Evaluation of Non-Chromated Bond Primers for Metal-to-Metal Structural Adhesive Bonding

Adhesive bond primers are used as part of a multi-step treatment to improve the adhesion of critical structural parts to aircraft.  The current DoD-approved primers contain hexavalent chromium as a corrosion preventative; however, hexavalent chromium is a human carcinogen and a hazardous air pollutant (HAP) with strict EPA and OSHA regulations.  In addition to the regulations, chromate-containing materials pose significant hazardous waste disposal costs and necessitate costly controls to prevent human and environmental exposure.

NAVAIR identified three commercially available bond primers containing no hexavalent chromium, two, 3M EW5000AS and Cytec BR6700-1, which contain alternative corrosion inhibitors and one, BR6747-1NC, with no corrosion inhibitors at all.  Before these materials can be used in critical structural applications, however, their strength and durability properties must be assessed.  Through the Navy’s Aircraft Equipment Reliability and Maintainability Improvement Program (AERMIP), NAVAIR tested these non-chromated bond primers against an approved, chromated product (Cytec’s BR6747-1).  The results of these studies, including performance in floating roller peel, single lap shear, wedge crack extension and scribed salt fog testing, will be presented.

Non-chrome primers performed comparably to the control in mechanical tests such as single lap shear and floating roller peel but did not perform as well in cyclic/static stress durability or wedge crack extension in hot/wet environments and salt fog exposure after being scribed.  Due to the nature of a bonded joint, as only the perimeter of the bond-line is exposed to the elements, corrosion protection may not be as essential as in exterior or exposed applications (i.e. paint primers, sealants, etc.).  Bonded joints using primers containing no corrosion inhibitors performed even better than non-chromated primers in both mechanical and environmental tests which raises questions about the need for corrosion protection in at least a subset of bonded joints.