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Understanding Corrosion Protection Requirements for Adhesive Bond Primers
Diane Kleinschmidt | Naval Air Warfare Center
The primary objectives of this project were to understand the function and performance of adhesive bond primers and the rationale and quantified need for corrosion inhibitors in current metal adhesive bonding applications. Chromated inhibitors (those containing hexavalent chromium) have been assumed to reduce the possibility of corrosion along the primer-substrate interface in bonded joints, which could lead to premature structural failures when the bondline is subjected to corrosive environmental stresses induced by exposure to moisture, atmospheric contaminants, salt air, and elevated temperatures. To determine the validity of these assumptions, the efforts in this project focused on qualitative and quantitative evaluation of bonded joint designs, assessment of corrosion and bondline degradation reactions occurring in fielded parts compared to model laboratory specimens, interactions between joint components (alloy, surface preparation, primer, adhesive), and performance differences between chromated bond primer systems and those containing nonchromated corrosion inhibitors or no inhibitors. Testing was conducted under both laboratory and marine atmospheric corrosive environmental conditions. The project focused on aluminum alloys bonded with 250°F-cure toughened epoxyfilm adhesives.
Aluminum adherends were prepared with phosphoric acid anodize (PAA) and Cytec Solvay Group (Cytec) BR 127 primer or grit-blast/sol-gel (GBSG) treatment with two Cytec primers, BR 6747-1 and BR 6747-1NC. PAA/BR 127 represents a baseline, high-performance treatment with a chromated, solvent-based bond primer widely used throughout the aerospace industry. BR 6747-1 (chromate inhibited) and BR 6747-1NC (noninhibited) are waterborne primer systems with nearly identical resin components compatible with the GBSG surface preparation commonly used for on-aircraft repair bonding applications. BR 6747-1 and BR 6747-1NC primers were used to compare performance against the baseline system and evaluate the effect of chromates in the bond primer. Results were validated using 3M EW-5000 (chromated), 3M EW-5000ET (non-chromate inhibitor), and EW-5000NC (experimental, noninhibited) primers.
A multidisciplinary team from across government and industry assessed the bond primer variants using several established and novel test techniques to gain insight into the role of bond primer inhibitors. The team conducted indoor exposure mechanical testing using the wedge crack extension test (WCET) per ASTM D37621 in a variety of static and dynamic corrosive environments, including hot/humid environments per ASTM D22472 (140°F with >98% relative humidity (RH)), cyclic corrosion per ASTM G853, and neutral salt fog per ASTM B1174 (140°F in addition to 95°F). Additional testing utilized the double cantilever beam (DCB) test per ASTM D34335 modified to be consistent with Boeing specification BSS72086. DCB specimens were exposed to marine atmospheric environments (Canaveral Air Force Station, FL, and Whidbey Island NAS, WA), as well as two laboratory environments, which were consistent with the WCET test environments. Multivariate statistical analyses were performed on data generated by laboratory and marine atmospheric exposure of WCET and DCB specimens.
Test methods to probe the hydration of the primer coatings, electrochemical performance, moisture transport mechanisms, permeation properties, and surface characteristics that could query any corrosion protection functions of corrosion inhibitors were investigated. Specific electrochemical methods used for evaluation included: 1) Electrochemical Impedance Spectroscopy (EIS) and Scanning Vibrating Electrode Technique (SVET) used to obtain electrochemical properties of surfaces of interest, 2) accelerated corrosion via exposure to static or dynamic electrochemical anodic stress (EC Stress), and 3) capacitance and dielectric loss measurements used to quantify moisture absorption levels in bonded joints EC properties of primed aluminum samples by Scanning Vibrating Electrode Technique (SVET) alone and in combination with accelerated EC Stress. Several of these methods were also used for more complex analysis of cohesive and adhesive failure surfaces. These include EC Stress of bonded joints, characterizing corrosion reactions ahead of and behind crack tips, and assessment of corrosion potential at undamaged bondline edges.
The project’s primary finding suggests chromated corrosion inhibitors in adhesive bond primers are less critical for bonded joint environmental durability than previously believed. Many of the traditional and novel tests conducted during the effort did not show significant differences between chromated and nonchromated bond primers in adhesive bondlines. Though certain rigorous tests used to qualify materials and processes for bonded joints did show chromated primers provide a positive contribution to environmental durability, even these tests revealed surface preparation is the dominant factor for aluminum bonded joint environmental durability performance. Results for bonded joint testing and electrochemical analysis are summarized at the beginning of their respective sections.
There is now greater understanding of the correlation of bond primer properties to environmental response and, consequently, a high level of confidence chromates play a smaller role in bondline environmental durability than previously believed. Aluminum surface preparations, and even key individual steps in these processes, appear to be more critical to bondline environmental durability than bond primer corrosion inhibition. Bond primers may be selected for evaluation irrespective of their corrosion inhibitor content and should be as assessed together with all other components that comprise the bonding system by current test methodologies using all materials and processes proposed for the application. Desired performance, the tests required to assess that performance and the level of acceptable risk are all critical considerations. A Bondline-Corrosion Risk Assessment Tool (B-CRAT) was conceptualized to assist in identifying potential risk factors associated with implementing nonchromated bond primers in applications for which chromate inhibitors were originally assumed to be necessary to provide required bondline corrosion protection.
Sufficient confidence was generated to warrant exploration of nonchromated bond primers for future field demonstrations of adhesive bonding applications on noncritical Department of Defense assets to help initiate a shift toward more environmentally friendly manufacturing and repair practices. Use of a noninhibited bond primer (BR 6747-1NC) for the T-45 aircraft rudder is proposed based on structural requirements, damage tolerance, and economic risk potential for repair and maintenance. Use of the same noninhibited bond primer for Navy depot-level installation of an aluminum F/A-18D aircraft doubler is supported as is a potential Army use of the bond primer for improved out-life/shelf life of ground support equipment and armor prior to bonding. The Air Force plans to install bonded patches on C-5 aircraft using BR 6747-1NC bond primer near similar bonded repairs conducted with BR 6747-1 to assess any in-service performance differences between these noninhibited and chromated variants of Cytec’s waterborne primer chemistry.
Points of Contact
Ms. Diane Kleinschmidt
Naval Air Warfare Center