhibitors.