High-Viscosity Pre-Penetrant Etching Materials

Fluorescent penetrant nondestructive inspection (NDI) processes are utilized to detect surface-breaking cracks and corrosion in aircraft structures. Most military aircraft structures are made of 7000 series aluminum alloys (with 7075/7050/7085 being most common). The parts are primed and painted to provide protection from corrosion. Removal of paint schemes or corrosion is often performed through mechanically abrasive processes such as sanding, grinding, or machining, which smear small amounts of material over fine cracks and corrosion, making them less detectable with the penetrant inspection method. Chemical etching is used throughout the NDI industry as a method to remove approximately 0.0002 in. (0.00508 mm) of smeared metal prior to penetrant inspections. The etching process typically requires multiple steps, which include precleaning the area, applying an etchant, applying a neutralizer, and applying a desmutting agent. The low- viscosity chemicals (~1 cP, similar to water) are prone to spilling and migrating into crevices in the structures (faying surfaces and fastener holes) near the inspection zone. ANI proposes to develop paste forms of viable etchant materials with paste-like viscosities (70,000 - 100,000 cP) to reduce the hazards of using these chemicals during inspections of parts that are still installed in the aircraft.