Bright Nickel Chrome Plating

Nickel is plated for many reasons. The appearance can be altered by modifying the nickel deposit itself or the thin chrome coating over it to produce bright, satin, or dark finishes. First and foremost, we tend to think of nickel as a bright finish. Nickel provides a decorative appearance due to its ability to cover imperfections in the basis metal (leveling). Nickel deposits can be made brilliant and when covered by a thin layer of decorative chromium, will maintain its brilliance even under severe conditions.

Satin Nickel

Satin nickel under bright or dark chromium offers a wide range of decorative appearances. By adding multi-layers of nickel, excellent corrosion protection can be achieved. This requires plating two or more different kinds of nickel (semi-bright and bright nickel for duplex). Nickel deposits can be modified to enhance wearability, reduce luster, or reduce stress by modifying the way the nickel is deposited or the additives used in the plating process. Depending upon the application, many of these requirements are specified simultaneously. Nickel is often plated for more than just one reason.

Bright or satin nickel plating can be used by itself or with other nickel deposits and chrome. It is used extensively in automotive applications like plated wheels, bright trim, truck exhausts, bumpers etc.  Other common applications include plumbing fixtures, furniture, tubular products, motorcycles, bicycles, tools and hardware. Most of these applications for nickel/chromium rely on these deposits to achieve the part’s decorative appearance with corrosion protection and wearability.

Watts Baths

The most commonly used nickel baths are Watts baths. These baths use a combination of nickel sulfate and nickel chloride, along with boric acid. This combination of nickel salts allows for a variety of characteristics of the deposited nickel. The nickel sulfate provides the majority of the nickel ions in the bath. The nickel chloride is needed for conductivity and good corrosion of the nickel metal anodes. Low chloride will created anode polarization. Modification of the concentrations of these components will change the physical properties of the deposit and the baths throwing power.

The total nickel metal (the sum of the nickel sulfate and nickel chloride metal) is a key factor in the ability of the bath to plate at higher current densities without burning. As the current density goes up so must the concentrate of the chloride and sulfate to compensate for the need for more nickel ions in the plating solution. Cost of adding additional nickel to the plating bath is always a consideration. Consequently most baths are controlled in a range where the total metal is the minimum to accommodate the required current density needed.

Boric Acid

Boric acid buffers the hydrogen ion concentration (pH) in the cathode film. This buffering action is needed to prevent the pH of the cathode film from exceeding pH 6.0 and forming nickel hydroxide which would be precipitated and co-deposited along with hydrogen, resulting in a burned deposit. An indication of low boric acid concentration is the appearance of pitting or roughness in high-current-density regions. Boric acid plays a very important role in establishing the upper limits of the applied current density.

In order to obtain the required physical properties such as uniform bright deposit, additives are needed in a Watts bath. These are typically organic compounds that modify the nickel deposit to achieve a desired property. Brighteners for semi-bright nickel are designed to give a uniform nickel deposit where sulfur does not co-deposit with the nickel. Brighteners for bright nickel typically include a carrier additive. The additive adds sulfur to the deposit, provides ductility, gives a uniform grain structure and a leveler to provide brilliance through a leveling mechanism.

Secondary Brightener

A secondary brightener works with the carrier to provide a high degree of luster. Additives for semi-bright nickel are designed to give a uniform nickel deposit where sulfur does not co-deposit with the nickel. It is critical that these organic components remain in balance; therefore, they are provided in proprietary packages from finishing suppliers. These packages are formulated to achieve the best combination of stability, brightness and ductility.

Factors in controlling a nickel bath include: pH, proper ratios of nickel sulfate, nickel chloride, boric acid, proper filtration, proper control of organic additives, control and removal of tramp metals (this varies with the base metal being plated), proper agitation- mechanical or air, and temperature.

Once the nickel is deposited often a thin film of chrome is added. This chrome film can be deposited from traditional hexavalent chrome baths, or trivalent chrome baths. The function of the chrome coating is to prevent the nickel form oxidizing. The nickel layer keeps its finish longer, and provides another layer of protection for the base metal. Additives can be used in the chrome deposit to change the color of the finish from clear to dark gray.

 Troubleshoot Your Nickel Plating