Handbook Properties and Selection Nonferrous Alloys and Spl Purpose Mtls (1992) WW Part 9 pptx

Instrument grades
I-70A 99.0 0.7 700 700 1000

700

700

400
O-50 99.0 0.5 700 700 1000

700

700

400
I-220B 98.0 2.2 1000

1500

1500

800

800

400
I-400B 94.0 4.25 min 1600

2500

2500

800

800

400

Structural grades of beryllium are indicated by the prefix S in their designations. Property requirements for
commercially available structural grades are presented in Table 3. In general, these grades are produced to meet ductility
and minimum strength requirements. S-200F, an impact-ground powder grade, is the most commonly used grade of
beryllium. This grade evolved from S-200E, which used attritioned powder as the input material. Recently, S-200FH has
been introduced to the marketplace; the suffix H designates consolidation by HIP. The properties of S-200F include a
minimum ductility of 2% elongation in all directions within the vacuum-hot-pressed billet, an increased of 1% over its
predecessor, S-200E. In addition, S-200F has yield and ultimate strengths that are somewhat higher than those of S-200E,
reflecting the general improvements in powder-processing techniques and consolidation methods.
Table 3 Mechanical property requirements for structural grades of beryllium

0.2% yield

strength
Ultimate
strength
Grade
MPa

ksi

MPa

ksi

Elongation,

%
S-65B 207 30 290 42 3.0
Grade S-65 also is an impact-ground powder product. This grade was formulated to meet the damage tolerance
requirements for use in the Space Shuttle. Grade S-65 sacrifices some strength for improved ductility. The 3% minimum
ductility requirement is achieved by using impact-ground powder in combination with tailored heat treatments. The heat
treatments produce a desirable morphology of iron-aluminum-beryllium-base precipitates.
Low levels of iron and aluminum are present in commercial grades of beryllium (see Table 2). Although these elements
cannot be eliminated economically, they can be balanced, and heat treatments can be applied to form discrete grain-
boundary precipitates of AlFeBe
4
. This minimizes the iron in solid solution or in the compound FeBe
11
, either of which is
embrittling. The precipitates also eliminate aluminum from the grain boundaries, thus precluding hot shortness at elevated
temperatures.
At moderate temperatures, beryllium develops substantial ductility. At 800 °C (1470 °F), for example, the elongation of
S-200F is in excess of 30%. Yield strength and ultimate strength decrease with increasing temperature, but usable strength
and modulus are maintained up to approximately 600 to 650 °C (1100 to 1200 °F). The changes in strength and ductility
of S-200F with temperature are shown in Fig. 4.

Fig. 4 Tensile properties of S-200F beryllium at elevated temperatures
Instrument grades of beryllium, which are designated by the prefix I, were developed to meet the specific needs of a
variety of precision instruments. These instruments generally are used in inertial-guidance systems where high
geometrical precision and resistance to plastic deformation on a part-per-million scale are required. The resistance to
deformation at this level is measured by microyield strength.
In addition to those grades developed to meet the needs of inertial guidance systems, grade I-70 was developed
specifically for optical components is satellite imaging systems. Because the large mirrors used in aerospace optics must

retain precise geometry throughout complex loading spectra, no differentiation was drawn between grades used for
optical instruments and those used for inertial-guidance instruments. Recently, however, O-50, a grade developed
specifically for the qualities of infrared reflectivity and low scatter has initiated the use of the prefix O to indicate an
optical grade. The property requirements for commercially available instrument grades are given in Table 4.
Table 4 Mechanical property requirements for instrument grades of beryllium
0.2% yield strength

Tensile strength

Microyield strength

Instrument grade

MPa ksi MPa ksi
Elongation,

%
MPa ksi
I-70A 172 25 240 35 2.0 . . . . . .
O-50 172 25 240 35 2.0 . . . . . .
I-220B 275 40 380 55 2.0 34.5 5
The property requirement that differentiates instrument grades from structural grades is microyield strength. In I-400,
ductility has practically been ignored in order to attain microyield strength values of 60 to 70 MPa (9 to 10 ksi). Grade I-
220 strikes a balance between ductility and microyield strength, with a 2% elongation requirement and a microyield
strength of 35 to 40 MPa (5 to 6 ksi). As greater insight into the relationship between properties and powder processing,
composition, and heat treatment is obtained, further property improvements should be made possible.
Wrought Products and Fabrication. Consolidated beryllium block can be rolled into plate, sheet, and foil, and it
can be extruded into shapes or tubing at elevated temperatures. At present, working operations typically warm work the
material, thereby avoiding recrystallization. The strength of warm-worked products increases significantly as the degree
of working increases. The in-plane properties of sheet and plate (rolled from a P/M material) increase as the gage

decreases, as shown in Table 5. As with most hexagonal close-packed materials, it develops substantial texture as a result
of these working operations. The texture in sheet, for example, generally results in excellent in-plane strength and
ductility, with almost no ductility in the short-transverse (out-of-plane) direction. Similar property trade-offs resulting
from the anisotropy caused by warm working are inherent in extruded tube and rod. Sheet can be formed at moderate
temperatures by standard forming methods. For some special applications, there has been interest in the improved
formability of ingot-derived rolling stock as opposed to material rolled from a block of P/M materials. The ingot-derived
stock has improved weldability and formability, primarily because of its reduced oxide content.
Table 5 In-plane properties of beryllium products rolled from a P/M source block

Thickness 0.2%
yield
strength
Ultimate
strength
mm in. MPa

ksi

MPa

ksi

Elongation,

%
11-15

0.45-0.60

275 40 413 60 3

6-11 0.25-0.45

310 45 448 65 4
In many instances, complex components must be built up from shapes made from sheet and foil. For example, beryllium
honeycomb structures have been made by brazing formed sheet, as have complex satellite structural components.
Beryllium tubing made by extrusion has also been used in satellite components. Space probes such as the Galileo Jupiter
explorer have used extruded beryllium tubing to provide stiff lightweight booms for precision antenna and solar array
structures.

Health and Safety Considerations
Beryllium has been commercially produced for more than 50 years, and its toxicity has been recognized and successfully
controlled for the last 30 years. Information on the toxicity of beryllium is contained in the article "Toxicity of Metals" in
this Volume.
The main concern associated with the handling of beryllium is the effect on the lungs when excessive amounts of
respirable beryllium powder or dust are inhaled. Two forms of lung disease are associated with beryllium: acute
berylliosis and chronic berylliosis. The acute form, which can have an abrupt onset, resembles pneumonia or bronchitis.
Acute berylliosis is now rare because of the improved protective measures that have been enacted to reduce exposure
levels.
Chronic berylliosis has a very slow onset. It still occurs in industry and seems to result from the allergic reaction of an
individual to beryllium. At present, there is no way of predetermining those who might be hypersensitive. Sensitive
individuals exposed to airborne beryllium may develop the lung condition associated with chronic berylliosis.
Exposure Limits. Two in-plant exposure limits have been set by the Occupational Safety and Health Administration to
prevent beryllium disease. The first is a maximum atmospheric concentration of 2 μg/m
3
of air averaged over an 8-h day.
The second is a short-exposure limit of 25 μg/m
3
of air for a duration of less than 30 min. The U.S. Environmental
Protection Agency (EPA) has set a nonoccupational limit of 0.01 μg/m
3

of air averaged over a 1-month period outside of
a beryllium facility. The EPA limits the emission of beryllium into the environment to 10 g in any 24-h period. To meet
these requirements, beryllium producers must adhere to industrial-hygiene standards and use air pollution control
measures when dusts, mists, and fumes might be created. Historically, these control measures appear to have been
effective in preventing chronic beryllium disease.
Precious Metals and Their Uses
A.R. Robertson, Englehard Corporation

