In-situ early detection of metal corrosion via turn-on fluorescence in smart epoxy coatings

Organic coatings (e.g., epoxy coatings) have been widely used to protect metal and metal alloys against corrosion. However protective coating fails with time, leading to corrosion of the metal substrate. When localized corrosion occurs, without being detected, it can result in disastrous failure of the metal structure. The purpose of this thesis is to develop a smart epoxy coating system that detects early stages of metal corrosion via indicators molecules embedded in the coating that fluoresce when triggered by ions liberated from corrosion. This fluorescence can be easily and non-destructively detected and thus further material damage can be prevented by providing necessary maintenance. In this thesis a spirolactam, [1H-isoindole- 1,91\u27-[9H]xanthen]-3(2H)-one, 3\u276\u27-bis(diethylamino)- 2-[(1 methylethylidene) amino] (FD1), was successfully used to sense early stages of metal corrosion, when embedded in the epoxy coating, via turn-on fluorescence. Despite that we unambiguously confirmed that FD1 forms a fluorescent complex with Fe3+ in a nonaqueous solution by using electrospray ionization mass spectrometry (ESI-MS), the predominant mechanism that FD1 is capable of detecting early metal corrosion is due to its acid-catalyzed hydrolysis to fluorescent protonated Rhodamine B hydrazide, as the consequence of the local pH decrease at the anodic sites of both steel and aluminum corrosion (water is always present). The turn-on FD1 fluorescence was easily, non-destructively detected under UV light before any visible sign of corrosion appeared. In addition, only a low FD1 concentration (0.5 wt%) in the coating was needed for effective corrosion detection. FM did not prematurely interact with the coating formulation components and was able to report early corrosion even when embedded in the filled epoxy coating in the presence of pigments