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The noble metals comprise elements like silver, gold, and the platinum group metals: platinum, palladium, rhodium, ruthenium, iridium, and osmium. They all have characteristically attractive appearances that are generally resistant to atmospheric and environmental tarnishing and, importantly, are rare and difficult to mine. Consequently, they have long been greatly valued by individuals as attractive materials. They also have some useful characteristics, particularly corrosion resistance and high temperature stability, and hence, they are critical in a number of key applications. This chapter reviews briefly these main characteristics and highlights their consequent usefulness.
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Noble Metals
Stuart Lyon,
Corrosion and Protection Centre, School of Materials,
University of Manchester,
Manchester, M13 9PL, UK
Author’s note:
The original article on this topic was written by G.W. Walkiden (Consultant); it was
subsequently revised by R.A. Jarman (Editor of the 3rd Edition of Shreir). This current article
has been fully re-written and extended.
Synopsis
The noble metals comprise the elements silver, gold, and the platinum group metals: platinum,
palladium, rhodium, ruthenium, iridium and osmium. They all have characteristically
attractive appearances that are generally resistant to atmospheric and environmental tarnishing
and, importantly, are rare and difficult to mine. Consequently, they have long been greatly
valued by individuals as attractive materials. They also have some useful characteristics,
particularly corrosion resistance and high temperature stability and, hence, they are critical in
a number of key applications. This article reviews briefly these main characteristics and
highlights their consequent usefulness.
Glossary
Noble metal: A metal generally characterised by having relatively positive potentials and low
corrosion rates when immersed aqueous environments by virtue of its relative thermodynamic
stability with respect to its simple hydrated ions in solution (and not solely by its passivity).
PGMs: The platinum group of metals that comprises: platinum, palladium, rhodium,
ruthenium, osmium and iridium; or (PGM) a single element of that group.
Sterling silver: An alloy containing at least 92.5% silver plus other (unspecified) alloying
additions, commonly copper. In the UK, this level of purity is the minimum standard
guaranteed by hallmark.
Troy ounce: A historic unit of measurement common in medieval Europe and said to derive
from the city of Troyes, France; approximately equal to 31.1 g. Currently used in the
weighing, trading and description of precious metals and jewels.
1. Introduction
The group of noble metals comprises adjacent elements from the 2nd and 3rd row of the
transition metals and from groups 8B (platinum group metals – PGMs) and 1B (silver and
gold) of the periodic table. They are characterised by their exceptional resistance to corrosive
attack by a wide range of liquid and gaseous substances, and their relative stability at high
temperatures under conditions where base metals would be rapidly oxidised. This resistance
to chemical and oxidative attack arises principally from the inherently high thermodynamic
stability of the noble metals. Apart from gold (which does not form an oxide in aqueous
media), under oxidising or anodic conditions a thin film of adsorbed oxygen or oxide may
form on the metal, which can enhance their corrosion resistance via passivity. In some
materials, for example silver in halide solutions, salt films can accomplish a similar
passivation effect.
Their high initial cost, combined with a mechanical strength which is generally inferior to that
of base metals, results in only a limited number of corrosion-related applications for the noble
metals and, where used, they are as sheaths, linings or coatings; e.g. electrodeposits. However,
noble metals have some critical applications, e.g. equipment for fine chemical and
pharmaceutical production, for fibre drawing, in the glass industry and for crystal growth.
Silver is produced in a number of countries, generally as a by-product from mining other
materials, particularly lead, zinc, copper, nickel and gold; although some significant primary
mines operate in Australia, Mexico and Russia [1]. The largest producers are in Latin
America (e.g. Peru, Mexico, Chile) while China, Australia and Eastern Europe also produce
significant quantities. Gold is mined as a primary product in a number of countries (e.g. South
Africa, Australia, Ghana) but is also produced as a secondary product from other mining
operations (e.g. Peru, Indonesia). By far the majority (>80%) of the PGMs are produced from
primary resources in South Africa and Russia with lesser amounts from secondary resources.
Table 1 shows estimated supply and demand figures for all the noble metals. Note the
relatively small volumes of material recovered. In years of excess of supply over demand
stocks are built up, which are then released when demand exceeds supply. This is
demonstrated for ruthenium demand in 2006. Noble metals are subject to large fluctuations in
price according to market demands.
Metal
Supply
(tonnes)
Demand
(tonnes)
Volume
(m
3
)
Average Price
($/oz : $/kg)
Silver
Gold
Platinum
Palladium
Rhodium
Ruthenium
Iridium
Osmium
26,900
3,400
218
259
27.1
26 (est.)
-
-
-
-
218
213
27.6
41.5
3.94
< 0.1
2569
176
9.18
21.6
1.98
3.11
0.174
< 0.0045
11.5
635
1140
320
4550
195
350
-
370
20,450
36,650
10,290
146,300
6,200
11,250
-
Table 1: Estimated supply/demand and average prices for noble metals in 2006 [1,2]
2. Properties
Although in the majority of their applications the choice of noble metals is determined by
their chemical rather than by their physical and mechanical properties, some consideration of
the latter is necessary. Thus, important physical and mechanical properties are reported in
Tables 2 and 3.