THE EIGHT PRECIOUS METALS, listed in order of their atomic number as found in periods 5 and 6 (groups VIII and
Ib) of the periodic table, are ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold. Atomic,
structural, and physical properties of the precious metals, which are also referred to as the noble metals, are listed in Table
1. Additional property data can be found in the articles "Properties of Precious Metals" and "Properties of Pure Metals" in
this Volume.
Table 1 Selected properties of precious metals
Value for indicated metal Property
Platinum

Palladium

Iridium Rhodium Osmium Ruthenium

Gold Silver
Atomic number 78 46 77 45 76 44 79 47
Atomic weight, amu 195.09 106.4 192.2 102.905 190.2 101.07 196.967

107.87
Crystal structure
(a)
fcc fcc fcc fcc hcp hcp fcc fcc
Electronic configuration (ground

state)
5d
9
6s 4d
10
5d
7
6s
2
4d
8
5s 5d
6
6s
2
4d
7
5s 5d
10
6s 4d
9
5s
2

Chemical valence 2,4 2,4 3,4 3 4,6,8 3,4,6,8 1,3 1,2,3
21.45 12.02 22.65 12.41 22.61 12.45 19.32 10.49 Density at 20 °C (70 °F), g/cm
3

(lb/in.
3

)
(0.774) (0.434) (0.818) (0.448) (0.816) (0.449) (0.697) (0.378)
1769 1554 2447 1963 3045 2310 1064.4 961.9 Melting point, °C (°F)
(3216) (2829) (4437) (3565) (5513) (4190) (1948) 1763.4
Boiling point, °C (°F) 3800 2900 4500 3700 5020 ±
100
4080 ± 100 2808 2210

(6870) (5250) (8130) (6690) (9070 ±
180)
(7375 ±
180)
(5086) (4010)
Electrical resistivity at 0 °C (32
°F), μΩ· cm
9.85 9.93 4.71 4.33 8.12 6.80 2.06 1.59
Linear coefficient of thermal
expansion, μin./in./°C
9.1 11.1 6.8 8.3 6.1 9.1 14.16 19.68
Electromotive force versus Pt-67
electrode at 1000 °C (1830 °F),
mV
. . . -11.457 12.736 14.10 . . . 9.744 12.34
(b)
10.70
(c)

Tensile strength, MPa (ksi)
207-241 324-414 2070-
2480

1379-
1586
. . . 496 207-
221
290 As-worked wire
(30-35) (47-60) (300-
360)
(d)

(200-
230)
(d)

(72)
(d)
(30-32) (42)
124-165 145-228 1103-
1241
827-896 . . . . . . 124-
138
125-
186
Annealed wire
(18-24) (21-33) (160-
180)
(120-130) (18-20) (18.2-
27)
Elongation in 50 mm (2 in.), %
As-worked wire 1-3 1.5-2.5 15-18
(d)

2 . . . 3
(d)
4 3-5
Annealed wire 30-40 29-34 20-22 30-35 . . . . . . 39-45 43-50
Hardness, HV
As-worked wire 90-95 105-110 600-
700
(d)

. . . . . . . . . 55-60 . . .
Annealed wire 37-42 37-44 200-240 120-140 300-670 200-350 25-27 25-30
As-cast 43 44 210-240 . . . 800 170-450 33-35 . . .
Young's modulus at 20 °C (70 °F),
GPa (10
6
psi)

171 115 517 319 558 414 77 74
Static
(24.8) (16.7) (75) (46.5) (81) (60) (11.2) (10.8)
169 121 527 378 . . . 476 . . . . . . Dynamic
(24.5) (17.6) (76.5) (54.8) (69)
Poisson's ratio 0.39 0.39 0.26 0.26 . . . . . . 0.42 0.37
(e)

Source: Engelhard Industries Division, Engelhard Corporation
(a)
fcc, face-centered cubic; hcp, hexagonal close packed.
(b)
At 800 °C (1470 °F).

(c)
At 700 °C (1290 °F).
(d)
Hot worked.
(e)
Annealed.

Precious metals are of inestimable value to modern civilization. Their functions in jewelry, coins, and bullion, and as
catalysts in devices to control auto exhaust emissions are widely understood. But in certain other applications, their
functions are not as spectacular and, although vital to the application, are largely unknown except to the users. Many
facets of daily life and influenced by precious metals and their alloys. For example, precious metals are used in dental
restorations and dental fillings (see the section "Precious Metals in Dentistry" in this article). Precious metal solders are
used in dentistry and in the jewelry and electronics industries. Thin precious metal films are used to form the electronic
circuits. Much of our clothing today is produced with the aid of precious metals that are used in spinnerettes for producing
synthetic fibers. Precious metals perform as catalysts in various processes; for example, widely used agricultural
fertilizers are produced with the aid of a platinum-rhodium alloy catalyst woven in the form of gauze, and auto emissions
are reduced through the use of platinum-group alloy catalysts. Electrical contacts containing palladium are essential to
telephone communications. Certain organometallic compounds containing platinum are significant drugs for cancer
chemotherapy.
Resources and Consumption
Metal specialists at the U.S. Bureau of Mines continually survey the market in silver, gold, and platinum-group metals to
determine present availability and usage and to forecast future trends. Mineral Commodity Profiles and Mineral Industry
Surveys covering these metals are issued periodically.
Much of the information in this section was obtained from Ref 1, 2, 3, 4, 5, 6, 7, and 8. For the latest available
information concerning these metals, it is recommended that the most recent issues of these publications be consulted.
Silver. In recent years, the United States has been a net importer of silver. Imports, including unrefined silver, supplied
about 89 million troy ounces of silver to the U.S. supply in 1988. Domestic mine production added 53 million troy ounces
to this total, and refining of old scrap increased the U.S. silver level to a total of approximately 217 million troy ounces
during this same period. The U.S. supply is obtained from primary and secondary sources. About 25% of primary silver is
obtained from predominantly silver ores; the remaining primary silver is a by-product of the refining of copper, lead, zinc,

and other metals. In addition, significant quantities of silver are derived as a by-product of gold mining. The top states for
mine production are Idaho, Nevada, Montana, Arizona, and Utah.
Five smelting and refining companies produce the major portion of domestic primary silver. The smelters and refineries
treat ores, concentrates, residues, and precipitates from company mines and plants in addition to materials purchased from
other sources. Silver scrap is recycled by several primary smelters and a considerable number of small secondary
refineries. In addition, secondary silver is recovered by several trading and fabricating companies and is recycled by end-
product manufacturers.
In recent years, government regulations relating to the environment and to control of emissions of hazardous compounds
have limited the operation of some base metal smelters that recover silver as a by-product. Silver in its ionic form, as in
waste discharged from electroplating plants, is considered a potential source of pollution and a health hazard. Because of
increasing concerns about environmental matters, governmental agencies can be expected to step up their efforts to
minimize discharges from processing plants.
The following table presents primary statistics for U.S. silver demand in 1987. A general indication of the annual demand
pattern for silver can be drawn from this data:

End use Demand,
troy ounces × 10
6


Electroplated ware 2.5
Sterling ware 3.8
Jewelry and arts 4.2
Photography 60.2
Dental and medical supplies 1.3
Brazing alloys and solders 5.6
Mirrors 1.0
Batteries 2.5
Contacts and conductors 22.7
Bearings 0.3

