Chromium conversion coatings, such as Alodine treatment, have been identified as a large, unacceptable source of hazardous waste generation and a danger to worker health and safety. Chromate conversion coatings have been utilized to promote adhesion and corrosion resistance of organic coating systems (primer-topcoat), but efforts to develop finishing systems that do not incorporate a pre-treatment have not demonstrated comparable performance. Additionally, the sodium dichromate seal used in the aluminum anodizing process produces hexavalent chromium. Removal and disposal of the additional hexavalent chrome from the process waste water exacerbates the cost and the potential health safety problem. The use of a non-chromate sealing system will permit an anodizer to eliminate one hazardous constituent.
The goal of this project is the development and implementation of (1) a non-chromate conversion coating for aluminum alloys and (2) an alternative sealing treatment to the currently used sodium dichromate in the anodizing process for use in combat and tactical vehicles, munitions, and aircraft.
The technical objective of this project was to demonstrate the efficacy of non-chromate conversion coatings and sealers for aluminum alloys without compromising corrosion resistance of Army applications and environments. Comprehensive characterization evaluations of candidate non-chromate conversion coatings (industrial sources and inhouse development) were conducted for aluminum alloys 2024, 5052, 5083, 5086, 6061, and 7075 (with special emphasis placed on the 5000 series primarily used in combat and tactical vehicles) for comparison with the Alodine chromate conversion coatings.
A comprehensive characterization of candidate nonchromate conversion coatings, from both industrial sources and in-house development, for aluminum alloys 2519,1083, and 7039 was conducted for comparison with the Alodine chromate conversion coatings. Test protocols included Auger and infrared spectroscopy analyses, electron spectroscopy chemical analysis, salt fog, electrochemical impedence spectroscopy, adhesion, stress corrosion cracking, outdoor exposure, and field tests.
Several sources of supply for the non-chromate sealer were identified and evaluated in the laboratory with respect to corrosion resistance, abrasion resistance adhesion, and fatigue life. Comparable tests were conducted in the actual production environment to demonstrate equivalent performance to chromium sealers. This project was completed in FY 1996.
Use of the non-chromate conversion coating and sealing system would allow government and industry facilities to eliminate one source of hazardous waste with concomitant cost savings associated with the reduction of waste treatment and disposal costs. Their use also would permit anodizers to comply with occupational health and safety standards.