State-of-the-art surface treatment for aluminum and titanium alloys generate large amounts of wastewater both within the surface treatment process and/or through the waste treatment process. Treatment processes themselves require the use and handling of hazardous acids and bases. Further, the increased emphasis on the reduction of volatile organic compounds (VOC) and toxic air emissions will increase the waste stream from surface preparation processes dramatically. The development of metal surface preparations that do not require the use or generation of hazardous substances will aid the Department of Defense significantly in meeting its pollution prevention goals.
The objective of this project was to identify, develop, and optimize non-wet chemistry approaches for the formation of stable morphologies on the surface of aluminum, titanium, and copper materials that allowed high-quality coating and adhesive bonding performance. Ultimately, the goal is to develop surface preparations for aluminum and titanium alloys that eliminate or minimize the use of hazardous materials found in conventional cleaning and surface treatment processes, such as hexavalent chromium, strong oxidizing acids, and concentrated bases or that minimize the generation of large amounts of contaminated wastewater.
This program involved laboratory research and development and process scale-up in two technical areas: plasma spray and sol-gel deposition. The new approaches represented a radical, but environmentally-benign, departure from existing technologies. Testing was conducted to verify the results from various contractors and to ascertain the level of repeatability with respect to both strength and durability and to allow fair comparisons between competing processes. These tests formed the basis for down selection. Promising solid state cleaning processes (96-116-USFA) were tested in conjunction with the best non-chemical surface preparation processes. Program technical risks include the ability to produce the proper surface chemistries and morphologies with the requisite thermodynamic and chemical stability as well as the needed mechanical strength without degrading the substrate mechanical properties.
Titanium adherents were treated by Boeing and Chemat using their respective sol-gel processes in order to support Navy/SERDP effort, project PP-66: “Aircraft Maintenance Chromium Replacement.” Most of the nonchromated alternatives being evaluated by the Navy for the titanium adhesive bonding portion of the project were developed under this SERDP project. These include the sol-gel processes and a plasma spray process. The Navy work continues, and the Air Force will conduct some long-term high-temperature exposure tests to support project WP-66.
Chemat Technology work in the area of sol-gel for adhesive bonding and corrosion protection was promising. Improvements to the plasma polymerized coatings have yielded encouraging long-term moisture durability results for aluminum adhesive bonding. This was accomplished by silanating the silica-like coatings after removal from the vacuum chamber. This project was completed in FY 1997.
Breakthrough technologies to prepare metal surfaces in various stages of manufacturing and remanufacturing will be of enormous benefit to aerospace and other industries in the U.S. and worldwide. It has been demonstrated that environmentally-benign alternative materials and processes can be less costly, more effective, and less time consuming than the technology replaced. In some instances, elimination of ozone depleting chemicals may be achieved.