Early detection of aluminum corrosion via "turn-on" fluorescence in smart coatings

In the present work we describe the successful application of spiro[1H-isoindole-1,9′-[9H]xanthen]-3(2H)-one, 3′,6′- bis(diethylamino)-2-[(1-methylethylidene)amino]("FD1") as a "turn-on" early aluminum corrosion detector in epoxy coatings. Electrospray ionization mass spectrometry (ESI-MS) and ¹H NMR investigations have revealed that the non-fluorescent FD1 is sensitive to low pH due to its acid-catalyzed hydrolysis to Rhodamine B hydrazide (RBH) that subsequently becomes protonated to its fluorescent ring-opened form. Both clear and filled FD1-containing smart epoxy coatings were capable of sensing acidic pH produced at the anodic site of localized aluminum corrosion at a low indicator concentration (0.5 wt%), which was demonstrated by the observation of fluorescent, bright-orange areas corresponding to localized pitting corrosion of the aluminum substrate illuminated by a handheld UV lamp. Therefore, early corrosion of aluminum can be easily and nondestructively detected via our "turn-on" fluorescence strategy.