Understanding Corrosion Protection Requirements for Adhesive Bond Primers

Ms. Diane Kleinschmidt | Naval Air Warfare Center

WP-2144

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.

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Technical Approach

The failure mechanisms of fleet hardware and laboratory-aged bond joint systems will be determined through instrument-based analyses. Specific technologies for interface stabilization, adhesion promotion, corrosion inhibition, and water permeation control will be evaluated for their independent and system-level contributions to prevent bond joint degradation processes through analysis in field and laboratory-aged samples and through tests and analysis of environmental durability using experiments with model structural adhesive bond systems. The team will identify and quantify the decay processes occurring in field-exposed, adhesively bonded components and accelerated test specimens to establish whether current test methodologies are valid. The team will also identify improvements needed in test protocols to replicate actual failure mechanisms.

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Benefits

This work is expected to lead to more efficient and effective adhesive joints in original manufacture and repair, to provide a methodology for assessing corrosion and structural risk levels in bonded structures, to provide insights into the corrosion mechanisms that occur in an occluded bondline, to demonstrate performance improvements in adhesive bond systems, and to accelerate the Department of Defense and industry qualification process for chromate-free bonding systems. (Anticipated Project Completion - 2015)

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Points of Contact

Principal Investigator

Ms. Diane Kleinschmidt

Naval Air Warfare Center

Phone: 301-342-8094

Fax: 301-342-8077

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