Effectiveness of Thymol Blue and Phenolphthalein in Sol–Gel Coatings for Sensing and Inhibiting Corrosion on Mild Steel

Abstract

Development of smart coatings incorporated with corrosion indicators that can warn of corrosion through signaling the alteration of pH or metal-ion concentration at the corrosion site, and simultaneously mitigate further corrosion, stands out as a highly efficient and economical approach to address corrosion issues. In this context, the present work provides a comprehensive comparison on the effectiveness of thymol blue (TB) and phenolphthalein (PhPh) in both sensing and inhibiting corrosion on mild steel. While most of the works primarily focused on independently investigating the corrosion indication behavior of different active agents, our study intends to offer valuable insights through the comparative analysis of TB and PhPh in making well-informed decisions when selecting the most effective indicator for a given set of application scenarios. Initially, TB and PhPh were dissolved in a 3.5% NaCl solution, and their ability to indicate and inhibit corrosion on mild steel was verified through a drop test and an electrochemical study, respectively. An electrochemical study and a salt spray test were carried out to evaluate the corrosion sensing and inhibiting capabilities of the coating produced on a mild steel substrate by integrating TB and PhPh in the sol−gel matrix. The synergistic effect of TB and PhPh in sensing and inhibiting corrosion was also studied by combining TB and PhPh in the sol−gel matrix. It was found that TB-based coatings showed approximately 8 times better inhibiting performance (in terms of charge transfer resistance) when compared to PhPh-based coatings, while the PhPh-based coating was around 16 times more sensitive (with respect to concentration and time) in indicating corrosion when compared to TB-based coatings on a mild steel substrate. Nevertheless, the combination of TB and PhPh in a 1:3 ratio incorporated into the sol−gel coating exhibited increased sensitivity in detecting corrosion and showed improved corrosion inhibition on mild steel.