Metal Finishing May/June 2013 : Page 32
TECHNICALLY speaking BY GREGORY MOROSE, TURI, UMASS LOWELL; DAYNA LAMB AND DAVE PINSKY, RAYTHEON; KENT DEFRANCO AND ZACHARY POWELL, LOCKHEED MARTIN; AND ALP MANAVBASI, METALAST Evaluation for Alternatives to Hexavalent Chromium Sealants P olysulfide sealants containing soluble hexavalent chromium compounds are currently being used in a variety of applications in aero-space/defense manufacturing. The applications mostly involve the ﬁlling of gaps and recesses to prevent water intrusion and collection in an attempt to prevent corrosion of the base metal. These sealants are most com-monly used on aluminum assemblies and are often over coated with a vari-ety of common coating systems with hexavalent chromium-based corro-sion inhibitors. Hexavalent chro-mium compounds are of concern because they are carcinogens, muta-gens, developmental toxicants, and have high acute toxicity. Workplace VENDOR PPG Aerospace 3M PPG Aerospace PPG Aerospace exposure to hexavalent chromium may cause health impacts, such as lung cancer and respiratory tract damage in workers who breathe air-borne hexavalent chromium, and skin damage from dermal exposure (OSHA, 2009). Regulatory and market drivers are motivating a global effort in the aero-space/defense industry to replace hexavalent chromium-containing materials with hexavalent chromium-free alternatives for various applica-tions. For example, the Occupational Safety and Health Administration (OSHA) implemented new Hexava-lent Chromium Standards for general industry in 2006 (29 CFR 1910.1026) where the average worker exposure to hexavalent chromium over the course of an 8-hour work shift was reduced to 5 ug/m3 (OSHA, 2009). The Defense Federal Acquisition Regula-tion Supplement (DFARS) was issued on May 5, 2011 (76 FR 22569), “Min-imizing the Use of Materials Contain-ing Hexavalent Chromium.” It states that no Department of Defense con-tract (for programs prior to Milestone A) may include a speciﬁcation or standard that results in a deliverable containing more than 0.1% hexava-lent chromium in any homogeneous material where acceptable substitutes are available , or requires use or removal during subsequent phases of the deliverable, unless an exception or approval applies. There are several exceptions to the DFARS rule, such as conversion coatings and hard chrome plating (DFARS 2011). How-ever, the DFARS rule applies to seal-ant and primer applications that con-tain hexavalent chromium. Despite the known hazards and restrictions, hexavalent chromium materials continue to be used in the aerospace/defense industry, due to technical performance and economic challenges of transitioning to hexava-lent chromium-free alternatives. The principal technical performance chal-lenge is that the long-term, corrosion-inhibiting properties of the hexavalent chromium-free alternatives are not CHEMISTRY Polysulﬁde Polysulﬁde Polysulﬁde Polythioether CORROSION INHIBITOR Hexavalent chromium Zinc phosphate Ammonium phosphite None VENDOR PN SPECIFICATION PS-870 MIL-PRF-81733D Type II Class 1 Grade A AC-735 MIL-PRF-81733D Type II Class 1 Grade B and AMS 3265 Class B PR-1775 AMS 3265 Class B PR-2001 AMS 3277 Type II, Class B Table 1: Sealants Included in the Design of Experiments VENDOR Akzo Nobel Aero-space Coatings Deft Inc. PRC-Desoto of PPG Aerospace MODEL 10P20-13 High solids epoxy primer + EC-213 HS epoxy primer cure solution 44GN098 1GK base and catalyst CA8211, 8211F37886MPY22K PRIMER/ TOPCOAT Primer Primer Topcoat HEXAVALENT CHRO-MIUM CONTAINING MIL SPEC PRF 23377, Yes Type 1, Class C MIL SPEC PRF 85582, No Type 1, Class N MIL SPEC PRF 85285, No Type 1 SPECIFICATION Table 2: Primers and Topcoat Included in the Design of Experiments 32 | metalﬁnishing | May/June 2013 www.metalﬁnishing.com
Evaluation of Alternatives to Hexavalent Chromium Sealants
By Gregory Morose, Turi, Umass Lowell; Dayna Lamb and Dave Pinsky, Raytheon; Kent Defranco and Zachary Powell, Lockheed Martin; and Alp Manavbasi, Metalast
Polysulfide sealants containing soluble hexavalent chromium compounds are currently being used in a variety of applications in aerospace/ defense manufacturing. The applications mostly involve the filling of gaps and recesses to prevent water intrusion and collection in an attempt to prevent corrosion of the base metal. These sealants are most commonly used on aluminum assemblies and are often over coated with a variety of common coating systems with hexavalent chromium-based corrosion inhibitors. Hexavalent chromium compounds are of concern because they are carcinogens, mutagens, developmental toxicants, and have high acute toxicity. Workplace exposure to hexavalent chromium may cause health impacts, such as lung cancer and respiratory tract damage in workers who breathe airborne hexavalent chromium, and skin damage from dermal exposure (OSHA, 2009).
