Metal Finishing Guide Book


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CONTROLLING THE PROPERTIES OF NICKEL DEPOSITS Methods that measure thickness, adhesion, and corrosion resistance of nickel coatings are available as means of quality control. Properties such as porosity, ductility, tensile strength, internal stress, hardness, and wear resistance are important to control the quality of electroplated articles. Some of these properties may be measured by the following methods. Thickness Micrometer readings are often used to determine the thickness of a coating at a particular point when the deposit thickness exceeds 125 μm (0.005 in.). Other methods for determining the thickness of electrodeposited coatings can be found in ASTM standards. ASTM standard B 487 describes a method based on metallographic examination of cross-sections of the plated object. Alternate tests involve magnetic (ASTM B 530) and coulometric (ASTM B 504) measurements of thickness. The STEP Test The simultaneous thickness and electrochemical potential (STEP) test is similar to the coulometric method of determining thickness. By including a reference electrode in the circuit, however, it is possible to measure the electrochemical potential of the material being dissolved. The test was developed to control the quality of multilayer nickel coatings. For example, with doublelayer nickel coatings, a large change in potential occurs when the bright nickel layer has dissolved and the underlying semibright nickel begins to be attacked. The potential difference is related to the overall corrosion resistance of the multilayer coating. The test has been standardized (ASTM B 764) and is specified for automotive plating. Corrosion and Porosity Testing Examination of the coated part after immersion in hot water for 2 to 5 hr for rust is one technique used in studying the corrosion resistance of plated steel. The number of rust spots in a given area is then used as the qualification for accepting or rejecting the piece. Modifications of this test include immersion for up to 5 hr in distilled water, in distilled water saturated with carbon dioxide, or in distilled water containing 0.5% by weight of sodium chloride at test temperatures of 82 to 85ºC (180 to 185ºF). Several salt spray tests have been used to simulate marine environments. These tests are commonly used to evaluate nickel and nickel-plus-chromium coatings on ferrous and nonferrous substrates. The salt spray tests are also used as accelerated quality control tests and are described in the following standards: salt spray (ASTM B 117); acetic acid-salt spray (ASTM B 267); and copper-accelerated acetic acid-salt spray (CASS Test: ASTM B 368). The ferroxyl test is another porosity test that is employed for coatings on ferrous metal substrates and involves the formation of Prussian blue color within exposed pits. The solution utilizes sodium chloride and potassium ferricyanide as reagents to develop the color. The only truly satisfactory method of establishing the relative performance of various coating systems is by service testing. Therefore, care should be exercised in interpreting the results of accelerated corrosion tests. Once an acceptable service life has been determined for a specific thickness and type of coating, the 304

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