Metal Finishing Guide Book

2013

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Layer (type of Nickel Coating) Specific Elongation, (5) Bottom (s) Middle (hgih-sulfur b) Top (b) — — Sulfur Content, (m/m) Thickness (Percentage of Total Nickel Thickness) Double-Layer Triple-Layer Greater than 0.15 Greater than 0.04 and less than 0.15 — Less than 40 10 Less than 40 Table IV. Requirements for Double - or Triple-Layer Coatings the availability of good technical standards (ASTM Standard B 456 and ISO Standard 1456) that provide the necessary guidance. Some of the requirements for double- or triple-layer nickel coatings are summarized in Table IV. These requirements specify the ductility (percent elongation) of the underlying semibright nickel layer, the sulfur content of each layer, and the thickness of each layer as a percentage of the total nickel thickness. For example, for double-layer nickel coatings on steel, the semibright nickel layer should be 60% of the total nickel thickness. This ratio is important for controlling the corrosion performance, the ductility and the cost of the double-layer coating (the semibright nickel process is generally less expensive than the bright nickel one). In addition to these general requirements, the standards give recommended thicknesses for nickel plus chromium coatings for various service conditions. The recommendations for coatings on steel from ASTM Standard B 456 are reproduced in Table V. The service condition number characterizes the severity of the corrosion environment: 5 being the most severe and 1 being the least severe. The classification numbers given in the second column of the table specify the coatings that are expected to meet the requirements of the condition of service. For example, the classification number: Fe/Ni35d Cr mp indicates that the coating is suitable for very severe service; it is applied to steel (Fe); the doublelayer (d) nickel coating is 35 μm thick and has a top layer of microporous (mp) chromium that is 0.3 μm thick. (The thickness of the chromium is not included Service Conditon Number SC 4 (extended time, very severe) SC 4 (very severe) SC 3 (severe) SC 2 (moderate) SC 1 (mild) a Classification Numbera Fe/Ni35d Cr mc Fe/Ni35d Cr mp Fe/Ni40d Cr r Fe/Ni30d Cr mc Fe/Ni40d Cr mp Fe/Ni30d Cr r Fe/Ni25d Cr mc Fe/Ni25d Cr mp Fe/Ni40p Cr r Fe/Ni30p Cr mc Fe/Ni30p Cr mp Fe/Ni40b Cr r Fe/Ni15b Cr mc Fe/Ni15b Cr mp Fe/Ni10b Cr r Nickel Thickness, Micrometers (mil) 35 (1.4) 35 (1.4) 40 (1.6) 30 (1.2) 30 (1.2) 30 (1.2) 25 (1.0) 25 (1.0) 40 (1.6) 30 (1.2) 30 (1.2) 20 (0.8) 15 (0.6) 15 (0.6) 10 (0.4) See text for explanation of Classification Numbers. Table V. Nickel Plus Chromium Coatings on Steel 297

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