Non-Destructive Testing of Corrosion Under Coatings

Surface corrosion on aluminum aircraft skins near joints and around fasteners is often an indicator of buried structural corrosion and cracking. Aircraft paints are routinely removed to reveal the presence of corrosion on the surface of metal structures and the aircraft is subsequently repainted. This process can be very expensive time consuming and results in the generation of air pollution and process waste. A method is needed to detect the early onset of corrosion on metal substrates covered by protective coatings so that aircraft paints do not have to be stripped without cause. By employing nondestructive techniques to inspect the aircraft exterior structure without removing coatings the amount of stripping and reapplication of coatings that occurs at the military rework facilities can be substantially reduced. The objective of this SERDP Project 1137 was to develop nondestructive inspection techniques to locate hidden corrosion on aircraft surfaces without requiring removal of the coating. Several techniques have been identified that can achieve this: imaging of the surface using spectral nondestructive evaluation (SNDE) and wide area spectral imaging (WASI) and scanning reference electrode measurement techniques using instrumentation such as the Scanning Kelvin Probe (SKP). This report describes progress made for years 1999 through 2004 for Corrosion Detection and Standards Development. Four principal techniques were investigated: spectral nondestructive evaluation (SNDE) wide area spectral imaging (WASI) electrochemical impedance spectroscopy (EIS) and a scanning probe in a conducting gas (SPG). The original document contains color images.