Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 385 of 707

light fixture components, and many other decorative articles. As with steel the zinc tends to corrode quite rapidly and must be electroplated with copper or brass for corrosion resistance and for antique finishing. Unlike steel, zinc diecastings can often have a porous surface, requiring the use of a copper strike in order to seal off this porosity prior to subsequent antiquing. BEGINNING THE ANTIQUING PROCESS: CLEANING Before chemical antiquing can begin on any substrate the surface must be free of oil, oxides, buffing compounds, mold-release compounds, soldering flux, finger- prints, or other foreign materials left over from the fabrication of the article. Once these materials are removed the surface is in a chemically active state and is ready for coloring, electroplating, or other operations. There are many cleaning options that could be considered. In selecting a metal-cleaning process, many factors must be taken into account including the identification of the substrate and the impor- tance of the condition of the surface or structure to the ultimate use of the part; the identification of the soil to be removed; the environmental impact of the cleaning method; the cost of the operation; and the nature of the subsequent chemical operations to follow the cleaning step. Because of the variety of cleaning options available, each option deserves careful consideration. In general one may rank the different cleaning options in the order of increas- ing degree of cleanliness as follows: abrasive blasting, cold solvent cleaning, vapor degreasing, emulsion soak cleaning, alkaline electrocleaning, alkaline soak cleaning followed by acid cleaning and finally ultrasonic cleaning. Each of these methods has its own advantages and disadvantages and is suited to particular types of soils. There is no universal cleaning method that works well on all types of soils. For example solid brass or copper items, which are soldered together, will have light oils and soldering flux on the surface, along with light tarnishing. These soils respond well to mild alkaline soak cleaners and may require mechanical agi- tation or scrubbing to remove all the flux. Cast bronze or brass items generally carry heavier oxides from the casting operation, but very little oil. Parts that can tolerate the surface roughening can be bead blasted with good success. Other cast parts, which ultimately require a bright, shiny finish, will be coated with buffing or polishing compounds that can be difficult to remove. In this case electro- cleaning or ultrasonic cleaning works well. These methods provide a combination of alkaline emulsification of oils along with a mechanical action of the ultrasonic energy or current flow to help to mechanically lift these soils from the surface. Stamped or spun steel parts usually have a layer of oil-based stamping lubricants on the surface. Because the steel can tolerate exposure to strongly caustic cleaners the preferred method is often a hot, caustic soak cleaner or electrocleaners, often followed by a milder alkaline cleaner to ensure free rinsing of the cleaning solutions. On the other hand, zinc diecastings are usually produced using a waxy mold release compound, which can be difficult to remove. In addition the zinc is a reac- tive metal that cannot tolerate a strongly caustic cleaner without being etched. Consequently, the best method here is a mold alkaline soak cleaner, electroclean- er, or, perhaps, ultrasonic cleaning at a moderate pH, which will not attack the zinc. Vapor degreasing can also be used with good success on machining or stamping oils or buffing compounds. In general it is safe to say that cleaning is the most important part of the entire fin- ishing process and is a prerequisite to uniform and adherent electroplating, antique finishing, and lacquer topcoats. Not only is cleaning the most important—it is also one of the least costly operations of the process line. Consequently, it pays to design 384

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