Regulatory and market drivers are motivating a global effort in the aerospace/defense industry to replace hexavalent chromium-containing materials with hexavalent chromium-free alternatives for various applications. For example, the Occupational Safety and Health Administration (OSHA) implemented new Hexavalent Chromium Standards for general industry in 2006 (29 CFR 1910.1026) where the average worker exposure to hexavalent chromium over the course of an 8-hour work shift was reduced to 5 ug/m3 (OSHA, 2009). The Defense Federal Acquisition Regulation Supplement (DFARS) was issued on May 5, 2011 (76 FR 22569), "Minimizing the Use of Materials Containing Hexavalent Chromium." It states that no Department of Defense contract (for programs prior to Milestone A) may include a specification or standard that results in a deliverable containing more than 0.1% hexavalent chromium in any homogeneous material where acceptable substitutes are available, or requires use or removal during subsequent phases of the deliverable, unless an exception or approval applies. There are several exceptions to the DFARS rule, such as conversion coatings and hard chrome plating (DFARS 2011). However, the DFARS rule applies to sealant and primer applications that contain hexavalent chromium.
Despite the known hazards and restrictions, hexavalent chromium materials continue to be used in the aerospace/defense industry, due to technical performance and economic challenges of transitioning to hexavalent chromium-free alternatives. The principal technical performance challenge is that the long-term, corrosion-inhibiting properties of the hexavalent chromium-free alternatives are not considered as effective as the hexavalent chromium-containing materials. The costs and resources required for investigating and evaluating the various hexavalent chromium free materials and manufacturing processes may be prohibitive for an individual company to undertake alone. To that end, the Toxics Use Reduction Institute (TURI) at the University of Massachusetts Lowell reached out to companies in the defense/aerospace industry to collaborate in addressing the challenges of adopting hexavalent chromium- free alternatives. There was considerable interest in collaboration, and consequently a Hexavalent Chromium- Free Sealant Evaluation Team was established with representatives from TURI, Lockheed Martin, Raytheon, Northrop Grumman, NASA, NAVAIR, AFRL, and AMCOM G4.
For the past year, the Evaluation Team has collaborated to jointly conduct research, testing, and analysis for using hexavalent chromium-free sealants, with various hexavalent chromium-containing and hexavalent chromium-free conversion coatings and primer materials for aluminum assemblies. The objectives of this collaborative effort were as follows:
• Evaluation of existing alternatives to metal finishing applications in the aerospace/defense industry that use hexavalent chromium sealants.
• Selection of appropriate testing and evaluation criteria to evaluate the corrosion resistance of sealant materials in various applications.
• Generation of screening level data to either: 1) justify the use of DFARS-compliant alternatives; 2) support a request for a DFARs exemption on an application by application basis; or 3) provide information to make decisions regarding further testing requirements for DFARS compliance.
• Development of a working relationship with research participants as a basis for continued collaborative research for addressing the use of hazardous substances.
Test Vehicle Design. Currently established industry specification test procedures for sealants do not provide sufficient and differentiable criteria for evaluating chromate and non-chromated materials in field applications where damage is expected to occur. Therefore, the team developed the necessary test vehicle configuration to evaluate the following five sealant applications simultaneously with induced damage in a corrosive environment: 1) wet installation of fasteners; 2) sealing over the head of a fastener; 3) butt joint sealing; 4) sealing of faying surfaces; and 5) the application of sealant to exposed material surface.
The test vehicle used for this evaluation consisted of three aluminum plates with a series of eight matching holes through which eight threaded fasteners were inserted and then held in place by eight nuts. The .-inch fasteners used were made of stainless steel alloy UNS S66286 (A286). This alloy created a galvanic mismatch with the aluminum plates, and is commonly used by the participating companies because of its relatively high ductility at below-zero temperatures, high strength, and non-magnetic properties. Four of the fasteners included in the test vehicle have flat heads (countersunk into the aluminum plate), and four of the fasteners have socket heads.
