Chapter-2

Silver Electroplating

2.1 Introduction

Silver was first plated at the beginning of the nineteenth century [1]. However,

the earliest patent for silver plating was granted to Elkington [2] • in 1840 and this

signaled the start of the electroplating industry. That bath is basically the one used in

silver plating today, that is, the double silver cyanide with excess free cyanide. Many

other baths have been proposed such as nitrate [3], iodide [4], thiourea [5],

thiocyanate [6], sulfamate [7], and thiosulfate [8]. A recent review summarizing

cyanide-free formulations [9] states that none of the baths listed has been accepted by

industry in preference to the common cyanide solution. A proprietary iodide bath has

been offered publicly.

Early usage of silver plating was for its decorative effect and for a suitable

deposit for tableware and hollow ware, in which cases its resistance to foods, and so

on (with the exception of those of high sulfur content) was the important

consideration. Within the past 40 years, several industrial uses have arisen. Such uses

are for bearings, where silver provides a surface resistant to galling at low loads, for

electronic circuit components, for slip rings, waveguides, and hot gas seals.

Improvements in the plating of silver have historically revolved around bright

deposits by the addition to a conventional bath of brightener and surfactant, and

changes in the amount of free cyanide and silver for increasing plating speed.

2.2 BATH FORMULATIONS

The composition of silver cyanide plating solutions will vary with the type of silver

plate required. Decorative silver is usually plated from baths with lower silver content

than those used for engineering applications where thicker deposits are required. The

higher silver concentrations allow the use of higher current densities and thus more

economic plating speeds. Typical bath compositions are shown in Table 1.

Silver cyanide forms a complex double cyanide with alkali metal cyanides,

KAg(CN)2. Thus, 1 g of AgCN requires about 0.5 g KCN in addition to the free

cyanide shown in Table 1. The bath is also commonly prepared by dissolving the

calculated amount of potassium cyanide in about half the final volume of deionized

water. After complete dissolution the silver cyanide is added slowly with constant

stirring to form the complex. Other required salts are then added and the solution

brought to operating volume with deionized water. The solution is then filtered,

preferably through activated carbon, before the addition of brighteners. Although

most large volume silver solutions are prepared in this fashion, many baths are

prepared with the readily available potassium silver cyanide. In this case, the bath

ingredients, other than the KAg(CN)2 and brighteners, are dissolved in a spare tank in

water equal to half the final bath volume. This solution is heated to 70°C and

activated carbon in the amount of 1.2 g/1 of bath is added. Stir 1 hr and filter into the

operating tank, make up almost to volume, then add the potassium silver cyanide

directly to the tank with stirring. With this procedure no silver is removed by the

carbon. Finally, add brighteners if desired and dilute to volume.

TABLE 1. TYPICAL BATH COMPOSITIONS

Conventional or Bright

Silver

High-Speed Silver for

Thicker Deposits

Silver (as metal) 20-45 35-115

Silver cyanide 31-56 44-153

Potassium cyanide (total) 50-78 68-235

Free potassium cyanide 35-50 45-160

Potassium carbonate 15-90 15-90

Potassium nitrate ---- 40-60

Potassium hydroxide ---- 4-30

Current density,

A/dm2 (with agitation)

0.5-1.5 0.5-10.0

0.5-1.5 0.5-10.0

Temperature, °C 20-28 38-50

Brighteners As required As required

Alkali silver cyanide provides the silver necessary for the deposition of silver on the

cathode. The mechanism of silver deposition is moot, but from a practical standpoint,

low silver content and high free cyanide favor improved throwing power, and higher

concentrations permit the use of higher current densities. Consequently, baths low in

silver contents are generally used for 2- to 5- µm deposits. The alkali cyanide

increases the conductivity (Table 2) [10] and the cathode polarization, contributes to

good anode corrosion, and, of course, serves to form the silver complex ion.

Potassium cyanide is used rather than sodium cyanide because of higher solution

conductivity, higher solubility of the resultant potassium carbonate buildup, and

higher limiting current densities provided. The potassium cyanide baths also offer a

wider operating bright range.

Potassium carbonate adds to the conductivity of the bath and increases anode

and cathode polarizations, which aids in increased throwing power. The

recommended minimum concentration is added initially and the concentration

gradually increases through decomposition of the cyanide. In regular production

practice the concentration of potassium carbonate may rise to 110 g/1 without adverse

effects. Higher concentrations may cause coarse, rough deposits. When the plating

process is properly engineered the carbonate content usually levels off within the

recommended operating range.

