Are Corrosion Inhibitors Needed in Adhesive Bond Primers for Bondline Durability?
Historically, chromated inhibitors in structural adhesive bond primers for aircraft, ground structures and support equipment, and armor applications are assumed to not only affect adhesion properties as related to varying environmental stresses (i.e. moisture, atmospheric contaminants, salt air, and elevated temperatures) but also reduce the possibility of corrosion leading to premature structural failures along the bond primer/adhesive interface.
Based on tests conducted in SERDP project WP-2144, hexavalent chromium corrosion inhibitors in adhesive bond primers were found to have no statistical significance for adhesive bondline environmental durability compared to non-chromated and non-inhibited primer variants. The team used qualitative and quantitative evaluations of bonded joint designs including assessments of corrosion and bondline decay reactions, interactions between joint components (alloy, surface treatment, primer, adhesive) to assess performance differences between hexavalent chromium Cr(VI) and non-Cr(VI) inhibited or non-inhibited bond primer systems. Testing was carried out under both laboratory and field corrosive environmental conditions. There is now a greater understanding of the correlation of bond primer properties to environmental response and, subsequently, a high level of confidence that Cr(VI) plays a smaller role in bondline durability than previously believed. Metal surface preparations, and even key individual steps in these processes, appear to be more critical to bondline environmental durability than bond primer corrosion inhibition.
Using chromated bond primers as controls, the project evaluated both non-chromated and non-inhibited primers using electrochemical and mechanical test methods. Aluminum alloys (2024-T3 and 7075-T6), phosphoric acid anodize (PAA) and grit blast/sol-gel (GBSG) surface preparations, and two 250°F-cure toughened epoxy film adhesive systems were used during the project. Environmental conditioning included four laboratory environments (ASTM B117 at 95°F, ASTM B117 at 140°F, ASTM G85 Annex 5 (‘Prohesion’), and elevated temperature (140°F) at >95% relative humidity) and two beach environments (Canaveral Air Force Station, FL and Whidbey Island Naval Air Station, WA). Accelerated electrochemical anodic stress cycles were also used to identify discreet electrochemical differences in corrosion response between the primers. Test methods included Electrochemical Impedance Spectroscopy (EIS), Scanning Vibrating Electrode Technique (SVET), the wedge crack extension test (WCET) per ASTM D3762, and a modified double cantilever beam (DCB) test per ASTM D3433. Specimens were post-analyzed for failure mode, moisture ingress, and corrosion. Indoor and outdoor exposure followed by mechanical testing yielded in excess of 2,000 data points for multivariate response screening modeling and statistical analysis.
Without evidence of an overwhelming technical driver requiring corrosion inhibitors in bond primers, the risks of transitioning to non-Cr(VI) or inhibitor-free products are associated with the usual material substitution considerations for critical applications. It appears bond primers should be selected for evaluation irrespective of their corrosion inhibitor content and solely by their ability to deliver the required performance for given applications as assessed by current test methodologies using all materials and processes proposed for the application. Sufficient confidence to explore the use of Cr(VI) free, or non-corrosion inhibited, primers for future field demonstrations of adhesive bonding applications on non-critical DoD assets is warranted to initiate a shift towards environmentally friendly manufacturing and repair practices.
Use of a non-inhibited bond primer (BR-6747-1NC) for the T-45 rudder is proposed by this effort based on structural requirements, damage tolerance, and economic risk potential for repair and maintenance. Navy depot-level installation of an aluminum F/A-18D doubler, potential Army use of non-Cr(VI) primer for improved out-life/shelf life of ground support equipment and armor prior to bonding, and Air Force installation of co-located Cr(VI) and non-inhibited primer bonded repairs under consideration on C-5 aircraft will generate data on in-service performance differences between the primer variants.