Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 342 of 707

have a finish which is evenly colored and distributed on the work surface, and in the case of brush plating will have good adhesion and cohesion. The selective plating process referred to as brush plating has preparatory procedures which have been developed for all of the base materials commonly encountered in industry These include steel, cast iron, stainless steel, alu- minum, copper-base alloys and nickel-base alloys, and titanium. When recom- mended procedures are followed, the strength of the bond between the brush plat- ed deposit and the base material is equivalent to the weaker of the cohesive strength of the deposit or the base material itself. Typically, the bond strength of a brush plated deposit is > 11,000 psi. Preparation of a base material usually begins with mechanical and/or chem- ical precleaning. This is followed by electrocleaning and then etching. Depending on the base material, a desmutting, activating and/or preplating step may be required. For instance, the procedure for brush plating a copper deposit on to 400 series stainless steel requires all of the steps previously mentioned . Brush plating solutions are quite different from tank plating solutions. Brush plating solutions have a higher metal content, are less likely to utilize a toxic mate- rial such as cyanide, are more likely to use metal-organic salts rather than met- al-inorganic salts and are more likely to be complexed and/or buffered with special chemicals than are tank plating solutions. Solutions used for brush plating must produce a good quality deposit over a wider range of current densities and temperatures than tank plating solutions. They must plate rapidly, operate with insoluble anodes, and produce a good deposit under variable conditions for a prolonged period of time. In addition, the solutions should be as nontoxic as possible, and they should not require chem- ical control by the operator. Formulations that are different from those used in tank plating obviously are required to achieve these objectives. The third group of solutions have been developed to meet the specific appli- cation requirements of portable processes such as selective anodizing, specialized black optical coatings, and electropolishing. SELECTIVE ANODIZING: Anodizing is a widely used electrochemical surface treatment process for alu- minum and its alloys. Depending on the particular type of anodizing process used, the resulting anodic coatings provide improved wear resistance, corro- sion protection, and/or improved adhesive properties for subsequent painting or adhesive repair. Selective anodizing is used when lim- ited, selective areas of large or complex aluminum assemblies need anodizing to either restore a previously anodized surface or to meet a specification requirement. The SIFCO Process of selective anodizing is a versatile tool which can be used for many different, demanding OEM and repair applica- tions. This portable process can be used both in the shop and in the field. Anodizing is the formation of an oxide film or coating on an aluminum 341

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