Several fluorescent materials have been used under epoxy coatings on 7075 aluminum alloy surfaces. These fluorescent materials either fluoresce upon oxidation and are thus corrosivity indicators or they fluoresce with the products of corrosion and are true corrosion indicators. Recent work has involved using a pigmented epoxy paint, such as that used on aircraft, to see if fluorescence is still visible. Although the intensity is reduced, there is still visible fluorescence. This paper reports the results of the most promising fluorescent markers in each of the two types mentioned above. Photographic data are presented with changes of fluorescent activity with time. These two types of indicators have the potential to detect corrosion or corrosive conditions under paint coatings and could greatly reduce the costs of corrosion maintenance as well as enhancing safety factors in aircraft.
Corrosion is an ongoing challenge to scientists and engineers because of the difficulty in detection and the cost of maintenance for this problem. Many industries are affected by corrosion, including the aircraft industry. It is especially severe for the Navy because of extreme operating conditions. Over $1 Billion is spent annually for corrosion maintenance by the Navy alone. If early detection was possible, savings of 20-30% could easily be recognized and that would be in addition to the improved safety for 1-3 personnel involved. The "smart coating" concept was illustrated in previous papers. The objectives of the smart coating are to detect corrosion, inhibit corrosion and facilitate coating removal for repair of damaged areas. This paper focuses on more work regarding the detection of corrosion by fluorescent materials on the surface of alloys or mixed with the primer coat of paint used on these alloys.
EXPERIMENTAL PROCEDURE
Materials tested for fluorescence were obtained from regular chemical suppliers. Aluminum alloy plates, epoxy paint, acrylic paint and clear polymer-adhesive film were obtained from commercial industrial suppliers. The aluminum alloy plates were 7075- T6. Some were used as received, while the others had a chrome conversion coat (The latter ones are designated as 7075ccc in this paper). The epoxy paint used was military spec primer(MIL-P-23377F). Photographs were taken with a digital camera in the black and white mode. Because of the black and white mode required for publication, the colored fluorescence can not be seen in the figures presented. An intense 386nm UV light was used for UV illumination and photo floodlamps were used for visible light photos. The fluorescent material was either applied directly to the alloy coupon and then spray painted or it was absorbed from a solution into porous silica and then the silica was mixed with paint and sprayed onto the coupon surfaces.
RESULTS
Oxine has been shown in previous work to detect corrosion on aluminum surfaces by forming a fluorescent complex with loose aluminum oxide. 3 Several years ago, samples of oxine on 7075 and 7075ccc plates were prepared by spraying a solution of oxine on these plates, allowing them to dry and then coating with acrylic or clear epoxy paint. After almost forty months of aging at room temperature, there is clearly a difference in the fluorescence of oxine on 7075 and 7075ccc surfaces as illustrated in Figures 1 and 2. These show the UV photographs of oxine under acrylic and epoxy primer paints on both 7075 and 7075ccc surfaces. Fluorescence shows up as light areas in black and white photos, regardless of the wavelength of emitted light. Under both types of paint, the fluorescence is greater on the 7075 plates, where corrosion should be more severe, than on the 707