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This page contains useful information for students and product designers.

The following material has been reproduced from the MFSA Quality Metal Finishing Guide,

Vol. 1 No. 1-B "Decorative Precious Metal Plating"

Decorative Precious Metal Plating

A program to improve and control the quality of a metal product should start at the desk of the designer. The metal finisher is restricted in what he can do by certain basic principles of mechanical finishing and of electroplating. The engineer should understand the limitations imposed by shape and size of components to facilitate quality finishing at an acceptable cost. The designer can exert as much influence on the quality attainable in finishing a part as can the electroplater himself. ASTM Standard B-5O7 can provide the designer with helpful information.

Significant Surfaces

An important term used in specifying metal finishes is "significant surfaces". In most products the same standard of quality is not required over every square inch of surface. Instead, the quality specifications apply and compliance is expected only for the so-called "significant surfaces" defined by mutual agreement between the producer and purchaser as follows:

Significant surfaces are defined as those normally visible (directly or by reflection) which are essential to the appearance or serviceability of the article when assembled in normal position, or which can be the source of corrosion products that deface visible surfaces on the assembled article. When necessary, the significant surfaces shall be the subject of agreement between purchaser and manufacturer and shall be indicated on the drawings of the parts, or by the provision of suitably marked samples.

Design for Mechanical Finishing

Metal products which are to be coated with copper/nickel/decorative precious metal or substrates utilizing non-nickel plating processes followed by the decorative precious metal finish are generally subjected to abrasive polishing with wheels or mass finishing techniques in preparation for the plating operations. This is done to aid in securing an attractive, uniform, mirror-like or satin appearance on the finished part. Mechanical finishing is an expensive operation. To reduce costs and assist the metal finisher in improving the appearance and quality of the product, the designer should consider certain rules applicable for parts requiring mechanical finishing.

Avoid blind holes, recesses and joint crevices which can retain polishing compounds and metal debris.
Avoid intricate surface patterns which will be blurred in polishing.
Significant surfaces should be exterior, reachable by ordinary polishing wheels or mass finishing media.
Avoid sharp edges and protrusions which cause excessive consumption of wheels.

In small parts, which are to be barrel processed, the above rules apply. This includes the requirement that the parts must be sturdy enough to withstand the multiple impacts of barrel rotation and will not entangle causing damage or incomplete finishing. Small flat parts, which tend to nest together, should be provided with ridges or dimples to prevent such nesting.

Design for Racking, Draining and Air Entrapment

Most metal parts weighing more than a few ounces or that require a high degree of surface finish or a jewelry finish, are not plated in bulk in barrels but are mounted on racks for processing in cleaning and electroplating tanks. Design considerations relating to racked parts are described below.

Consult the plating department to make certain that parts can be held securely on a plating rack with good electrical contact without masking a significant surface. Many difficult racking problems can be solved by design modification.
Provide for good drainage of processing solutions from racked parts. Certain shapes tend to trap solution which then causes contamination by carry over, possible corrosion of the part and waste of materials. Carry over aggravates the problem of waste disposal and adds excessive cost due to chemical losses. In design, avoid rolled edges, blind holes, and spot-welded joints. Drain holes are especially important in irregular shapes and tubular parts.
Avoid shapes which can trap air on entry into processing tanks if this air could block access of solution to areas requiring treatment. Wherever air can be trapped, hydrogen or oxygen gas may also accumulate during a cleaning or plating step.

Design for Good Distribution of Electrodeposit

Experience and cost accounting show that simple shapes are always finished more uniformly and more economically than complex shapes. This rule is number one for the designer.

One of the most important factors determining the quality of a coating is its thickness on significant surfaces. Fundamental laws of electrochemistry (current distribution) operate to prevent perfectly uniform deposition of an electrodeposited coating on a cathode of any practical shape and size. Portions of the work which are nearer the anodes tend to receive a heavier deposit. Sharp edges or protrusions at all current densities tend to steal a disproportionate share of the current. The goal of the designer and the plater is to make thickness variations as small as possible. At the same time, uneconomical wastage of metal by excessive build-up of both non-significant and significant areas must be avoided. The same difference in plated thickness found within a plated article also exists from piece to piece on the rack of plated work and within the barrel load. Metal distribution is of particular importance in precious metal plating due to both cost sensitivity from overplating and the lack of adequate corrosion protection in underplated areas.

It is possible to estimate metal distribution ratios from models or mock-ups, but there are also empirical rules. These can guide the designer to improved uniformity of thickness, hence, improved quality with greater economy. These general principles and various sketches illustrate what has been learned from practical experience.

