Decorative Nickel Plating Processes and Multilayer Coatings

The technology of decorative nickel plating has been improved continuously over the years. Prime examples include development of the organic bright nickel solutions, introduction of semibright nickel plating processes, development of multilayer nickel coatings, and the use of microdiscontinuous chromium in combination with multilayer nickel. The major result of these developments has been a remarkable improvement in the corrosion performance of decorative nickel plus chromium coatings without the need to increase deposit thickness.

Bright nickel plating solutions are modifications of the Watts formulation given in Table 2, but they contain organic and other additives that act to produce a fully bright finish suitable for immediate chromium plating without mechanical finishing. Portions of the addition agent molecules may be incorporated into the deposit, resulting in a hard, fine-grain coating that contains incorporated sulfur. The sulfur causes the deposit to be electrochemically more reactive than sulfur-free matte, polished, or semibright nickel deposits. Decomposition products of the additives accumulate in solution with time and are removed by purification with activated carbon. In modern solutions, continuous filtration through active carbon removes deleterious decomposition products without significant removal of the addition agents themselves.

Several substances--organic and inorganic--are used at appropriate concentrations to achieve brightness, leveling, and control of internal stress. (Leveling is the ability of the deposit to become smoother than the surface on which it is deposited as the thickness of the nickel is increased.) The substances used as additives in bright nickel plating solutions may be described by the following three terms: carriers, auxiliary brighteners, and brighteners. The terminology is not standardized, however, and alternative terms mentioned in the literature are shown in parentheses.

Carriers (brighteners of the first class, secondary brighteners, control agents, ductilizers) are usually aromatic organic compounds. They are the principal source of the sulfur codeposited with the nickel. Their main function is to refine grain structure and provide deposits with increased luster compared with matte or full deposits from baths without additives. Some of these additives can be used in Watts solution or high-chloride versions of the Watts solution (for example, solutions with 115 g/L nickel chloride). This class of brightener widens the bright range when used in combination with the auxiliary brighteners and brighteners discussed below. Some examples of carriers are saccharin (o-sulfobenzoic imide), paratoluene sulfonamide, benzene sulfonamide, benzene monosulfonate (sodium salt), ortho sulfobenzaldehyde (sodium salt), and naphthalene 1,3,6-trisulfonate (sodium salt). Carriers are used in concentrations of about 1 to 25 g/L (0.1 to 3 oz/gal), either singly or in combination. They are not consumed rapidly by electrolysis, and consumption is primarily by dragout and by losses during batch carbon treatment. (Batch treatment involves interrupting production and transferring the plating solution to a separate treatment tank where it is treated with activated carbon, filtered, and returned to the main tank.) The stress-reducing property of carriers is increased if they contain amido or imido nitrogen. For example, saccharin is a most effective stress reducer and often helps to decrease or eliminate hazes. It is generally used as sodium saccharin at a concentration of 0.5 to 4.0 g/L (0.07 to 0.5 oz/gal).

Auxiliary brighteners may be either organic or inorganic. Their functions are to augment the luster attainable with the carriers and brighteners and to increase the rate of brightening and leveling. Some examples are sodium allyl sulfonate; zinc, cobalt, cadmium (for rack and barrel plating); and 1,4-butyne 2-diol. The concentration of these additives may vary from about 0.1 to 4 g/L (0.01 to 0.5 oz/gal). The rate of consumption depends on the type of compound and may vary widely. These compounds may be of aromatic or aliphatic types and usually are heterocyclic or unsaturated. The inorganic metallic ions--zinc, cobalt, cadmium--are not often used anymore as auxiliary brighteners.

Brighteners (brighteners of the second class, primary brighteners, leveling agents), when used in combination with carriers and auxiliary brighteners, produce bright to brilliant deposits having good ductility and leveling characteristics over a wide range of current densities. Some of the compounds used as brighteners include reduced fuchsin, phenosafranin, thiourea, 1,4-butyne diol, n-allylquinolinium bromide) and 5-aminobenzimidazolethiol-2. Materials of this type generally are used in concentrations of 0.005 to 0.2 g/L (0.0006 to 0.02 oz/gal); an excess of brighteners may cause serious embrittlement. The rates of consumption of these materials may vary within wide limits.