Ag
Au
Ru
Rh
Pd
Os
Ir
Pt
Relative atomic mass
Room temperature structure
Density (Mg m-3)
Melting point (K)
Boiling point (K)
Resistivity (Ω cm) x 10-6
-1
-1
47
107.9
fcc
10.49
1234
2438
1.59
419
79
196.9
fcc
19.32
1337.6
3130
2.06
311
44
101.1
hcp
12.45
2523
4423
6.80
105
45
102.9
fcc
12.41
2236
3970
4.33
150
46
106.4
fcc
12.02
1825
3237
9.93
76
76
190.2
hcp
22.61
3300
5285
8.12
87
77
192.2
fcc
22.65
2716
4701
4.71
148
78
195.1
fcc
21.45
2045
4100
9.85
73
Table 2: Physical properties of the noble metals [3,4].
Metal:
(annealed)
Melting point
(K)
Proof Stress at
0.2% (MPa)
Ultimate tensile
strength (MPa)
Elongation (%)
Young’s
modulus (GPa)
Silver
Gold
Ruthenium
Rhodium
Palladium
Osmium
Iridium
Platinum
+5% rhodium
+10% rhodium
+20% rhodium
+30% rhodium
+40% rhodium
Pt-10Rh +
dispersed ZrO2
1234
1337.6
2523
2236
1825
3300
2716
2045
2098
2123
2173
2193
2213
2123
40
30
370
80
50
brittle
235
45
55
65
80
95
100
240
180
120
430
700
190
brittle
1100
150
210
310
480
510
580
355
60
70
3
15
40
brittle
10
40
35
35
30
30
30
30
82
72
420
315
115
555
515
170
170
180
215
-
-
190
Table 3: Room temperature mechanical properties of the noble metals and some alloys [3]
2.1 Silver and gold
Silver in the fully annealed state is a soft, ductile metal which is easily fabricated into the very
wide range of forms employed in industry by the normal metal-working techniques such as
drawing, spinning, rolling, etc. Silver work-hardens appreciably during fabrication. The
mechanical strength of silver is markedly affected by an increase in temperature, and falls to
about 25% of the initial value for cold, hard-worked silver when the metal is heated to just
over 200 °C. It has the highest electrical and thermal conductivities of all metals and these
properties are sometimes utilised for specialist applications. Silver is available in several
grades including: fine silver, the normal commercial product containing a minimum of
99.95% silver, and chemically pure silver containing a minimum of 99.99% silver, used for
catalytic and special purposes where the presence of certain trace impurities may adversely
affect its resistance to corrosion.
Silver is available in many standard forms - sheet, strip, foil of thicknesses down to 0.013 mm,
rod, wire down to 0.013 mm diameter, gauze, tubes, bi-metal as silver-clad copper or
phosphor-bronze and many others. It is easily fabricated by the normal techniques of rolling,
spinning, drawing, etc. and readily joins to itself by fusion-welding using argon-arc welding.
Flame welding may be used, but the resulting welds are often not as satisfactory owing to the
possibility of oxygen absorption while the metal is molten, followed by embrittlement by
hydrogen. Fine silver filler rods may be used, and hammering the weld fillet down to the
contour of the surrounding metal produces a very strong joint.
Gold is an extremely soft and ductile metal, and exhibits little work hardening during
deformation. Applications of gold are almost entirely restricted to thin linings or
electrodeposits on base-metal equipment. Gold is available for industrial purposes in a grade
containing a minimum of 99.9% gold in a wide range of forms-sheet, foil, tube, wire, etc. It is
easily fabricated, and when it is being joined to itself may be fusion-welded with an oxy-
hydrogen flame or hammer-welded at temperatures well below the melting point.
2.2 Platinum group metals
Pure platinum is soft, ductile and easily fabricated although its mechanical properties are
affected by the degree of cold working and the presence of impurities or alloying constituents.
In its applications it is frequently alloyed with other PGMs; the melting points of its alloys
with Rh, Ir, Os and Ru being higher than the parent metal, those with Pd being lower. In most
cases the strength, rigidity, hardness and resistance to corrosion are improved by alloying.
Contamination with certain base metals (e.g. iron), however, can lead to embrittlement and
failure of platinum and its alloys and, for example, if platinum is handled at high temperatures
with steel tongs, they should be platinum-tipped to avoid iron pick-up.
The platinum group metals have a range of often unique properties that find critical use in a
number of key applications, for example in automotive catalysts, chemical catalysts,
electronics and jewellery; however, such applications are outside the scope of this article.
Apart from palladium, the other platinum group metals are seldom used in isolation for
corrosion resistant applications mainly because of their cost and availability. Palladium has
very similar properties to platinum but is less corrosion resistant. Its remarkable ability to
absorb large quantities of hydrogen is used in hydrogen permeation membranes and in similar
applications such as solid-state hydrogen reference electrodes. Iridium and osmium are
characterised by their extreme densities, their high melting points and their low ductility and
high hardness, which is sometimes utilised in specialist corrosion and wear-resistant
applications for hard alloys: for example the traditional use of iridium-osmium tipped
fountain pens and precision pivots and bearings. Rhodium, ruthenium and iridium are used as
alloying additions to platinum in order to improve mechanical and physical properties.
Iridium is used in specialist equipment for fine crystal growth (e.g. in electronics) especially
from 1600-1900°C. Both iridium and osmium form oxides that are volatile at modest
temperature which, in the case of osmium, forms and volatilises at room temperature.