Avoid concave or perfectly flat significant surfaces. Convex or crowned areas receive more uniform coatings. Use a 0.4 mm per 25.4 mm (0.015 inch per inch) crown - minimum.
Edges should be rounded to a radius of at least 0.4 mm (1/64 inch) preferably 0.8 mm (1/32 inch).
Re-entrant angles or corners should be filleted with a generous radius. Make such radii as large as possible.
Avoid concave recesses, grooves, or slots with width less than one-half the depth.
Minimize the number of blind holes because these must usually be exepted from minimum thick-ness requirements. Where necessary , limit their depth to 50% of their width. Avoid diameters less than 6 mm (7/32 inch).
Countersink threaded holes to minimize electroplate thickness at their peripheries and facilitate insertion of fasteners after plating.
If fins or ribs are required, reduce their height and specify a generous radius, 1.6 mm (1/16 inch) at each base. Round off tips with radii of at least 1.6 mm (1/16 inch). Multiple parallel fins should have spacing between centers equal to four times the width of the fin. Broad hollow ribs are preferred over slender solid ones.
Adopt recessed in preference to raised letters and insignia, but round off edges and provide gentle contours.
Integrated studs for fasteners should be shortened as much as possible and inside angles at each base should be rounded generously. Tips should be similarly rounded.
Studs or bosses with hollow centers should be shortened as much as possible and angled 90 degrees from the major plane of the part. All bosses should face the same direction.
Assist the plater by clearly marking significant surfaces in part drawings.
Avoid use of a variety of basis metals in any one part to be plated. The contact of dissimilar metals may interfere by galvanic action with covering power or with adhesion of the deposit.

DESIGN FEATURES THAT INFLUENCE ELECTROPLATABILITY

The effect of the basic design of a product or component upon the effectiveness or durability of the plating used has been the subject of much study and research. Many failures for which the plater has been blamed can be attributed to the original design.

A major contribution to the plating industry was made by the Zinc Institute, Inc. when it sponsored a design study by Battelle Memorial Institute which has resulted in the establishment of basic design principles to be applied to zinc die castings. The principles can be applied to other substrates.

HISTORICAL PERSPECTIVE

The history of the deposition of precious metals can be traced to Luigi V. Brugnatelli, who probably first performed his electrodeposition of gold around 1800 using the Voltaic Pile as discovered by his friend, Allisandro Volta. However, an insult from Napoleon Bonaparte caused Brugnatelli to confine the publication of his works to his own journal, which effectively caused the loss of this information for almost forty years.

John Wright, from Birmingham, England, found that potassium cyanide was a suitable electrolyte for gold and silver electroplating. His work, combined with that of the Elkington cousins and other developments by Barratt, resulted in the issue of several patents in 1840. These discoveries and patents are the foundation of modern gold and silver plating.

This alliance, and its discoveries, was almost certainly precipitated by the invention of copper electroplating (referred to as electroforming and electrotyping in the literature) developed in the latter part of 1838, early 1839. This led to the adaptation of galvanic current for the deposition of gold and silver.

Gold and silver plating proceeded at a fast pace. By 1842, Elkington & Mason had a successful silverware manufacturing facility where George Elkington produced, using the new gold electroplating processes, spectacle frames, pen nibs, etc. of such quality and low cost that he dominated the entire trade in Birmingham. Charles Christofle (licensee of Elkington), in France, had overnight commercial success with his electroplating company producing hollowware and flatware for the government and society of France. He did provide the table setting for the official receptions of Napoleon III.

It was in Russia that the first large scale electroplating of copper, silver and gold took place. Hundreds, perhaps thousands, of statues, icons and other religious artifacts, as well as numerous copper plates for cathedral domes, were gold plated. One cathedral dome alone required almost 500 kilograms (about 16,000 troy ounces) and appears to be the first specification plating. Two parts from every 100 were checked by strip and weigh methods; if both parts failed to meet the specification, the entire batch was rejected.

From that time on, the gold and silver plating industry in Europe and the United States grew in size with practically no new developments. The passing of the Victorian grand object'd'art relegated gold plating to faux jewelry, spectacle frames and other inexpensive novelties and silver plating to the flatware and hollowware industries. Much of the technology used to plate large items, heavy deposits and massive electroforms was lost. This quiescent period lasted almost eighty years, for it was not until World War II and the advances in the electronics industries that interest was renewed in gold electroplating (with the exception of the Providence, RI and NY areas). It was not until the 1950's that the rediscovery of bright cyanide gold plating solutions and the techniques for electrodepositing fine grained, uniform, thick gold plate occurred. In the 1960's, acid gold and gold alloy systems were developed to produce deposits with specific physical properties, ductility, wear and corrosion resistance, deposit purity, etc., etc. Non-cyanide gold electrolytes were also introduced during this time period. Chemical developments since have provided plating solutions that show improved performance in the areas of deposition speed, throwing power, covering power, distribution, and a variety of colors.