Modern bright nickel plating solutions employ combinations of additives similar to those described and are formulated to produce bright deposits over a wide range of current densities. The deposits have excellent leveling or scratch-filling characteristics, produce deposits with fair ductility and low internal stress, produce bright deposits in areas of low current density, permit use of high average current densities and bath temperatures, are less sensitive to metallic contaminants than some of the solutions first commercialized, permit continuous purification of the plating solution by use of activated carbon on filters, produce breakdown products that can be removed by activated carbon, and are not overly sensitive to anode effects.

Multilayer Decorative Plating. The single-layer bright nickel coatings produced from solutions containing organic additives are less resistant to corrosion than polished nickel coatings. The lower corrosion resistance is due to the presence of small amounts of sulfur that originate from the organic additives present in solution. The amount of sulfur that is incorporated depends on exactly how the process is formulated and controlled. Single-layer bright nickel coatings are suitable for use in mildly corrosive service using a nickel thickness of 10 to 20 pm (0.39 to 0.79 mil). For severe and very severe conditions of exposure, especially where longtime resistance to corrosion is required, multilayer nickel coatings with microdiscontinuous chromium are used. The principal types are double- and triple- layer coatings.

Double-layer coatings involve the electrodeposition of two layers of nickel, one semibright and one bright, before the application of chromium. The first layer (semibright) is deposited from a Watts-type formulation containing one or more sulfur-free organic additives. Semibright nickel deposits contain less than 0.005 wt% S and are semilustrous, smooth, and fine-grain over a wide current density range. The deposits have a columnar structure and good ductility. The typical composition and operating conditions for a semibright nickel plating bath are given in Table 2. Deposit internal stress increases with increasing nickel chloride content; deposits also tend to be nonuniform in color and leveling at high chloride levels. The concentrations of the organic additives for semibright nickel solutions are usually fairly low, from 0.05 to 0.5 g/L (0.006 to 0.06 oz/gal). Examples of these additives are 1,4-butyne diol (or other aliphatic compounds with olefinic or acetylenic unsaturation), formaldehyde, coumarin, and ethylene cyanohydrin. There are two families of semibright nickel plating processes that are usually referred to as coumarin and noncoumarin types. The latter were introduced more recently and offer advantages. Semibright nickel plating solutions usually contain anionic surfactants and antipitting agents, singly or in combination.

The bright nickel layer deposited on top of the semibright one may range in thickness from 5 to 8 pm (0.2 to 0.3 mil), or about 20 to 35% of the total nickel thickness. Ideally, it should be plated from a bath that is compatible with the semibright additive, or additives, because in most double-layer systems the semibright additive functions as either a brightener or an auxiliary brightener in the bright nickel bath.

Triple-layer coatings are similar to double-layer coatings except that a thin, high-sulfur-containing layer is deposited between the semibright and bright layers. The thin layer must contain greater than 0.15 wt% S. Some of the requirements for double- and triple-layer nickel coatings are summarized in Table 3. Why multilayer coatings improve corrosion performance is discussed in the section "Corrosion Performance" in this article.

Table 3 Requirements for double- or triple-layer nickel coatings

Type of nickel coating(a)

Specific elongation,

%

Sulfur content, wt%

Thickness as a percentage of total nickel thickness

Double-layer

Triple-layer

Bottom (s--semibright)

Greater than 8

Less than 0.005

Greater than 60 (but at least 75 for steel)

Greater than 50 (but not more than 70)

Middle (b--high-sulfur bright)

Greater than 0.15

10 max

Top (b--bright)

Between 0.04 and

Greater than 10, but less than 40

Equal to or greater than 30

(a) s,semibright nickel layer applied prior to bright nickel; b, fully bright nickel layer that contains the amount of sulfur specified

Microdiscontinuous Chromium. Decorative, electrodeposited nickel coatings, whether single- or multilayer, are most often used in combination with electrodeposited chromium. The thin layer of chromium, initially applied over nickel to prevent tarnishing, now provides added resistance to corrosion because of the developments discussed in this and the next section.

