Raytheon Electronics Systems, Bedford, Massachusetts, submitted an unsolicited Task Change Proposal (TCP)-526R1 to the Air-to-Air Joint System Program Office (JSPO), ASC/YA, Eglin Air Force Base, Florida, for qualifying new low volatile organic compound (VOC), chromium-free coating systems for the Advanced Medium Range Air-to-Air Missile (AMRAAM). Current AMRAAM corrosion prevention materials are undesirable because of chromium and high VOC content. A second project was included in this TCP to identify chromium-free corrosion preventive materials for AMRAAM manufacturing; however, this component of the project will not be discussed except to relate the parallel benefits of this project with the low VOC, chromium-free coating systems.

The objectives for qualifying low VOC coatings for AMRAAM manufacturing were as follows:

1) Performance properties for low VOC coatings shall be equivalent or better than the baseline materials.

2) Coatings with minimal VOC and chromium content shall be chosen.

3) Low VOC coatings are to be cost effective relative to baseline materials.

ASC/YA committed to TCP-526R1 as part of its continuing pollution prevention program. The pollution prevention program was begun to study AMRAAM materials and processes used in manufacturing missiles, launchers, and associated equipment for the improvement of environmental, safety, and health considerations. AMRAAM has an active, ongoing program to eliminate or minimize the use of hazardous materials, with an emphasis on the Environmental Protection Agency’s list of 17 industrial toxins in accordance with Air Force Acquisition Policy 94A-003. A significant initial effort in TCP-526R1 included identifying, to the fullest extent possible, the uses of chromium as part of AMRAAM manufacturing. Finishing materials were identified as a significant source of chromium in AMRAAM manufacturing. The TCP found that chromium-containing finishes could be replaced without significant risk to AMRAAM production and reliability. As a stakeholder in manufacturing at Air Force Plant 44, Tucson, Arizona, the JSPO has actively supported chromium elimination opportunities as part of the Joint Group on Acquisition Pollution Prevention. The AMRAAM Technical Data Package and technical orders are being updated to incorporate the benefits of this pollution prevention program.

The qualification test plan for the low VOC coatings project was divided into four parts. The project began with an industry trade study that evaluated AMRAAM coating performance requirements and gathered data on possible low VOC coating candidates. All major AMRAAM exterior surfaces are finished using primer-pretreatment, Sentry brand MIL-C-8514; epoxy-polyamide corrosion inhibiting primer, MIL-P-23377F type I, class 1 Courtaulds, part number 513X390; and aliphatic polyurethane topcoat, MIL-C-83286, Courtaulds, part number 822X363. Every test included specimens painted with these baseline coatings to bench mark the test. Low VOC coating candidates selected from the industry trade study were tested in three steps: screen testing and selection, prevalidation testing, and final validation.

Group 1 Screen Testing and Downselection. Nine metal pretreatments or surface preparations, eight corrosion inhibiting primers, three topcoat materials, and two powder coatings were selected from the industry trade study. These coatings were tested in initial group 1 screen testing.

Test panels painted with the various combinations of pretreatments and corrosion inhibiting primers were strenuously tested for adhesion to titanium, fluid resistance, and SO₂/ salt fog corrosion resistance. SO₂/ salt fog corrosion resistance was measured in a 500-hour test according to ASTM G85 Annex 4. The primer coated test panels were not topcoated. These tests were severe and designed to provide insight into final validation corrosion resistance testing. The test substrates for SO₂/ salt fog corrosion resistance testing included titanium, chemical conversion coated 6061 aluminum, passivated 301 stainless steel, cadmium plated 4340 steel, ion vapor deposited aluminum on 4340 steel, and electroless nickel plated 6061 aluminum. Adhesion testing was performed on titanium (a major AMRAAM substrate) specimens to quickly expose weaknesses in these coating materials. Primer adhesion was measured on sequential specimens during 14-days water immersion. Fluid resistance testing was performed on chromated aluminum substrates. The test fluids for resistance testing were hydraulic fluid, engine oil, and JP-8 aviation fuel.

Topcoat materials and powder coatings were tested for ultraviolet (UV) light resistance and evaluated for color, gloss, and appearance qualities. Using the baseline primer and pretreatment materials as a control substrate, fluid resistance of topcoat and powder coat materials was tested. Candidate topcoats and powder coatings were evaluated and compared to the baseline polyurethane topcoat performance.

Raytheon and the JSPO reviewed these results and selected three pretreatments, five corrosion inhibiting primers, three topcoat materials, and two powder coatings.

Group 2 Screen Testing and Downselection. Next, the selected low VOC coatings were tested in a more measured approach. Group 2 testing included film durability and physical performance testing, corrosion resistance with weathering, and coating capability testing.

