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 smart indicator in epoxy-based coatings for the early detection of steel corrosion. The FD1 indicator was used for epoxy-based coatings on steel for its desirable property of "turn-on" fluorescence upon forming a complex with ferric ions produced at the anodic site during steel corrosion and because it does not prematurely fluoresce when mixed with the coating precursors (i.e., epoxy resin and amine). This indicator, after incorporation into a filled epoxy coating at a concentration as low as 0.5 wt %, was observed to become fluorescent in areas where corrosion started before any obvious sign of metal damage was observable. FD1 fluorescence was apparent both in areas around a scribed portion of the coating where the metal was exposed and in undercoating corrosion, where the coating surface was intact. This nondestructive method of early corrosion detection can help signal when maintenance is needed before the metal suffers serious damage.
