Infrared Reflectance Imaging for Environmentally Friendly Corrosion Inspection Through Organic Coatings

Abstract : Prematurely stripping aircraft for corrosion inspection or maintenance purposes causes excess pollution in the form of hazardous air pollutants (HAP), volatile organic compounds (VOC) emissions, Resource Conservation Recovery Act (RCRA) waste, and carcinogenic chromates. With government policies driving waste reduction, these waste streams are a concern. The National Emission Standard for Hazardous Air Pollutants (NESHAP) has been the principal compliance driver over the last decade for the aerospace industry, in particular NESHAP 40 CFR Part 63. Hazardous material (HAZMAT) reduction is driven by the Clean Air Act (CAA) and RCRA through pollution prevention (P2) efforts. Many P2 projects impact both CAA and RCRA concurrently. The recently developed infrared reflectance imaging technique (IRRIT) has been identified as a technology that can potentially reduce these waste streams. IRRIT utilizes mid-wave infrared (MWIR) (3-5 micrometers of light) to image corrosion through coatings. By stripping coatings only when IRRIT inspection indicates that corrosion is present, pollution from premature aircraft stripping can be minimized. It was found during demonstration and validation testing that the IRRIT system consistently identified corrosion through coatings more accurately than an unassisted visual inspection. The contributing factor for such a large deviation of inspection results between visual and IRRIT was due to the detection methods utilized for each technique. The IRRIT method directly images corrosion by-product through the paint system due to reflectance contrast differences of the substrate. The visual method relies on the identification of paint surface irregularities/blistering (i.e., paint degradation) as a result of substrate volume changes associated with corrosion formation.