15 ADVANCED MATERIALS & PROCESSES | MARCH 2024 with trivalent chromium conversion coatings. Each of the two testing methods mentioned above has its pros and cons. Salt-spray testing is performed per ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus[5], and can give results in a matter of hours or days (Fig. 7). This is the standard corrosion test specified in MIL-DTL-5541 for the evaluation of conversion coated aluminum substrates[6]. However, salt-spray testing is not representative of actual conditions that a launch system will see, and it can be overly aggressive. Outdoor exposure testing is a good complement to salt spray testing. The author is shown at the Seaside Atmospheric Exposure Test Facility (Fig. 8), where parts and test panels can be exposed to the outdoor environment. The facility is located adjacent to their launch pad and is representative of the local atmospheric conditions. The primary drawback of outdoor exposure testing is that it can take months or years to see how a coating system will fare in real-world coastal conditions. Fortunately, NASA has generated considerable data at their beachside facility over many decades, and much of this information is made available to the industry. A recent round-robin study was performed along with Q-Lab’s Homestead, Fla. facility and the Kennedy Space Center (KSC). The carbon steel panels used for the study were painted and scribed at the same time, using the same technique, and by the same person. After just three months of outdoor exposure, there was a distinct difference in the degree of corrosion between the three sites. The KSC facility is located on the beach and showed the greatest degree of corrosion. The test panels hung at the author’s site just 4 miles south and approximately 1000 feet inland from the beach showed less corrosion. Q-Lab’s panels showed the least amount of corrosion of the three sites, as that facility is several miles inland from the coast (Figs. 9 and 10). Fig. 6 — Modern-day reusable rocket launch and land sequence. Fig. 7 — Q-FOG cyclic corrosion tester. Image courtesy of Q-Lab. Fig. 8 — Seaside Atmospheric Exposure Test Facility, Merritt Island, Florida. Fig. 9 — Relative Florida locations of round robin test participants, north to south: Site 1, Site 2, and Site 3.
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