Platinum is available as sheet, foil down to 0.0064 mm thick, tube, rod, wire down to 0.0064
mm diameter, wire down to 0.001 mm diameter, and clad on thin sections of base metals, e.g.
copper, nickel, Inconel, etc. Platinum, palladium and the normal alloys of platinum used in
industry are easily workable by the normal techniques of spinning, drawing, rolling, etc. To
present a chemically clean surface of platinum and its alloys after fabrication, they may be
pickled in hot concentrated hydrochloric acid to remove traces of iron and other contaminants;
this is important for certain catalytic and high-temperature applications. In rolling or drawing
thin sections of platinum, care must be taken to ensure that no dirt or other particles are
worked into the metal, as these may later be chemically or electrolytically removed, leaving
defects in the platinum.
When platinum or its alloys are being joined, properties of the weld or solder must be such
that it is no less corrosion or oxidation-resistant for the application in question than the parent
metal. Platinum and its alloys are readily joined to themselves and to certain base metals, e.g.
iron, nickel, and copper. The principal methods for joining platinum are as follows:
1. Fusion welding, using a platinum or alloy filler rod of the same composition as the
parent metal and a shielded electric arc or an oxy-hydrogen flame (an oxy-acetylene
flame may cause carbon pick-up by the molten metal). The weld fillets are then cold
hammered to the contours of the surrounding metal to provide a strong joint.
2. Platinum and rhodium-platinum alloys when cleaned are readily hammer-welded to
themselves and to each other at temperatures in the range 800-1000°C. The welds so
produced are completely homogeneous.
3. Fine gold, copper, silver-palladium or platinum-palladium-gold-copper alloys may be
used to solder platinum to itself and to its alloys, or to steels, nickel, etc. No fluxes are
used, and soft solders should not be employed.
2.2.1 Platinum-rhodium
Rhodium alloys readily with platinum in all proportions, although the workability of the
resulting alloy decreases rapidly with increasing rhodium content. Alloys containing up to
about 40% rhodium, however, are workable and find numerous applications. Alloys that
contain more than 40% rhodium, while very difficult to fabricate, are almost immune from
attack by oxidising acids. The Pt-10Rh alloy is particularly resistant to attack by free wet
chlorine such as that produced by the combustion of halogenated organic vapours.
The resistance of rhodium-platinum alloys to corrosion is about the same as or slightly better
than that of pure platinum, but they are much more stable at high temperatures. They have
excellent resistance to creep above 1,000°C, a factor which largely determines their extensive
use in the glass industry, where continuous temperatures sometimes exceeding 1,500°C are
encountered. Rhodium additions to platinum reduce appreciably the volatilisation of pure
platinum at high temperatures.
2.2.2 Alloys of platinum with other PGMs
Iridium alloys with platinum in all proportions, and alloys containing up to about 40% iridium
are workable, although considerably harder than pure platinum. The creep resistance of
iridium-platinum alloys is better than that of rhodium-platinum alloys at temperatures below
500°C. Their stability at high temperatures, however, is lower, owing to the higher rate of
formation of a volatile iridium oxide. Additions of ruthenium increase the hardness of
platinum substantially, but the limit of workability is reached at about 15% ruthenium. Apart
from a somewhat greater tendency to oxide formation at temperatures above 800°C, the
resistance to corrosion of ruthenium-platinum alloys is comparable to that of iridium-platinum
alloys of similar composition.
2.2.3 Dispersion strengthened alloys
Platinum-rhodium alloys are used extensively in high temperature applications for crucibles
and related equipment, especially in the glass industry. The increasing cost of rhodium and
platinum provides a strong driver to reduce overall costs and this has led to the development
of a range of oxide dispersion strengthened alloys and composite alloys using, for example,
platinum coatings on palladium cores [5]. The most common formulation is of zirconia
dispersed Pt-10Rh alloy [6] with somewhat higher room temperature strength than Pt-10Rh
but and 2-3 times the creep rupture properties of Pt-40Rh at 1,400°C, Table 3.
3. Thermodynamic Behaviour
The behaviour of the noble metals (indeed all metals) in different environments is determined
by three principal factors:
1. Their relative thermodynamic stability (nobility).
2. The formation of passive protective films.
3. Their tendency to form complex ions in solution.
In the absence of species that form soluble complex ions the noble metals are extremely
resistant to corrosion by aqueous solutions of alkalis, salts and acids. However, the resistance
of silver to oxidising acids is generally lower than that of the other noble metals, while in
halogen acids it forms a protective film of insoluble halide. Silver also differs from the other
noble metals in forming a sulphide tarnish film in the presence of reduced sulphur compounds.
Where complexing species are present and stabilise the metal ions in solution (for example
cyanide) then the noble metals will corrode.
3.1 Silver
Silver, with a standard electrode potential EAg+/Ag = 0.79 V, is exceeded in nobility only by
gold and the platinum-group metals. The Pourbaix diagram for silver, Figure 1, shows that at
potentials below about 0.4 V, and in the absence of complexing ions, silver is immune to
attack over almost the whole pH range.
Figure 1: Pourbaix (E-pH) diagram for silver at a metal ion concentration of 10-5M [4].
The situation is different in the presence of complexing agents, such as cyanide, or with
species with which silver forms an insoluble salt, such as chloride. Thus, the presence of
halides (with the exception of fluoride) substantially increases the zone of passivity, due to the
formation of halide salt films as passivation layers [5]; at 25 °C: AgCl = 1.7×10-10; AgBr =
5.0×10-13; AgI = 8.5×10-17; AgF is soluble. Silver will also passivate in solutions containing
sulphate. However, in the presence of sulphide, silver can form Ag2S tarnish films, even in the
absence of air.
