Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 228 of 707

"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 engi- neering applications have been included in Table III. In addition, electrolytes for industrial plating, including all-chloride, sulfate-chloride, hard nickel, flu- oborate, 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 elec- trodeposited 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 con- ditions. The operating conditions significantly influence the mechanical properties of elec- trodeposited 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 infor- mation 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° (900O C F). Nickel deposits from sulfamate solutions are stronger at cryogenic tem- peratures 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 nickel in chemical solutions, whereas reducing conditions retard corrosion. Nickel also has the ability to protect itself against certain forms of attack by developing a passive oxide film. When an oxide film forms and is locally destroyed as in some hot chloride solutions, nickel may form pits. In general, nickel is resistant to neutral and alkaline solutions, but not to most of the mineral acids. Corrosion resistance in engineering applications is controlled by optimizing nick- el thickness. The thickness of the nickel is dependent on the severity of the corro- sive environment. The more corrosive the service conditions the greater the thick- ness of nickel required. Thickness generally exceeds 0.003 in. (75 µm) in engineering applications. Nickel Plating and Fatigue Life Thick nickel deposits applied to steel may cause significant reductions in the com- posite fatigue strength in cyclical stress loading. The reduction in fatigue strength is influenced by the hardness and strength of the steel and the thickness and inter- 227

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