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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
Introduction
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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