Silver, therefore, is thermodynamically stable in reducing acids, e.g. hydrochloric acid, acetic
acid, phosphoric acid, provided oxidising substances are absent. Oxidising acids, e.g. nitric
acid, hot sulphuric acid at concentrations exceeding 80% and reducing acids containing
oxidising agents, will be corrosive to silver, and the diagram shows that an extensive zone of
corrosion occurs at elevated potentials in the acid region. When silver is passivated by a
halide film, as is formed for example in hydrochloric acid, the film is tenacious, self-healing
and highly insoluble. However, such films are easily reduced, for example by galvanic
coupling to a less noble metal such as zinc, aluminium and passive stainless steels and nickel-
based alloys. In such instances silver will continuously corrode. In highly alkaline solution,
silver corrodes only within a narrow region of potential, provided complexants and oxidants
are absent. It is thus suitable to handle aqueous solutions of sodium or potassium hydroxides
at all concentrations; it is also unaffected by fused alkalis.
3.2 Gold
The high resistance of gold to attack by a very wide range of corrosive media results from its
exceptional thermodynamic stability in aqueous conditions. The Pourbaix diagram for gold,
Figure 2, shows immunity from attack over the whole range of pH values and, uniquely,
gold’s zone of thermodynamic stability includes the entire region of water stability; thus it is
immune from corrosion in aerated water. Gold, however, is easily complexed, and its
solubility in hydrochloric acid containing an oxidising agent (e.g. nitric acid) results from a
combination of high redox potential and the formation of chloroauro complex ions (AuCl4-).
The unstable Au+ ion and the easily reducible Au3+ ion also readily form stable complexes.
Gold is unaffected in alkaline solutions, but in the presence of cyanides the soluble Au(CN)2-
ion is readily formed by air oxidation. This reaction forms the basis for the extraction of gold
from its ores on an industrial scale.
Figure 2: Pourbaix diagram for gold at a metal ion concentration of 10-5M [4].
3.3 Platinum group metals
All the six platinum-group metals are highly resistant to corrosion by most acids, alkalis, and
other chemicals. As may be seen from the potential-pH diagram, Figure 3, platinum is
immune from attack at almost all pH levels, although it will corrode slowly in aqua regia
(concentrated hydrochloric + nitric acids). It should also be noted that platinum is
significantly less noble than gold and is covered by an oxide film in air (unlike gold, which is
oxide free).
Figure 3: Pourbaix diagram for platinum at a metal ion concentration of 10-5M [4].
Platinum is unaffected by most organic compounds, although some compounds may
catalytically decompose or become oxidised on a platinum surface at elevated temperatures,
resulting in an etched appearance of the metal. Carbon and sulphur do not attack platinum at
any temperature up to its melting point.
Figure 4: Pourbaix diagram for palladium at a metal ion concentration of 10-5M [4].
Compared with platinum, palladium is significantly less noble as can be seen from its
Pourbaix diagram, Figure 4, where an extended passive oxide region is notable (PdO) and
simple Pd2+ aquo-ions are stable at low pH and high potential (compare with platinum).
However, palladium is relatively stable in the presence of aqueous solutions of all pH values
with the exception of strong oxidising agents and complexing substances. Non-oxidising acids,
e.g. acetic, oxalic, hydrofluoric and sulphuric acids, have no effect on the metal at ordinary
temperatures. Strongly oxidising acids, however, e.g. hydrochloric acid containing nitric acid,
rapidly attack palladium. Dilute nitric acid attacks palladium only slowly, but the metal is
rapidly corroded by the concentrated acid. Alloys of palladium with platinum, however, retain
most of the corrosion resistance of platinum. In ordinary atmospheres palladium is resistant to
tarnish, but some discoloration due to sulphide-film formation may take place in industrial
atmospheres containing sulphur dioxide. Alkaline solutions, even in the presence of oxidising
agents, are without significant effect.
The resistance of rhodium and iridium to chemical attack is very similar to that of platinum
although with a wider domain of stability particularly at low pH in oxidising solution.
Additions of rhodium or iridium to platinum generally raise the overall corrosion resistance of
the alloy to a very wide range of reagents. Like the other noble metals, in the absence of
complexing agents both rhodium and iridium are stable in aqueous solutions at all pH values.
Both metals are unattacked by alkalis, acids or oxidising agents in aqueous solution, although
oxidising molten salts (e.g. potassium nitrate) are more corrosive. Iridium in particular has
excellent resistance to fused lead oxide, silicates, molten copper and iron at temperatures up
to 1,500°C.
Ruthenium and osmium are decidedly less noble than the other four elements in the platinum
group. Both exist in numerous valency states and very readily form complexes. Ruthenium is
not attacked by water or non-complexing acids, but is easily corroded by oxidising alkaline
solutions, such as peroxides and alkaline hypochlorites. Osmium forms a volatile (and toxic)
oxide at room temperature in air and should be handled with care.
4. Corrosion and Electrochemistry
There is great interest in the electrochemistry of the noble metals; many of them having
scientifically interesting and sometimes unique properties that can result in commercially
useful attributes, for example in electro-catalysis. However, it is impossible to provide a fully
comprehensive review in this section and it is not even attempted. Thus, the general
electrochemistry of the materials is only superficially considered here. From the perspective
of corrosion, one of the most interesting and potentially valuable effects is the substantial
reduction in the corrosion rate of passive alloys (i.e. stainless steels) as a consequence of the
addition of small amounts of noble metal. This is treated in detail in a separate article on
cathodic modification of stainless steels.