Conventional or regular chromium deposits are low-porosity coatings, whereas microdiscontinuous chromium deposits have a high, controlled degree of microporosity or microcracking. Controlled microporosity or microcracking in the chromium is achieved by depositing a special nickel strike on top of the bright nickel layer just prior to chromium plating. When it is plated over with chromium, the thin layer of nickel, usually about 1 to 2 pm (0.04 to 0.08 mil), helps create microcracks or micropores in the chromium. Microporosity may also be achieved without the use of a special nickel layer by means of the Pixie process, a patented process that involves postplating treatment of the chromium to increase porosity on a microscopic scale. Traditionally, the chromium is deposited from conventional hexavalent processes, but within the last ten years, trivalent chromium plating processes have grown in popularity.

Microcracked chromium is produced by depositing the thin layer of nickel from a special bath formulated to produce nickel with a high internal tensile stress. When the chromium deposit is chromium plated, the thin nickel and the chromium then crack. Varying the conditions under which the nickel layer is deposited can provide variations in the crack density over a range of from 30 cracks/mm (750 cracks/in.) to 80 cracks/mm (2000 cracks/in.). The nickel bath usually consists of a basic nickel chloride electrolyte with additives that provide additional stress, such as the ammonium ion. Boric acid is not used, but other buffers such as the acetate ion may be added. Proprietary organic additives are also used to enhance the brightness and the ability of the deposit to crack, especially in the low-current-density areas. Temperature and pH are controlled to vary the crack density; low temperature (23 °C, or 73 °F) and high pH (4.5) favor higher crack densities; high temperature (36 °C, or 97 °F) and low pH (3.5) favor lower crack densities. Cracking of the chromium deposit must occur subsequent to chromium plating. Aging or the use of a hot water dip may be necessary to promote the formation of all microcracks.

Microporous chromium is produced from Watts-type nickel baths using air agitation and containing very fine inert particles, usually inorganic, and the normal additives used for bright nickel plating. Chromium, plated over the resulting nickel-particle matrix, deposits around the particles, creating pores. The nickel baths are operated much like bright nickel solutions, with the exception that filtration cannot be performed. In some instances, auxiliary additives permit reduction of the particle concentration in the plating bath and still provide high pore densities. Pore densities can vary according to the concentration of particles, agitation rates, and additives. Generally, a minimum pore density of 100 pores/mm2 (64,000 pores/in.2) is specified. In either case, chromium thicknesses should not be allowed to exceed about 0.5 pm (0.02 mil) or the cracks and pores will start to heal.

Microcracked chromium deposits can also be produced directly from chromium baths by increasing thickness, or by depositing chromium over chromium. The latter, dual-layer chromium technique is no longer popular.

Corrosion Performance. The remarkable corrosion resistance of modern decorative nickel-plus-chromium coatings depends on the use of multilayer nickel in combination with microdiscontinuous chromium. The improved performance of multilayer nickel coatings is due to the fact that the combination of layers of nickel have different electrochemical reactivities. If one measures the corrosion potentials of various nickel deposits in the same electrolyte, one finds that the bright nickel deposits display more active dissolution potentials than do the semibright nickels. If bright and semibright nickel deposits (for example, in the form of foils separated from the substrate) are electrically connected in the electrolyte, electrons will flow from the bright nickel to the semibright nickel. The result is that the rate of corrosion of the bright nickel is increased, whereas the rate of corrosion of the semibright nickel is decreased. In a composite coating consisting of bright nickel over semibright nickel, this is manifested by enhanced lateral corrosion of the bright nickel layer and delayed penetration of the semibright nickel layer.

