Metal Finishing Guide Book

2013

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The deposits are smooth and have a columnar structure unlike the banded structure characteristic of fully bright deposits. The solution was developed to facilitate polishing and buffing; semibright nickel deposits are easily polished to a mirror finish. Efforts to eliminate polishing led to the combination of semibright and bright nickel deposits. Experience has shown that a multilayer nickel coating has greater resistance to corrosion than a single-layer coating of equivalent thickness. Single Layer and Multilayer Nickel Coatings Single and multilayer nickel coatings are used to produce decorative coatings that resist corrosion. Single-layer bright nickel deposits are specified for mildly corrosive service. Double-layer coatings are specified for use in severe and very severe service. In double-layer coatings, the first nickel layer is deposited from a semibright bath. The second layer is then deposited from a bright bath. Triplelayer coatings may also be specified for severe and very severe service. In this case, a special thin layer of bright, high-sulfur nickel is deposited between the initial layer of semibright 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 sacrificial action. For optimum corrosion performance, it is critical that the semibright nickel layer contain no codeposited sulfur. Microdiscontinuous Chromium Electrodeposited chromium is applied on top of the decorative multilayer nickel coatings to prevent tarnishing of the nickel when exposed to the atmosphere. The chromium coating is relatively thin compared with the nickel, because electrodeposited chromium is not intrinsically bright and will become dull if thickness is increased beyond an acceptable level. Studies of the corrosion performance of multilayer nickel plus conventional chromium coatings revealed a tendency to form one or two relatively large corrosion pits that would rapidly penetrate to the basis metal. This was believed to be due to the relatively low porosity 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 development of microdiscontinuous chromium deposits of two types: microporous and microcracked. These deposits greatly improve corrosion performance by distributing the available corrosion current over a myriad number of tiny cells on the surface 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 μm thick (1.5 mils) electroplated with either microporous or microcracked chromium and applied uniformly resisted corrosion in severe service for more than 16 years. Specifying Decorative Nickel Coatings The specification of decorative nickel coatings is often misunderstood, despite 296

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