The test vehicles were 0.25-inchthick aluminum plates using alloys UNS A96061 (6061) and UNS A97075 (7075). These alloys were selected because they were common alloys used by the members of the Evaluation Team. The aluminum plates are 2 inches wide and 4.5 inches long. The test vehicle design is shown in Figure 1.
Design of Experiments. A total of 48 test vehicles were assembled to provide a design of experiment that included the following materials: two types of aluminum alloys, four types of sealants, two types of primers, two types of conversion coatings, and one type of topcoat. The hexavalent chromium- containing materials were selected because they were commonly used by members of the Evaluation Team and intended as a baseline for comparison. The hexavalent chromium- free materials were selected because they had relatively similar performance characteristics to the hexavalent chromium-containing materials. The design of experiments included the four types of sealants shown in Table 1.
The design of experiments included two types of primers and one type of topcoat as shown in Table 2.
The design of experiments included the following two conversion coatings to improve the adhesive bonding and anti-corrosion properties of the aluminum test panels:
• Iridite 14-2 as a baseline control conversion coating containing hexavalent chromium.
• Metalast TCP-HF HPA 100 hexavalent chromium-free conversion coatings containing trivalent chromium.
Research Approach. During September 2012, the Metalast conversion coating was applied to 20 test vehicles at the Metalast facility in Minden, Nev., and the Iridite conversion coating was applied to 28 test vehicles at the Northrop Grumman facility in Linthicum, Md.
Each test vehicle had only one type of sealant applied. During October 2012, the test vehicles were assembled over a three-week period at the Raytheon facility in Tucson, Ariz., using the following method:
1. Butt Joint Scribe: Scribes were generated by an "Erichsen Scratch Stylus Acc to Sikkens Model 463" with a 1mm-wide carbon tip. Scribed an "X" on the top side of the bottom plate.
2. Faying Surface: Applied a smooth even coat of sealant (0.005 inches) to both sides of the mating panels using an orange stick or glass rod. Five-mil bond wires were used to control the thickness.
3. Threaded Fasteners: Fasteners were used as received from the supplier and not cleaned. Evenly coated all eight fasteners with sealant using orange stick or gloved finger. Inserted fasteners into the freshly mated panels. Installed nuts and torqued to 40 in-lbs.
4. Butt Joint: Applied sealing compound to the butt joint to completely fill the gap using an orange stick or glass rod.
5. Clean: Wiped excess sealant from the entire test vehicle prior to proceeding.
6. Fastener Heads: Used an orange stick or glass rod to apply sealant to completely cover over and around two of each type of fastener head, as well as the corresponding nuts on each test vehicle.
7. Flat Plate Surface: Applied a strip of sealant on the back side approximately 0.005 inches x 0.25 inches. Two strips of tape (approximately 0.005 inches thick) were attached and the sealant was applied in between and then smoothed with a squeegee. The tape was then removed.
8. Curing: Cured the assembly for 48 hours at ambient laboratory temperature with the relative humidity between 35 and 48%.
9. Prime and Paint: Applied primers and topcoat to the test vehicles according to manufacturer instructions.
10. Additional Scribes: Scribed through the primer/topcoat at edge of test vehicle in four locations. Scribed through the primer/topcoat and the sealant along the length of the strip of sealant on the back side of the plate. In addition, scribes were applied to four fastener heads (two flush heads and two protruding heads) and two nuts on each test vehicle.
The completely assembled, painted, and scribed test vehicle is shown in Figure 2.
To stress the sealant joints, the test vehicles were then mechanically and thermally preconditioned at the NAVAIR facility in Patuxent River, Md. The preconditioning was executed according to MIL-PRF-81733D Section 220.127.116.11.1 Cyclic Loading for Class 1 materials. The purpose of this testing was to simulate a very severe operating environment of especially harsh aerospace/defense applications. The preconditioning consisted of cyclic loading between 0 and 5,000 pounds for 250 cycles. The test vehicles were subjected to 250 cycles at a temperature of -65°F after a 30-minute soak at the same temperature with no load.
