ByBeverly A. Graves, Editor

To learn more about nickel plating, check out these Products Finishing articles on different types of nickel plating: Nickel Plating; Nickel Alloy Plating; and Plating Plastics at Guardian Automotive.

Any introductory course starts out with a brief history of the subject, and this article will be no different. According to a technical report of the International Nickel Co., Ltd., Joseph Shore applied for a patent for nickel plating in 1840; however, it was not until 1842, in Frankfort, Germany, that Böttger succeeded in depositing nickel from a solution of nickel ammonium sulfate.1

One of the first United States patents was granted to Adams in 1869 for a solution of nickel ammonium chloride, and in 1878 Weston obtained a patent for the addition of boric acid to nickel plating solutions. Several patents were granted for various bath improvements, until O.P. Watts developed a rapid nickel plating bath in 1916. This nickel plating bath is still predominantly used throughout the world and was the first to make it possible to exceed current densities of 5 asf by a factor of 10.

Nickel plating solutions based on sulfamate solutions were introduced by Cambi and Piontelli in a report for the Lombard Institute of Sciences.

Types of Nickel Plating Solutions

Sulfate Solutions. The most common nickel plating bath is the sulfate bath known as the Watts bath. Typical composition and operating conditions are shown in Table I. The large amount of nickel sulfate provides the necessary concentration of nickel ions. Nickel chloride improves anode corrosion and increases conductivity. Boric acid is used as a weak buffer to maintain pH.

The Watts bath has four major advantages: 1) Simple and easy to use; 2) Easily available in high purity grades and relatively inexpensive; 3) Less aggressive to plant equipment than nickel chloride solutions; and 4) Deposits plated from these solutions are less brittle and show lower internal stress than those plated from nickel chloride electrolytes.

High Chloride Solutions. Chloride baths have an advantage over sulfate baths in deposition speed; not necessarily in current density, but in improved current distribution.

All-Chloride Solutions. The advantages of all-chloride nickel plating solutions include the following: 1) Low voltage; 2) Good polishing characteristics; 3) Heavy coatings can be deposited; 4) Low pitting; 5) Improved cathode efficiency; and 6) No need to cool the plating solution. See Table I for composition and operating parameters.

However, there are disadvantages to this bath as well: 1) Highly corrosive; 2) Nickel chloride is sometimes less pure than nickel sulfate (particularly important in bright nickel plating); 3) Mechanical properties of the deposit are not as good as those from the Watts bath.

Fluoborate Solutions. In nickel fluorborate baths, the electrolyte is maintained at a pH of 2.0-3.5 using fluoroboric acid. Metal content is maintained at up to 120 g/liter of nickel, which is much higher than in a Watt's bath. Because of this, higher current densities are necessary.

Nickel coatings deposited from this type of bath have properties similar to those deposited from Watt's baths; however, these coatings are usually specified for heavy nickel applications and electroforming.

Anode dissolution in a nickel fluoborate bath not containing chloride is better than in a nickel sulfate solution with nickel chloride.

Disadvantages of fluoborate baths include the following: 1) High cost of chemicals; 2) Throwing power less than that of sulfate solutions.

Sulfamate Solutions. This bath is based on the nickel salt of sulfamic acid, and the pH is adjusted using sulfamic acid, nickel oxide or carbonate. When intensive agitation is used in solutions with a high nickel concentration, current densities up to 500 asf can be achieved.

Nickel coatings from this type of bath usually have very low stress values and high elongations. Another advantage is that it is possible to operate the sulfamate bath without difficulties related to anode dissolution at low chloride levels or even without chloride. The principle advantage of this bath is that it can be operated at nickel concentrations of 180-200 g/liter. This allows for the use of high current densities without losing the properties of the coating.

Types of Nickel Plating

Bright Nickel. Bright nickel plating baths are used in the automotive, electrical, appliance, hardware and other industries. Its most important function is as an undercoating for chromium plating, helping finishers achieve a smooth bright finish as well as a significant amount of corrosion protection.

Bright nickel plating baths use combinations of organic agents to achieve bright nickel deposits. There are two classes of these organic additives. The first class is the aromatic sulfonic acids, sulfonamides and sulfonamides that contain the functional group =C-SO-2. Saccharin is a widely used example of this type of brightener. Nickel deposits plated using these additives are mirror bright initially; however as the nickel builds, brightness diminishes. This first class of brighteners incorporates sulfur into the bright nickel, reducing corrosion resistance.

Brighteners in the second class, also called levelers, have inorganic metal ions and organic compounds. These may include butynediol, coumarin, ethylene cyanohydrin and formaldehyde. These are used as leveling agents because they increase surface smoothness as the nickel deposit thickness increases.

