Objective

While the bonded area of a joint may seem impervious to the effects of corrosion due to the physical protection provided by adjoined structures, bond edges, defects, and damages in these critical joints are susceptible to the same environmental stresses as exterior structures. Bond edges may be a very small portion of the bond area in large structural joints, but damage at edges propagates further into the bondline over time. Also, edges can be a large percentage of the bond area in certain applications, such as honeycomb core bonding. At the same time, adhesives and primers at structural joints must carry loads critical to successful performance of a weapons system. Early testing of commercially available non-hexavalent chromium (Cr[VI]) bond primers using current test protocol has indicated overall reduced corrosion performance with variable mechanical performance compared to Cr(VI)-inhibited bond primers.

The objective of this project is to understand the function and performance of adhesive bond primers and the rationale and quantified need for corrosion inhibitors in current bonded joint applications through qualitative and quantitative evaluation of bond designs, corrosion reactions occurring in fielded parts, interactions between joint components (alloy, surface pretreatment, primer, adhesive), and property and performance differences between Cr(VI) and new or existing non-Cr(VI) bond primer inhibited systems in both the laboratory and field. The project will focus on metal structures bonded with 250°F and 350°F service temperature toughened film and paste epoxy adhesive systems.