Evaluation of Hexavalent Chromium Free Bond Primers for Aerospace and Defense Applications

Bond primers containing hexavalent chromium (hex chrome) compounds are currently being used in aerospace and defense manufacturing with different combinations of surface preparations, adhesives, and substrates such as metals and composites.  The bond primers are used to improve the adhesion of structural bonds and historically have included hex chrome as a corrosion inhibitor.  Bond primers may also be relied on to preserve part surfaces for an extended period of time prior to bonding.

However, regulatory mandates have accelerated a global effort to replace hex chrome-containing materials because of their toxicity. In 2011, the Defense Federal Acquisition Regulation Supplement mandated that no U.S. Department of Defense contracts can include a specification that results in a deliverable containing more than 0.1 percent hex chrome in any homogeneous material where acceptable substitutes are available. It also prohibits the use or removal of hex chrome-containing materials during subsequent phases of the deliverable, unless an exception or approval applies. Bond primer applications that contain hex chrome are not an exception.

In 2014, a Hexavalent Chromium Free Bond Primer Evaluation Consortium was established to identify, test, and evaluate various safer commercially available bond primer materials. The team includes representatives from the following organizations: Air Force, Boeing, Bombardier, GE Aviation, Harris, Lockheed Martin, NASA, Naval Air Systems Command, Northrop Grumman, Piper, Pratt & Whitney, Raytheon, Sikorsky, Textron Aviation, Toxics Use Reduction Institute (TURI) at University of Massachusetts Lowell, and UTC Aerospace Systems. The goal of the project was to execute a Joint Test Protocol to comparatively evaluate commercially available hex chrome free bond primer alternatives to an existing hex chrome containing baseline.  Ultimately, the project aim was to provide the data necessary for each individual consortium member to justify the use of DFARS and REACH compliant hex chrome free bond primers for aerospace applications.  This collaborative project was designated as a NASA Technology Evaluation for Environmental Risk Mitigation (TEERM) project. 

Evaluation Process

A Joint Test Protocol was developed by the consortium members to best evaluate the most recent and promising commercially available alternative bond primers.  Based on the specifications covered, the primer types, and the corrosion inhibiting technology, the following bond primer materials listed in Table 1 were selected for inclusion in the evaluation.

 

Table 1: Bond Primer Materials Included in the Evaluation

Vendor	Product	Primer Type	Corrosion Inhibition
Cytec	BR 127	Baseline – Solvent Based	Strontium Chromate
Cytec	BR 127NC	Solvent Based	No Corrosion Inhibitor
Cytec	BR 252	Water Based, Low VOC (approx. 30g/l)	Non-Chromate Corrosion Inhibitor (zinc oxide, zinc phosphate)
Cytec	BR 6747-1NC	Water Based	No Corrosion Inhibitor
3M	EW-5000 ET	Water Based	Non-Chromated Corrosion Inhibitor (zinc molybdate)
3M	EW-5005	Water Based	Non-Chromate Corrosion Inhibitor (zinc and aluminum phosphates)

 

To evaluate the bond primers, the materials and processes selected included three surface preparations, two substrate materials, and two adhesives. The surface preparations included were: phosphoric acid anodize (PAA) in accordance with ASTM D3933, 80 micron aluminum oxide grit blasting followed by Sol-Gel (AC-130-2) (GBSG), and 80 micron aluminum oxide grit blast only (GB). The substrate materials included were: 2024-T3 and 7075-T6 aluminum alloy panels.  The film adhesives included were: 3M AF163-2K (knit carrier, nominal weight 0.045 lbs/ft²) and Cytec FM 73 (knit carrier, nominal weight 0.045 lbs/ft²).

The Consortium determined it would be advantageous to utilize a single supplier with the capability to prepare all of the specimens, from surface preparation through bond primer application.  A request for proposal was developed and two suppliers responded with proposals. After Consortium review, Triumph Fabrications from Ft. Worth, Texas was selected for the tasks. 

The Consortium selected two tests for evaluating the alternative bond primers: neutral salt fog (NSF) corrosion testing in accordance with ASTM B117, and Wedge Crack Extension Testing (WCET) in accordance with ASTM D3762.  Scribed test specimens were used for the corrosion test, and inspections were conducted at one week intervals during the 40 days (960 hours) required in ASTM E1826 and BMS5-89. WCET specimens were exposed to two environmental conditions: NSF per ASTM B117 and an elevated temperature and humidity/wet (ETW) environment of 140°F and >95% relative humidity.  

