Electroless Nickel Plating

Electroless nickel plating is used to deposit nickel without the use of an electric current. The coating is deposited by an autocatalytic chemical reduction of nickel ions by hydrophosphite, aminoborane, or borohydride compounds. Details of the process can be found in the article "Electroless Nickel Plating" in this Volume. When sodium hypophosphite is the reducing agent, the deposit generally contains between 3 and 11% P. The boron contents of electroless nickel range from 0.2 to 4 wt% and from 4 to 7 wt% when the reducing agents are an aminoborane and sodium borohydride, respectively.

Electroless nickel is an engineering coating, normally used because of excellent corrosion and wear resistance. Because of these properties, electroless nickel coatings have found many applications, including those in petroleum, chemicals, printing, mining, aerospace, nuclear, and automotive industries.

Electroless nickel can be heat treated to hardnesses comparable to those of electrodeposited chromium. The maximum hardness can be attained in 1 h at about 400 °C (750 °F) or 10 h at 260 °C (500 °F). The hardness of as-plated nickel-phosphorus alloys varies from 500 to 650 on the Vickers scale. As-plated nickel-boron deposits are generally harder than the nickel-phosphorus ones. The ability of electroless nickel deposits to maintain their hardness under elevated-temperature service conditions increases with increasing phosphorus or boron content, but decreases rapidly above 385 °C (725 °F). Nickel-boron coatings tend to better withstand wear at elevated temperatures and are therefore more widely used under these conditions.

Wear rates of electroless nickel are summarized in Fig. 16. As this figure indicates, the wear loss of heat treated electroless deposits is lower than either electroplated nickel or non-heat-treated electroless deposits.

Coefficients of friction of electroless nickel in the as-deposited condition (EN) and heat treated at 400 °C (750 °F) (EN400) and at 600 °C (1110 °F) (EN600) are compared in Table 26 to the three chromium alloys depicted in Fig. 16. Additional information on the friction and wear characteristics of electroless nickel deposits can be found in Ref 36.

Table 26 Coefficients of friction of chromium versus electroless nickel

Coating

Coefficients of friction

Counterface diamond

Counterface plain carbon steel

CrA

0.040

0.88

CrB

0.035

0.82

CrC

0.030

0.81

EN

0.180

0.96

EN400

0.300

0.95

EN600

0.060

0.90

Source: Ref 36 Chromium Plating

Plating Baths. Most chromium plating is done from hexavalent chromic acid (CrO3) baths, but trivalent systems are gaining in popularity. Hexavalent chromium plating baths consist of chromic acid and small amounts of a sulfate catalyst (SO42-). Recently, mixed catalyst baths containing fluoride compounds in addition to chromic acid and sulfate have been employed. Proprietary self-regulating baths control the concentration of the catalyst automatically. Bath formulations and process controls are discussed in the articles "Industrial (Hard) Chromium Plating" and "Decorative Chromium Plating" in this Volume.

Applications and Properties. As implied above, chromium plating is divided into decorative and hard coatings. Decorative coatings are applied over a base deposit of nickel or copper plus nickel to provide color and tarnish resistance. Coating thicknesses are usually less than 1 pm (0.04 mil). Decorative chromium coatings are most often found on automobiles, furniture, and kitchen appliances.

Hard chromium coatings are generally deposited directly on the base material without a nickel undercoat in thicknesses ranging from 1 to 500 pm (0.04 to 20 mils). Hard coatings provide resistance to wear, heat, abrasion, and/or corrosion. Typical applications for hard coatings include hydraulic pistons and cylinders, piston rings, wearing parts in business machines, aircraft engine parts, yarn and thread guides for textiles, plastics molds, and various parts of nuclear reactors where galling is a particular concern.

Microcracks are present in most electroplated hard chromium deposits. Figure 19 shows a typical microcrack structure. The density of the microcracks in chromium deposits varies from 0 to more than 1200 cracks/cm (3000 cracks/in.), depending on bath chemistry, current density, and temperature. The number of microcracks increases with the concentration of the catalyst in the plating bath. The depth of a microcrack is less than about 8 pm (0.3 mil) on a deposit that is 130 pm (5 mils) thick with crack counts of about 800 cracks/cm (2000 cracks/in.).

Fig. 19 Photomicrographs of chromium deposits (plated in a high-efficiency etch-free bath) after etching. (a) and (b) Deposit plated at 78 A/dm2 (5 A/in.2) and at 55 °C (130 °F). (a) 540x. (b) 2300x. (c) Cross section of a chromium deposit plated at 93 A/dm2 (6 A/in.2) and at 58 °C (135 °F). The specimen was polished before etching. 880x. Both deposits contain 800 microcracks/cm (2000 microcracks/in.).

Fig. 19 Photomicrographs of chromium deposits (plated in a high-efficiency etch-free bath) after etching. (a) and (b) Deposit plated at 78 A/dm2 (5 A/in.2) and at 55 °C (130 °F). (a) 540x. (b) 2300x. (c) Cross section of a chromium deposit plated at 93 A/dm2 (6 A/in.2) and at 58 °C (135 °F). The specimen was polished before etching. 880x. Both deposits contain 800 microcracks/cm (2000 microcracks/in.).

Electroplated hard chromium is chemically resistant to most compounds and offers excellent corrosion protection in various environments. Electroplated chromium for atmospheric corrosion applications should be between 20 and 25 pm (0.8 and 1 mil) thick. For corrosion resistance in chemical exposures, electroplated chromium should be 50 to 75 pm (2 to 3 mils) thick. A detailed review on the corrosion resistance of electroplated chromium can be found in Ref 40.

The hardness of hard chromium varies from about 900 to 1100 on the Knoop and Vickers hardness scales. These values are considerably higher than the hardness of bulk chromium. Deposits from trivalent solutions are softer than those that are plated from hexavalent chromium solutions. However, after heat treating at about 700 °C (1290 °F), a hardness comparable to that of hard chromium can be achieved (Ref 41).

Chromium deposits are characterized by high internal tensile stresses that can reach 1000 MPa (145 ksi). These stresses can reduce the fatigue properties of coated components. Hydrogen is also codeposited with chromium and can diffuse into components, causing hydrogen embrittlement. Heat treatments are typically required to relieve the stresses and hydrogen effects, but can reduce the hardness.

The coefficients of friction of hard chromium against hard materials are generally the lowest of any electrochemically deposited coatings. The actual values vary considerably, depending on the test method, the mating surfaces of the materials, and the degree of lubrication. Some values of static and sliding coefficients of friction are listed in Table 27. In general, hard chromium has a lower wear rate than either electroplated or electroless nickel, which are the two competing materials. Wear rates are compared in Fig. 16.

Table 27 Coefficients of friction for hard chromium electrodeposits

Couple

Static coefficient

Sliding coefficient

Chromium-plated steel versus itself

0.14

0.12

Chromium-plated steel versus steel

0.15

0 0

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