Fluorescent coatings for corrosion detection in steel and aluminum alloys
Coatings are often used as a means of protecting aluminum alloy and steel structures in industry. The assessment of corrosion under these coatings can be challenging. Corrosion sensing coatings can exhibit properties that allow undercoating corrosion to be identified before it can be seen with the naked eye. This would be very advantageous and could potentially result in tremendous savings in time and money when structures undergo routine maintenance. Our work involved the study of corrosion sensing coatings with incorporated fluorescent indicators that can be used to sense the undercoating corrosion on metal substrates. The fluorescent indicator in the coated-aluminum system was a negative indicator, i.e. the indicator in the coating was initially fluorescent and subsequently non-fluorescent due to the reduced pH at the anodic sites of corrosion. The fluorescent indicator in coated-steel system was positive, in the sense that the coating changed from non-fluorescent to fluorescent over the cathodic areas due to increased pH. The corrosion sensing coating was composed of commercial epoxy-polyamide and the indicator: 7-amino-4-methylcoumarin (7-AMC) for the coated-aluminum alloy system and 7-diethylamino-4-methylcoumarin (7-DMC) for the coated-steel system. The feasibility of using 7-AMC for sensing early undercoating corrosion was demonstrated by using fluorescent observations, Electrochemical Impedance Spectroscopy (EIS), Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) tests. EIS results estimated that with continuous immersion the undercoating corrosion occurred within 24 hours after immersion in the salt solution. When corrosion occurred, the corrosion was invisible under natural light. However, small spots appeared in the fluorescent image, changing from initially fluorescent to non-fluorescent where the anodic sites were identified by SEM and EDS. In other words, the fluorescent indicator could sense the early undercoating corrosion, although blistering can be a competing mechanism associated with corrosion under some conditions. The sensitivity of the 7-AMC corrosion detection system was tested by applying anodic current to the metal and measuring the charge at which fluorescence quenching was detected. The critical charge for a detectable pit under the coating was approximately 2x10⁻⁵ C, which implied a critical radius of a single corrosion spot or set of spots of approximately 10 [mu]m. The fluorescent properties of 7-AMC, its effect on the protectiveness, its sensitivity to pH and its concentration in the coating are explored as well. Fourier transform spectroscopy (FTIR) was used to characterize the structure of the coating with and without 7-AMC. The results suggested that there is no structure change occurring after adding 7-AMC into the coating. Fluorescence behavior, electrochemical behavior, microscopic evidence, and visual observations of coated steel specimens with 7-DMC are compared based on exposure to saltwater conditions. Some of the challenges associated with the use of these types of coatings will be presented. This includes the interference from the increased production of ferrous and ferric ions. All of this information is aimed at the development of corrosion sensing coatings that can reveal undercoating corrosion before it is visible to the naked eye.