4.1 Silver
4.1.1 Anodic processes
Silver generally corrodes anodically below the reversible oxygen potential, unless an
insoluble oxide or salt is formed. When silver is used as an anode in sulphuric acid solutions,
its behaviour shows an analogy with that of lead. Silver sulphate, Ag2SO4, is first formed, and
this acts as a passive film [7]. When the potential is raised the sulphate is oxidised to AgO,
which may be cathodically reduced back to Ag2SO4 at a potential lower than that required for
its initial formation. When made anodic in nitrate solutions, silver generally dissolves
quantitatively as Ag+, and this forms the basis of the electrorefining techniques widely used in
industry. Similar considerations apply to the anodic behaviour in cyanide solutions where
silver forms a complex cyanide. In chloride solutions silver anodes become covered with a
layer of silver chloride [8]. The anodic oxidation and reduction of silver in alkaline solutions
is of interest in battery applications and a few % of alloying additions of palladium or gold
improve the capacity of silver oxide electrodes [9].
Silver is not generally resistant to sulphidising environments, as is commonly demonstrated
by the atmospheric tarnishing of silver (see below), a sulphide film of varying thickness
generally forming. The aqueous corrosion of silver in sulphide solution was studied by
electrochemical and analytical methods in order to elucidate the mechanisms [10]. Under
anodic polarisation, silver initially forms Ag2S, which is then further oxidised to Ag2O at
higher potentials. The limiting anodic current density was found to be proportional to the
sulphide concentration in solution and this was thought to be due to diffusion of SH- ion to the
silver surface. The reaction rate increased as the pH increased, which is consistent with the
change in speciation of SH- ion in solution; little effect on corrosion rate was found with
dissolved oxygen content. X-ray diffraction and Raman spectroscopy indicated the formation
of Ag2S as the main reaction product.
4.1.2 Atmospheric corrosion and tarnishing
Silver is traditionally used decoratively and functionally in fine cutlery and tableware and is
valued for its high lustre and long life. It has also been widely used in electrical contact
applications due to its high conductivity. Although silver is stable to oxidation it forms a
tarnish film in atmospheres containing inorganic or organic sulphur species and chlorides.
This dulls the surface and detracts from appearance; the film also increases contact resistances.
Atmospheric levels below 0.1 ppm of H2S, SO2 and HCl are sufficient to tarnish silver at
measurable growth rates Dry nitrogen atmospheres stops tarnish film growth, which suggests
that the process is electrochemical and occurs in a thin adsorbed water layer on the metal
surface and requires a air as oxidant [11]. Long chain organic molecules containing sulphur
were also found to strongly interact with silver surfaces, giving tarnish films of similar
composition to those found in H2S [12].
The sulphidation of silver was studied in a tubular reactor with well-controlled mass transfer
characteristics. In dry air H2S slowly reacted with silver at a constant rate independent of flow.
The mechanism is this surface controlled and was found to involve atmospheric oxygen; the
rate being increased significantly in the presence of an alternative oxidant (NOx). In humid air
corrosion was over 1000 times faster with the kinetics now controlled by mass transfer of H2S
in the gas phase [13]. In a series of comprehensive studies, the effect of carbon oxy-sulphides
(COS) was also investigated and it was concluded that the sulphidation of silver by COS was
at least as significant as by H2S; the detailed mechanisms of film growth were also determined
[14].
Significant research effort has been expended on preventing (or at least reducing) the kinetics
of the tarnishing process. These apply one of three strategies: coating the silver, alloying the
silver or use of a tarnish (corrosion) inhibitor. In the electronics sector, electroplating with
gold is commonly used in order to limit corrosion of contacts and consequent increase in
contact resistance and is generally a successful solution. However, if the excellent appearance
of decorative silver is required, gold plating (i.e. a gilt finish) is not acceptable.
A number of treatments have been considered for tarnish protection, including palladium or
rhodium plating (importantly the latter causes very little colour change on silver), tarnish
inhibitors incorporated into commercial polishes (commonly using organic thiols [15]),
treatments with lacquers or plasma-polymerised coatings and formation of self-assembled
monolayers [16]. In a comparative evaluation of their performance the commercial tarnish
inhibitors were found not to provide significant protection, while the most effective treatment
was an organic lacquer, following by rhodium plating and the self-assembled monolayer
treatment [17].
The search for a tarnish resistant silver alloy has been on-going for at least 70 years with, until
recently, comparatively little success. Although alloys of silver that substitute some of the
copper content for palladium, nickel, zinc or tin do tarnish more slowly than standard sterling
silver [18], they have no compelling advantage. Novel formulations for decorative silver
alloys are constrained by the need to stay within the purity level for hallmarked sterling silver
(i.e. more that 92.5% silver content). Until recently, no novel alloy had significant proven
advantages in this application. However, an alloy containing germanium has been shown to be
relatively resistant to sulphide tarnishing and is now marketed as “Argentium” [19]. The
improved surface properties are due to preferential oxidation of germanium, resulting in a
continuous protective oxide layer, and the high diffusivity of germanium, which permits the
layer to be re-established rapidly if damaged.
4.2 Gold
4.2.1 Anodic processes
Gold is thermodynamically stable to corrosion in ordinary aerated non-complexing
environments and, of course, it neither tarnishes in the atmosphere nor in biological fluids. It
is therefore ideal for applications in the decorative, medical and dental (in-vivo) and electrical
(contacts and connectors) areas. It is also used extensively (as is platinum) as a non-reactive
electrode substrate for laboratory electrochemical studies of redox reactions of species in
solution, however, such applications are not within the scope of this discussion.
