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Thickness Layer (type of Nickel Coating) Bottom (s) Middle (hgih-sulfur b) — Top (b) — Greater than 0.15 Greater than 0.04 and less than 0.15 Table IV. Requirements for Double - or Triple-Layer Coatings sification 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 double-lay- er (d) nickel coating is 35 µm thick and has a top layer of microporous (mp) chromi- um that is 0.3 µm thick. (The thickness of the chromium is not included unless it differs from 0.3 µm.) 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 semi- bright electrodeposited nickel. The type of chromium is given by the following sym- bols: r, for regular or conventional chromium; mp, for microporous chromium; mc, for microcracked chromium. The standards provide additional information to assure the quality of elec- trodeposited decorative nickel-plus-chromium coatings. In essence, the avail- able standards, which summarize many years of corrosion experience, show that multilayer nickel coatings are significantly more corrosion resistant than single- layer 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. Service Conditon Number SC 4 (extended time, very severe) SC 4 (very severe) SC 3 (severe) Classification Numbera SC 2 (moderate) SC 1 (mild) 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 a See text for explanation of Classification Numbers. Table V. Nickel Plus Chromium Coatings on Steel 226 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) — Less than 40 10 Less than 40 Specific Sulfur Elongation, (5) Content, (m/m) (Percentage of Total Nickel Thickness) Double-Layer Triple-Layer