Coin, medallions, and commemorative objects

4.2
Catalysts 2.5
Other 4.5
Total U.S. demand 115.3

Several factors can affect the supply and demand of silver. One factor is the appreciable amount of silver required for
monetary purposes; another is the speculative or investor market in refined silver bars and sacks of domestic coins. In
addition, there has been some interest in the collection of commemorative medallions and limited-use objects fabricated
from silver. Whether the latter end use will continue to grow or will decline in the future is a matter of considerable
interest. In any event, depending on prices, a large potential secondary supply of silver is available in the form of coins,
silverware, jewelry, and commemorative objects.
Gold. Because of its aesthetic beauty and enduring physical properties, gold is important not only to industry and the arts,
but also as a commodity having long-term value. In the past, gold was considered to be mainly a monetary metal.
However, starting in the late 1950s, more gold was used by manufacturers and investors than was used for monetary
purposes. Since 1968, gold has become, to a considerable extent, a free-market commodity, with prices free to adjust to
supply and demand. Despite this open market, almost half of the total world supply of gold, estimated at 2.4 billion troy
ounces, is in various government vaults tied down by agreements among large industrial nations.
About 1.7 million troy ounces of gold per year are mined and reclaimed from old scrap in the United States (see, for
example, the section "Recycling of Electronic Scrap" in the article "Recycling of Nonferrous Alloys" in this Volume).
This amount falls far short of the amount required by U.S. industry. The requirements for bullion and coins are almost
equal to those of industry. Because of this general demand for gold, the net inflow of this metal from foreign sources is
large. In 1988, for example, 3.0 million troy ounces were imported, mostly in the form of refined metal.
About 50% of U.S. refinery production comes from gold ores, and the remainder from by-products of the refining of
copper and other base metals. Refinery production in the United States includes gold from domestic mines, from imported
ores and base bullion, and from domestic and foreign scrap. In recent years about 5 to 10% of U.S. refinery production
has been derived from foreign ores, base bullion, and scrap.
About 5 to 10% of the total U.S. supply of gold comes from old scrap, which is defined as metal discarded after use. New
scrap, which is generated during manufacturing, is usually reclaimed by the fabricator and is not considered part of the

market supply.
The United States has appreciable gold resources, some of which are marginally profitable to recover. The price is now
high enough to encourage growth of production at a modest rate, but environmental restraints on placer mining and the
high cost of developing lode deposits currently dictate that the United States will continue to import most of its gold.
The largest foreign producer of gold, the Republic of South Africa, produced about 20 million troy ounces of the
estimated 1988 total world production of 59 million troy ounces. Other important gold producers are the Soviet Union,
Canada, South America, Asia, and Oceania. According to the U.S. Bureau of Mines, world resources are adequate to meet
the forecast demand for this metal to the year 2000.
Data for U.S. gold demand in 1988 illustrates the general pattern of consumption of gold in fabricated products:

End use Demand
troy ounces × 10
3


Jewelry and arts 1774.0
Dental supplies 247.0
Industrial products 1176.0
Total U.S. demand

3197.0
Source: U.S. Bureau of Mines
The use of gold for jewelry accounts for approximately 55% of the gold consumed. Dental uses generally amount to 7 to
8% of annual demand. Industrial requirements are generally centered in the electronics industry. However, even though
the total number of end uses for gold in electronics continues to be high, considerable emphasis has been placed on
reducing the use of gold in present applications because of its increasing cost. Bars, medallions, coins, and related
products amounted to less than
1
4
of 1% of the total U.S. gold consumption in 1988.

Platinum-Group Metals. The six closely related metals in the platinum group commonly occur together in nature.
These transition elements are near neighbors in periods 5 and 6 of group VIII in the periodic table. Ruthenium, rhodium,
and palladium each have a density of approximately 12 g/cm
3
, osmium, iridium, and platinum each have a density of
about 22 g/cm
3
(see Table 1).
The platinum-group metals are among the scarcest of metallic elements, and for this reason their cost is high. They occur
as native alloys or in mineral compounds in placer deposits, sometimes together with gold; they also occur in lode
deposits in basic or ultrabasic rocks, where they may be found together with nickel and copper. Most of the world supply
of platinum-group metals is currently extracted from lode deposits in the Republic of South Africa, the Soviet Union, and
Canada.
Another major source of platinum-group metals is old scrap obtained from obsolete equipment, spent catalyst, and
discarded jewelry. This source is increasing in importance with the broadening industrial applications of these metals.
Material of each type can be concentrated by a number of different methods, then refined by any of several chemical
processes that conclude with a heating step to convert precipitates to metal in a porous, somewhat powdery form called
sponge. Sponge is the most common form in commercial metal transactions, although ingots and shot are also traded.
In lode deposits, platinum-group metals are often associated with nickel and copper sulfide, which may be the principal
products of mining (as in Canada and the Soviet Union) or important coproducts (as in the Republic of South Africa). In
the ores, the proportions of the six platinum-group metals vary from one lode deposit to another. Canadian deposits in the
Sudbury District contain approximately equal amounts of platinum and palladium; South African deposits contain more
than twice as much platinum as palladium. Generally, platinum and palladium together account for about 80 to 85% of the
platinum-group metals present in any given ore, followed by (in order of decreasing presence) ruthenium, rhodium,
iridium, and osmium.
The composition of placer deposits differs somewhat from that of lode deposits. Placer deposits are characterized by the
nearly complete absence of palladium and the common presence of gold. It appears the palladium and, to a certain extent,
platinum, rhodium, and ruthenium are dissolved away during placer formation; in the well-established placer deposit in
Witwatersrand, South Africa, only osmium and iridium are present (as the alloy osmiridium). At present, the only
economically important placers are found in Colombia, South Africa, and the Soviet Union; together they account for

about 2% of total world production.
The United States depends almost entirely on foreign sources for platinum-group metals. In 1988, net import reliance as a
percentage of apparent U.S. consumption was approximately 93%. Apparently U.S. consumption during 1988 is
estimated at 2.7 million troy ounces. The sources of U.S supply between 1984 and 1987 were the Republic of South
Africa, 44%; the United Kingdom, 16%; the Soviet Union, 9%; all others, 31%.
One company in Texas recovers platinum-group metals as by-products of the copper-refining process. In addition, about
24 smaller refineries located mostly on the East and West Coasts, handle or in some way process domestic scrap.
However, most of them treat only platinum and palladium, and only three or four refine all six metals.
At present, U.S. mine production and reserves of platinum-group metals are small; untapped domestic resources appear to
be large but are not well explored. The heavy dependence the United States places on foreign sources for these critical
metals has strategic implications as well as a substantial impact on the U.S. balance of payments. The need for
exploration and development of U.S. resources is apparent.
The U.S. demand for platinum-group metals is projected to grow at an annual rate of about 2.5%, with an estimated 1990
demand of 3.3 million troy ounces. Demand in the rest of the world is forecast to grow more slowly than in the U.S.; it is
expected to reach a level of about 9.7 million troy ounces in the year 2000. World reserves and resources appear to be
more than adequate to meet this demand.
The U.S. demand patterns in 1988 for three of the most-used platinum-group metals were:

1988 Demand, troy ounces × 10
3
End use
Platinum

Palladium

Rhodium

Automotive 609,000 160,000 65,000
Chemical 61,976 81,343 3,091
Dental and medical 10,871 227,747 142

Electrical 108,660 386,710 3,508
Glass 19,896 350 2,748
Jewelry and decorative

11,932 7,356 5,254
Petroleum 36,730 26,111 45
Miscellaneous 76,394 135,581 22,787
The United States and Japan currently use about 60% of the platinum-group metals produced. Western Europe and the
Soviet Union essentially divide the remaining 40%. Automotive emission requirements (which require the use of a
platinum-palladium catalyst) began in 1974 in the United States and presently are substantial (this application alone
accounted for 43% of total consumption in 1987). In Japan, about three-fourths of the platinum goes into jewelry, whereas
in the United States and Western Europe, about 5 to 15% of this metal is used in jewelry. Substantial increases in the use
of platinum-group metals can be expected in the European Economic Community with the gradual implementation of
restrictions on automobile emissions from 1988 through 1993.