Potassium nitrate has been recommended for high-speed plating baths [11].

Additions of 40 to 60 g/1 are said to aid in anode corrosion at higher current densities

and lower free cyanide content.

Hydroxides have been used for promoting anode corrosion for high-speed

plating [11] and stabilizing the bath against cyanide decomposition [12].

Chlorides, formates, acetates, phosphates, borates, and sulfates have been

reported to increase the hardness of the silver as deposited. This was explained as

associated with the use of a carbon disulfide brightener [10].

Addition agents are generally added to the silver bath to produce fine-grained,

smooth deposits and, in the case of decorative silver, mirror-bright deposits. Details of

these additions are discussed separately in the sections on decorative and engineering

applications.

2.3 OPERATING CONDITIONS

The conventional silver plating baths are generally operated at room temperature (21-

27°C) and at current densities of 0.5 to 1.5 A/dm2. Agitation is usually supplied by

cathode-rod motion augmented by solution circulation by means of a pump or

mechanical mixers.

Higher operating temperatures and vigorous agitation are required for

TABLE 2.

THE EFFECT OF INCREASING CONCENTRATION OF ADDED INGREDIENTS

ON THE RESISTIVITY OF A CYANIDE SILVER PLATING SOLUTION

Effect of Potassium

Carhimate

Effect of Sodium Cyanide Effect of Potassium Nitrate

Concentration

g/1

Resistivity

ohm/cm3

Concentration

g/1

Resistivity

ohm/cm3

Concentration

g/1

Resistivity

ohm/cm3

12.9 9.6 7.5 14

75 6.2 30 11 75 9

130 5.0 45 9 120 6.6

high-speed silver plating. Current densities as high as 10 A/dm2 can be used with a

high silver content in a solution operated at 50°C. The limiting current density

depends mainly on the degree of solution flow past the cathode. In other words, under

the recommended plating conditions for high-speed plating, the limiting current

density will depend on the capabilities of the agitation system.

The average silver plating installation includes equipment for continuous or

intermittent solution filtration with arrangements for treatment with activated carbon.

Periodic filtration is required of all silver plating baths, coupled with carbon treatment

when it becomes necessary to remove harmful organic impurities. High-speed baths

for plating bearings, and so on, should be filtered continuously.

2.4MAINTENANCE AND CONTROL

Cyanide silver plating solutions are relatively easy to maintain and control. Anode

and cathode current efficiencies are essentially 100% under normal operating

conditions and, therefore, the solution remains in balance for long periods with

respect to metallic content. Small amounts of impurities of less noble metals present

no serious problem because the silver plates out preferentially. However, as with all

plating baths, good housekeeping habits should be observed to prevent heavy metallic

or organic contamination. The introduction of iron in large amounts through the use of

steel anodes or dragin from the strike solutions can cause off-color granular deposits

and interfere with bright plating solutions. Iron can be removed by cooling the

solution to 3°C and filtering off the precipitated ferrocyanide [13].

Solution control is based on routine chemical analysis and appearance of the

plate. Standard analytical procedures [14-16] are used for control of silver, free

cyanide, carbonate, hydroxide, and nitrates if present. Frequency of analysis depends

on the work schedules and can be determined only by experience. Carbonate

formation [17], cyanide decomposition [18], dragin from strike solutions, and dragout

from the plating bath are the main causes for change in composition.

Sodium carbonate in excess of 60 g/1 can be removed by cooling the solution

to 3°C. Potassium carbonate, which is used in most silver electroplating installations,

cannot be removed by freezing out because of its high solubility. The carbonate can

be precipitated by the addition of calcium nitrate, or calcium or barium hydroxide. For

solutions containing nitrate or hydroxide the use of calcium compounds is most

economical. One gram of potassium carbonate will be removed by about 1.2 g of

calcium nitrate or 0.5 g of calcium hydroxide. Barium cyanide is more generally

suitable becausethe soluble end product is potassium cyanide. One gram of potassium

carbonate will be precipitated by 1.4 g of barium cyanide. However, the high cost of

this chemical has limited its use. Filtration following this treatment is mandatory. The

effect of added nitrate, hydroxide, or cyanide resulting from these reactions must be

taken into consideration. Moreover, if the carbonate is high, no attempt to reduce it

drastically should' be made because the barium or calcium carbonate is voluminous

and difficult to filter out.