Accelerated corrosion testing was conducted on 44 test vehicles by Lockheed Martin Aeronautics at their Fort Worth, Texas, facility. The test vehicles were exposed to sulfur dioxide (SO2 ) salt fog for 1008 hours (6 weeks) per ASTM G85 Annex 4. This test consisted of spraying salt fog with introduction of SO2 gas directly into the chamber periodically. Two types of inspections – non-destructive and destructive – were performed on the test vehicles during the accelerated corrosion testing.
Long-term corrosion testing is being conducted for four test vehicles at the NASA Beachside Atmospheric Test Facility located at Kennedy Space Center, Fla. The beachfront laboratory is used to conduct real-time corrosion experiments. It provides remote monitoring of surrounding weather conditions, including wind speed and direction, and rainfall. The beachfront testing is occurring over a 12-month duration between November 2012 and November 2013. The results of this testing will be presented elsewhere.
Non-destructive Inspection Results. Non-destructive inspections were used to examine the type and degree of corrosion on the test vehicles and were performed on all of the test vehicles at the following intervals of exposure to the salt fog: 336 hours (two weeks), 672 hours (four weeks), and 1008 hours (6 weeks). Non-destructive inspection of the test vehicles provided valuable information regarding the outside appearance of the test vehicles. However, the destructive inspection provided better insight for the corrosion inhibiting properties of the sealants.
Destructive Visual Inspection Results. The test vehicles were dismantled to conduct the destructive inspection so that the amount of corrosion could be recorded. Destructive inspections occurred for four test vehicles after two weeks, for another four test vehicles after four weeks, and for the remaining 36 test vehicles at the end of the six weeks of testing. The test vehicles were dismantled by removing the fasteners, separating the plates of the test vehicles, as well as using toluene to strip the sealant and corrosion by-products to reveal the inner surfaces that were protected by sealant.
Figure 3 shows the corrosion on the faying surface of Test Vehicle #40 after 336 hours of salt fog exposure. The test vehicles were divided into three categories: 1) 6061 aluminum alloy with hex-chrome free (HCF) primer; 2) 7075 aluminum alloy with HCF primer; and 3) 7075 aluminum alloy with hex-chrome (HC) primer. Within the three categories of test vehicles, there were two areas of interest: 1) butt joint and faying surface related areas and 2) fastener countersunk related areas, which created a total of six groups to analyze each sealant/conversion coating combination. Because of the large amount of surface area that each sealant had to protect for the butt joints and faying surfaces category, it was regarded as the primary performance indicator of sealant corrosion inhibition.
Table 3 presents the destructive inspection results for test vehicles that completed 1008 hours (six weeks) of SO2 salt fog exposure. Ratings for the amount of corrosion in each area of interest were recorded as a percentage of corrosion for each area examined. Corrosion was only recorded if there was deterioration of the metal (pitting). A rating of 0% would reflect that no corrosion was present, and a rating of 100 % would indiate complete corrosion of that area of interest. For each sealant/conversion coating combination, there were two test vehicles that were averaged together to provide a single corrosion rating.
This study was not performed to any military or industry standard specification. The Evaluation Team developed this screening level test approach specifically for differentiating between hexavalent chromium-containing and hexavalent chromium-free sealants in field applications subject to a corrosive environment where damage can be expected. The testing was modeled after MIL-STD-81733, but was modified to better evaluate the performance differences between hexavalent chromium-containing and hexavalent chromium-free sealants, by inducing failures in the limited timeframe available to conduct the test. The testing included a large number of variables, a limited number of test vehicles, and no replicates. Consequently, there is not enough data to derive statistically significant results for the many variables. The results and conclusions of this study are based on the conditions of this limited testing effort and are not intended to be an endorsement or disapproval of the various products included in the test.
The following paragraphs summarize findings from this test effort:
Butt Joints and Faying Surfaces. When tested with an Iridite 14-2 hexavalent chromium-containing conversion coating, regardless of primer type or alloy type, the corrosion-inhibiting performance of the AC-735 and PR-1775 hexavalent chromiumfree sealants was comparable to the PS-870 hexavalent chromium containing sealant.
Countersink Areas. When tested with an Iridite 14-2 hexavalent chromium- containing conversion coating, regardless of primer type or alloy type, the corrosion-inhibiting performance of the PR-1775 hexavalent chromiumfree sealant was comparable or better to the PS-870 hexavalent chromium-containing sealant. For the countersink areas, there was high variability in the corrosion results. Although the fasteners were mated with nuts, the holes were not threaded. This situation, coupled with the severe preconditioning, may have led to variable water intrusion and subsequent corrosion.