References cited in this section
1.

R.G. Reese, Jr., Silver, in Mineral Commodity Profiles, U.S. Bureau of Mines, 1983
2.

R.G. Reese, Jr., Silver, in Mineral Facts and Problems, Bulletin 675, U.S. Bureau of Mines, 1985

3.

J.M. Lucas, Gold, in Mineral Facts and Problems, Bulletin 675, U.S. Bureau of Mines, 1985
4.

Platinum-Group Metals in the Third Quarter 1988, Miner. Ind. Surv., 1988
5.

Platinum-Group Metals in the Fourth Quarter 1988, Miner. Ind. Surv., 1988
6.

Gold and Silver in June 1989, Miner. Ind. Surv., Aug 1989
7.

Metal Statistics 1988, 81st Annual Ed., Am. Met. Mark., 1988
8.

Eng. Min. J., Vol 190 (No. 3), March 1989

Trade Practices
Precious metals are bought and sold in troy ounces or, in markets where the metric system is used, in kilograms. One
kilogram is equal to 32.15 troy ounces. The troy system of weights is based on the troy ounce of 480 grains, 31.1 grams,
or 20 pennyweight. One troy ounce is equal to 1.097 avoirdupois ounces.
Silver and Gold. The term fineness refers to the weight portion of silver or gold in an alloy, expressed in parts per
thousand. For example, 1000 fine silver (also called fine silver) is pure silver, or 100% silver, and 1000 fine gold is 100%
pure gold. Gold bullion that is commercially traded is at least 995 fine or higher. Sterling silver is 925 fine, or 925 parts
(also 92.5%) silver and 75 parts (or 7.5%) copper. Until 1964 the U.S. coin silver was an alloy of 90% Ag (900 fine) and
10% Cu. Silver bullion that is traded has a silver content ranging from 999 fine to 999.9 fine. Gold and copper are silver
impurities in any fineness of silver bullion.
Another way of indicating gold purity is by the karat, which is a unit of fineness equal to the
1
24
th part of pure gold. In
this system, 24 karat (24 k) gold is 1000 fine or pure gold. The most popular jewelry golds in the United States are:

Karat designation

Gold content
24 k 100% Au (99.95% min)

18 k
18
24
ths, or 75% Au
14 k
14
24
ths, or 58.33% Au
10 k
10
24
ths, or 41.67% Au

Each category differs from the others in number, type, and proportions of the base metal additions. Gold alloys used in
jewelry are always specified by karats, whereas those used in dentistry and for electronic purposes are designated by
percentage.
Jewelry golds range from light yellow through deep yellow to reds and greens, and also include a family of whites. Each
alloying element has a different effect on the color of gold:
• Silver: As the proportion of silver increases, gold changes in hue from yellow to greenish-yell
ow to
white
• Copper: As copper content increases, gold becomes redder in appearance
• Nickel: Nickel has the effect of whitening gold. The so-called white golds substitute nickel for silver
• Zinc: Zinc is considered a decolorizer. Some red golds (copper-con
taining alloys) are converted to a
substitution of zinc for some of the copper and silver
Trade practice rules for the jewelry industry in the United States, set by the Federal Trade Commission, require that any

article labeled gold contain at least 10 k gold, with a tolerance of
1
2
k. In gold cladding (gold adhered to base metal
stock), the ratio of the weight of the material is indicated, together with the karat of the cladding. For example, one-tenth
12 k gold filled stock is a base metal surfaced with a layer or layers of 12 k gold alloy that make up 10% of the weight of
the composite article. Such an article, if assayed into, would be found to contain 5% gold.
The designation gold filled is limited by stamping regulations to articles in which the weight of the coating is at least
1
20

th of the total. Lower ratios may be stamped rolled gold plate. The quality mark cannot be applied to articles surfaced
with an alloy of less than 10 k. In the cases of the gold-filled and rolled gold plate materials, the karat gold is bonded to
the base metal substrate by soldering, brazing, welding, or mechanical means.
Platinum-Group Metals. Platinum and palladium are traded on the New York Mercantile Exchange in respective lots
of 50 and 100 troy ounces; on the Chicago Mercantile Exchange, they are traded in units of 100 troy ounces. On the New
York Exchange, the metals can be in the form of bar or sheet.
The required purity of platinum may vary according to the end use or application. Although commercial-grade platinum
must be at least 99.8% pure, platinum with a purity of at least 99.9% is required for alloying, laboratory ware, and
contacts. Platinum of even higher purity, sometimes with controlled impurities, is used for other specialized applications
such as thermocouples and resistance thermometers. The present U.S. thermometric standard platinum, designated Pt 67,
is 99.999% pure.
Federal regulations stipulate that an article of trade can be marked platinum only if it contains at least 98.5% platinum-
group metals, of which no more than 5% may consist of platinum-group metals other than platinum; that is, the material
must contain a minimum of 93.5% Pt. Special stamping provisions cover some jewelry alloys. All platinum jewelry sold
in the United Kingdom must be hallmarked.
Alloys used for dental purposes are rather complex in composition, and metallurgical considerations are the
dominant factors in their design. Various specifications have been established by the American Dental Association and by
the federal government, but these do not cover all of the dental alloys (see the section "Precious Metals in Dentistry" in
this article).

Special Properties
The precious metals have unusual combinations of properties that often are superior to those of other materials. In some
cases, these property combinations make them the only materials that can meet the specialized requirements of an
advanced technology or industrial application. The initial investment in these metals or their alloys may be high, but it is
offset by long, reliable service and by ease of refining. Also, the refining process is marked by a high recovery rate
because the precious metals are virtually indestructable. This high rate of recovery can make their use, for many
applications, economical as well as efficient.
The precious metals share a number of properties that distinguish them from other metals or alloys, including corrosion
resistance, good electrical conductivity, catalytic activity, and excellent reflectivity. However, each of these metals also
has distinctive individual characteristics.
Silver, a bright white metal, is very soft and malleable in the annealed condition. It does not oxidize at room
temperature, but it is attacked by sulfur. Nitric, hydrochloric, and sulfuric acids attack silver, but the metal is resistant to
many organic acids and to sodium and potassium hydroxide.
In commercial applications, the special chemical properties, superior thermal and electrical conductivity, high reflectivity,
malleability, ductility, and/or corrosion resistance of silver justify its high initial cost. In addition, uses have been
established in photography, brazing, batteries, medicines, dentistry, mirror backings (silver-backed mirrors may become a
more significant use as solar energy technology is developed), bearings, catalysts, coinage, and nuclear control rods.
The use of silver in photography is based on the ability of exposed silver halide salts to undergo a secondary image
amplification process called development. In silver solders, the controlling factor is the rather low melting temperature of
the alloys and their ability to wet various base metals at temperatures below the melting points of the metals to be joined.
Such alloys do not dissolve or attack steel in normal usage, are ductile, have sufficient strength over a wide range of
temperatures, and are capable of joining a wide variety of materials. Silver alloys are finding more and more use as
replacements for the lead-tin solders traditionally used in residential plumbing.
Silver that contains varying amounts of dispersed cadmium oxide (≤20% CdO) is used in medium- and heavy-duty
electrical contacts (see the article "Electrical Contact Materials" in this Volume). In this composite material, silver imparts
its good electrical and thermal conductivity as well as its low surface contact resistance, and the dispersed cadmium oxide
improves resistance to sticking and welding and provides good resistance to arc erosion (good arc quenching). The
susceptibility of fine silver contacts to sulfidation precludes their use in low-current, low-voltage, and low-contact-force
applications. In general, they should not be used below 10 V (except at high currents) or in situations where a voltage