The main cause of roughness of electrodeposited silver under normal

operating conditons is the presence of suspended solid particles in the plating bath.

Periodic or continuous filtration is essential for this reason. Continuous filtration is

especially valuable in high-speed plating of thick deposits. Care must be exercised to

avoid the introduction of air through the filtering system because of its undesirable

effect in promoting pitting of the silver plate. Occasional treatment with activated

carbon may be necessary to remove harmful organic contaminants. Brighteners

removed by the carbon are replaced after the treatment.

Brighteners can be controlled by Hull cell tests under the direction of an

experienced operator or as recommended by suppliers of proprietary brightener

additives.

2.5ANODES

Silver anodes are usually supplied with a purity of 99.97% or higher. Two grades of

anode silver are generally available, a regular grade and a high-quality grade. For the

best plating results the high-quality-grade anodes should be used. These anodes are

made from selected melts and are recommended by the anode suppliers for optimum

performance in the plating bath. Impurities in silver anodes can cause the formation of

black film and flaking which result in plate roughness. Anodes are supplied in the

form of bar, balls, shot, and special shapes, as required.

Development of films and blackening of silver anodes sometimes occur in

plating solutions. Assuming that high-quality silver anodes are used, the "black

anodes" can be caused by low free cyanide in the plating bath, too low pH, a high

anode current density, high iron content, or the presence of sulfur compounds or

impurities such as organic decomposition products from brighteners in the bath [19].

The ratio of anode to cathode area should not be less than 1-1 to avoid films due to

anode polarization. The addition of nitrate and hydroxide to high-speed plating baths

aids in preventing "black anodes" due to high-current-density conditions.

The use of high-quality anodes is important not only to prevent black film

formation due to anode impurities but also to avoid flaking or shedding ofanode

particles into the solution during dissolution. The first indication of flaking may be the

appearance of roughness on the work caused by adherence of fine silver particles. An

investigation of the cause of particle separation [20, 21] has contributed to the

production of high-quality anodes. In critical conditions it is common practice to bag

the anodes or anode assemblies (steel baskets containing silver balls or chips) with

woven synthetic fabrics such as polypropylene, polyethylene, or other alkali-resist-ant

materials. All fabric coverings should be free of sizing that will cause contamination

of the solution. Fabric suppliers should be informed of the solution temperature and

brighteners used. The weave of the fabric should not restrict solution flow to cause

anode polarization. The fabric selected should be approved by the vendor of bright

plating solutions.

2.6 MATERIALS OF CONSTRUCTION

Plating equipment is generally the same as that used in other cyanide plating

systems. Steel tanks should be lined with rubber, neoprene, or plastics inert to highly

alkaline solutions. The supplier of new equipment (tanks, or pumps and filter systems)

should be apprised of the bath composition, temperature, and brighteners that will be

used. Rigid polyvinyl chloride (PVC), polypropylene, and reinforced polyester tanks

are frequently used.

Unlined steel or stainless steel tanks are unsatisfactory for the silver plating

solutions because of the possibility of stray currents which may lead to nonuniform

current distribution and plating of nonadherent silver deposits on the tank wall that

may become a source of roughness on the plated part.

Anode hooks and rack contacts should be stainless steel. Standard coated racks

recommended by the supplier are satisfactory. Stainless steel or plastic lined filter

systems are generally employed.

2.7PREPARATION OF BASIS METAL

The preliminary treatments of cleaning and etching of any metal before silver

plating follow standard procedures, except that a silver strike is required. Because

most basis metals are less noble than silver (including gold in the cyanide system),

they will precipitate silver by immersion from the regular silver plating baths and

result in poorly adherent deposits. The strike baths contain low metal concentration

and high free cyanide. This composition lowers the tendency for electrochemical

displacement by the basis metal and provides a low-efficiency hydrogen activation

process. The established procedure for steel is to use a double strike, first in a

solutioncontaining a lower silver content and some copper cyanide, and second in a

conventional strike solution. The second strike solution is also used as the first strike

bath for copper and copper alloys, nickel or nickel-silver, etc.

