Self-Indicating Coatings Using Fluorescent Indicators for Corrosion Detection in Mild Steel

Simultaneous corrosion self-indicating and inhibiting coatings provide an effective approach for monitoring and mitigating corrosion. While several studies have explored various corrosion indicators for self-indicating coatings, most have focused solely on indicative property, overlooking the inherent inhibition capabilities of these indicators within the same coatings. Moreover, metal ion and pH indicators behave differently on corrosion self-indication, yet no studies have directly compared these two types. This study addresses the gap by comparing both the corrosion self-indicating behavior and inherent inhibition capabilities of two indicators: the metal ion indicator, 8-hydroxyquinoline (8HQ), and the pH indicator fluorescein (FLUO). These were incorporated into the sol–gel coating both directly and after encapsulation within halloysite nanotubes (HNT) and deposited on a mild steel substrate. Corrosion indication performance was evaluated using confocal microscopy and MATLAB image processing, while corrosion inhibition was assessed through electrochemical studies and salt spray tests. Results showed that 8HQ was more effective when directly added, whereas FLUO performed better when encapsulated. Also, encapsulated FLUO exhibited better corrosion inhibition (2.59 × 104 Ohm·cm2) compared to encapsulated 8HQ (6.0 × 103 Ohm·cm2). Thus, this study highlights the importance of comparative evaluations, offering valuable insights into selecting suitable indicators by elucidating their advantages and limitations.