Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 386 of 707

the cleaning operation to do a thorough job on the metal surface and to perform all the recommended maintenance to the tanks. This practice represents an inexpensive insurance policy against poor quality finishes in subsequent steps. As part of the cleaning operation, many parts benefit greatly from an acid cleaning to remove light oxides and to lower the pH of the surface. Here, several dif- ferent materials can be used, including sulfuric, hydrochloric, or fluoboric acids or sulfuric acid salts, depending on the base metal and the desired activity of the acid. TESTING FOR CLEANLINESS The final evaluation of the effectiveness of a cleaning process should come from a performance test. The simplest and most widely used is the water-break test. It consists of processing the article or a standard test panel through the cleaning sequence in the normal manner, then dipping the part into clean water and observing how the water runs off the surface. A part that still carries residual oils will cause the water to bead up on the surface and form water breaks; whereas a part that is uniformly free of oil will allow the water to drain off uniformly with no water breaks. An oil-free surface will stay uniformly wet and the water will "sheet" off the surface rather than bead up. Another method (useful on steel parts only) involves the use of an acid copper autoplating solution. Here, the cleaned surface is immersed in a dilute acid copper solution. A uniformly oil-free surface will allow metallic copper to be autoplated onto the surface in a uniform manner with no skips or bare spots. Any uncoated areas would indicate the presence of residual oils on the surface. Once the part has been properly and completely cleaned of all foreign materials, it is ready to proceed to the next step in the antiquing process. ELECTROPLATING As mentioned earlier many parts do not require electroplating. Obviously any sol- id brass, bronze, or copper substrate would not necessarily be brass or copper plated as well. Once the surface is clean it would be ready for coloring in the appropriate solution. Other parts, however, such as steel or zinc diecast surfaces, do require an electroplated layer on the surface prior to being colored. Here, conven- tional plating techniques are used. The best quality plated finishes usually begin with a copper strike, followed by a generous brass or bronze deposit of approximately 0.0002 to 0.0003 in. thickness. The copper strike is an excellent way to seal off any porosity present in the base metal and make the surface more receptive to an adherent brass deposit of low porosity. Most commercial brass plating baths contain cyanide. Noncyanide baths have enjoyed limited utilization because they often lack solution stability and produce deposits, which are darker in color and rougher than those of conventional cyanide baths. In addition, because they contain organic chelating agents, they can be more difficult to work with in waste treating the rinsewaters. A conventional cyanide bath forces the finisher to treat and decompose the cyanide residues in the rinsewaters, but the zinc and copper are often more easily precipitated. In this area the suppliers of the chemicals normally offer technical assistance in the correct oper- ation and maintenance of the brass plating tanks. It is important to perform rou- tine maintenance in order to keep these baths operating efficiently. COLORING OF THE SURFACE Once the surface has been plated with brass, it is ready to be colored by using one of several types of antiquing solutions available. 385

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