drop of 0.2 V will be troublesome; in addition, they are not suitable for application in low-level audio circuits because of
the electrical noise they would introduce.
Silver is used in engine bearings because it has good lubrication properties as well as moderate hardness, good thermal
conductivity, and low solubility in iron.
The good mechanical properties of certain silver-tin-mercury and silver-tin-copper-mercury alloys, and the small
dimensional changes that occur during setting of these alloys, are the basis for the extended use of silver in dental
amalgams (see the discussion of dental amalgams in the article "Properties of Precious Metals" that follows).
Sterling silver (silver-copper alloy) retains its long-established position in uses where elegant appearance is of paramount
importance. For jewelry and tableware, high reflectivity makes silver particularly attractive. Much work has been done in
developing a nontarnishing sterling silver, but no such alloy has yet been produced. Various thin protective coatings, such
as rhodium, have been used on silver objects that are not likely to scratch.
Silver-clad copper, brass, nickel, and iron are produced for a variety of uses, ranging from electrical conductors and
contacts to components for chemical equipment. Silver is also used in various chemical processes, including catalytic
applications such as the production of formaldehyde or the oxidation of ethylene.
Silver coatings are applied to glass and ceramics by spreading a special silver paste on the material and then warming it to
red heat. These coatings are widely used in electronic devices and automotive applications. Chemical methods for
applying conductive coatings to plastics and glass are also used as the base for electroplate. Organometallic solutions
containing silver are applied and fired in the production of conductors, electrical grounds and shields, resistance heaters,
electrode terminals, and conductive bases for electroplating.
The rapid diffusion of oxygen through silver at elevated temperatures can be an advantage or disadvantage depending on
the application. This phenomenon has been used to advantage in the internal oxidation of base metal alloying constituents
(such as cadmium, rare earths, cerium, or calcium) in silver alloys. The resulting silver composite containing fine, well-
dispersed oxide particles has been used in electrical contact applications (see the article "Electrical Contact Materials" in
this Volume).
Electrodeposited silver is used widely for electrical, electronic, industrial, and decorative applications. Heavy
electrodeposits can be used for surfacing chemical equipment and for bearings.
Gold is a bright, yellow, soft, and very malleable metal. Its special properties include corrosion resistance, good
reflectance, resistance to sulfidation and oxidation, freedom from ionic migration, ease of alloying with other metals to
develop special properties, and high electrical and thermal conductivity. Because gold is easy to fashion, has a bright
pleasing color, is non-allergenic, and remains tarnish free indefinitely, it is used extensively in jewelry. For much the

same reasons, it has long been used in dentistry in inlays, crowns, bridges, and orthodontic appliances (see the section
"Precious Metals in Dentistry" in this article).
Gold is used to a considerable extent in electronic devices, particularly in printed circuit boards, connectors, keyboard
contactors, and miniaturized circuitry (see Packaging, Volume 1 of the Electronic Materials Handbook published by
ASM INTERNATIONAL). Because electronic devices employ low voltages and currents, it is important that the coated
components remain completely free from tarnish films and that they remain chemically and metallurgically stable for the
life of the equipment.
Gold is a good reflector of infrared radiation; for this reason, gold films are used in radiant heating and drying devices as
well as in thermal barrier windows for large buildings. A much publicized use of gold reflective coatings has been for
protecting space vehicle components and space suits from excessive solar radiation that could raise temperatures
substantially.
Fired-on gold organometallic compounds are used to decorate porcelain and glassware. Chemically inert gold rupture
discs are used in chemical process equipment. Because of its good resistance to corrosion and wear, the gold alloy 70 Au-
30Pt has been used in the perforated spinnerettes through which cellulose acetate fibers are extruded. Gold has also been
used in other industrial applications, such as sliding electrical contacts, fine-wire gold connectors for the semiconductor
industry, vacuum and sputter-deposited films or coatings for interconnecting links in thin-film integrated circuits, gold
brazing alloys for joining jet engine components, and gold alloys in thermocouples for both cryogenic service down to
liquid helium temperature and high-temperature use up to 1300 °C (2370 °F).
Among the gold alloys, the Au-Ag-Cu-Pt-Pd alloys are used in dentistry because of their good mechanical properties,
response to age-hardening treatments, nobility, and moderate melting points. The Au-Ag-Cu (yellow) golds and Au-Ni-
Cu-Zn (white and suntan) golds have a relatively good resistance to tarnishing and corrosion, and adequate mechanical
properties; these attributes, along with social custom and the available colors of the materials, account for their use for
jewelry, eyeglass frames, and rings. These alloys are also used for certain rubbing contacts in small electrical devices.
Gold-silver alloys containing about 70% Au, generally with a few percent platinum, resist both oxidation and sulfidation,
and have other properties useful for low-current electrical contacts.
Pure gold is readily electrodeposited and it, as well as rhodium and palladium, is used for surfacing certain high-
frequency conductors for service in environments where silver corrodes. A substantial quantity of electrodeposited gold is
used for surfacing plug-type electrical connectors. Platinum metals may be required at higher temperatures to minimize
diffusion and adhesion (sticking). Gold alloys are also electrodeposited on jewelry and other items where appearance is
important. For some pieces, several layers of gold and other metals are deposited successively, and the article is

subsequently heated to produce an alloy by diffusion.
Pure gold has high reflectivity in the red and infrared spectral ranges and therefore is sometimes used for surfacing
infrared reflectors. Although pure gold resists nitric, sulfuric, and hydrochloric acids as well as many other corrosives, its
applications are limited because of its susceptibility to attack by halogens, its softness and relatively low melting point,
and, to some extent, its cost. However, gold sometimes is used as a lining for small calorimeter bombs and as a corrosion-
resistant solder. The hard 70Au-30Pt alloy has been used for rayon spinnerettes, but it generally has been replaced in this
application by Pt-10Rh.
Platinum-Group Metals. All six platinum-group metals are closely related and commonly occur together in nature.
Their most distinctive trait in the metallic form is their exceptional resistance to corrosion. Of the six metals, platinum has
the most outstanding properties and is the most used. Second in industrial importance is palladium, which is the lightest
metal of the group.
Rhodium occasionally is fabricated in the unalloyed form, but it is more commonly used as an alloying element with
platinum, and to a lesser extent, with palladium. In the unalloyed form, iridium is fabricated into large crucibles that are
used in the production of single crystals of yttrium-aluminum garnet and gadolinium-gallium garnet, a substrate for
bubble memory devices. It also finds considerable use as an alloying element for platinum and rhodium. Ruthenium is
mainly used as an alloying element for platinum and palladium. When alloyed with platinum and palladium, rhodium,
iridium, and ruthenium (in order of increasing effectiveness) act as hardening agents. Osmium forms a toxic oxide at
ambient temperature and is therefore a difficult metal to utilize. A naturally occurring alloy of osmium and iridium called
osmiridium is very hard and has been used for fountain pen tips and phonograph needles. The important properties of the
platinum-group metals are outlined below.
Platinum is a white, very ductile metal that remains bright in air at all temperatures up to its melting point. Platinum has
the following engineering characteristics:
• High melting point (1769 °C, or 3216 °F)
• Readily strengthened by alloying with compatible precious metals
• Can be electroplated
• Virtually nonoxidizable
• Resists molten glass and molten salts in oxidizing atmospheres
• Low vapor pressure
•
Low electrical resistivity and, conversely, a high temperature coefficient of electrical resistivity; this