1. Strike bath No. 1 for steel

Silver cyanide 1.5-2.5 g/1

Copper cyanide 10-15 g/1

Potassium cyanide 75-90 g/1

Temperature 22-30°C

Current density 1.5-2.0 A/dm2

Voltage 4-6

2. Strike for nickel and nonferrous metals, also second strike for steel

Silver cyanide 1.5-5 g/1

Potassium cyanide 75-90 g/1

Temperature 22-30°C

Current density 1.5-2.0 A/dm2

The strike also serves to cover work made up of more than one metal, such as

soldered parts and assemblies. The required time is 8 to 25 sec for bright decorative

silver plating, and 15 to 35 sec for thicker deposits.

As an added precaution to prevent loss of adhesion due to electrochemical

displacement, the work is made cathodic before immersion it the strike and plating

solutions. On automatic plating lines where this procedure is not practical, use the

strike sequence for steel for all metals. When there is little danger of dragin from a

contaminated strike solution, the rinse between the strike and plating solution can be

omitted.

A recommended practice for plating adherent thick deposits of silver on steel is

reviewed in the engineering section of this chapter, as are special practices for

decorative work.

2.8DECORATIVE SILVER PLATING

Most of the decorative silver plating applications concern electroplating small

household items, such as hollow ware, flatware, jewelry, and the like. Because of the

variety of such items, the types of basis metal may be diverse and often the item to be

plated is a combination of two or three metals joined by solder.

Replating silvered items is even more of a chore, because usually the silver is

stripped and then repairs are made, often with several solder compositions. For items

having basis metals of copper, high or low brass, lead alloys, or Britannia metal (high

tin alloy), plus solder, the initial soakcleaner should be a relatively low pH alkaline

cleaner formulated for removal of buffing soils. Time of such soaking may be up to

10 min, if required, but shorter times are preferable. This is followed by a mild

cathodic electrolytic alkaline cleaner at 2.0 to 2.5 A/dm2, after which comes an acid

dip in hydrochloric (10%), fluoboric (10%) for lead alloy and soldered items, or a

proprietary acid solution. Following these operations a thin strike of either copper or

nickel is recommended to obtain the coverage on solder seams or lead alloy parts. If

the alloy is high in tin, use a nickel fluoborate strike. The parts are now given a silver

strike in the following bath:

Free potassium cyanide 75-90 g/1

Silver 1.25 g/1

The strike should be operated at or slightly below room temperature, at 1 to 2

A/dm2 and for 20 sec. Silver plate immediately after striking. The best baths for this

type of plating are the proprietary bright ones at 0.5 to 1 A/dm2. Since silver plating is

approximately 100% efficient, the time required to obtain a thickness of 5 pm at 1

A/dm2 is 7 min 24 sec or 37 min for 25 pm. The proprietary bright baths minimize the

buffing of the final plate, which not only saves metal but also assures a more uniform

thickness on finished items. Rinsing following plating should provide for dragout

recovery. Some proprietary plating baths leave a water-break surface after plating.

This may increase water spotting in the final drying step, so many platers use a film

breakdown step in the rinse line or return to the soak cleaner, and acid dip before the

final rinsing. The preparation of nickel-brass (nickel-silver used for flatware) is done

in a manner similar to that described previously, except that more alkaline soak

cleaners and elctrolytic cleaners are used. A nickel strike is optional, but a Wood's

nickel strike followed by a brief Watts plate will assure adhesion. For very heavy

silver on leaded nickel-brass a fluoborate nickel strike has made it possible to

eliminate a pimpling effect which occurred using an antimony-hardened bright silver.

Brass is handled much the same as nickel-brass flatware, except that the silver

deposit is normally only about 5 pm. Consequently, it is common to plate 5- to 15µm

of a leveling bright nickel before the silver plate.

Preparation of steel is described thoroughly in the section on engineering

applications. Stainless steel represents a special case, where the first plating step

should be a Wood's nickel strike of at least 3-min duration at 4 to 6 A/dm2, except

when the silver deposit is thin. When the silver deposit is only 2 to 3 pm, the

possibility of pores can accelerate corrosion when a more base metal undercoat, such

as nickel, is interposed. In such cases, a strike in a highly acid (pH 2.1) gold bath is

recommended. The platiiig of beryllium copper requires proper surface preparation

and adouble silver strike. The composition of the second strike should be

Free potassium cyanide 70 g/1

Silver 2.5 g/1

Conventional solutions containing a carbon disulfide or thiosulfate brightener

are used for plating flatware. These baths should be operated at 1.5 to 2.0 A/dm2.

