Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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thin layer of bright, high-sulfur nickel is deposited between the initial layer of semi- bright nickel and the top layer of bright nickel. The very thin layer should comprise about 10% of the total nickel coating thickness and must contain greater than 0.15% sulfur (as compared with 0.06% to 0.10% normally found in fully bright deposits). Multilayer nickel coatings provide improved protection because the active, sulfur-bearing bright nickel layer protects the underlying sulfur-free layer by sac- rificial action. For optimum corrosion performance, it is critical that the semi- bright nickel layer contain no codeposited sulfur. Electrodeposited chromium is applied on top of the decorative multilayer nick- el coatings to prevent tarnishing of the nickel when exposed to the atmosphere. The chromium coating is relatively thin compared with the nickel, because elec- trodeposited chromium is not intrinsically bright and will become dull if thick- ness is increased beyond an acceptable level. Studies of the corrosion perfor- mance of multilayer nickel plus conventional chromium coatings revealed a tendency to form one or two relatively large corrosion pits that would rapidly pen- etrate to the basis metal. This was believed to be due to the relatively low poros- ity of the top layer of chromium. It was concluded by many investigators that a pore-free chromium electrodeposit should improve corrosion resistance. The pore-free chromium plating processes developed in the early 1960s were short-lived when it was observed that the chromium layer did not remain pore-free in use. Other investigators concluded that chromium deposits with high porosity or crack densities on a microscopic scale would be preferable. This led to the devel- opment of microdiscontinuous chromium deposits of two types: microporous and microcracked. These deposits greatly improve corrosion performance by distrib- uting the available corrosion current over a myriad number of tiny cells on the sur- face of the coating. Corrosion proceeds uniformly over the entire surface instead of concentrating at one or two pits and, as a result, the rate of pit penetration is slowed dramatically. Double-layer nickel coatings 40 ┬Ám thick (1.5 mils) electro- plated with either microporous or microcracked chromium and applied uni- formly resisted corrosion in severe service for more than 16 years. Microdiscontinuous Chromium Specifying Decorative Nickel Coatings The specification of decorative nickel coatings is often misunderstood, despite 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 semi- bright nickel layer, the sulfur content of each layer, and the thickness of each lay- er as a percentage of the total nickel thickness. For example, for double-layer nick- el 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 corro- sion environment: 5 being the most severe and 1 being the least severe. The clas- 225

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