Machinery's Handbook, 31st Edition
CONVERSION COATINGS AND COLORING 1639 (1.16 specific gravity), 8 fluid ounces (23.6 cc); and water, 1 gallon (3.9 liters). The parts are immersed until a uniform color is obtained; they are scratch brushed while wet, and then dried and lacquered. A light brown color is obtained using a room-temperature solution of barium sufate, 0.5 ounce (14g); ammonium carbonate, 0.25 ounce (7g); and water, 1 gallon (3.8 liters). The following solutions are used to color alloys that contain less than 85 percent copper. To color brass black, parts are placed in an oblique tumbling barrel made of stainless steel and covered with 3 to 5 gallons (11.2 to 19.8 liters) of water. Three ounces (85 g) of copper sulfate and 6 ounces (170 g) of sodium thiosulfate are dissolved in warm water and added to the barrel’s contents. After tumbling for 15 to 30 minutes to obtain the finish, the solution is drained from the barrel, and the parts are washed thoroughly in clean water, dried in sawdust or air-blasted and, if necessary, lacquered. To produce a blue-black color, the parts are immersed in a 130–175 ° F (54–79 ° C) solution of copper carbonate 1 pound (450 g); ammonium hydroxide (0.89 specific gravity), 1 quart (940 cc); and water, 3 quarts. Excess copper carbonate should be present. The proper color is obtained in 1 minute. To color brass a hardware green, immerse the parts in a 160 ° F (71 ° C) solution of ferric nitrate, 1 ounce (28 g); sodium thiosulfate, 6 ounces; and water, 1 gallon. To color brass a light brown, immerse the parts in a 195–212 ° F (91–100 ° C) solution of potassium chlorate, 5.5 ounces; nickel sulfate, 2.75 ounces (170 g); copper sulfate, 24 ounces (680 g); and water, 1 gallon (3.8 liters). Post treatment: The treated parts should be scratch brushed to remove any excess or loose deposits. A contrast of colors may be obtained by brushing with a slurry of fine pumice, hand nabbing with an abrasive paste, mass finishing, or buffing to remove the color from the highlights. In order to prolong the life of parts used for outdoor decorative purposes, a clear lacquer should be applied. Parts intended for indoor purposes are often used without additional protection. Coloring of Iron and Steel.— Thin black oxide coatings are applied to steel by immersing the parts to be coated in a boiling solution of sodium hydroxide and mixtures of nitrates and nitrites. These coatings serve as paint bases and, in some cases, as final finishes. When the coatings are impregnated with oil or wax, they furnish fairly good corrosion resis- tance. These finishes are relatively inexpensive compared to other coatings. Phosphate Coatings are applied to iron and steel parts by reacting them with a dilute solution of phosphoric acid and other chemicals. The surface of the metal is converted into an integral, mildly protective layer of insoluble crystalline phosphate. Small items are coated in tumbling barrels; large items are spray coated on conveyors. The three types of phosphate coatings in general use are zinc, iron, and manganese. Zinc phosphate coatings vary from light to dark gray. The color depends on the carbon content and pretreatment of the steel’s surface, as well as the composition of the solution. Zinc phosphate coatings are generally used as a base for paint or oil, as an aid in cold- working, for increased wear resistance, or for rustproofing. Iron phosphate coatings were the first type to be used; they produce dark gray coatings and their chief application is as a paint base. Manganese phosphate coatings are usually dark gray; however, since they are used almost exclusively as an oil base, for break in and to prevent galling, they become black in appearance. In general, stainless steels and certain alloy steels cannot be phosphated. Most cast irons and alloy steels accept coating with various degrees of difficulty depending on alloy content. Anodizing Aluminum Alloys.— In the anodizing process, the aluminum object to be treated is immersed as the anode in an acid electrolyte, and a direct current is applied. Oxi dation of the surface occurs, producing a greatly thickened, hard, porous film of alumi - num oxide. The object is then immersed in boiling water to seal the porosity and render the film impermeable. Before sealing, the film can be colored by impregnation with dyes or pigments. Special electrolytes may also be used to produce colored anodic films directly
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