Since the surface may be oxidized before final finishing and flatware can be buffed

easily (liquid, greaseless), high brightness is not required. Special plating applications

may dictate the use of high-speed solutions with thiosulfate brightener and an elevated

temperature (32-37°C).

For many years, until the 1950s, two brightening agents were widely used in

silver plating. Carbon disulfide [22] in cyanide solutions was, and still is, used for

decorative baths. The other was thiosulfate which is still used in high-speed and

flatware baths. Neither brightener gives mirror-bright deposits, but the resulting plate

requires much less buffing. Excessive amounts of carbon disulfide brightener can

cause lowered throwing power, poor deposit distribution, black spots, pinholes, and

areas of no plate and even extreme roughness. Excess brightener can be removed by

an activated carbon treatment.

Many addition agents have been proposed, such as gums [23], sugars,

unsaturated alcohols [24], and sulfonated aliphatic acids [25]. Most of these agents are

sulfur-bearing organic compounds or reaction products of sulfur and organic

compounds.

Fully bright silver plating on a consistent basis became possible with the

development of a ketone-carbon disulfide reaction product [26], a modification of

Weiner's [27] selenite [28, 29] bath, and the development of antimony polyalcohol

addition agents [30, 31]. Such full bright baths give mirror-bright deposits over a

reasonable current density range for decorative plating. Certain of these bright baths

are claimed to give better scratch resistance and thus' greater hardness. Use of all

should result in lowered postplating finishing costs, in most applications other than

flatware. The combination of full bright systems with accompanying bath characteris-

tics yields better deposit distribution, much better throwing power, and slightly less

tendency to tarnish. All mirror-bright solutions are proprietary, thus full information

should be obtained from the vendor. These solutions normally require a moderate

silver concentration and relatively high free cyanide. As is true of most other plating

solutions, failure to maintain sufficient anode area can produce conditions which will

cause these mirror-bright systems to fail. Good control is required.

There has been much discussion, especially in Europe, with respect to the

relation of hardness and resistance to wear. The alloy (Sb) hardened silverhas an

essentially permanent hardness, as high as 200 kg/mm2 (Vickers), while the selenite

and ketone-carbon disulfide silver deposits are in the 120-kg/mm2 range. Originally,

it was thought that the very hard deposits would wear longer, but this was not borne

‘out by tests, admittedly often confusing, because wearability, like solderability, is

difficult to measure. It is now pretty well agreed that very high hardnesses are

undesirable. However, it is claimed [32] that silver processes providing hardnesses of

130 to 150 kg/mm2 (Vickers) of the permanent type are preferable. The controversy

has not been particularly recognized by platers of flatware on this side of the Atlantic,

who generally use thiosulfate- or CS2-brightened baths at a higher current density

than employed for consistent full bright processes, but they do buff (often with liquid

compounds) and find the wearability as good as or better than that of antimony-

hardened silver deposits.

2.9SPECIFICATIONS FOR FLATWARE

General standards applying to the plating of flatware and hollow ware have

existed for many years. Flatware is generally specified with regard to the numer of

troy ounces of silver per gross (144) of teaspoons as follows:

Federal specification plate 280 g or 9 tr oz/gross

Quadruple plate 250 g or Ekr oz/gross

Triple plate 187 g or 6 tr oz/gross

Double plate 124 g or 4 tr oz/gross

Par plate 62 g or 2 tr oz/gross

Since the area of an average teaspoon is about 52 cm2, the thickness of

quadruple plate would be about 32 Am. Equivalent thicknesses are provided on the

other basic items of tableware (place fork, dessert or place spoon, knife, tablespoon).

The items getting occasional usage are generally given commensurately less silver.

In the case of hollow ware, the specifications for plating are less definitive.

They are generally as follows:

Federal specifications plate 32.5 mg/cm2 (20dwt/ft2)

Hotel plate 16.8-25.1 mg/cm2 (10-15 dwt/ft2)

Commercial plate 2.4-6.8 mg/cm2 (2-4 dwt/ft2)

In the plating of silver, the system of weights for the metal remains today that

of the troy scale of weights (a dwt is a pennyweight, or one-twentieth of a troy ounce).

Federal specification RR-T-451a covers in detail the standard thickness for the plating

of tableware and hollow ware.

2.9 BEARINGS AND RELATED INDUSTRIAL PRODUCTS

The excellent mechanical properties of silver, as reported by the National

Bureau of Standards [33], suggested its use as bearing material and led to the

development of high-speed plating of silver on steel-backed sleeve bearings. Since

this time silver has been widely used as an intermediate material for heavy-duty

bearings and to prevent galling or seizing of metal surfaces under light loads.

