Dual functional Coatings: Comparative Study of Corrosion Indicators for Corrosion Reporting and Self-healing on MgAZ31 Alloy

The current study presents a comparative evaluation of the corrosion-sensing and the inherent corrosion inhibition performance of sol-gel-based smart coatings on Mg AZ31 alloy, using three distinct active materials such as phenolphthalein (PhPh), 8-hydroxyquinoline (8HQ), and fluorescein (FLUO). Direct incorporation of 2% PhPh in the sol-gel matrix provided early corrosion indication with moderate inhibition. However long-term performance could not be expected from this coating, as PhPh would leach out of the coating with time or degrade under sunlight. Therefore, all active materials were encapsulated within halloysite nanotube (HNT) smart containers and incorporated into the sol-gel matrix, which was subsequently applied onto Mg alloy substrate. Though the encapsulation of PhPh within HNT could not provide corrosion indication on Mg alloy due to the encapsulation under acidic condition, encapsulated 8HQ and FLUO based coatings demonstrated significant potential for corrosion sensing and inhibition. Confocal microscopy images, after 1 h salt spray, revealed distinct fluorescence signals, with 8HQ (a metal ion indicator) based coating localizing its signal within scribed areas and FLUO (a pH indicator) based coating producing signals at scratch edges. FLUO exhibited superior fluorescence intensity due to its higher quantum yield when compared to 8HQ. Further, FLUO based coating achieved a higher charge transfer resistance of 1.3 ×106 Ohm·cm² when compared to 8HQ based coating (3.8 ×105 Ohm·cm²) providing better corrosion inhibition. In conclusion, while PhPh was found to be unsuitable for corrosion sensing with encapsulation into HNT, FLUO@HNT demonstrated a superior performance as both a corrosion indicator and inhibitor, outperforming 8HQ@HNT in fluorescence intensity and barrier-forming efficacy.