PREPARATION FOR ELECTROPLATING

Many of the items that are precious metal plated are first surface finished by means of mechanical polishing or mass finishing. The MFSA Quality Metal Finishing Guide, Volume 1, Number 5, entitled "Mass Finishing", will provide a more than adequate introduction on this subject.

As with all electroplating operations, the first step after any polishing operation is to clean and activate the basis metal so proper adhesion and defect-free appearance of the finished part may be achieved.

Since the introduction and adoption of the Montreal Protocol, the gradual phasing out of CFC's and other ozone depleting chemicals has resulted in the virtual elimination of degreasing by means of chloro and/or fluorocarbon type solvents.

Cleaning, degreasing and activation are carried out in aqueous cleaning solutions, utilizing various alkalis, surface active agents, detergents, chelators, saponifiers and emulsifiers. Activation of the surfaces to be plated is usually provided by either dilute mineral acids or by dry acid salt solutions.

After cleaning, most of the items that will finally receive their noble metal topcoat are electroplated with copper and/or nickel in order to achieve the fully bright or satin finished appearance.

COPPER ELECTROPLATING

A wide range of types of functional or decorative copper electrodeposition processes are presently available. The type of copper to be used is defined by regulatory laws, speed needed, substrate utilized, and appearance requirements. The general types of copper electrodeposition utilized at the present time are:

ACID COPPER DEPOSITION

Decorative, bright, high leveling for rack or barrel, electroforming, rotogravure and other functional coatings, high speed functional for wire, strip and rods, electronic copper (ductile, high throw baths) acid copper strike undercoat for plastics, electrowinning and electrorefining of copper

ALKALINE COPPER DEPOSITION

Alkaline, cyanide-free copper for rack or barrel, Alkaline, cyanide copper deposition, Alkaline, cyanide copper strike undercoat.

All but the cyanide copper baths deposit from a divalent copper ion, taking twice as many ampere hours to deposit the same amount of copper. However, since the acid copper baths and some of the alkaline baths are able to operate at higher cathode current densities, the negative aspect can be easily removed.

All the types of alkaline copper baths utilize some type of complexed ion in order to tie up the copper ion and prevent it from precipitating out. This complex typically can be cyanide, tartrates, pyrophosphates, or phosphonates. Acid copper, on the other hand, deposits right out of a plain copper ion, whether the anion in the bath is sulfate, fluoborate, or other. Acid copper is also the process, due to the additives used, that can level out imperfections and scratches in the substrate. Therefore, it is often recognized that acid copper is the preferred deposit for decorative applications.

A copper strike puts on a barrier copper coating which allows subsequent deposits to go on trouble-free. In the case of plastics, the strike increases the thickness of the electroless coatings, allowing for conductivity and stability under higher voltages. A copper strike is essential for the subsequent acid copper plating of zinc based die castings and tin based castings.

DECORATIVE NICKEL

Decorative bright nickel electrodeposits are often used as an undercoat for precious metal coatings. Nickel undercoats provide brightness to improve reflectivity and leveling to smooth out surface defects. They also improve corrosion resistance, reduce porosity and can act as a diffusion barrier to prevent the base metal from migrating into the precious metal topcoat.

Today's bright nickel processes are vastly superior to earlier processes. Current nickel processes provide outstanding leveling characteristics and excellent physical properties. Most employ a WATTS electrolyte which can be modified to satisfy specific plating requirements. For example, a high chloride WATTS type bath is employed where plating speed is important; a low chloride WATTS is more suitable for applications that require excellent ductility and low stress. Typical basic formulations are shown below.

Watts Bath
Nickel Sulfate 40.0 ounces/gallon
Nickel Chloride 8.0 ounce s/gallon
Boric Acid 5.5 ounces/gallon
High Chloride
Nickel Sulfate 8.0 ounces/gallon
Nickel Chloride 30.0 ounces/gallon
Boric Acid 5.5 ounces/gallon
Low Temperature or Mixed Bath
Nickel Sulfate 35.0 ounces/gallon
Nickel Chloride 15.0 ounces/gallon
Boric Acid 6.0 ounces/gallon

ADDITION AGENTS

The brightness and leveling of bright nickel deposits are, perhaps, the most desirable properties for the jewelry industry. Organic addition agents are used to provide these characteristics. The additive systems are usually a combination of ingredients called Primary and Secondary Addition Agents. These ingredients work synergistically to provide very bright deposits that can substantially smooth out surface defects common to the base metal, thereby providing a brilliant surface that is substantially free of surface defects. Supplier's recommendation must be carefully followed to provide optimum performance.