Semi-Bright Nickel. At first, coumarin was used to obtain a high-leveling, ductile, semi-bright and sulfur-free nickel deposit from a Watts nickel bath. However, coumarin-free solutions are now available. A semi-bright nickel finish is semi-lustrous, as the name implies. However, it was specifically developed for its ease of polishing and buffing. Or, if subsequently bright nickel plated, buffing can be eliminated. Brightness and smoothness are dependent on operating conditions (see Table I).

The reason semi-bright nickel finishes are so easily buffed and/or polished is that the structure of the deposit is columnar, whereas the structure of a bright nickel finish is plate-like (lamellar). However, the structure can be changed with additives, a change in pH, current density or even an increase in solution agitation. This is not a problem unless it affects properties such as internal stress.

Internal stress can be compressive or tensile. Compressive stress is where the deposit expands to relieve the stress. Tensile stress is where the deposit contracts. Highly compressed deposits can result in blisters, warping or cause the deposit to separate from the substrate. Deposits with high tensile stress can also cause warping in addition to cracking and reduction in fatigue strength.

Watts baths and high-chloride type baths can produce high tensile stress. During bright-nickel plating, stress-reducing additives are used, but these codeposit sulfur materials that affect the physical and/or engineering properties of the deposit. Saccharin is often used as a stress reducing agent. Nickel sulfamate baths can deposit pure low-stressed finishes without using additives.

Other Types of Nickel. To obtain other types of finishes such as satin nickel, organic additives are used and deposition conditions are altered. Deposits from a Watts bath are usually 7-10 mm thick, with the appearance dependent on the temperature and/or pH. At higher temperatures and a pH of 4.5-5.0, nickel deposits are matte. At 122F and a pH of 2.5-3.5, deposits are bright.

Black nickel plating is lustrous and has a black or dark gray color. Plating is done with little or no agitation. Occasionally it is necessary to remove hydrogen gas (bubbles) from the part's surface using wetting agents. The pH of the bath ranges from 5-6, and the temperature varies from ambient to 140F. Current density remains at approximately 0.5 A/dm2.

The coatings average 2 mm thick and corrosion resistance is limited, therefore they are usually lacquered or coated with oil or grease. If the black nickel must have good corrosion resistance, an undercoating such as bright or dull nickel, zinc or cadmium is necessary.

Barrel Nickel Plating

Barrel plating solutions are relatively similar to rack plating solutions; however, operating conditions may differ, although not radically. The pH is usually maintained at about 4, unless plating zinc diecasting, in which case a pH higher than 4 may be necessary. However, anode corrosion is better at a lower pH, and anode area is limited. The anode area should be as large as possible to avoid the liberation of oxygen and chlorine.

Temperatures can vary for barrel nickel plating from 86-104F for some solutions and 104-140F for others. Current density can also vary. For a typical barrel, approximately 24-32 inches long and 16 inches in diameter, the load is 300-600 amps per load or between 1-1.5 A/dm2. Other considerations are the barrel loading, surface area and coating thickness.

There are some special considerations for barrel plating: 1) Parts must be able to move about freely in the barrel; 2) Precise surface preparation is essential, including thorough rinsing; and 3) When the electrolytes are used to full capacity, low-current-density treatment should be used continuously.

Properties of Nickel Deposits

Thickness. Corrosion resistance is often intimately related to the thickness of the coating; however, the functional requirements of the coating are also important. Micrometer readings are used most often to determine coating thickness. ASTM standard B487 describes a method of measuring coating thickness based on metallographic examination of cross-section of the plated part. Other ASTM tests include ASTM B530 and ASTM B504. The ASTM web site (www.astm.org) has information on the tests mentioned in this article.

Hardness. Certain addition agents, such as saccharin or napththalene sulfonic acid, can increase the hardness of a nickel deposit. Wetting agents may also increase hardness. Nickel deposits plated from Watts nickel baths, sulfamate or fluoborate baths can rise to 650 HV (HV is Vickers hardness). Heavy nickel baths produce deposits with hardnesses between 250-350 HV.

Hardness is not only a result of addition agents but is also affected by the plating bath composition, temperature, current density and other operating conditions. ASTM B578 is a test for the microhardness of plated coatings.

Ductility. Ductility can be measured using two ASTM test procedures, B489 and B490. Ductility can also be measured using a tensile testing machine; however this test is specific to measuring plated thin foils.

Information about other properties such as adhesion, brittleness, dull deposits and burning are covered in PFOnline's Nickel Troubleshooting Guide which is this month's online exclusive (for the url see the contents page).

This primer by no means even scratches the surface of nickel plating. There have been volumes written on the subject. It is hoped that this article will give you some information on the basics and some reference materials as to where you can go to find more information about the process.