Metal Finishing Guide Book


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it differs from 0.3 ��m.) The type of nickel is designated by the following symbols: b, for electrodeposited bright nickel (single-layer); d, for double or multilayer nickel coatings; p, for dull, satin, or semibright unpolished nickel deposits; s, for polished dull or semibright electrodeposited nickel. The type of chromium is given by the following symbols: r, for regular or conventional chromium; mp, for microporous chromium; mc, for microcracked chromium. The standards provide additional information to assure the quality of electrodeposited decorative nickel-plus-chromium coatings. In essence, the available standards, which summarize many years of corrosion experience, show that multilayer nickel coatings are significantly more corrosion resistant than singlelayer bright nickel coatings. Microdiscontinuous chromium coatings provide more protection than conventional chromium, and the corrosion protection afforded by the use of decorative electroplated nickel-plus-chromium coatings is directly proportional to the thickness of the nickel. ���Total quality improvement��� goals cannot be achieved without understanding and complying with the requirements contained in technically valid standards. ENGINEERING NICKEL PLATING Engineering or industrial applications for electrodeposited nickel exist because of the useful properties of the metal. Nickel coatings are used in these applications to modify or improve surface properties, such as corrosion resistance, hardness, wear, and magnetic characteristics. Although the appearance of the coatings is important and the plated surface should be defect-free, the lustrous, mirrorlike deposits described in the preceding section are not required. Engineering Plating Processes Typical compositions and operating conditions for electrolytes suitable for engineering applications have been included in Table III. In addition, electrolytes for industrial plating, including all-chloride, sulfate-chloride, hard nickel, fluoborate, and nickel-cobalt alloy plating have been discussed by Brown and Knapp.1 Mechanical Properties The mechanical properties are influenced by the chemical composition and the operation of the plating bath as indicated in Table III. The tensile strength of electrodeposited nickel can be varied from 410 to 1,170 MPa (60 to 170 psi) and the hardness from 150 to 470 DPN by varying the electrolyte and the operating conditions. The operating conditions significantly influence the mechanical properties of electrodeposited nickel. Figures 2, 3, and 4 show the influence of pH, current density, and temperature on the properties of nickel deposited from a Watts bath. Additional information on how the properties of electrodeposited nickel are controlled is available.2 The mechanical properties of electrodeposited nickel vary with the temperature to which the coatings are exposed as shown in Figure 5. The tensile strength, yield strength and ductility of electrodeposited nickel reaches low values above 480��C (900OF). Nickel deposits from sulfamate solutions are stronger at cryogenic temperatures than deposits from the Watts bath. Corrosion Resistance Engineering nickel coatings are frequently applied in the chemical, petroleum, and food and beverage industries to prevent corrosion, maintain product purity, and prevent contamination. As a general rule, oxidizing conditions favor corrosion of 340

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