Determination of corrosion on aluminum alloy under protective coatings using fluorescent probes

A new method is described using fluorescence probes for determination of metal ions in corrosion processes of Al 2024T3. Three probes, lumogallion, N, N′-bis-(salicylidene)-2,3-diaminobenzofuran (SABF) and Phen Green™ were used to study their response to Al 3+, Mg 2+ and Cu 2+, respectively, in aqueous solution and corrosion processes. Fluorescence intensity of lumogallion and SABF increased with increasing concentration of Al 3+ and Mg 2+, respectively. The emission intensity of Phen Green™ was observed to decrease with increasing Cu 2+ concentration. The variation in emission intensity is dependent on ion concentration and showed a linear relationship. The fluorescence emission intensity of 0.50 M NaCl corrosion solution increased with exposure time. Soluble Al 3+ and Mg 2+ can be determined in corrosion solution and Al 3+ and Mg 2+ reached 0.126 and 2.344 μM in 1 week, respectively. Cu 2+ cannot be detected in corrosion solution. The anodic dissolution of Al matrix and Al–Cu–Mg particles developed Al 3+ and Mg 2+, but Cu was left at the metal surface. Fluorescence microscopy was used to investigate topography of coating surfaces, localized corrosion and corrosion processes. Uniform emission background produced by excitation of fluorescent probes indicated that lumogallion was well-dispersed in the epoxy/polyamide primer. Visible fluorescence image changes revealed corrosion processes of Al 2024T3 under protective coatings. Energy dispersed X-ray (EDX) and scanning electron microscopy (SEM) were employed to confirm the results obtained.