Anionic surfactants are often used in bright nickel baths to emulsify oils and prevent pitting. They can be measured by monitoring the surface tension of the nickel plating bath. Low foaming wetting agents are used in air agitated baths and reduce surface tension to about 40 dynes/cm. Since cathode rod agitation baths have a greater tendency to pit because of higher organic loading and less agitation than air agitated baths, higher foaming surfactants with greater wetting properties are used. The surface tension of these baths are generally less than 35 dynes/cm.

AGITATION

Agitation is very important with respect to plating speed, as well as brightness and leveling. The greater the agitation, the faster a bright nickel bath will plate and level out surface defects. Cathode rod agitation is often used when bright nickel plating for jewelry and other applications. While air agitation would provide faster speed and better leveling, it is normally not used when precious metal plating is the final finish. This is because most jewelry is not positively racked (held rigid) and the violent agitation would knock parts off the racks and into the plating tank.

EQUIPMENT

Bright nickel baths require tanks that are polypropylene or steel lined with approved rubber or Koroseal. Anodes are generally electrolytic squares or sulfur depolarized rourids that are placed in titanium baskets. Rolled carbon bars can also be used. All anodes must be bagged with either laundered cotton or a synthetic, such as polypropylene or dynel, to prevent roughness. Air agitation is distributed through a properly designed sparger and supplied by a low pressure blower. Mechanical agitation is generally supplied by moving the cathode bar back and forth. Heating coils, utilizing steam or electricity, are required to maintain proper operating temperature and are generally made of titanium, tantalum or TEFLON. Filters are generally used to remove particulate matter and prevent roughness. Normally, the filters are packed with activated carbon to remove organic degradation products, oils or impurities that have been dragged down the plating line. It is extremely important that all materials be approved by your plating supplier.

DECORATIVE BLACK NICKEL

Black nickel plating solutions provide a distinctive finish suitable for a wide variety of articles. Any black finish, ranging from matte to a brilliant luster, can be obtained. Matte finishes have been found suitable for industrial and military instruments, cameras, microscope and binocular parts. Articles requiring lustrous finishes include tubular furniture, plumbing fixtures, buttons and trophies. Often these surfaces can be mechanically relieved to provide highly desirable antique finishes suitable for casket hardware, jewelry, buckies and lamps. The appearance of the antique finish is altered by the substrate on which the black nickel coating is applied, e.g., copper, zinc, nickel, etc.

Corrosion resistance of black nickel coatings is generally poor, so a clear coating is either sprayed or applied electrophoretically to the piece. The clear coating prevents tarnish and base metal deterioration. In cases where the application of a clear coating is impractical, a light oil can be applied to the significant surfaces with a soft cloth.

Black coatings can be achieved by chemical or electrolytic treatment of a dull or bright nickel surface or by plating a black coating directly from a proprietary electrolyte. In the latter case, addition agents and/or post treatments are used to intensify the blackness. Different shades of blackness can be achieved, ranging from a warm, slightly dark metallic finish to a jet black deposit.

Processes that provide black coatings directly from a plating tank utilize the same equipment as other decorative processes. Likewise, the articles are cleaned and prepared in exactly the same manner. Lined tanks are required. Heaters, air spargers, anodes and bags should be used according to the recommendations of the specific supplier of the process. Reputable plating suppliers will also recommend specific cycles that are suitable for the user's specific needs.

NICKEL CONTACT DERMATITIS

In Europe, the use of nickel in components that come into contact with the skin, or that are inserted into the human body, is currently under review and potentially subject to future legislation. In October of 1994, the European Council of Research Ministers adopted Directive 94/27/EC, a supplement to the 1976 Directive 76/759/EEC, limiting the use of nickel. It is anticipated this Directive will be implemented by the end of 1997. The Directive will then need to be adopted by each member of the European Community.

Directive 94/27/EC requires that nickel may not be used in the applications below, nor may products not conforming to the directive be marketed.

Post assemblies that are inserted into pierced ears or other human body parts, unless the nickel content is less than 0.05%.
In products intended to come into direct contact with the skin, e.g., earrings, necklaces, bracelets, anklets, snaps, fasteners, zippers, etc., if the rate of release is more than 0.5 micrograms per cm2 per week.
In products such as those that have a non-nickel coating (a lacquer or heavy gold electroplate for example) whose rate of nickel release will not exceed 0.5 micro-grams per cm2 per week for a period of at least two years of normal wear.

