Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 234 of 707

The STEP Test The simultaneous thickness and electrochemical potential (STEP) test is similar to the coulometric method of determining thickness. By including a reference elec- trode in the circuit, however, it is possible to measure the electrochemical poten- tial of the material being dissolved. The test was developed to control the quali- ty of multilayer nickel coatings. For example, with double-layer 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 relat- ed 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 85° to 185° F). 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 accelerat- ed 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 fer- rous 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 ser- vice life has been determined for a specific thickness and type of coating, the per- formance of other candidate coatings may be compared against it. Hardness measurements involve making an indentation on the surface (or cross section for thin coatings) of the deposit. The indenter has a specified geom- etry and is applied with a specified load. In the case of industrial nickel coatings, the most common hardness determination is the Vickers method of forcing a dia- mond point into the surface under a predetermined load (normally 100 g). This provides a measure of that surface to permanent deformation under load. The fig- ure obtained is not necessarily related to the frictional properties of the material nor to its resistance to wear or abrasion. The measurement of microhardness of plated coatings is discussed in ASTM B 578. Hardness The magnitude of internal stress obtained in deposits is determined by plating onto one side of a thin strip of basis metal and measuring the force causing the strip to bend. One method used in commercial practice involves plating the exterior sur- face of a helically wound strip and measuring the resultant change of curvature. Internal Stress 233 C (180

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