Metal Finishing Guide Book


Issue link:

Contents of this Issue


Page 380 of 903

REQUIREMENT Type II Coating Weight 1,000 minimum (milligram/squar e feet [mg/ft2]), unsealed ACCEPTANCE CRITERIA Type IIB Type III 200���1,000 4,320 minimum for every 0.001 inch of coating Corrosion Resistance Minimal corrosion after 336 hours Not specified Paint Adhesion No intercoat separation between the paint system and the anodic coating or between the anodic coating and the base metal. Not applicable Coating Thickness (mils [0.001 in]) 0.07���1.0 0.02���0.7 0.5���4.5 Table 1. MIL-A-8625F Performance Requirements for Anodic Coatings on Aluminum Alloy Components ing process, the micropores of the anodized (oxide) layer are hydrated. Because the hydrated form of the oxide has a greater volume than the unhydrated form, this process fills the micropores and subsequently improves corrosion and staining resistance [Ref. 4]. The most commonly used sealing process, a sodium dichromate solution, has been used for many years and has proven to be both effective and robust. A standard dichromate sealer bath can last a year or longer with standard loading. However, this solution contains hexavalent chromium (Cr6+). Note: Cr6+ is a hazardous material that is listed on the Office of the Secretary of Defense (OSD) Emerging Contaminants Action List, the Environmental Protection Agency���s (EPA���s) list of toxic industrial chemicals to be voluntarily reduced or eliminated, and in Superfund Amendments and Reauthorization Act (SARA) Title 313 [Toxic Release Inventory (TRI)]. Additionally, the Department of Defense (DoD) issued a policy directive in April 2009 that restricts the use of Cr6+ compounds on military vehicles and weapon systems [Ref. 5]. Therefore, the DoD has an ongoing need to identify and validate a chromium-free sealer to reduce the use of Cr6+ in anodizing operations, thereby reducing associated environmental and compliance burdens. MIL-A-8625F is the military specification that establishes the requirements for anodizing and sealing processes used for military components [Ref. 6]. This specification delineates three alternative sealing processes that are approved for use: boiling deionized (DI) water, solutions containing cobalt acetate, and solutions containing nickel acetate. However, sufficient data to promote the use of these and other chromium-free sealing processes do not exist. To address these issues, the Air Force Research Laboratory (AFRL) Energy and Environment Team (RXSCP) tasked Concurrent Technologies Corporation (CTC) to identify and demonstrate/validate an alternative to sodium dichromate sealers used in anodizing operations at OO-ALC. The successful implementation of a chromium-free sealing technology will reduce environmental compliance bur373

Articles in this issue

view archives of Metal Finishing Guide Book - 2012-2013