Film durability and physical performance testing on coated aluminum specimens included chip resistance, flexibility testing, and impact resistance. These tests were measured before and after UV exposure. Flexibility testing was performed at room temperature and at -50 degrees Fahrenheit (° F). These tests were conducted to evaluate these coatings’ responses to typical AMRAAM service environments.

Corrosion resistance testing was evaluated using a novel accelerated cyclic weathering procedure. Painted stainless steel, aluminum, and nickel plated specimens were exposed to the cyclic weathering test described in ASTM D5894. These specimens were exposed to 168 alternating hours of water condensation and UV radiation. During the next 168 hours, the specimens were exposed to cycles of salt fog and dry air. The salt fog solution contained 3.5 grams per liter (NH₄ )₂SO₄ and 0.5 gram per liter NaCl. The specimens were examined at the half way point of the accelerated weathering and corrosion test and after the test was concluded at 2,016 hours.

The final series of tests in Group 2 required adhesion testing of low VOC coating materials to various composite substrates. These composite materials are used in assembling important painted exterior AMRAAM surfaces. Polyimide; polysiloxyrane composites; and composite bonding primer, CYTEC BR-127, were topcoated, and coating adhesion was measured after exposure to 10 days of humidity.

The results of Group 2 testing led to the selection of coatings and processes for prevalidation listed in table 1.

Prevalidation. Quantitative measurements were performed as part of prevalidation testing. Adhesion of the liquid coating systems and powder coatings was measured according to ASTM D4541. Corrosion resistance was measured on aluminum using electrochemical impedance spectroscopy. Touch-up compatibility was tested using MIL-C-83286 and MIL-C-85285 polyurethane topcoats. Adhesion to polysulfide sealing compounds was measured as part of the AMRAAM chromium-free corrosion preventive materials project. Finally, electrical attenuation was assessed on painted antenna composites. All the replacement liquid and powder coatings demonstrated equivalent or superior performance when compared to the baseline coating system in this round of testing.

Final Validation. Scrap AMRAAM hardware was finished using the appropriate liquid and powder coating system for the substrate. The hardware was finished using full production finishing processes. The painted hardware was again exposed to 500 hours of SO₂/ salt fog corrosion resistance testing. Functional testing of an AMRAAM target detection antenna was performed after undergoing production painting. Again the substrates coated using the low VOC coatings demonstrated equivalent performance to the baseline coatings.

The low VOC coatings and processes listed in table 1 were selected because they were the best performing coatings evaluated in Group 1 and Group 2 testing. Several coatings and coating combinations performed successfully, but these coatings were the most suitable for continuation to final validation. Figures 1 and 2 are photographs showing AMRAAM hardware painted with the baseline coating system and hardware painted with the liquid and powder replacement coating systems. This hardware is shown before and after 500 hours of SO₂/ salt fog corrosion resistance testing.

Corrosion testing and adhesion testing performed throughout the low VOC coating project demonstrated excellent coating system integrity without the use of MIL-C-8514 phosphoric acid, vinyl butyrate wash primer. Pretreatment primer replacements proved to be of little or no value when abrasive blasting was used to prepare AMRAAM substrates for painting. MIL-C-8514 primer may be used when abrasive blasting is detrimental.

High-solids, chromium, and lead-free epoxy primer, MIL-P-53022 type II, HS-022/2.8, performed equivalent to the baseline corrosion inhibiting primer for all substrates, provided surface preparation was performed correctly. Any defect in electroless nickel plating was found to result in pitting of the plated aluminum substrate. This observation was also true for specimens painted with the baseline corrosion inhibiting primer. Because of the corrosion inhibition demonstrated by this high-solids, chromium, and lead-free epoxy primer, the primer has been qualified for barrier coating applications when installing dissimilar metals as part of the AMRAAM chromium-free corrosion preventive materials project.

Two different manufactured brands of high-solids polyurethane topcoat were qualified. Both formulations proved equivalent, if not superior, to the baseline polyurethane topcoat. The Spraylat formulation of topcoat has a lower VOC content and still had excellent appearance.

Two different manufactured brands of low temperature curing epoxy powder coating were qualified. Both formulations proved to be superior to the baseline liquid coating system. The powder coatings have no VOC or hazardous air pollutants. The typical cure for these coatings is 30 minutes at 275 ° F. In physical performance testing and in electrochemical impedance spectroscopy and corrosion resistance tests, the powder coatings demonstrated superiority in protecting AMRAAM substrates. The only drawback to these coatings was cosmetic color fading following UV exposure.

The authors have concluded that the coating systems listed in table 2, when applied to major AMRAAM metallic substrates, have equivalent or superior performance to the baseline AMRAAM coating system. Particularly, low-temperature curing epoxy powder coatings have demonstrated superior damage resistance and corrosion resistance. These conclusions have been presented to the Joint Pollution Prevention Advisory Board as a future streamlining initiative for Raytheon Electronic Systems Andover, Massachusetts, and Hughes Missiles Systems Company, Tucson, Arizona.