The anodic oxidation of gold itself is of interest (indeed it is amongst the most studied of
oxidation processes) as it can elucidate generic mechanisms for film formation and metal
dissolution under anodic dissolution. In general terms, on anodic polarisation in non-
complexing electrolytes, for example, sulphate, perchlorate, hydroxide, etc., gold will
reversibly passivate by forming an oxide/hydroxide film above about +1.4V (SHE) at pH 0.
Thus, in sulphuric acid gold dissolves transiently to the Au(I) species but rapidly passives
forming a film of hydroxide [20], containing Au(III) species consisting generally of Au(OH)3.
Evidence for lower oxides is no longer convincing. Conway’s extensive and thorough review
describes in detail adsorption and oxidation on noble metal surfaces [21]. Unlike conventional
corrosion, anodic oxidation on gold proceeds predominantly by surface chemical processes
involving sub-monolayer adsorption of hydroxide and oxide species (hydroxide electrolytes)
that eventually grow by coalescence and thickening, eventually forming a macroscopic
multilayer hydrous oxide film, probably by field assisted ion migration. In the presence of
other anions, a competitive chemisorption with, for example, HSO4-, ClO4-, Cl-, occurs which
inhibits the onset of surface oxidation. Studies using scanning tunnelling microscopy (STM)
and atomic force microscopy (AFM) on single-crystal gold surfaces has shown the
development of single-atom dimensioned pits, and have demonstrated that atomic re-
arrangement (or dissolution) is more difficult from atom terraces than from edges [22].
In solutions where gold forms a soluble complex ion, gold may dissolve or may passivate
(depending on the concentration of the complexing ion and the potential). On anodic
polarisation in acidic chloride solutions gold initially dissolves, due to formation of the AuCl4-
ion, at potentials greater than about +1.3V (SHE, pH=0), depending on the chloride ion
concentration; no evidence is seen for dissolution in the Au(I) state. At higher potentials
greater than about +1.6V, gold passivates forming an oxide film [23]. In alkaline cyanide
solutions (pH=13), which are of obvious interest for gold extraction and in electroplating,
dissolution proceeds above about -0.66 V (SHE) by competitive adsorption of the cyanide ion
on gold followed, stepwise, by single-electron transfer then adsorption of another cyanide ion
leading to formation of the Au(CN)2- ion. No convincing evidence for formation of the Au(III)
state, Au(CN)4-, is found at higher potentials, instead passivation terminates dissolution at
potentials above about +0.38V (SHE, pH=13) due to formation of an Au(OH)3 film [24].
In addition to cyanide and chloride, gold forms complexes with a number of other common
species in acid or alkaline conditions including: thiosulphate, thiocyanate, thiourea, sulphite,
bromide and iodide and the anodic dissolution of gold in these environments was reviewed by
Nicol in 1980 [25]. In view of the toxicity of cyanide, there is obvious interest in using
alternative leaching agents for gold recovery and both thiourea, thiocyanate and bromide
processes have been considered. Of more academic interest is the surface chemistry and
electrochemistry of gold in the presence of sulphur-containing species where many detailed
studies have been carried out however, these are beyond the scope of this section.
4.2.2 Gold extraction
Since the late 19th Century, most recovery of gold from crushed ore has been via the
MacArthur-Forrest cyanidation process. This utilises the stability of the gold cyanide complex
ion (Au(CN)2: K = 10-38) to dissolve gold, which may then be recovered by electrodeposition
or via zinc dust (Merrill-Crowe process) [26,27]. The principle dissolution reaction is given
by Elsener’s equation: 4Au + 8CN + O2 + 2H2O 4Au(CN)2 + 4OH
In order for efficient extraction, and to minimise loss of cyanide as volatile HCN, the leaching
takes place ideally close to pH 10.5. The cyanidation process requires an oxidant to be present
(usually dissolved oxygen) and the efficiency of extraction is reduced if insufficient oxidant is
present. Other factors that reduce leaching efficiency (present in so-called “refractory” ores)
include the presence of sulphides, copper, iron and zinc. Pre-treatment of crushed ores is often
required as is the use of additional oxidants (chemical or biological – i.e. sulphur oxidising
bacteria).
Once leached the gold has to be recovered from the relatively dilute cyanide solution. One of
the largest gains in efficiency of gold recovery from more dilute cyanide liquors is in the
absorption of the gold complex on activated carbon and a variety of processes are in use: e.g.
the carbon-in-pulp process [26]. For cyanide processes there is also the not inconsiderable
problem of cyanide disposal. To counter this various other processes have been considered
with some in active development. These include the use of bromine, chlorine, thiourea and
thiosulphate and while these have advantages, much research needs to be done to optimise
them. Currently, therefore, the cyanide process is generally still the most economical.
4.2.3 Dealloying and nanoporous materials
Dealloying as a corrosion phenomenon has been known for over 100 years, the classical case
being dezincification (i.e. selective dissolution of zinc from brass leaving a copper-rich,
mechanically weak, layer). Dealloying typically occurs in an alloy where at least two of the
components have relatively well-separated equilibrium potentials in the environment. In such
cases, where the alloy is polarised between these values, the less noble component of the alloy
is selectively dissolved, leaving the remaining more noble component. This mechanism
invariably produces layers that have profoundly altered mechanical properties usually of very
low fracture toughness (i.e. they are brittle). These have been found to have profound
influence on many stress-corrosion cracking processes in alloys [28,29]. Although it was not
called such, and the mechanism was not then known, dealloying was first developed as a
process to produce porous nickel (by selective dissolution of aluminium from a nickel-
aluminium alloy Raney nickel), which is used a catalyst in various chemical processes [30].
