Chromium VI is a carcinogen, and Federal, state, and local agencies [the Environmental Protection Agency, California’s Air Quality Management Districts (AQMD), etc.] have issued regulations (i.e., Clean Air Act (CAA), Clean Water Act, AQMD rules, etc.) that limit or prohibit the use of chromated materials. Specifically, the 1990 CAA Amendment, the National Emission Standards for Hazardous Air Pollutants, and the San Diego AQMD electrolytic chromium rule restricted the emissions from these processes beginning in 1994. In addition, directives from the Office of the Chief of Naval Operations require significant reductions in hazardous waste generation, of which production and depot-level maintenance of chromated processes, such as chromic acid anodizing and the use of chrome-containing materials (e.g., adhesive bonding materials and pretreatments), are major contributors. To comply with these rules while maintaining aircraft operational readiness, chrome-free alternatives have to be developed and transitioned to fleet use.
The objective of this project was to develop replacement materials for chromates used in aerospace materials and processes on naval aircraft and weapon systems. Nonchromate alternative materials and processes were investigated to replace existing chromated anodizing, pretreating, and adhesive bond processes.
This project identified the best alternatives to chromic acid anodizing (a common inorganic pretreatment for aluminum) from existing and developmental methods. The alternatives included thin sulfuric, phosphoric, and Boeing Aerospace’s boric-sulfuric acid anodizing. This approach also was taken for the development of non-chromate conversion coatings (non-CCC). CCC replacement had been difficult because of the diverse mechanisms by which the CCC provided protection and performance properties. Available non-CCCs, while passing the minimum requirements for an A1 conversion coating, did not inhibit corrosion as effectively as standard CCCs containing hexavalent chromium. In order to reduce risk in the development of a non-CCC, a twopronged approach was pursued. First, the best available non-CCCs, after testing and optimization, were readied for transfer to the Naval Aviation Depot (NADEP), North Island, San Diego, CA, to meet regulatory deadlines. Second, promising longer-term solutions, under laboratory development, were evaluated and optimized.
Selected alloys were evaluated to determine which replacement systems provided equivalent corrosion resistance and paint adhesion while maintaining the existing mechanical properties provided by chromic acid anodizing. The most promising alternative (the sulfuric/boric acid anodizing process) was optimized for aircraft applications through a demonstration at NADEP at North Island. After successful service demonstration, the MIL-A-8625 anodize specification was modified and the process was transitioned to fleet use and implementation. This project was completed in FY 1997.
The elimination of chromic acid anodizing, CCCs, and chromated adhesive bonding materials significantly reduces the total amount of chromium emitted from naval operations. Replacement of chromic acid anodizing also eliminates the need for expensive emission control equipment (estimated at $700K in capital costs and $250K in annual operating costs per depot facility) and required CAA and AQMD legislation. Furthermore, these alternatives reduce the amount of chromium disposal from naval operations (estimated at 12 tons/year per facility). In addition, without the use of adequate replacements, aircraft operational readiness would be curtailed by excess ive environmental degradation. This is particularly important considering the cost of naval aircraft and weapon systems as well as the severely deleterious environment in which the Navy operates. This technology is being coordinated with commercial aerospace, chemical, and equipment manufacturers.