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Re: A discussion on Electrolytic and Electroless Nickels

A discussion on Electrolytic and Electroless Nickels

Nickel is a silver-white metal generally used to enhance the value, utility, and lifespan of industrial equipment and components by protecting them from corrosion. Nickel is also commonly used in the chemical and food processing industries to prevent iron contamination. Because the properties of nickel can be controlled and varied over broad ranges, there are numerous applications for nickel-plating, including:
Corrosion prevention, coating of critical tolerance components, magnetic properties, appearance, food processing surfaces and the controlling of lubricity, hardness, reflectivity, emissivity, and absorptivity etc.

Electrolytic Nickel

Electrolytic nickel is electro-plated, and can be deposited soft or hard, dull or bright. Hardness can range from 150-500 Vickers. Appearance can range from a dull gray to an almost white, bright finish.
Sulfamate nickel is deposited from a nickel Sulfamate bath, and is the most widely used electrolytic nickel. It is often used as a final plating layer and also as an underplate for precious metals. Sulfamate nickel is a pure deposit that allows soldering and brazing during later assembly steps. It is normally a dull gray to dull silver color depending upon the machined finish of the plated part.
Watts nickel is deposited from a nickel sulfate bath. Watts nickel normally yields a brighter finish than does Sulfamate nickel since even the dull Watts bath contains a grain refiner to improve the deposit. Watts nickel may also be deposited as a semi-bright finish. Semi-bright Watts nickel achieves a brighter deposit because the bath contains organic and/or metallic brighteners. The brighteners in a Watts bath level the deposit, yielding a smoother surface than the underlying part. The semi-bright Watts deposit is easily polished to a high luster; however, the brighteners may lead to problems with soldering and brazing, and this should be considered when selecting a finish for your application.
A bright nickel bath contains a higher concentration of organic brighteners that have a leveling effect on the deposit. Sulfur-based brighteners are normally used to achieve leveling in the early deposits and a sulfur-free organic, such as formaldehyde, is used to achieve a fully bright deposit as the plating layer thickens. Organic brighteners yield a lustrous deposit; however, the possible negative effects on later assembly operations such as brazing and soldering should be considered.
Black nickel is a decorative coating that is often applied over an under plating of electrolytic or electroless nickel. Black nickel offers little in the way of additional protection, and is easily scratched or stained. A thin layer of black nickel gives an attractive cosmetic finish. Thicker layers of black nickel attain a matte black finish that is often used to achieve high emissivity in aerospace applications.

Electroless Nickel

Electroless nickel plating is an autocatalytic process and does not use externally applied electrical current to produce the deposit. The electroless process deposits a uniform coating of metal, regardless of the shape of the part or its surface irregularities; therefore, it overcomes one of the major drawbacks of electroplating – the variation in plating thickness that results from the variation in current density caused by the geometry of the plated part and its relationship to the plating anode. An electroless plating solution produces a deposit wherever it contacts a properly prepared surface, without the need for conforming anodes and complicated fixturing. Since the chemical bath maintains a uniform deposition rate, the plater can precisely control deposit thickness simply by controlling immersion time.
Low-phosphorus electroless nickel yields the brightest and hardest deposits. Hardness ranges from 60-70 RC. Low-phosphorus content improves soldering and brazing performance. The most common specifications pertaining to electroless nickel plating are ASTM 733 B, and MIL-C-26074.
Medium-phosphorus or mid-phos is the most widely used electroless nickel bath. If the phosphorus content is not specified, the finisher will likely choose mid-phos. The hardness is typically 40-42 RC. Hardness may be improved by heat-treating into the 60-62 RC range.  Porosity is lower, and conversely corrosion resistance is higher than low-phos electroless nickel.
High-phosphorous electroless nickel is dense and dull in comparison to the mid and low-phosphorus deposits. High-phosphorus exhibits the best corrosion resistance of the electroless nickel family; however, the deposit is not as hard as the lower phosphorus content. High-phosphorus is a virtually non-magnetic coating.
Two additional types of electroless nickels are often used for special applications. Nickel boron is often used as an underplate for metals that require firing for adhesion. Nickel PTFE composite or nickel-Teflon® is used for anti static and lubricity, particularly in UHV environments.

David French & Associates, Inc.
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