Selective dissolution in, for example, Cu-Au and Ag-Au alloys has been well studied in order
to determine generic aspects of the dealloying mechanism in these and other more
commercially important alloys. Increasingly, controlled dealloying is being used in its own
right to produce tailored nanoporous gold substrates for chemical sensing and catalytic
applications. The ordered intermetallic alloy Cu3Au is an excellent template to study
dealloying phenomena. At low overpotentials for copper dissolution, scanning tunnelling
microscopy has demonstrated 2-dimensional clustering of gold while at higher overpotentials
the surface is mostly covered by gold and effectively passivates in 2-dimensions although
small regions of material continue to dissolve generating 3-dimensional roughness and
porosity [31]. Above a threshold potential, global surface roughening occurs, which is
strongly influenced by adsorption of species such as sulphate, chloride and alkyl-thiols on the
surface mobility of the gold atoms. This process has been modelled using a continuum model
such that porosity during dealloying develops on a length scale that is characteristic of the
surface aggregation (diffusion) of the noble metal atoms [32].
The formation of nanoporosity in Ag0.7Au0.3 and Ag0.65Au0.35 alloys during dealloying in
perchloric acid has been studied. Without halide addition, the pores were of the order of 8 nm
while with chloride, bromide and iodide the pore size changed, respectively, to 17, 16, and 67
nm. This coarsening can be interpreted as an increase in surface mobility of the gold atoms in
the presence of halides [33]. Gold substrates with tailored nanostructures can thus be prepared
by controlled electrochemical dealloying. Although development is still required, in particular
to resist coarsening of the porosity during application, nanoporous gold substrates have many
potential applications; for example: in electro-catalysis [34], fuel cells [35] and chemical and
biochemical sensing [36].
4.3 Platinum group metals
4.3.1 Anodic processes
The platinum group metals are characterised by their intrinsic thermodynamic stability and in
this they are similar to gold. The key difference between them is that gold remains oxide-free
in aerated non-complexing aqueous environments; however, the PGMs are predicted to retain
an oxide film that contributes to their passivity. The PGMs have many important properties
and applications in catalysis that are beyond the scope of this section.
For many years it was assumed that platinum and other PGMs were oxide-free in air and in
aerated solutions however, the advent of rapid cyclic sweep voltammetry techniques has
allowed the elucidation of the redox processes between platinum and oxygen, and this
research was reviewed extensively by Conway in 1995 [21]. After initial adsorption of
oxygen-containing species, a place exchange mechanism takes place between Pt and O to
effectively develop a 2-dimensional, then 3-dimensional oxide film. Oxygen adsorption,
reduction and place exchange occur via a variety of steps with the rate-determining processes
for each part of the mechanism determined; however, a full discussion is well beyond the
scope of this review. Like gold oxide formation on, and dissolution of, platinum occurs as a
function of crystallography. On low-index surfaces, oxide formation passivates the surfaces
more easily, resulting in a lower dissolution rates at higher potentials, while nanofaceted
surfaces dissolve more rapidly, which is evidence that atomic edges and corners are the main
locations of dissolution [37]. Perhaps the most interesting finding is that although, like gold,
anion adsorption plays a role it appears to be much less important, which reflects the
increased stability of PGMs to complexing species such as halides, cyanides, etc.
Platinum group metals are, therefore, extremely resistant to dissolution in almost all aqueous
environments under almost all conditions. For example, and in contrast to gold and silver,
high pressures and temperatures are required for the dissolution of platinum in cyanide to
occur at significant rates. Table 4 shows the corrosion resistance of the PGMs in various
environments; importantly, PGMs are one of the few non-polymeric materials than can
successfully handle fluorides and HF without significant problem.
Environment
Ru
Rh
Pd
Os
Ir
Pt
HF (40%, 20°C)
HCl (36%, 100°C)
H2SO4 (96%, 100°C)
HNO3 (62%, 100°C)
HCl+HNO3 (aqua regia, 100°C)
H3PO4 (100°C)
HClO4 (100°C)
KCN (100°C)
NaOCl (100°C)
A
A
A
A
A
A
D
A
A
B
A
A
A
B
A
B
C
D
D
B
C
D
C
A
A
A
D
D
D
D
A
A
A
A
A
A
A
B
A
A
D
A
A
C
A
Table 4: Corrosion resistance of PGMs in various environments: (A) – No attack; (B)
Minor attack but can be used; (C) Major attack and cannot be used; (D) Rapid attack [38,39].
4.3.2 Platinum extraction and secondary recovery
As noted above, ambient temperature and pressure cyanide leaching is not effective for the
recovery of PGMs from crushed ore bodies. Thus, after mining and concentration by gravity
and/or froth flotation, the materials are smelted to a copper-nickel matte (sulphide-rich
intermediate), which is further separated into copper and nickel anode materials that are
purified electrolytically. The PGMs remain within the (insoluble) anode slime and were
traditionally recovered using wet chemical methods including sequential precipitation.
Current technology uses a liquid-liquid solvent extraction technique that is more efficient,
offering enhanced recovery and lower cost [40].