Antigalling applications include silverplated threads on stainless steel bolts, on

titanium compressor blades [34], and as a sealing medium for hot gas seals [35].

Aerospace material specification 2410E [36] covers this type of application.

Primary requirements of electroplated silver for heavy-duty bearing use are

adhesion, ductility, and soundness of deposits for thicknesses up to 1.5 mm. The

development of high-speed silver plating was directed toward these requirements by

Mathers and Gilbertson [37], who reported on the adhesion of thick silver deposits on

steel. Later investigations of Simon and Lumley [38] pointed to the use of vigorous

agitation for high-current-density silver plating. Further advancements were made

during World War II with the development of high-speed silver plating of aircraft

bearings [39-41].

The adhesion of silver to the steel backing for bearing use is of prime

importance. The preplate procedure recommended by Hart and Heussner [4] is as

follows: Degrease, anodic alkaline clean, anodic sulfuric acid etch, followed by an

activating 1.2 N hydrochloric acid dip. A nickel strike is interposed between the steel

and the conventional silver strike used before plating the silver deposit. The nickel

strike provides a higher degree of adhesion and greater reliability than plating silver

directly on steel. A low-pH (2.0-2.0) Watts-type nickel bath was recommended by

Schaefer [11] for this procedure.

When thick deposits of silver with mechanical strength are required for a

bearing material or electroforms, the rapid plating process becomes practical. Higher

plating speeds are made possible by increasing the agitation, raising the temperature,

adjusting the solution composition, and carefully controlling the symmetry between

anode and cathode. Filtration of the bath is important to prevent roughness caused by

dirt or anode particles adhering to the cathode. Continuous filtration and bagging of

anodes are mandatory if high-quality, thick deposits are to be produced.

Wide variations in solution compositions and operating conditions for high-

speed silver plating have been proposed. Typical compositions are as follows:

A[11] B[42]

Silver cyanide 45-50 g/1 75-

110 g/1

Potassium cyanide (free) 45-50 g/1 50-

90 g/1

Potassium hydroxide 10-14 g/I 0-

30 g/1

Potassium carbonate 45-80 g/1 15

g/1 (min)

Potassium nitrate 40-60 g/1

0

Brightener as required

Temperature, °C 42-45 38-49

The functions of the nitrate and hydroxide anions are not completely

understood. Both aid in anode corrosion and the hydroxide aids in stabilizing the bath

against cyanide decomposition [12].

Agitation and electrode positioning are usually determined empirically for a

practical plating rate of 5 to 10 A/dm2. Rapid agitation is accomplished by a

combination of cathode motion or rotation and pumping of solution past the cathode

surface. The elctrodeposition cell should be designed to maintain uniform agitation.

When moderate agitation and lower current densities (1.0-2.0 A/dm2) are used in the

presence of thiosulfate as the brightener, the solution temperature should be kept at

28°C maximum to avoid nodular roughness.

The brightener commonly used for these high-speed baths is ammonium

thiosulfate. The brightener gives the desired metallurgical properties with practical

solution control. Suggested rates of addition are 0.02 to 0.05 g/1 every 24 hr [11]. On

starting the bath after a shutdown period, 0.02 g/1 is added. Continuous or more

frequent additions may be beneficial. Potassium thiosulfate is preferred by Orr [43].

Silver plate for bearings or other antigalling use often requires a heat treatment

or anneal to meet hardness specifications or to improve adhesion. The problems

associated with the proper annealing procedure carry over to silver plate subjected to

high-temperature engineering applications. Oxygen rapidly diffuses through silver at

high temperatures and, in the case of silver plate, will oxidize the underlying metal,

causing blistering of the deposit. Typical heat-treating specifications over 204°C

require the heating and cooling medium to be a neutral or reducing atmosphere.

Federal specification QQ-S-365a (Amendment 2, February 24, 1967) requires

"copper-alloy-basis metal articles on which a nickel undercoat is not used and other

basis metals whereon a copper undercoat is employed should not be used for

continuous service in excess of 150°C. Adhesion of the silver plating is adversely

affected because diffusion forms a weak silver-copper eutectic at the basis metal-

coating interface."