It is recommended that any manufacturer of these products, many of which are plated with decorative precious metals, especially those manufacturers who export their goods to Europe, will need to:

(a) Carefully review the Directive, and (b) Consider alternatives to the use of nickel as an undercoat to the precious metal.

Note: Legislation exists limiting the use of nickel in Denmark, Germany and Sweden. Teflon* is a Trademark of DuPont.

NON -NICKEL ALTERNATIVES

The most obvious alternate to nickel is acid copper. Acid copper can provide equal brightness and leveling to that obtained from bright nickel solutions when rack plating. Acid copper can also provide a suitable satin or matte surface for those applications. Recent developments have vastly improved the performance of barrel acid copper plating providing similar performance to (barrel) nickel plating.

The most common precious metals plated, namely gold and silver, will migrate into copper. It is, therefore, necessary to plate a barrier layer which will retard or prevent this migration. These barrier layers are either palladium, palladium/cobalt alloy, yellow bronze, white bronze or a combination, such as yellow bronze plus palladium.

YELLOW BRONZE AND WHITE BRONZE

Both these processes are deposited from high, free cyanide plating solutions and offer challenges to the electroplater because of the alloy deposited. Traditionally, bronze plating used ammonia to maintain good color and soluble alloy anodes. Modern alloy baths contain alternate complexors and, therefore, do not require the use of ammonia and, for better control, utilize full additive systems and inert stainless steel anodes.

Typical bath formulations are shown in the Table below.

BRONZE PLATING SOLUTIONS

Yellow

Copper g/l 15 to 36
Tin g/l 5t o 9
Zinc g/l 2 to 5
Free Cyanide g/l 35 to 58
Temperature 100 to 120 degrees F
pH 12.0 to 13.5
Current Density 20 to 50 ASF
Alloy Composition
Copper 72 to 88%
Tin 12 to 28%
Zinc 6 to 10%

White

Copper g/l 12 to 18
Tin g/l 10 to 16
Zinc g/l 2 to 5
Free Cyanide g/l 45 to 60
Temperature 110 to 130 degrees F
pH 1l.0 to 12.0
Current Density 10 to 50 ASF
Alloy Composition
Copper 54 to 65%
Tin 25 to 40%
Zinc 6 to 10%

PALLADIUM AND PALLADIUM ALLOYS

Palladium and palladium alloys, notably palladium-nickel and palladium-cobalt, are beginning to be used more extensively in the jewelry industry and other decorative precious metal applications. Low porosity and bright white deposits are used to enhance the appeal of many decorative items.

Deposits of pure palladium and palladium-cobalt are used as barrier layers between bright acid copper deposits and gold deposits in the production of non-nickel containing products. A deposit thickness of 7 to 10 microinches (0.175 to 0.25 microns) will greatly limit the migration of gold into the copper electroplate and vice versa. Palladium and palladium alloys are also used as final white finishes. The color of the deposits obtained is bright white and is often used as a rhodium replacement. For decorative applications, palladium and its alloys can be deposited from ammoniacal systems in which palladium is present as an amine complex. Other complexing systems are also used when specific deposit properties are required.

Since ammoniacal solutions will readily tarnish copper, copper alloys and nickel, and copper contamination in quite small quantities will cause darkening of the deposit, it is good plating practice to strike the work prior to plating with palladium and/or palladium alloy processes.

GOLD PLATING

Today's decorative gold plating processes are applied to a very diverse range of consumer products. Typically, plated items include watch cases and bands, plumbing fixtures, writing instruments, jewelry, eyeglass frames, cigarette lighters, fashion accessories, and lighting fixtures. Deposit thicknesses will vary with the specifications and application.

Watch cases, watch bands and writing instruments are often plated to thicknesses of 2 to 5 microns (80 to 200 microinches). The watch industry often uses duplex gold by first applying a thick deposit of alloy gold, typically in the range of 12 to 16 karat, followed by a hard acid gold deposit of up to 0.5 microns (20 microinches). This final layer of gold also provides the final color.

Plumbing, bathroom and kitchen accessories usually fall into two segments. Those that are plated with gold to thicknesses ranging from 0.5 microns minimum to 3 microns or more (20 microinches to 120 microinches) and marketed to the luxury markets. The other segment utilizes flash deposits of 0.075 to 0.125 microns (3 to 5 microinches), followed by a topcoat, either clear powder coat or electrophoretically applied lacquers.

Fashion jewelry typically uses deposits ranging in thickness from a flash of 0.075 microns to 2.5 microns or more (3 to 100 microinches) of hard gold deposit. The requirements for most other industries are similar.

The gold deposits for these diverse applications are obtained from chemistries which are segmented into four (4) types, as follows.