PAA test specimen surfaces were solvent cleaned, alkaline cleaned with Turco 4215, deoxidized with AmChem 6-16, and anodized in accordance with ASTM D3933. Amchem 6-16 was made up of Turco Deoxidizer 6 Makeup and Nitric Acid 50-70%. The panels were stored in a controlled environment until primed.  The bond primers were applied within 24 hours of the completion of the PAA process.

GB test specimen surfaces were solvent cleaned, blasted with virgin aluminum oxide grit (180grit/80micron) using compressed conditioned air at a pressure of 40-60 psi at an approximate angle of 45 degrees to the surface, and surface blown with compressed conditioned air (approximately 35 psi) to remove as much residual grit as possible. Panels were stored in a controlled environment until primed.  The bond primers were applied within 8 hours of grit blasting.

GBSG test specimen surfaces were treated as described in the GB treatment followed by application of Sol-gel (3M AC-130-2) with an induction time of 30 minutes and applied within 1 hour of grit blasting. The panel surface was kept wet with the sol-gel for a minimum of 3 minutes and drained for 5-10 minutes followed by drying for 1 hour at room temperature.  Panels were stored in a controlled environment until primed. The bond primers were applied within 24 hours of AC-130-2 processing.

Application of the bond primers and adhesives was done in accordance with manufacturer’s instructions. Representatives of the bond primer and adhesive suppliers, 3M and Cytec, were on site at Triumph Fabrications and observed the application process.

For corrosion testing of the scribed test specimens, four replicates of each combination of materials were used to assess corrosion performance. For the WCET, a Design of Experiments was used including three replicates for each combination to improve statistical accuracy of the results.  It was not a balanced design of experiments because values were missing for GBSG/EW-5000-ET and GB/EW-5005 surface preparation/bond primer combinations. Specimens for the WCET were oriented at a 30 degree angle with the crack facing up and the wedge area at the bottom to prevent moisture collection in the crack initiation location (see Figure 1 above).

Corrosion Testing Results

Given the high variability often experienced in corrosion testing and the limited number of test specimens for each material system (i.e. substrate, surface preparation, adhesive, exposure), the data generated in this test program was analyzed by data subsets rather than individual data points. The comparative results and data trends provided some understanding of the relative corrosion performance of the alternative bond primers.  

For the substrate materials, greater consistency and better corrosion resistance performance was witnessed on all 7075-T6 specimens as compared to 2024-T3 aluminum specimens.  This was anticipated based on previous test results and the higher copper content of 2024-T3 leading to higher susceptibility to corrosion. 

For the surface preparations, all specimens with the PAA surface preparation exhibited greater consistency and increased corrosion protection as compared with the GBSG and the GB specimens.  The GB surface preparation indicated poor corrosion protection with all specimens, and based on these results, is not recommended for structural bonding. The GBSG surface preparation provided better corrosion resistance than GB; however, specimens were often inconsistent within the sample set and did not exhibit the same level of corrosion protection as the PAA specimens.

When comparing bond primer performance, the PAA surface prep on 7075-T6 alloy data was the most consistent in this study and determined to be of the most value. Therefore, discussion in this section was generally limited to these data subsets.

The corrosion test results for the six adhesive bond primers using PAA surface treatment on 7075-T6 aluminum alloy were consistent within the specimen sets. None of the individual specimens with BR 252 or BR 127-NC bond primers passed the 960 hour threshold for corrosion outside the scribe.   However, all specimens with BR 127, BR 6747-1NC, EW 5000-ET, and EW 5005 passed the 960 hour threshold for corrosion outside the scribe, (See Figure 2 and Table 2). As noted in Figure 2, the primers which passed the 960 hour threshold continued to perform without corrosion beyond the scribe out to 1,008 hours.

 

Table 2: Corrosion Resistance Results for Test Specimens with PAA on 7075-T6 aluminum.

Surface Prep	Alloy	Bond Primer	Panel 1	Panel 2	Panel 3	Panel 4	% Past 960 hrs
PAA	7075	BR 127	2,265	2,265	2,265	2,265	100%
PAA	7075	BR 252	336	336	336	336	0%
PAA	7075	EW 5000-ET	2,265	2,265	2,265	2,265	100%
PAA	7075	BR6747-1NC	1,200	1,200	2,265	2,265	100%
PAA	7075	EW-5005	2,265	1,360	1,360	1,360	100%
PAA	7075	BR 127-NC	168	168	168	168	0%

 

The 2024-T3 alloy results experienced higher variability within specimen sets as shown in Table 3. Two of the BR 127 (chromated control bond primer) specimens did not pass the 960 hour threshold but two specimens far exceeded the threshold with no corrosion outside the scribe after 2265 hours. This anomaly was investigated but no conclusion for the inconsistency was determined. Consistent results were observed from two other bond primers that exceeded the threshold requirement, EW 5005 and the EW 5000-ET.  Only one of the four specimens for the BR 6764-1NC passed the 960 hour threshold.

