Comparing Electroless Nickel and Electrolytic Nickel Deposits Part 1
All nickel coatings are not created equal. The appearance, deposit uniformity, and metallurgical properties vary greatly. Therefore any intermediate processing operations, the atmosphere the coating will be exposed to, as well as the end use of the product must be considered when selecting a coating. This is part of a series that will give an overview of various electroless nickel phosphorus and electrolytic nickel coatings.
DEFINTION: Electroless nickel coatings are deposited by a chemical reaction occurring on the surface of the part without the use of electrical current. Electrolytic nickel coatings, which include Watts, Sulfamate, and Woods Nickel, require electrical current to coat a substrate.
PURITY: Electroless nickel deposits an alloyed coating. Heatbath’s “EN” coatings are actually a nickel phosphorus alloy containing between 1 – 14% phosphorus in the deposit. The amount of phosphorus co-deposited is referenced by the type of electroless nickel. For example, a low phos electroless nickel bath will deposit a coating with 96 – 99% nickel and 1 – 4% phosphorus. A mid phos electroless nickel bath will deposit a coating of 91 – 94% nickel and 6 – 9% phosphorus. The high phos electroless nickel bath will deposit 11 – 14% phosphorus. For comparison, electrolytic nickel coatings such as Watts, Woods, or Sulfamate Nickel are over 99% nickel. Other alloying elements may be present in low levels in both electrolytic and electroless nickel deposits.
DENSITY: The density of the deposit will be affected by the alloy. High phos electroless nickel will have the lowest density, in the range of 7.6 – 7.9 g/cm3. As the level of phosphorus decreases, the density of the deposit will increase. Mid phos electroless nickel will typically have a density of 8.0 – 8.2 g/cm3 and low phos electroless nickel will be in the 8.4 – 8.8 g/cm3 range. This can be compared to pure nickel deposits with a density of 8.9 g/cm3, which can include electrolytic nickel coatings.
PLATE DISTRIBUTION: Electroless nickel is far superior to electrolytic nickel for uniform plate distribution. This is an important feature for very complex configurations or tubular parts where the recessed areas and inner dimensions are critical surfaces. The uniform plate distribution is due to the autocatalytic reaction from the electroless nickel solution that will deposit a coating wherever the solution contacts the substrate. However, the benefit of uniform plate distribution does not produce a leveled deposit that bright electrolytic nickels can, so any surface imperfections such as scratches or pits in the substrate may still be visible after electroless nickel plating. Electrolytic nickel requires special, and often expensive, fixtures with auxiliary anodes to deposit a uniform coating in deep recesses and inner dimensions, and shields to prevent over-plating at the edges and protruding surfaces.
APPEARANCE: Electroless nickel deposits are considered functional and rarely used for decorative purposes. The appearance can range from a satin to bright finish, as dictated by the formulation and maintenance of the bath. Along with the inability to level, the consistency of the appearance may vary over the life of the bath in electroless nickel deposits. This is particularly evident with bright deposits that deteriorate to semi-bright or even dull deposits as the bath ages. For applications with stringent appearance criteria, electrolytic nickel may be preferred.
New applications for nickel coatings continue to expand. Selecting the best coating for each application is critical in the design phase of the project. This is just an introduction to the many differences between types of nickel coatings available.
More information on Heatbath Nitec® products for Electroless Nickel coatings.
More information on Heatbath Lustra-Ni® products for Electrolytic Nickel coatings.
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