Machinery's Handbook, 31st Edition
1638 Etching and ETCHING FLUIDS sixty-fourths, it is very important to use a thin resist that will cling to the metal and prevent any under-cutting of the acid: the resist should also enable fine lines to be drawn without tearing or crumbling as the tool passes through it. One resist that has been extensively used is composed of about 50 percent of asphaltum, 25 percent of beeswax, and, in addition, a small percentage of Burgundy pitch, black pitch, and turpentine. A thin covering of this resisting material is applied to the clean polished surface to be graduated and, after it is dry, the work is ready for the graduating machine. For some classes of work, paraffin is used for protecting the surface surrounding the graduation lines which are to be etched. The method of application consists in melting the paraffin and raising its temperature high enough so that it will flow freely; then the work is held at a slight angle and the paraffin is poured on its upper surface. The melted paraffin forms a thin protective coating. Conversion Coatings and the Coloring of Metals Conversion Coatings.— Conversion coatings are thin, adherent chemical compounds that are produced on metallic surfaces by chemical or electrochemical treatment. These coatings are insoluble, passive, and protective, and are divided into two basic systems: oxides or mixtures of oxides with other compounds, usually chromates or phosphates. Conversion coatings are used for corrosion protection, as an adherent paint base; and for decorative purposes because of their inherent color and because they can absorb dyes and colored sealants. Conversion coatings are produced in three or four steps. Pretreatment often in- volves mechanical surface preparation followed by decreasing and/or chemical or electrochemical cleaning or etching. Then thermal, chemical, or electrochemical surface conversion processes take place in acid or alkaline solutions applied by immersion spraying, or brushing. A post treatment follows, which includes rinsing and drying, and may also include sealing or dyeing. If coloring is the main purpose of the coating, then oiling, waxing, or lacquering may be required. Passivation of Copper.— The blue-green patina that forms on copper alloys during atmo spheric exposure is a passivated film; i.e., it prevents corrosion. This patina may be pro duced artificially or its growth may be accelerated by a solution of ammonium sulfate, 6 pounds (2.7 kg); copper sulfate, 3 ounces (85 g); ammonia (technical grade, 0.90 specific gravity), 1.34 fluid ounces (39.6 cc); and water, 6.5 gallons (24.6 liters). This solution is applied as a fine spray to a chemically cleaned surface and is allowed to dry between each of five or six applications. In about 6 hours a patina somewhat bluer than natural begins to develop and continues after exposure to weathering. Small copper parts can be coated with a passivated film by immersion in or brushing with a solution consisting of the following weight proportions: copper, 30; nitric acid, concen trated, 60; acetic acid (6%), 600; ammonium chloride, 11; and ammonium hydroxide (technical grade, 0.90 specific gravity), 20. To prepare the solution, the copper is dissolved in the nitric acid before the remaining chemicals are added, and the solution is allowed to stand for several days before use. A coating of linseed oil is applied to the treated parts. Coloring of Copper Alloys.— Metals are colored to enhance their appearance, to produce an undercoat for an organic finish, or to reduce light reflection. Copper alloys can be treated to produce a variety of colors, with the final color depending on the base metal composition, the coloring solution’s composition, the immersion time, and the operator’s skill. Cleaning is an important part of the pretreatment; nitric and sulfuric acid solutions are used to remove oxides and to activate the surface. The following solutions are used to color alloys that contain 85 percent or more of copper. A dark red color is produced by immersing the parts in molten potassium nitrate, at 1200– 1300 ° F (649–704 ° C), for up to 20 seconds, followed by a hot water quench. The parts must then be lacquered. A steel black color can be obtained by immersing the parts in a 180 ° F (82 ° C) solution of arsenious oxide (white arsenic), 4 ounces (113 g); hydrochloric acid
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