The extent to which bright nickel protects the underlying semibright nickel layer by sacrificial action is dependent on the difference between the corrosion potentials of the semibright and bright nickel. The difference should be at least 100 mV (as measured by the simultaneous thickness and electrochemical potential, or STEP test, described in the section "Quality Control of Nickel Plating" in this article), differences in potential are beneficial, especially in low-current-density areas of complicated parts. If the difference becomes too great, appearance suffers because of the accelerated corrosion of the bright nickel layer; that is, there is an optimum value that represents a compromise between preventing basis metal attack and controlling superficial corrosion. The result is that penetration of the coating and exposure of the underlying substrate occur slowly. Multilayer nickel coatings are thus more protective than single-layer bright nickel coatings of equal thickness.

The rate of pit penetration through the nickel layers varies inversely with the number of microdiscontinuities in the chromium layer. Pit penetration may occur rapidly with low-porosity, conventional chromium. When corrosion takes place at a pore in conventional chromium, the large cathodic area of chromium surrounding the pore accelerates the corrosion of the nickel, and pitting may occur rapidly. With microdiscontinuous chromium, a large number of microscopic pores or cracks are deliberately induced in the chromium deposit so that corrosion can start at many sites. The available corrosion current has to be spread over a myriad number of tiny corrosion cells, so that the rate of corrosion of the nickel is greatly reduced. For example, the approximate depth of pitting of nickel after 16 h of CASS testing (ASTM B 368, "Copper-Accelerated Acetic Acid Salt Spray [Fog] Testing") was 10 to 20 pm with conventional chromium and 1 to 6 pm with microdiscontinuous chromium.

Corrosion studies conducted by plating suppliers, nickel producers, and groups such as ASTM Committee B-8 have confirmed that multilayer nickel coatings are significantly more protective than single-layer bright nickel coatings, that microdiscontinuous chromium coatings provide more protection than conventional chromium coatings, and that the corrosion protection of decorative, electroplated nickel-plus-chromium coatings is directly proportional to nickel thickness and to the ratio of semibright and bright nickel in multilayer coatings. Table 4 is based on the results of a study conducted at the LaQue Center for Corrosion Technology, Wrightsville Beach, NC, and it summarizes the types of coatings that protected standard panels from corrosion for more than 15 years outdoors in a severe marine atmosphere.

Table 4 Coating systems on steel giving best performance after 15 years of outdoor marine exposure and 96 h of CASS testing

Type and thickness of coating, ^m

ASTM performance ratings(a)

Copper

Nickel(b)

Chromium(c)

Outdoor marine, 15 years

CASS, 96 h

38d

1.5 mc

10/8

10/8

12

26d

1.5 mc

10/9

10/8

38d

0.25 mp

10/7

10/7

12

26d

0.25 mp

10/9

10/7

Note: CASS testing ("Copper-Accelerated Acetic Salt Spray [Fog] Testing") is conducted according to ASTM B 368.

(a) A two-number system has been adopted by ASTM for rating panels after corrosion testing. The first, the protection number, is based on the percentage of the base metal that is defective due to corrosion. A rating of 10 on steel indicates that the panel did not rust. The second, the appearance number, is similarly based on percentage of defective area, but it rates the extent to which corrosion of the base metal as well as superficial corrosion, detract from the overall appearance. Appearance ratings of 7, 8, or 9 indicate that 0.25 to 0.5%, 0.1 to .25%, or 0 to 0.1% of the area, respectively, is defective due to superficial staining and corrosion.

(b) d, double layer. The double-layer nickel coatings in the program differed in reactivity. For details see G.A. DiBari and F.X. Carlin, Decorative Nickel/Chromium Electrodeposits on Steel--15 Years Corrosion Performance Data, Plating and Surface Finishing, May 1985, p 128.

(c) mc, microcracked; mp, microporous. The type of microcracked chromium used in this study is based on the addition of selenium compounds to a conventional chromium bath to obtain microcracking. Consistent crack patterns were obtained at the chromium thicknesses given in the table.

Standards and Recommended Thicknesses. ASTM B 456 provides information on specific requirements for decorative nickel-plus-chromium coatings to achieve acceptable performance under five different conditions of service. The standard defines several classes of coatings that differ in thickness and type, and it classifies the various coating systems according to their resistance to corrosion. The standard specifies the requirements for double- and triple-layer nickel coatings (Table 3), and it gives the classification numbers of coatings appropriate for each service condition number. For example, Table 5 specifies decorative nickel-plus-chromium coatings on steel.