When silver plate is used for low load bearing or antigalling surfaces, the

thickness required is usually 7.5 min or less. Deposits in this thickness rangeare plated

from conventional silver plating baths, since higher throwing power may be desired

and plating speed is not critical. Laboratory studies [44] have shown that

superimposing alternating current, or the periodic reversal of direct current, raises the

limiting current density of a highspeed bath operated at 20 to 25°C without the use of

agitation. Current densities as high as 1.5 A/dm2 can be used with improved plate

distribution while maintaining plate quality and structure.

Silver deposits 25 to 38 ism thick are used for pressure-activated hot gas seals

in rocket engine systems [35, 45]. The seals are fabricated from heat-treated INCO

718 alloy. The mating surfaces are Hastelloy C flanges. The seals operate at high

temperatures in an oxidizing atmosphere and the silver plate is permeable to oxygen.

The underlying metal is plated with 1.3 1.4m of gold after proper surface preparation

and then baked under vacuum to diffuse the gold into the basis metal before silver

plating. This diffused gold-INCO 718 interface resists oxidation and subsequent

blistering of the silver during high-temperature operation. After this diffusion layer is

formed and silver is plated, a heat treatment in an argon atmosphere is used to soften

the silver, producing optimum sealing properties and adhesion of the silver plate. An

overlay of 0.13 to 0.25µm of rhodium prevents sticking of the silver plated materials

to the mating Hastelloy C surfaces.

An application of plating silver on an iron-nickel alloy for glass-to-metal seal

arrangement included a thin coating of indium over the silver plate to resist diffusion

of oxygen at high temperatures [46].

A unique four-step process for electroplating aluminum bus bar was

developed by Westinghouse Electric Corporation [47]. Ultrasonic irradia-tion was

used in the zincate immersion solution, as well as the silver plating solution. This

method resulted in a process that produced better adhesion and a silver coating equal

to or better than conventional processes. The effect of ultrasonic agitation, coupled

with a flowing solution in the cell, increased the limiting current density.

2.10 ELECTRICAL CONTACTS AND ELECTRONIC CIRCUITS

Silver plate has been and is used in the electrical and electronic industries

because of its outstanding electrical conductivity. The conductivity of the deposit will

vary according to the silver process employed. In ,eneral, the specific resistivity will

be in the range 0.017 to 0.024 ohm-mm /m (1.8-2.4 microhm-cm) as compared to

0.0162, 0.0178, and 0.029 for high-purity solid silver, copper, and aluminum,

respectively.

Krusenstjern and co-workers [48] report that sulfur- and selenium-con-taining

electrolytes yield deposits with 85 to 90% of the conductivity of pure silver.

Antimony-brightened deposits, however, have a conductivity of 10 to 25% that of

pure silver. A bath without addition agents gave specificconductivities of 0.0167 to

0.0196 ohm-mm2/m. Consequently, metallic brightening agents should be avoided for

high-conductivity applications.

Deposits 7.5 µ m thick are used on plug and socket contacts; thicker deposits

(500 m m or more) are used on slip rings and heavy-duty switch gear [49].

The major drawback of silver on contacts is its tendency to form sulfide films

which significantly increase contact resistance. For light-pressure, low-voltage

contacts, overlays of gold or rhodium are used [50, 51]. It has been reported [52] that

silver-plated contacts overplated with 4 Am of gold still had a large amount of

porosity. Harding [50] found that 10 /AM of gold was necessary over silver.

Consequently, there has been a strong trend not to use silver for the more sensitive

electronic circuit contacts, although Mil-G.45204-B, Amendment 2, still permits a

silver strike and underplate. The formation of sulfide films on heavy-duty switches is

less important because the heavy contact pressure and high voltage break through the

tarnish.

Silver plate is widely used in the production of waveguides for radar use

because radio frequency conductivity is directly related to electrical conductivity. The

use of periodic reverse (PR) current was proposed for this application when it was

shown that the conductivity of silver plated with PR current is higher than that plated

by conventional methods [53]. The bath used for PR plating of waveguides is a

moderate, high-speed formulation (40 g/1 silver cyanide, 40 g/1 potassium cyanide)

with 9-sec cathodic at 1.9 A/din2, 4-sec anodic at 1.2 A/dm2. This PR silver yielded

99% conductivity based on the International Annealed Copper Standard; direct

current yielded 94%. Brighteners that affect electrical resistivity should be avoided in

waveguide applications. The technique for plating these devices has been described

by Foster and Eddy [54].

