Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 277 of 707

REQUIREMENT Type II Coating Weight (milligram/squa re feet [mg/ft2]), unsealed Corrosion Resistance Minimal corrosion after 336 hours Paint Adhesion No intercoat separation between the paint system and the anodic coating or between the anodic coating and the base metal. Coating Thickness (mils [0.001 in]) Table 1. MIL-A-8625F Performance Requirements for Anodic Coatings on Aluminum Alloy Components rosion-resistant surfaces. A photo of the C-5 Galaxy aircraft, as well as the main landing gear from this aircraft, is presented in Figure 1 [Refs. 1, 2, 3]. A sealing operation usually completes the anodizing process. During the seal- 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 sodi- um 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 val- idate 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 276 0.07–1.0 0.02–0.7 0.5–4.5 Not specified Not applicable Type IIB ACCEPTANCE CRITERIA Type III 1,000 minimum 200–1,000 4,320 minimum for every 0.001 inch of coating

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