Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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surface using reverse current (part is positive) and a suitable electrolyte. Principal types of anodized coatings are chromic, sulfuric, hard coat, phosphoric and boric-sulfuric. The SIFCO Process of selective electroplating has been expanded to provide a portable method of selectively applying these anodized coatings for a variety of localized-area applications. The five types of anodizing film differ markedly in the electrolytes used, the typical thickness of the coating formed, and in the purpose of the coating. Also, the five types of anodized coatings are formed under distinctively different operating conditions. The electrolytes used for selective anodizing are available in water based solutions, or may be in the form of anodizing gels. Solutions are available for all five types of anodizing and gels are available for chromic acid, phosphoric acid and boric-sul- furic acid anodizing. The operating conditions for the gels are the same as for their respective solutions, and they apply coatings of the same quality. The gel is used when anodizing near critical components that may be damaged by splashed or running anodizing solutions. The gel stays over the work area and does not stray into inappropriate places such as aircraft instrumentation, equip- ment and crevices where corrosion would start. The gels produce coatings com- parable to solution electrolytes and have the advantage of staying on the select- ed work surface. The gels are ideally suited for work in confined areas where it would be difficult to clean up. In military and commercial applications, anodized coatings are usually applied for dimensional reasons (salvage), corrosion protection and/or wear resistance pur- poses. Selective anodizing meets the performance requirements of MIL-A-8625 for type I, II and III anodized coatings. In the consumer marketplace, anodizing is often utilized for cosmetic appearance reasons. SELECTIVE PLATING TOOLS: Tools used in Selective Plating processes are known as plating tools, stylus or styli. They are used to prepare, as well as brush plate, anodize and electropolish work surfaces. The tools consist of the following elements: a handle with electrical input connectors, an anode, an anode cover, and in some cases, a means of solution flow. Additionally, the tool must have a high current carrying capability and must not contaminate the solution. Only insoluble anodes are used in selective plating. The reason for this is simple. Products of the anodic reac- tion would build up on a soluble anode when subjected to the high current densities necessary for selec- tive plating applications. The reac- tion products would be contained by the anode cover resulting in a decrease in current to unacceptable levels. For this reason, soluble anodes are not used. Graphite and platinum are excel- lent materials for selective plating anodes. The purer grades of graphite are economical, thermally and electrically conductive, noncontaminating, easi- ly machined and resistant to electrochemical attack. Platinum anodes, although more expensive, are used in some cases. These 342

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