ASTM B 117 Screening of Nonchromate Conversion Coatings on Aluminum Alloys 2024, 2219, 5083, and 7075 Using DOD Paint Systems

摘要

This study examines the corrosion resistance of eight nonchromate conversion coatings versus hexavalent chromium-based Alodine 1200S controls on scribed coated test panels of aluminum alloys 2024, 2219, 5083, and 7075. Five representative Department of Defense (DoD) primer/topcoat organic coating systems were evaluated for each of the conversion coating/alloy combinations. The five DoD coating systems were as follows: MIL-PRF-23377 high solid epoxy primer with MIL-PRF-85285 high solid polyurethane topcoat; MIL-PRF-85582cl waterborne epoxy primer with MIL-PRF-85285 topcoat; MIL-PRF-85582nc waterborne epoxy chromium-free formulation primer with MIL-PRF-85285 topcoat; MIL-P-53030 water-reducible epoxy primer with MIL-C-53039 chemical agent resistant, single- component polyurethane topcoat; and MIL-P-53O22 epoxy primer with MIL-C-53039 polyurethane topcoat. Scribed panels were exposed using American Society for Testing and Materials (ASTM) B 117 (neutral salt fog) for 3000 hours and were photographed and measured at 1, 2, and 3 weeks. Subsequent measurements were taken every 500 hours until 3000 hours were reached or until failures from scribe creepback and/or via blistering occurred using method ASTM D 1654. The testing resulted in the following conclusions: pretreatment performance varies among varying alloys; DoD coating systems formulated with chromium +6 (Cr+6) additives provided significantly enhanced corrosion resistance versus coatings without Cr+6; Alodine 5200 and NAVAIR TCP Cr+3 performed best overall in ASTM B 117 salt fog among non-Cr+6-based pretreatments with performance at or near Cr+6-based Alodine 1200S across all alloys and coating systems examined; commercially available Alodine 5200, in conjunction with the chromium-free organic coatings tested in this study, provides a completely chromium-free coating system with good corrosion resistance. (29 tables, 13 figures, 17 refs.).