Metal Finishing Guide Book

2012-2013

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cleaning, pretreatment & surface preparation PREPARATION OF NONFERROUS METALS FOR PAINTING BY EARL GROSHART CONSULTANT, SUMNER, WASH. Metals to be painted should be cleaned to remove oily soils and loose materials. It may not be necessary to remove the natural oxides if they are solid, such as the blue scale on heated steel; however, for many active metals it may be advantageous to remove the natural oxides and replace them with artificially formed oxides by ���anodizing.��� For the preparation of aluminum, see both ���Paint Pretreatments for Aluminum��� and ���Anodizing as a Pretreatment for Aluminum,��� in this section of the Guidebook. The preparation of cadmium, chromium, cobalt, copper, lead, magnesium, molybdenum, nickel, silver, stainless steels, titanium, zinc, and alloys of zinc-nickel and tin-zinc are covered here. Each metal requires its own processing, but a few general steps are the same for all metals. GREASE REMOVAL ���Degreasing,��� i. e. , the removal of oils, greases, waxes, and corrosion-inhibiting compounds, which have in the past been removed in vapor phase degreasers with chlorinated solvents, should now be removed in an environmentally approved manner. These include closed vapor degreasers, solvent (vat) washing, parts washers with cleaning agents in water, and emulsion degreasers. There are a number of closedloop vapor degreasers that can safely be used with chlorinated solvents. These satisfy and offer the least disruption to production that in the past has used vapor degreasers. Solvent washing with a hydrocarbon solvent, usually a blend of several solvents and a combined low-vapor pressure (below 45 mm Hg at 20��C for the aerospace industry), has replaced some degreasers. If properly covered, these will meet environmental requirements. The parts washers and the emulsion cleaners can be selected so that they leave the work with a water-break-free surface and, thus, would not require further cleaning. The solvent cleaning methods will require alkaline cleaning to provide this water-break-free surface if other processing, such as conversion coating, is to be applied. If not further wet processed, the emulsion cleaning and washer cleaning will require an extra drying step prior to painting. HAND SOLVENT WIPING As painting is generally done after the cleaned parts have been exposed, i. e. , after assembly or storage, most parts will require a hand solvent wipe just prior to painting. This is especially true prior to painting with the new environmentally designed high-solids and water-reducible paints. Because they have little or no organic solvent, they cannot tolerate even small amounts of organic contamination on the surface of the substrate. To exacerbate the problem further, as the Clean Air Act of 1990 becomes more defined for various industries, fewer solvents are being allowed for cleaning and their vapor pressures are going down, making them more difficult to remove. Solvents and wipe cloths should be clean. A level of 100-ppm dissolved contamination in the solvent with clean, lint-free wipes should be maintained. Before the solvent dries, it should 142

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