Another reason for the use of silver for plating the complex forms encountered

in waveguide design is the relatively high throwing power of the silver cyanide

plating solution. Comparative throwing power values have been reviewed by Foulke

and Johnson [55]. Silver is not used extensively in the printed circuit industry because

of what is known as "silver migration." It has been shown that under a positive direct-

current potential within a damp resin component, silver will "migrate" across

insulating paths and, on drying, silver metal will be found in the body of the

insulation, producing low-resistant leakage paths [56]. Silver plating should not be

used when circuit boards are to meet Mil-Standard 275B or Mil-P-55110.

2.11 POSTPLATE TREATMENTS

The formation of tarnish films on silver, varying in color from light brown to

black, presents the most serious problem to the silver plating industry.Whereas silver

is practically unaffected by oxygen, it tarnishes readily in the presence of sulfur-

containing media. Since urban atmospheres normally contain traces of sulfur

compounds, protection from tarnish becomes very important for decorative and

electrical applications. Apart from the poor appearance, the sulfide film diminishes

the solderability and increases contact resistance of silver-plated electrical

components.

Since the early days of silver plating many methods have been proposed for

the prevention of tarnishing, by the use of both alloying elements and surface

treatment. Alloying the silver with tin, indium, cadmium, or palladium has not been

successful because of change of color on aging, lower electrical conductivity, and

difficulty in obtaining uniform deposits. Surface treatments for retarding silver

tarnishing include deposition of more noble metals over the silver plate, passivation

treatments, and the application of clear waxes or lacquers. The organic coatings are

not suitable for electrical contacts or cutlery but find some use in decorative

applications. Care in the selection of these coatings is necessary to make sure there is

no yellowing with age. Overlays of gold or rhodium are commonly used to deter

silver sulfide formation. A rhodium plate 0.31 to 1.3 /AM thick has been used to a

large extent as a protective coating for jewelry and silver contacts. Gold overlays also

offer protection for silver-plated electrical contacts. However, Harding [50] reports

that many instances of tarnishing have occurred through pores in a 7- Am overlay of

gold. The creep of sulfide tarnish on gold-plated silver contacts has also been

discussed by Antler [51].

Thick overlays of gold, on the other hand, can be very useful. If, for example,

25 Am of a highly conductive corrosion-resistant metal is required, it would be more

economical to plate 15 Am of silver with an overlay of 10 Am of gold, than one 25-

Am layer of gold.

Chromate passivating treatments have been developed which are quite

effective in protecting silver plate from tarnishing [57, 58]. The best protection is

afforded by films which are developed with the aid of direct current or a galvanic

couple (Al—Ag or Ag—Zn). The work is made cathodic at about 2 A/dm2 or in

contact with aluminum or zinc for 1 to 5 min in the following solutions:

Potassium dichromate 25-50 g/l

Potassium carbonate 25-50 g/1

Potassium hydroxide 25-50 g/1

It is operated at 22 to 30°C using stainless steel anodes. Although corrosion of

the silver is not permanently prevented, a worthwhile improvement in tarnish

resistance is achieved with minimum effects on contact resistance and solderability.

However, when a low contact resistance is critical, tests should be carried out to

ascertain whether this treatment impairs electrical properties.

The packaging of silver-plated parts is very important for temporary

prevention of tarnish. Paper products in contact with the parts should be free of sulfur

compounds and, preferably, be impregnated with antitarnish chemicals. Transparent

plastic bags are especially suitable as a protective package.

An accelerated tarnish test has been used by Goldie [59] and Dettner [60] to

evaluate postplate treatments for silver. The test consists of immersion of the treated

sample in 2% potassium polysulfide solution at 20 to 25°C. A properly passivated

silver part must withstand a 15- to 30-min dip without breakdown of the passivated

surface. Dettner reviews the various methods of tarnish prevention and accelerated

tests. The Silver Institute reported a number of antitarnish methods [61], including the

use of mercaptans [62-65].

2.12 ANALYSIS AND TESTS

The Hull cell is used for brightness control .Often the amount of silver is

specified in terms of weight per piece. Large items are often weighed before and after

plating. Grab samples of a number of smaller pieces are stripped and the weight per

piece found by the weigh-strip-weigh method. For Britannia metal, make anodic in

sodium cyanide solution; for nickel-silver, brass, or copper, use a solution containing

19 parts sulfuric acid and one part nitric acid, both concentrated.

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