combination makes it eminently suited for measuring elements in resistance thermometers
• Stable electrical contact resistance
• Stable thermoelectric behavior (the Pt-
10Rh versus Pt thermocouple is the defining instrument on the
International Practical Temperature Scale of 1968)
• High thermionic work function
• Special magnetic properties when alloyed with cobalt
• High thermal conductivity
• High resistance to spark erosion (hence its use in spark plugs)
• Excellent catalytic activity
• Coefficient of thermal expansion matching that of common glass
Platinum resists practically all chemical reagents and is soluble only in acids that generate free chlorine, such as aqua
regia.
Palladium is a white, very ductile metal with properties similar in many respects to those of platinum. Palladium has the
following engineering characteristics:
• A density of 12.02 g/cm
3
, which is approximately 56% that of platinum and 63% that of gold. It can be
used in place of lower-cost gold alloys without sacrificing the good corrosion resistance of gold
• High melting point (1554 °C, or 2829 °F)
• Excellent ductility
• Easily cold worked
• Outstanding ability to form extensive ductile solid solutions with other metals
• Can be electroplated, electroformed, and deposited via electroless methods
• Effective whitener for gold
• Good catalytic activity
Palladium resists tarnishing in ordinary atmospheres, but it does tarnish slightly upon outdoor exposure to sulfur-
contaminated environments. When palladium is heated in air to 400 to 800 °C (750 to 1475 °F), a thin oxide film is
formed; this film decomposes at higher temperatures, leaving the metal with a bright appearance. Hydrochloric acid and
sulfuric acid attack palladium slightly; nitric acid, ferric chloride, and most halogens attack it readily. Palladium absorbs

hydrogen, which will diffuse at a relatively rapid rate when the metal is heated. This reaction is the basis for laboratory
apparatus for purifying hydrogen. Palladium has found increasing use in dental porcelain fused to metal alloys (for
example, Au-Pt-Pd-Ag alloys).
Rhodium is a hard white metal. It is fairly ductile when hot. Rhodium is the whitest platinum-group metal and remains
bright under all atmospheric conditions at ordinary temperatures. It resists hot aqua regia. High oxidation resistance and a
high melting point (1963 °C, or 3565 °F) permit rhodium to be used for fabricating items for use at high temperature.
Rhodium has high specular reflectivity and the highest electrical and thermal conductivities of any platinum-group metal.
Iridium is a white metal that has limited malleability at room temperature; however, it can be worked at elevated
temperatures. Iridium oxidizes visibly when heated in air (to temperatures of 600 to 1000 °C (1100 to 1850 °F), but it
remains bright at higher temperatures. Acids or aqua regia do not attack it, but molten salts do. Iridium has exceptional
corrosion resistance, and this property coupled with a high-temperature (≤ 1650 °C, or 3000 °F) strength comparable to
that of tungsten and a high melting point (2447 °C, or 4437 °F) permit its use in crucibles for melting nonmetallic
substances at temperatures as high as 2100 °C (3800 °F). Iridium has a high modulus of elasticity (517 GPa, or 75 × 10
6

psi). It is the only known metal that can be used for short periods of time at temperatures up to 2000 °C (3650 °F) in air
without undergoing catastrophic failure. Iridium is catalytically active and is the heaviest of all metals (22.65 g/cm
3
).
Ruthenium is a very hard white metal that cannot be worked cold. It can be worked after being heated to a failure high
temperature, but only with extreme difficulty. Ruthenium resists common acids, including aqua regia, at temperatures up
to 100 °C (212 °F). Ruthenium has a high resistance to contamination by lead. Like iridium, it is principally used as a
hardener for platinum and palladium. Ruthenium has a high melting point (2310 °C, or 4190 °F), is exceptionally hard,
and has a high elastic modulus (414 GPa, or 60 × 10
6
psi). In the absence of oxygen, ruthenium exhibits good resistance
to attack by molten lithium, sodium, potassium, copper, silver, and gold. It has low electrical contact resistance at
temperatures up to 600 °C (1100 °F) and resists any tendency of the contacts to weld together at these temperatures.
Osmium is a white hard metal that is not malleable at room or elevated temperatures. It forms a toxic oxide at ambient
temperatures. Osmium is used as an alloying element to provide other precious metals with extreme hardness and

resistance to corrosion. Osmium has the highest melting point of all the platinum-group metals (3045 °C, or 5513 °F) and
the second-highest density (22.61 g/cm
3
).

Commercial Forms and Uses
Semifinished Products. Silver, gold, platinum, palladium, and rhodium can be drawn to rod and wire as small as 25
μm (0.001 in.) in diameter. Iridium can be drawn to diameters as small as 75 μm (0.003 in.). Some of the platinum alloys
containing iridium or rhodium can be drawn to diameters of 7.5 μm (0.0003 in.).
Sheet, strip, ribbon, and foil in a broad range of alloys, sizes, and thicknesses can be produced. Silver, gold, platinum, and
some of its alloys can be rolled to thicknesses as small as 2.5 μm (0.0001 in.), but tolerances cannot be guaranteed. Clad
materials can be obtained as wire, sheet, strip, and formed parts, with a great variety of substrate materials.
Tube is manufactured in a wide range of sizes and in round, half-round, and square sections. Seamless tube made of
platinum, palladium, gold, and most alloys of these metals is manufactured in sizes ranging from 0.4 mm (0.016 in.)
outside diameter × 0.1 mm (0.004 in.) wall thickness up to 44 mm (1.750 in.) outside diameter × a 55 mm (0.200 in.) wall
thickness. Tube in larger sizes or made of less ductile materials such as platinum alloyed with rhodium (≥25% or iridium
(>25%) is manufactured only as seamed tube with a 3 to 75 mm (
1
8
to 3 in.) inside diameter × a 0.25 to 2.5 mm (0.010 to
0.100 in.) wall thickness. Pure rhodium and pure iridium are usually furnished as seamed tube with a 3 to 40 mm (
1
8
to
1
1
2
in.) inside diameter × a 0.25 to 0.6 mm (0.010 to 0.025 in.) wall thickness and as single lengths about 150 mm (6 in.)
long. Base metal tube is available with an outer cladding or an inner lining of platinum, gold, silver, or any of the
commercial precious metal alloys.

