Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Another method is based on the flexure of a thin metal disc. See ASTM B 636 for the method of measuring internal stress with the spiral contractometer. Most of the tests that have been used for evaluating the ductility of plated coat- ings are qualitative in nature. Two bend tests are described in ASTM B 489 and B 490. Both of these procedures require a minimum amount of equipment. Another method for measuring the ductility of thick deposits is to determine the elonga- tion of a specimen in a tensile testing machine. This method is limited to relatively thick foils of controlled geometry and thickness. A method specifically designed for plated thin foils has been used and is known as the hydraulic bulge test. A mechanical bulge test is also available. Ductility In general, the adhesion between a nickel coating and the basis material should exceed the tensile strength of the weaker material. As a result, when a force is applied to a test specimen, which tends to pull the coating away from the basis met- al, separation occurs within the weaker material rather than at the boundary between the basis metal and the nickel coating. A number of qualitative tests have been used that utilize various forces applied in a multitude of directions to the com- posite basis metal and coating, such as hammering, filing, grinding, and deform- ing. Quantitative tests have also been described in the literature. Achieving good adhesion requires a sound bond between the substrate and the coating. A sound metallurgical bond may be achieved on most materials by proper surface prepa- ration prior to plating. The selection of grinding, polishing, pickling and condi- tioning treatments for a variety of basis metals varies from one material to anoth- er, and depends on the initial surface condition of the metal. The activating treatments that follow polishing and cleaning operations are listed in Table VIII for the most commonly plated basis metals. ASTM standards provide addition- al information. Nonconductive plastics and other materials can be plated by metallizing the material, using etching and catalyzing techniques (ASTM B 727). Adhesion NICKEL ANODE MATERIALS Important developments in nickel anode materials and their utilization have taken place. Of utmost significance was the introduction of titanium anode bas- kets in the 1960s. Today the use of expanded or perforated titanium anode bas- kets filled with nickel of a selected size has become the preferred method of nick- el plating. Titanium anode baskets are preferred because they offer the plater a number of advantages. Primary forms of nickel can be used that provide the least costly nickel ion source. Anode replenishment is simple and can be auto- mated. The constant anode area achieved by keeping baskets filled improves current distribution and conserves nickel. Several forms of primary nickel are currently being used in baskets. These include electrolytic nickel squares or rectangles and button-shaped material that contains a small, controlled amount of sulfur. Nickel pellets produced by a gas-refining process and similar pellets containing a controlled amount of sul- fur are being utilized. Prior to the introduction of titanium anode baskets, wrought and cast nickel anode materials were the norm. They are still used, but not to the extent they were before 1960. The wrought and cast anode materials comprise rolled bars containing approximately 0.15% oxygen; rolled nickel containing approximately 0.20% carbon 235

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