Table 5 Decorative nickel-plus-chromium coatings on steel

Service condition number (typical applications)

Coating designation(a)

Minimum nickel thickness, ^m

SC 5--Extended very severe (exterior automotive where long-time corrosion protection is a requirement)

Fe/Ni35d Cr mc

35

Fe/Ni35d Cr mp

35

SC 4--Very severe (exterior automotive, boat fittings)

Fe/Ni40d Cr r

40

Fe/Ni30d Cr mp

30

Fe/Ni30d Cr mc

30

SC 3--Severe (patio and lawn furniture, bicycles, hospital furniture and cabinets)

Fe/Ni30d Cr r

30

Fe/Ni25d Cr mp

25

Fe/Ni25d Cr mc

25

Fe/Ni40p; Cr r

40

Fe/Ni30p Cr mp

30

Fe/Ni30p Cr mc

30

SC 2--Moderate service (stove tops, oven liners, office furniture, golf club shafts, plumbing fixtures and bathroom accessories)

Fe/Ni20b Cr r

20

Fe/Ni15b Cr mp

15

Fe/Ni15b Cr mc

15

SC 1--Mild (toaster bodies, interior automotive accessories, trim for major appliances, fans, light fixtures)

Fe/Ni10b Cr r

10

(a) b, electrodeposited single-layer bright nickel; d, double-layer or multilayer nickel coating; r, regular or conventional chromium; mc, microcracked chromium; mp, microporous chromium. The numerals in the designations denote the thickness of the nickel coating in microns. The thickness of the chromium is assumed to be 0.3 pm unless otherwise specified. When permitted by the purchaser, copper may be used as an undercoat for nickel, but it cannot be substituted for any of the part of the nickel specified. Results of several test programs have raised doubt about whether coating systems involving regular chromium are satisfactory for SC 4 and SC 3.

The service condition number characterizes the severity of the corrosion environment, 5 being the most severe and 1 being the least severe. The classification number is a way to specify the details of the coating in an abbreviated fashion. For example, the classification number Fe/Ni30d Cr mp indicates that the coating is applied to steel (Fe) and consists of 30 pm of double-layer nickel (d) with a top layer of microporous (mp) chromium that is 0.3 pm thick. (The thickness value of the chromium is not included in the classification number unless its thickness is different from 0.3 pm.) The type of nickel is designated by the following symbols: "b" for electrodeposited single-layer bright nickel, "d" for double- or multilayer nickel coatings, "p" for dull, satin, or semibright nickel deposits, and "s" for polished dull or semibright electrodeposited nickel. The type of chromium is given by the following symbols: "r" for regular or conventional chromium, "mp" for microporous chromium, and "mc" for microcracked chromium.

Decorative nickel-iron alloy plating processes were introduced to conserve nickel and to lower anode material costs by substituting a portion of the nickel with iron. Decorative nickel-iron alloy deposits have full brightness, high leveling, excellent ductility, and good receptivity for chromium. Nickel-iron can be plated on steel, brass, aluminum, zinc die castings, or plastic substrates in either barrel or rack equipment. The operation and the proprietary additives used in commercially available processes are similar to those in conventional bright nickel plating. In addition, the bath requires special additives to stabilize the ferrous and ferric ions so that hydroxide compounds do not form and precipitate. The stabilizers are either complexers or reducing agents, depending on the nature of the proprietary process. The processes should be controlled within the limits recommended by plating supply houses. Deposits on steel or copper that is subsequently chromium plated have had good acceptance for interior applications as a substitute for bright nickel. Decorative nickel-iron alloy deposits are not often used for outdoor applications where corrosion conditions are severe, because the deposits tend to form a fine, superficial brown stain relatively quickly. The rate at which this occurs depends on the iron content of the deposits, and those with less than 15% Fe have been used in outdoor applications.

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