Precious metal powders are produced for a wide range of electronic and industrial applications. Electronic powders
are chemically precipitated to produce particle sizes of less than 10 μm and tend to be high in surface area. They are used
as inks in hybrid circuits. Flake powders tend to produce shinier, smooth films; spherical particles more often result in
dull-appearing surfaces. Platinum, palladium, 40Pt-20Pd-40Au, 10Pt-20Pd-70Au, 7.5Pt-22.5Pd-70Au, and 75Au-25Pd
powders have been used for electronic purposes. Trials are usually necessary to determine the most suitable powder from
the standpoint of both cost and performance.
Powders intended for industrial uses are composed of mixtures of particles that range in size from about 2 to 3 μm to as
large as 840 μm (20 mesh), depending on the size required. These powders are suitable for use in powder metallurgy
parts, as protective coatings against hostile industrial environments, as raw materials in alloy manufacture, and for various
other uses.
Industrial Uses. Requirements and materials for more than 65 industrial applications of precious metals are cited in
Table 2. Additional information on selection and application of precious metal contacts is included in the article
"Electrical Contact Materials" in this Volume.
Table 2 Industrial applications of precious metals
Application Special requirements Metal or alloy
Electrical and electronic devices
Spark plug electrodes Resistance to corrosion and erosion Thoriated Pt-4W, Ir, ODS Pt, Pd-Au
Jet engine glow plugs Relight on flameout Rh-Pt
Leads for thermistors Freedom from oxidation Pt and Ag plus binder
Doping contact Au and doping alloy Transistor junctions
Nondoping contact Ir-Pt
Resistors and potentiometers High resistivity, low temperature
coefficient, and low contact resistance
8W-Pt, 5Mo-Pt, 10Ru-Pt, Au-Pd-Fe, dental-
type alloys
Resistance wire and resistance film High resistivity, low temperature
coefficient, and low contact resistance
Au-Pd-Pt
Electrodes for ceramic condensers Applicability, nonoxidizing,
solderability

Ag or Pt, with bonding agent
Electrodes for air condensers Corrosion resistance Ag and Au
Conductors in printed circuits Corrosion resistance, solderability,
wear resistance (Rh)
Ag, Au, Rh, Pd (Ag may lead to ionic
shorting)
Connectors (such as terminals, lugs, and tabs) Low contact resistance, solderability Ag, Au; Pd electro- or electroless plate
High-temperature wiring Conductivity, oxidation resistance,
low contact resistance
Pt-clad base metal, solid Ag, Ag-Mg-Ni
Fuses High conductivity and oxidation
resistance
Ag-Au
Solid leads in mercury contact devices Negligible solubility, freedom from
oxidation
Pt where wetting required; also 10Ir-Pt. Ir
where no wetting desired. Rh-plated steel for
collector rings
Bonding in vacuum devices requiring
vacuum-tight low-vapor-pressure seals
Desired melting point and low vapor
pressure
28Cu-72Ag, 20Cu-80Au, 40Ni-60Pd, Au-Pd
Brazing alloys for tungsten Ductility, high melting point, vapor
pressure
Platinum
Instrument applications
Sensing elements for resistance thermometers Stable and known resistance, high
temperature coefficient
Ultrapure Pt

Stable temperature relation 10Rh-Pt vs Pt, 6Rh-Pt vs 30Rh-Pt, 13Rh-Pt vs
Pt, 5Rh-Pt vs 20Rh-Pt, Au-Pd vs Rh-Pt, Au-
Pd vs Ir-Pt
For sensing ultrahigh temperature in
oxygen-free atmosphere
Ir-Rh vs Ir
Thermocouples
High electromotive force Au-Pd vs Rh-Pt, Au-Pd vs Au-Pd-Pt
Thermocouple connectors Low-resistance joints with base metal
wires
Platinum plate
Galvanometer suspensions Corrosion resistance, strength, and
conductivity
40Cu-60Pd (slow cooled), 14 k Au, Ag-Cu
Galvanometer pivots Hardness and corrosion resistance 60 Os-Ru alloy
Contact parts in low level switches Low electrical contact resistance, good
wear resistance
Rh electroplate; 69 Au-6Pt, 25Ag; Pt, Pd, and
hard dental alloys
Slip rings, brushes for selsyns Low contact resistance, good wear
resistance, and minimum friction
18 k Au, dental alloys, 60Pd-40Cu, Ag, Au
electroplate, Rh electroplate
Sensing elements for gas analyzers Catalytic action proportional to gas
content
Pd-Pt, platinum metal
Glass and ceramics industries
Tanks and crucibles for optical glass Insolubility, high melting point,
noncontaminating
Pure platinum

Bushings and valves for fiberglass Insolubility, high strength 10Rh-Pt, 20Rh-Pt ODS Pt-Rh
Crucibles for continuous melting glass frit Noncontaminating Platinum
Crucibles for melting optical salt crystals Insolubility, high melting point,
noncontaminating
Platinum
Metallized glass and ceramics, metal film
bonded to ceramic by heat
Nonoxidizing, desired color Liquid-bright Au and Pt pastes
Metallized glass and ceramics, metal film
produced by vacuum sublimation
Desired properties Au, Pd, Rh, Ag, and alloys
Heater windings for glass, ceramic, and ferrite
research
Nonoxidizing, high melting point, low
vapor pressure
Pt, 20Rh-Pt, and 40Rh-Pt
Chemical industry
Septum in a hydrogen purification system Selective transmission Pd, 60Pd-40Ag
Catalyst for removal of oxygen from H
2
Activity at low temperature Pd on alumina
Septum in an oxygen purification system Selective transmission Pure silver
Catalyst for production of nitrogen or
nitrogen-hydrogen heat-treating atmosphere
from ammonia
Activity and long life Platinum metal
Catalyst for production of formaldehyde from
methanol
Activity Silver
Catalyst for production of ethylene oxide from

ethylene
Activity Silver
Catalyst for destruction of odoriferous or
hazardous contaminants
Activity Platinum metal
Catalyst for ammonia plus air to yield HNO
3
Long life, high efficiency Rh-Pt
Catalyst for ammonia, air and methane to
yield HCN
Long life, high efficiency Rh-Pt
Rayon spinnerettes Corrosion resistance, strength,
ductility
Rh-Pt, Pt-Au
High-temperature HCI containers Corrosion resistance Platinum
Electrochemical applications
Insoluble anode for electrolytic protection Non-film-forming, high corrosion
resistance
Platinum, 20Pd-Pt, and 50Pd-Pt
Insoluble anode for production of persulfates
and perchlorates, and for electroplating
Corrosion resistance in chlorides,
sulfates; proper anodic reaction
Platinum and 5Ir-Pt
Positive plates in primary and secondary
batteries
Corrosion resistance, conductivity,
and depolarization
Ag-Ag
2

O
2

Fuel cell electrodes Catalytic activity, corrosion resistance Platinum metals
Container for tantalum capacitors Corrosion resistance, high
conductivity
Silver
Aerospace applications
Brazing alloys in stainless steel systems for
handling rocket fuels and oxidizers
Corrosion resistance, compatibility Au-Cu-Ni, Au-Ni-Cr
Special uses
Crucible for molten lead Insolubility and high melting point Ir under oxygen-free atmosphere
Crucible for molten bismuth Insolubility and high melting point Ru under oxygen-free atmosphere
Crucible for molten NaOH High corrosion resistance Silver
Container for high-temperature sulfur and
sulfur gases
High corrosion resistance Gold
Container for high-temperature SO
2
Corrosion resistance, ductility Pure Pt, pure Au, Au-Pt alloy
Container for high-temperature (1000 °C, or
1830 °F) H
2
S
Corrosion resistance, ductility Gold, platinum
Container for S and H
2
S (<1000 °C, or <1830
°F)

Corrosion resistance, ductility Gold
Neutron absorber High absorption cross section Iridium
Intense gamma ray source Radiation energy; moderate half-life Iridium
Magnet Highest known energy product and
corrosion resistance, ductility
23Co-Pt
Laboratory ware Corrosion and heat resistance Platinum, 0.6Ir-Pt, 3.5Rh-Pt
Reflectors
Visible and infrared reflecting surface High efficiency Ag where protected; Rh where exposed
Ultraviolet and infrared reflecting surface High and uniform reflectivity Rhodium
Red and infrared reflecting surface High long-wave reflectivity Gold
Safety devices
Over-pressure protector (frangible disk) Reproducible tensile properties,
corrosion resistance
0.6Ir-Pt, Ag, Au
Fuse wire for temperature-limiting fuse Required and constant melting point
oxidation resistance
Gold