 

Table 3: Corrosion Resistance Results for Test Specimens with PAA on 2024-T3 aluminum. 

Surface Prep	Alloy	Bond Primer	Panel 1	Panel 2	Panel 3	Panel 4	% Past 960 hrs
PAA	2024	BR127	2,265	504	504	2,265	50%
PAA	2024	BR252	840	504	504	840	0%
PAA	2024	EW 5000-ET	1,200	1,200	1,200	1,360	100%
PAA	2024	BR6747-1NC	840	672	672	1,008	25%
PAA	2024	BR127-NC	840	840	840	840	0%
PAA	2024	EW-5005	1,360	1,360	1,200	1,200	100%

 

Wedge Crack Extension Testing Results

The results of the Wedge Crack Extension Testing after 9 weeks indicated that test vehicles with PAA surface preparation had the lowest crack lengths.  The PAA test vehicles had an average crack growth of 0.22 inches, the GBSG test vehicles had an average crack length of 0.69 inches, and the test vehicles with GB had an average crack length of 1.71 inches.

The PAA results for each of the bond primers can be seen in Figure 3. For the PAA test vehicles only, three of the hexavalent chromium free bond primers, BR 6747-1NC (0.21 inches), EW 5000-ET (0.20 inches), and EW 5005 (0.20 inches), had smaller crack length than BR 127 (0.22 inches) after 9 weeks.  Also, the PAA test vehicles with FM 73 adhesive had a smaller crack length (0.19 inches) than the PAA test vehicles with AF163 adhesive (0.24 inches).

For the GBSG test vehicles, two of the hexavalent chromium free bond primers, BR 6747-1NC (0.26 inches) and EW 5005 (0.26 inches), had smaller crack length than BR 127 (0.57 inches) after 9 weeks.

Upon completion of the wedge crack testing, the test vehicles were disassembled for failure mode analysis; failure modes were characterized as either adhesive or cohesive.  In general, high cohesive failure rates were found in test vehicles with low crack length values and high adhesive failure rates were found in test vehicles with high crack length values.  For example, all of the EW5005 samples experienced cohesive failure and also had low crack length. 

Conclusions

Based on the results of the corrosion testing of scribed specimens using a phosphoric acid anodize surface treatment, three hexavalent chromium free bond primers, BR 6747-1NC, EW 5000-ET, and EW 5005, demonstrated that they can surpass the 960 hour threshold before exhibiting panel corrosion away from the scribe. 

Based on the results of the WCET with both types of substrate materials, PAA surface treatment, and FM 73 adhesive, the same three bond primers, BR 6747-1NC, EW 5000-ET, and EW 5005, demonstrated better bondline durability performance based on smaller crack length and greater cohesive failure mode than the bond primer with hexavalent chromium.  Results of the GBSG test vehicles indicated that BR 6747-1NC and EW 5005 had smaller crack length and greater cohesive failure mode than the hex chrome-inhibited BR 127. The EW 5000-ET bond primer was not evaluated for GBSG surface treatment.  GB treated test vehicles exhibited high crack length and poor failure mode for all bond primers.

In conclusion, the results of this study demonstrated that with a PAA surface treatment, EW-5000-ET, EW-5005, and BR 6747-1NC are all viable alternative bond primers achieving both the required corrosion resistance and adhesion strength.  With a GBSG surface treatment, EW-5005 and BR6747-1NC are considered viable alternative bond primers for the bonded joint requirement only.

The Consortium members recommend additional testing of these products to their individual material, requirements, and specifications for the given application prior to qualification or implementation. The Consortium members do not endorse the use of any of the products tested and want to clarify that this testing did not qualify any products for use by any of the Consortium members.

Dayna Lamb is with Raytheon, Gregory Morose is with the University of Massachusetts Lowell/TURI, Kent DeFranco is with Lockheed Martin, Diane Kleinschmidt is with NAVAIR, and Kurt Kessel is with ITB Inc.