Use of a Fluorescent Indicator in Monitoring Underlying Corrosion on Coated Aluminum 2024-T4

A fluorescent indicator (FI) can be used to monitor the underlying corrosion on metal substrates based on sensing pH changes caused by corrosion reactions. One coumarin derivative was chosen as FI and added to a coated-aluminum system. The initial state of the coating system is fluorescent under UV light and it changes to nonfluorescent at the site of corrosion. The effects of the fluorescent indicator on the fluorescent intensity and protectiveness of the coating system are discussed. Electrochemical impedance spectroscopy (EIS) was used to estimate the protectiveness of the coating and interface corrosion reactions between the metal and coating. This was correlated with the fluorescent intensity change of the coating. Based on the impedance measured by EIS, the coating failed shortly after immersing into the 3.5% sodium chloride solution. After 24 h, pH-induced fluorescent quenching points began to appear. Scanning electron microscopy and energy-dispersive spectroscopy of selected specimens indicated that the quenching point was the corrosion site. Several coating thicknesses were examined, and based on these tests the optimal coating thickness was determined to be about 80 mu m.