Coatings. Several cladding or coating processes are used to produce composite articles with precious metal surfaces.
Table 3 lists the most important coating processes for precious metals, with characteristics, common thickness ranges, and
typical applications of each.
Table 3 Precious metal coatings
Method Characteristics Thickness
range
Examples of applications
Mechanical and thermal bonding (cladding)
Brazing, hot pressing, hot
and cold rolling, puddling,
casting

100% density, good adhesion, high
wear resistance, uniform thickness
≥
2.5 μm
(
≥
0.1 mil)
Precious-metal-clad base metals for jewelry,
electrical contacts, chemical apparatus, or other
industrial uses; applicable for all malleable precious
metals and alloys
Vacuum coating
Vacuum metallizing Fairly uniform coating, transparent
layers, good adhesion
0.025-12.5
μm (1-500
μin.)
For decorative purposes, reflectors (rhodium on
glass), condensers for electronic devices (mostly
metals on paper, plastic, or lacquered surfaces);
applicable for Ag and Au. Nucleation with Ag
required prior to applying Zn on plastic condensers
Cathode sputtering Very even coating, good adhesion,
high density
1.2-125 μm
(0.05-5 mils)

For improved corrosion resistance, silver in surgical
gauzes, gold on thin Al alloy foils, diaphragms,
mirrors

Electrochemical and chemical coating
Electroplating Reasonably dense and usually
well-adhering deposits;
mechanical and physical properties
depend greatly on plating
conditions
0.15-125 μm
(6-5000 μin.)

Decorative uses, improved corrosion and wear
resistance, electrical contacts; applicable to a wide
range of elemental precious metals and some of
their alloys
Fired-on films
Formulated organometallic
solutions, thermal
decomposition
Thin, well-adhering film 0.05-0.25 μm
(2-10 μin.)
Ceramic and electronic uses, printed circuits,
decorations; applicable to bright Au, Ir, Pt, Pd, and
Ag, mostly on nonmetallic surfaces
Resins containing very fine
suspended metal particles
with a low-melting
inorganic glass flux
Thick, adhering films 12-40 μm
(0.5-1.5
mils)
Electronic applications

Chemical decomposition
coating
Thin, well-adhering film Usually very
thin
Mirrors

Jewelry. Gold, the first jewelry metal, still is the most popular. The popularity of gold is maintained by tradition, its
distinctive color, and the karat mark. Color and karat are the primary factors to be considered in the selection of a
particular gold. Yellow is the most popular color, but red, green, and white karat golds are also available. The 14 k golds
are the most popular in the United States, although significant quantities of all kinds of jewelry are made of 18 k gold. At
the same time, there is significant use of 10 k gold, especially for rings set with synthetic colored stones. Gold-plated
jewelry generally is produced for mass market jewelry lines rather than for fine jewelry lines.
Hand crafting is usual where only a few exclusive creations are made, but where many duplicates will be required, die
forming or casting is appropriate. For simple rings, mechanical forming methods are justified where more than a thousand
units are required; casting is more cost effective for smaller quantities. When complex shapes such as watchcases are
produced from clad or filled stock, intricate and expensive dies are required to maintain uniformity of the cladding.
Platinum is frequently used to make settings for the finest jewelry. In addition to its high intrinsic worth, the workability
and strength of platinum ensure reliable retention of jewels, and its white color enhances the brilliance of diamonds.
Manufacturers use Pt-10Ir for either wrought or cast items; in some instances, Pt-5Ru may be used. The 15 and 20% Ir
alloys are preferred for some of the more delicate pieces, such as small chain.
Where any excess weight is objectionable, as in earrings, palladium is preferred to platinum because of its lower density.
Palladium has platinumlike characteristics that have led to its increased use, particularly in quality jewelry.
All of the white metals platinum, palladium, and white gold are frequently finished with rhodium plate for whiteness
and wear resistance. Sterling is the standard silver jewelry alloy in spite of its tendency to tarnish.

Precious Metals in Dentistry

THE CHANGES IN THE USE of precious metals in dentistry over the last two decades has been dramatic. The use of
gold has declined substantially; it has been replaced in many dental applications by palladium or by nonmetallic dental
materials. Reference 9 provides information on the materials science specific to the dental industry. Reference 10 is a

comprehensive review of the multidiscipline approach involved in considering materials for the oral environment.
This section is a brief review of the dentistry materials presently in use, with an emphasis on the applications of precious
metals. The corrosion characteristics of dental alloys are thoroughly reviewed in the article "Tarnish and Corrosion of
Dental Alloys" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook, and reference is
made to this article in several tables.

References cited in this section
9. R.W. Phillips, Skinner's Science of Dental Materials, 8th ed., W.B. Saunders Company, 1982
10.

B.R. Lang, M.E. Razzaoug, and H.F. Morris, Ed.,
International Workshop on Biocompatibility, Toxicity and
Hypersensitivity to Alloy Systems Used in Dentistry, University of Michigan School of Dentistry, 1986

Classification of Dental Alloys
A variety of alloys are available for dental applications:
• Direct filling alloys
• Crown and bridge alloys
• Partial denture alloys
• Porcelain fused to metal (PFM) alloys
• Wrought wire alloys
• Implant alloys
• Soldering alloys
Of these categories, precious metals are used in direct filling alloys, crown and bridge alloys, PFM alloys, and soldering
alloys. Figures 1 and 2 show a number of typical restorations fabricated from some of these alloys.

Fig. 1 Different classes of inlay p
reparation. Classes I and II involve one or two surfaces of a posterior tooth.
The restorations can be made of a soft gold alloy (80Au-10Ag-9Cu-
1Pd), but are usually made of silver

amalgam. Alternate materials are either composite resin or dental porcelain
bonded onto the remaining tooth.
Classes III, IV, and V inlays are generally made of composite resin. Source: J.F. Jelenko & Company

Fig. 2 Gold alloy dental crowns. (a) Three-
quarter crown, which covers three surfaces of a tooth. (b) Full
crown, which covers the entire tooth. These types of restorations as well as bridgework (multiple crowns) are
made from gold alloys containing 40 to 78 wt% Au. Source: J.F. Jelenko & Company
Direct Filling Alloys. Compositions for direct filling restorations usually consist of silver-tin-copper-zinc alloy
amalgams. Pure gold in the form of cohesive foil, mat, or powder is used only in very limited applications.
Amalgams are produced by combining mercury with alloy particles by a process referred to as trituration. About 42 to
50% Hg is initially triturated with the high-copper types; increased quantities of mercury are used with the low-copper
types. High-speed mechanical amalgamators mix the materials in a matter of seconds. The plastic amalgam mass after
trituration is inserted in the cavity by a condensation process. This is accomplished by pressing small amalgam
increments together until the entire filling is formed. For amalgams using excess mercury during trituration, the excess
mercury is condensed to the top of the setting amalgam mass and scraped away. Table 4 presents compositions for a
number of different amalgam alloys.
Table 4 Compositions of selected dental amalgam alloys
Composition, wt% Alloy
(a)


Ag Sn Cn Zn
Low-copper amalgam
1 75.0

24.6

0.1 0.3
2 72.0

26.0

1.0 1.0
3 72.8

26.2

2.4 1.0
4 69.0

26.6

3.5 0.9
5 68.0

26.0

5.1 0.9
High-copper amalgam