Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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hex-chrome alternatives Alternatives to Dichromate Sealer in Anodizing Operations BY ROBERT B. MASON,A CEF; SARAH CLARKA; AND MELISSA KLINGENBERGA, PH.D; ELIZABETH BERMAN, PHDB; AND NATASHA VOEVODIN, PHDC. aConcurrent Technologies Corporation, Largo, Fla.; Dayton, Ohio; and Johnstown, Pa. bUnited States Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio cUnited States Air Force Research Laboratory/University of Dayton Research Institute Dayton, Ohio ABSTRACT Ogden Air Logistics Center (OO-ALC) is the primary facility within the United States Air Force for maintaining and overhauling aircraft landing gear. Aluminum landing gear components are anodized at OO-ALC to provide enhanced corro- sion resistance, paint adhesion, and wear resistance; a sodium dichromate seal- ing operation usually completes the anodizing process. During sealing, the pores of the anodized (oxide) layer are hydrated, which fills the pores and provides improved corrosion resistance. However, this sealer contains hexavalent chromi- um, which is listed on the Environmental Protection Agency's list of industrial toxic chemicals that are targeted for voluntary reduction or elimination. The spec- ification that outlines the sodium dichromate sealing process delineates three alternative processes that are approved for use: 1) boiling de-ionized water, 2) cobalt acetate, and 3) nickel acetate. While some research to support the use of these and other sealing processes has been gathered under past efforts, additional work must be conducted to fully integrate non-chromate sealers into OO-ALC's anodizing operations. To meet this need, the Air Force Research Laboratory tasked Concurrent Technologies Corporation to identify viable alternatives to the sodium dichromate sealer, conduct testing on these alternatives, and recommend the most promising sealer(s) for implementation based on the test results. This paper will describe the requirements for anodizing and sealing operations with- in OO-ALC, as well as the sealing technologies that are available and a path for- ward to demonstrate/validate the most promising alternatives for the specific needs and applications of OO-ALC. Anodizing is a mature and commonly used electrochemical process that controls and enhances the formation of protective oxide coatings on aluminum (Al) and Al alloys. The process is used for a wide variety of commercial and military applications. For example, anodizing operations are routinely performed at the Ogden Air Logistics Center (OO-ALC) at Hill Air Force Base, Utah, as part of the depot maintenance process, on numerous vehicles and weapon systems within the United States Air Force (USAF) inventory. Specifically, OO-ALC is the primary facility within the USAF for maintaining and overhauling aircraft landing gear. Al landing gear components are anodized at OO-ALC to provide wear- and cor- 275

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