HARD ACID GOLD

Hard acid gold is defined as the premier gold plating process. First developed in the early 1950's. These processes, based on potassium gold cyanide and operating at pH values as low as 3.5, are able to produce a multitude of basic colors, ranging from rich yellow Old English through the Swiss Normalized gold color standards, and include the full range of Hamilton colors.

The deposit purity, as plated, ranges from 90 to 99%. The deposits also exhibit excellent wear resistance, durability, and color stability. The hardness and brightness of the deposits is obtained from the inclusion of various metal complexes in the plating solution matrix. These metals (cobalt, nickel, iron, indium and others), either individually or in combination, are co-deposited with the gold and provide a range of colors. Typical formulations are reviewed in Table 1.

TABLE 1 - HARD ACID GOLD SYSTEMS

Constituent Concentration g/l
Gold 2 to 10
Conducting Salt 60 to 150
pH 3.2 to 4.4
Temperature 70 to 120 E
Nickel Complexed 0 to 10
Cobalt Complexed 0 to 8
Indium Complexed 0 to 1
Iron Complexed 0 to 3

In the late 1980's, further development provided for the inclusion of organic additives within the solution matrix. These additives offered lower gold concentration, increased the brightness range, extended the current density range, and improved productivity. Other benefits of these organic additives are improved distribution, enhancement of the color and richness of the deposits. These hard acid gold systems enjoy now, and are expected to enjoy for the foreseeable future, a predominant share of the decorative gold plating market.

ALKALINE NON-CYANIDE GOLD

The major discovery of this unique chemistry during the 1950's resulted in the development of cyanide-free sulfite systems offering outstanding benefits in metal distribution, ductility, electroforming, and the ability to build brightness. These systems contain gold sulfite, free sulfite, complexing agents, stabilizing agents, and color additives; offer an excellent range of colors as indicated in Table 2, and were well received by the optical frame industry. Cyanide-free sulfite systems are particularly attractive to the European optical frame industry, where the pink color of rolled gold eyeglass frames is very fashionable.

TABLE 2 - CYANIDE-FREE SULFITE SYSTEMS - COLOR ADDITIVES

Color Metal(s) Additives
Green Cadmium, Silver, Zinc
Pink - Red Copper
Pale Yellow - Gray Palladium
Champagne Palladium , Copper
Pale Yellow - White Nickel

These non-cyanide systems produce thick, ductile deposits that are extremely useful in high build and electroforming applications. Typical formulations are shown in Table 3.

TABLE 3 - CYANIDE-FREE SULFITE SYSTEM - TYPICAL FORMULATIONS

Constituent Concentration g/l
Gold 10
Metal Complex 5
Stabilizer 15
Free Sulfite 50
Complexor 5 +
pH 9to10
Temperature 95 to 130 E
Current Density 10 Amps./sq. ft.

ALKALINE CYANIDE GOLD

Alkaline cyanide gold systems originated in 1840 and have been associated with having the widest color range. Over 600 shades were plated in the 1930's and 1940's. The formulations, based on potasium gold cyanide or potassium auricyanide, free potassium cyanide and coloring additives, are extremely versatile, offering flash coats, thin and thick deposits, alloy deposits often used in duplex gold systems, and electroforming. However, as improvements have been made in other systems, the future use for alkaline cyanide gold will mostly be limited to alloys, duplex systems, and electroforming. Typical formulations are shown in Tables 4 and 5.

TABLE 4 - FLASH GOLD PLATE

g/l
Gold as Potassium
Gold (I and II) Cyanide 0.8 to 6
Free Potassium Cyanide 2 to 15
Di Potassium Phosphate 2 to 25
Metal Coloring Additives 0.025 to 3.0
pH 10 to 12.5
Temperature _F 140 to 160
Current Density Amps./sq. ft. 10 to 50

COLORING ADDITIVES

Green to White Silver, Tin
Zinc, Cadmium
Yellow to White Nickel
Pink to Red Copper, Nickel

TABLE 5 - HEAVY GOLD PLATE

g/l
Gold (as Gold Potassium Cyanide) 8 to 20
Silver & Silver Potassium Cyanide 0.3 to 2
(For bright deposits)
Potassium Cyanide 15 to 100
pH 12 to 12.5
Temperature _F 60 to 80
Current Density
Rack Amps./Sq. Ft. 3 to 8
Barrel Amps./Sq. Ft. 1 to 2

ALLOY GOLD PLATING

Similar formulations to those shown in Table 5 are utilized to produce low karat alloy gold electrodeposits ranging from 12 karat to 23 karat (50% to 98% of gold in deposit). Alloying metals are usually silver, nickel, copper and cadmium. The free cyanide content of these alkaline solution formulations is often relatively low.