Comparison of Aluminum Alloys. The 6061 aluminum alloy consistently exhibited better corrosion resistance compared to 7075 when similar conversion coatings and sealants were used. This difference in corrosion resistance was especially evident in the countersink areas of the test vehicles and was an expected result of the inherent corrosion resistance in the specific alloy.
Comparison of Conversion Coatings. The Iridite 14-2 conversion coating provided the best performing corrosion resistance result for the 6061 butt joint/faying surfaces, the 7075 butt joint/faying surfaces, and the 7075 countersink areas. Metalast TCP conversion coating provided the best performing corrosion resistance result for the 6061 countersink areas.
Comparison of Secondary Finishes. For similar sealant and conversion coating combinations on 6061 aluminum alloy test vehicles, no major difference was observed in corrosion resistance of sealant protected areas for primer-only or primer and topcoated test vehicles. However, on 7075 aluminum alloy test vehicles, in general, a primer and topcoat provided more corrosion resistance for sealant protected areas than a primer-only finish. No difference in corrosion resistance of sealant protected areas was observed for 7075 test vehicles with hex-chrome-free, primer-only or hex-chrome, primer-only secondary finishes.
Alloy 6061 with Hex-Chrome-Free Primer. For corrosion resistance in the butt joints and faying surfaces, PS-870 sealant with Iridite conversion coating, PR-1775 sealant with Iridite conversion coating, PR-1775 sealant with Metalast conversion coating, and AC-735 sealant with Iridite conversion coating provided the best performing results. For corrosion resistance in the countersink areas, PR-2001 sealant with Metalast conversion coating, PR-2001 sealant with Iridite conversion coating, and PR-1775 sealant with Metalast conversion coating provided the best results.
Alloy 7075 with Hex-Chrome-Free Primer. For corrosion resistance in the butt joints and faying surfaces, AC-735 sealant with Iridite conversion coating, PR-1775 sealant with Iridite conversion coating, and PS-870 sealant with Iridite conversion coating provided the best performing results.
For corrosion resistance in the countersink areas, PR-1775 sealant with Iridite conversion coating provided the best result.
Alloy 7075 with Hex-Chrome Primer. For corrosion resistance in the butt joints and faying surfaces, all sealant and conversion coating combinations performed relatively similar to each other.
For corrosion resistance in the countersink areas, PR-1775 sealant with Iridite conversion coating provided the best result. The results of the countersink area ratings for this group of test vehicles are similar to the results of the 7075 test vehicles with hex-chrome-free primer. (Lockheed Martin, 2013.)
Next Steps. The Evaluation Team will complete the long-term corrosion testing and will then document the detailed results of the research – including the long-term corrosion testing results, in a future Toxics Use Reduction Institute technical paper.
Tony Phillips, Kent Defranco, Zachary Powell, Natalie Cowart, Susan Lewis, and Scott Jones from Lockheed Martin, for technical input and accelerated corrosion testing.
Dave Pinsky, Amanda Gravanda, Dave Betzhold, and Dayna Lamb from Raytheon, for technical input, test vehicle mechanical drawings, and test vehicle assembly operations. Greg Morose at the Toxics Use Reduction Institute at the University of Massachusetts Lowell, for project management and statistical analysis of testing results.
Steve Davidson, Eric Schoch, Hal Ozpaker, and Paul Cohen from Northrop Grumman Corp., for application of the trivalent conversion coating to the test vehicles and for technical input.
David Carignan from the University of Massachusetts Lowell, for machining the aluminum plates for the test vehicles.
Alp Manavbasi and Kevin Baranowski, for application of the trivalent conversion coating to the test vehicles.
Casey Yeary, Paul Robinson, and Mark Feathers, from the U.S. Army, for technical input.
Alan Fletcher from the U.S. Air Force, for technical input. Diane Kleinschmidt, from the U.S. Navy, for test vehicle preconditioning and for technical input, and Kurt Kessel, from NASA, for beachfront testing of the test vehicles and for technical input.
1. Occupational Safety and Health Administration , Hexavalent Chromium, OSHA 3373-10 2009.
2. Defense Federal Acquisition Regulation Supplement DFARS, May 5, 2011 (76 FR 22569), "Minimizing the Use of Materials Containing Hexavalent Chromium".
3. Hex-Chrome Free Sealant Project, Lockheed Martin Aeronautics Company, MPLR 101436, March 25, 2013.