Materials Engineering Department
BS in Materials Engineering
Raytheon Company currently uses a Forest Products Laboratory (FPL) paste etchant for preparing aluminum surfaces for adhesive bonding, and FPL is a source of hazardous hexavalent chromium. The goal of this study was to evaluate a less-toxic P2 paste etchant as a possible replacement. Coupons of 2024-T3, 6061-T6, and 7075-T6 grades of aluminum alloy were solvent-degreased, abrasively cleaned, and etched at room temperature using P2 paste following a strict protocol adopted from Raytheon. Coupons were then left exposed to air for assigned time intervals (or “outlife” times) of 0, 1, 4, 8, 16, and 63 or 72 hours. The aluminum alloy coupons were then adhesively bonded together using Loctite EA9394 adhesive with an approximate bonding area of 0.5” by 1.0” and 0.010-inch thickness into single-lap joints for testing as per ASTM D1002. Samples were placed in a tensile-test fixture on an Instron with a pull rate of .05 in/min to measure bond shear strength. Decrease in shear strength was plotted as a function of outlife for each alloy, and statistical analysis was carried out to identify outlife times for each part which bond strength decreased significantly. The information obtained will further the development of future P2 etch processes, providing maximum allowable outlife times prior to commencing structural adhesive bonding operations.