These processes are used as a final finish such as white gold, or as part of a duplex system as a method of producing thicker deposits at lower cost. Typical duplex systems are used by the watch case industry, where alloy gold is used as an undercoat, followed by at least 1 micron (40 microinches) of hard acid gold of the desired color.

NEUTRAL CYANIDE GOLD

Alkaline gold systems are modified to provide more accurate control of the complexed alloy systems by using alternate chelating agents in place of free cyanide. For example, a typical formulation may use complexors like EDTA. As these solutions operate at or near a pH of 7.0, they are designated as neutral cyanide gold processes. These processes are designed for ease of operation and for use in flash operations only. Typical formulations are shown in Table 6.

TABLE 6 - NEUTRAL CYANIDE GOLD PROCESSES

g/l
Gold (as Gold Potassium Cyanide) 0.75 to 2.0
Conducting Salt 15 to 30
Complexor 6 to 10
pH 6.5 to 7.5
Temperature _F 110 to 140 _F
Current Density Amps./Sq. Ft. 10

COLOR ADDITIVES

Yellow to White Nickel
Green Silver
Red Copper
Pink Silver/Copper
Nickel/Copper

STANDARDS

Classifications are set by international agencies worldwide. In the United States, the Federal Trade Commission classifies gold deposits by thickness.

Gold Flash, Gold Wash and Gold Tone refer to gold deposits that are less than 7 microinches (0.175 microns) in thickness. Gold Electroplate or Gold Electroplated refers to gold electrodeposits of at least 7 microinches (0.175 microns or more) minimum.

To carry Heavy Gold Electroplate or Heavy Gold Electroplated designation, the gold deposit must be at least 10 karat in fineness and be 100 microinches (2.5 microns) in thickness. If the gold is produced from a specific process, this may also be stated, for example, as Heavy Gold Electroplate (x karat) or Heavy Gold Electroplate (x process). If products are to be exported to overseas markets, the relevant local standards should be used, i.e., the ISO Standards in Europe.

GILDING

Gilding, or the art of coating metals and non-metals with gold, is indeed an ancient art. This art was certainly practiced by the Egyptians as can be seen by the artifacts recovered from the Tombs of the Valley of Kings. Cleopatra was reported to have the beams of her palace gilded with gold. There is some evidence to show that gilding was practiced by the Babylonians. Broaches and pins of gilt have been found on the site believed to be that upon which Babylon sat.

Arab artisans gilded copper and silver by the mercury almagam method (vermeil). This method involves the application of a coating of mercury and then burnishing on a layer of gold leaf. In the mid-nineteenth century, many items including spectacle frames, candelabra and the dome of the cathedral church of St. Isaac's in St. Petersburg, were gilt by this very dangerous method, often resulting in death and/or severe disability of the artisan. It is reported that some 60 craftsmen died from the resultant mercury poisoning from the gilding of the cathedral dome of St. Isaac's.

During the eighteenth and early nineteenth century, large amounts of "water gilding" was carried out using dilute solutions of gold chloride. G. R. Elkington improved this process and was issued patents for developing an immersion gold (or gilt) bath based on gold chloride neutralized with potassium bicarbonate. Elkington's process was vastly superior to traditional water gilding and approached the quality and thickness of the mercury gilding deposits. To this day, gilding is still produced primarily on non-metallic substrates of wood or plaster religious artifacts.

BRUSH PLATING

Brush plating is the modern equivalent of gilding. The general principle of brush plating is to connect the work to the negative side of the D.C. power source - a battery, small or larger rectifier, or even a portable D.C. generator.

The anode, usually non-soluble, is encased in a porous material such as felt. The porous pen or brush is dipped into the specially formulated plating solution. The pen or brush is then applied to the surface of the part to be plated (the cathode) using a brushing action. In large applications rollers, similar to paint rollers, are employed. Perhaps the most spectacular example in North America of brush plating is the domes of the Greek Orthodox Church of Markham, located near Markham, Ontario, Canada. The copper domes of this cathedral, including the orbs, crosses and other fixtures, were all brush plated with gold. The effect is both dramatic and spectacular. Other examples include the elevator doors of the MGM in Las Vegas, NV and the independent man atop the RI State House.

Other precious metals can be, and often are, brush plated. These include silver, for spot repair of silver plated hollowware; Rhodium, providing a bright white finish in recesses of jewelry, and Ruthenium, for contrast effects, etc.

Consult your precious process supplier for details to meet your specific needs.

