Surface corrosion on aluminum aircraft skins and around joints and fasteners is often the precursor to buried corrosion. Aircraft paints are routinely removed to reveal the presence of corrosion on the surface of metal structures, and the aircraft is subsequently repainted. Aircraft painting and repainting operations result in significant emissions of volatile organics, organic and inorganic hazardous air pollutants, and hazardous waste.
The objective of this project is to develop nondestructive evaluation (NDE) techniques to detect the presence of corrosion under an organic film in order to reduce the amount of painting and depainting that is normally performed.
Inspection and measurement techniques to be developed include (1) a spectral NDE technique employing an optical reflectance probe in the near/mid infrared (IR) region combined with Directional Hemispherical Reflectance (DHR) and Fourier Transform Infrared (FTIR) integrated detector; (2) Wide-Area Spectral Imaging (WASI) using spectral filters and high-resolution focal plane cameras to allow rapid initial assessment of sub-paint corrosion; and (3) a Scanning Kelvin Probe (SKP) electrochemical method employing a calibrated capacitance probe to indirectly measure corrosion potential across a surface. Challenges to be overcome include probe positioning and electrical noise. Baseline measurements of unexposed coatings and typical corrosion products were performed to establish a database of standards. Aged aircraft components then were evaluated. Measuring systems will be optimized at varying levels of corrosion and modified for field use. Prototypes will be verified in conjunction with the Naval Air Warfare Center Aircraft Division (NAWCAD). Finally, a transition plan for cost-effective applications will be prepared.
A portable IR Reflectance Camera System has been developed to detect corrosion under military and commercial aerospace coatings up to 12 thousandths of an inch in thickness. The system is also capable of detecting fatigue cracks of approximately 0.030 thousandths of an inch in length emanating from fastener holes and corrosion pits down to 0.001 of an inch under coatings. IR images of high fidelity were produced in real time on a television monitor for instant observation with the capability of storing the data as a movie (i.e., MEPG file) or capturing the data in the form of a picture, as a JPEG or TIFF file.
By minimizing the number of times the aircraft exterior coatings are stripped and reapplied, substantial pollution prevention and cost saving opportunities are provided. The NDE techniques developed in this project will be able to target and map specific areas that require maintenance due to corrosion, allowing for a migration from schedule-based to condition-based maintenance. In addition, verification of the condition of the primer and surface preparation once the topcoat has been removed will eliminate a portion of the rework that now routinely occurs.