SILVER PLATING

Silver electroplating is as old as the plating industry itself, first patented and produced in 1840 (British Patent 8447, 1840) by the Elkington cousins. The basic electrolyte has changed little since then. The only developments until recently (for decorative applications) has been that of grain refiners and brighteners, as today's metal deposits are required to be bright in order to reduce or eliminate the cost of polishing.

Typical formulations are given below.

Constituent Concentration g/l (oz./gal.)

I II III

Silver (Metal) 5 to 40 (0.6 to 5.5) 5 to 40 (0.6 to 5.5) 4.50 to 120 (1 to 14)
Potassium Cyanide (free) 16 to 130 (2 to 18) 45 to 160 (5.5 to 20)
Sodium Cyanide (free) 16 to 110 (2 to 15)
Potassium Carbonate 15 to 90 (2 to 6) 16 to 80 (2 to 10)
Sodium Carbonate 15 to 45 (2 to 3)
Potassium Hydroxide 2 to 6 (.25 to .75) 4 to 30 (0.5 to 4)
Temperature _F 68 to 86 68 to 76 100 to 120
Current Density ASF 1 to 40 5 to 15 5 to 100

The plating solution is best made using potassium silver cyanide available in high purity from supply houses and readily soluble. This material has a silver content of 54.0% ± 0.2%. Silver cyanide is also available with a silver content of about 80%; it is not soluble in water. While considered pure, its purity is often less than that of silver potassium cyanide. The use of the silver potassium cyanide salt eliminates the possible source of impurities which may affect the performance of the operating bath. As is common with all alkaline precious metal solutions, immersion coatings on less noble metal surfaces are a common occurrence. This will cause poor adhesion. It is, therefore, desirable to apply a silver strike in order to minimize or eliminate this effect. Typical strike formulations are shown below.

Constituent For Ferrous Metal For Non-Ferrous Metal

I II

Silver (Metal) 1.5 to 2.5 g/1 1.5 to 2.5 g/l 1.5 to 2.5 g/l
Potassium Silver Cyanide 1.5 to 3.0 g/l 5 to 8 g/l 1 to 2 g/l
Sodium Cyanid 60 to 90 g/l 60 to 90 g/l
Potassium Cyanide 20 to 30 g/l
Potassium Hydroxide 18 to 22 g/l

Rinsing the work between silver striking and silver plating is not required. Non-cyanide solutions are now being used primarily for electronic and functional applications. However, as research continues, fully bright, decorative processes will be available in the not too distant future.

RHODIUM PLATING

Of the platinum group metals, rhodium has found wide acceptance in decorative precious metals applications. Rhodium has several desirable properties - it's brilliant white color, high reflectivity, and hardness which make it very popular with the jewelry and faux jewelry industries. Rhodium can provide excellent tarnish protection for sterling silver and silver plated flatware and hollowware from quite thin deposits. Typically, rhodium electroplate is deposited on precious and faux jewelry, sterling and silverplate to a thickness of 0.05 to 0.125 microns (2 to 5 microinches). This thickness of rhodium is produced in about 20 to 60 seconds from phosphate, sulfate or phosphate-sulfate baths.

Typical formulations are shown below.

Phosphate Sulfate Phosphate/Sulfate
Rhodium g/1 1.5 to 2.0 1.3 to 2.0 1.5 to 3.0
Phosphoric Acid Pure ml/l 40 to 80
Sulphuric Acid Pure m/l 25 to 80 20 to 80
Temperature _C 20 to 50 20 to 50 20 to 60
Agitation None - None - Moderate
Moderate Moderate
Current Density ASF 20 to 100 20 to 100 10 to 60

PLATINUM PLATING

The electrodeposition of platinum is not as well established as that of rhodium. The recent surge in demand for platinum and platinum finishes for the jewelry industries will result in the platinum processes becoming more established. Recent developments eliminating stress, solution polarization, anode polarization, and porosity have led to the ability to electroform this valuable metal.

RUTHENIUM PLATING

Electroplated ruthenium is gaining acceptance in the jewelry, giftware and other industries. It has a unique dark finish varying in shade from a distinctive gray through to black. Deposits of electroplated ruthenium are the hardest of all the platinum group metals.

ELECTROFORMING

The electroforming of precious jewelry is gaining wide acceptance worldwide. Gold can be electro-formed from cyanide alloy solutions, providing karat deposits ranging from 10 karat to 24 karat. Typically, the alloys deposited are gold/silver alloys or gold/copper/cadmium alloys. The use of the computer and sophisticated chemical control has permitted the mass production of quality karat jewelry.

Silver requires simple equipment and many items are electroformed in silver, ranging from jewelry to statuettes.

As previously mentioned, platinum is now being electroformed from relatively simple equipment but with very close control of the solution chemistry and operating conditions.

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