Efficient secondary recovery of platinum group metals from, for example, vehicle exhaust
catalysts, and recovery and separation from other metals from, for example, electronic circuit
boards, is critical for their sustainable use. In such cases, higher temperatures and pressures as
well as more corrosive solutions, can be used. Several leaching processes have been proposed,
the majority using strong hydrochloric acid containing an oxidant (i.e. aqua regia) [41,42] of
high temperature and pressure leaching with cyanide [43,44]. The pressure cyanide processes
are better developed and can, under ideal conditions, extract over 95% of the platinum and
palladium and about 85-90% of rhodium.
4.3.3 Cathodic processes: hydrogen evolution
The platinum group metals are well-known for their outstanding electrocatalytic activity,
especially for the evolution of hydrogen. The exchange current density for the overall
hydrogen evolution reaction: 2H+ + 2e- H2(g)
varies by about 10 orders of magnitude from mercury and lead to platinum, Table 5.
Environment
Metal
Exchange current density,
io A cm
-2
1M H
2
SO
4
at 20°C
0.1M HCl at 20°C
Palladium
Platinum
Rhodium
Iridium
Nickel
Gold
Titanium
Aluminium
Lead
Mercury
Platinum
Palladium
Silver
Gold
Iron
Nickel
Copper
Lead
103.0
103.1
103.6
103.7
105.2
105.4
108.2
10-10.0
1012.0
1012.3
10-2.6
10-3.2
10-5.6
10-5.6
10-6.0
10-6.0
10-6.8
10
-13.2
Table 5: Exchange current densities for the evolution of hydrogen on various metals in
various environments [45,46].
The large exchange current density on PGMs implies a correspondingly low overpotential for
hydrogen evolution. This property finds extensive applications in electrocatalysis in general
and in hydrogen storage, release, hydride batteries and fuel cells, where nano-sized PGM
particles are used; both to increase surface area and reduce the quantity of expensive material
used. An important application is in noble metal alloying of passive metals (stainless steels
and titanium) where small amounts (0.2%) of noble metals (typically Pt, Pd or Ru) causes
spontaneous passivation in acids by increasing the cathodic rate of hydrogen evolution. This
“cathodic modification” effect is discussed in more detail in a separate section.
The PGMs are somewhat susceptible to a reduction in their catalytic activity as a consequence
of “poisoning” with other surface active species, including metallic impurities with lower
exchange current density, which will tend to plate out onto cathodically polarised surfaces
[47]. On the other hand, noble metal oxides, in particular ruthenium and iridium oxide, are
almost as effective cathodic electrocatalysts as platinum and palladium, however are almost
immune from poisoning by metal ions in solution [48,49].
5 High Temperature Properties
5.1 Silver and gold
Owing to their relatively low melting points and mechanical strengths, silver and gold find
very few applications at elevated temperatures. Silver below its melting point has
considerable resistance to oxide-film formation, but molten silver dissolves appreciable
quantities of oxygen, which precipitates as silver oxide or bubbles dispersed throughout the
metal when the metal solidifies. Gold is not subject to oxide-film formation at any
temperature up to its melting point, but may be covered by a thin adsorbed layer of oxygen.
The absence of an oxide film enables gold to be pressure-welded at room temperatures.
5.2 Platinum group metals
The excellent resistance of platinum, rhodium and iridium to oxidation at high temperatures
finds numerous applications in technology, in particular in the form of platinum-based alloys.
Osmium and ruthenium form volatile oxides and are therefore not suitable for high
temperature use on their own.
Oxide
Molar Mass
Density
Pilling-
Bedworth ratio
Properties at high temperature
RuO
2
RuO4
RhO
Rh2O3
PdO
OsO2
OsO4
IrO2
PtO
Pt3O4
PtO2
133.1
165.1
118.9
253.8
122.7
222.2
254.2
224.2
211.1
649.3
227.3
6.97
8.2
8.31
11.29
4.95
11.69
14.9
8.89
10
2.31
-
1.87
1.66
2.33
6.09
2.24
1.56
2.68
2.63
Dissociates at 930-950°C
Sublimes at 40°C
Dissociates above 1000°C
Dissociates above 1100°C
Dissociates above 870°C
Dissociates at 650°C
Boils without decomposition at 131°C
Decomposes at 400°C
Dissociates above 1100°C
Decomposes when heated
Thermally unstable
Table 6: Selected properties of platinum group metal oxides [50]
Platinum, while it does not form a measurable oxide film, is covered by thin adsorbed layer of
oxygen [51], which volatilises at an increasing rate as its temperature rises above 1,000°C via
a volatile metastable oxide [52]. In the presence of flowing oxygen or air the rate of
volatilisation is considerably increased. Rhodium, iridium and palladium exhibit oxide-film
formation, the last at temperatures as low as 600°C [53]. However, palladium oxide
dissociates above 870°C, the metal then appearing bright up to its melting point. Absorption
of oxygen without film formation occurs, however, and the palladium increases in weight.
Platinum looses more mass via this volatilisation mechanism compared with rhodium and
iridium from 900°C to 1,200°C, but their volatilities are about the same at temperatures
around 1,300 °C. Below 1,100 °C alloys of platinum with rhodium and palladium loose less
mass than pure platinum however, but palladium-platinum alloys absorb oxygen detrimentally.
Rhodium-platinum alloys at high temperatures show no preferential loss of either metal, and
are widely used. Iridium-platinum alloys show greater loss of weight on heating in air,
because of the greater rate of oxidation of iridium and the higher volatility of the oxide of this
metal. Iridium is thus lost preferentially from iridium-platinum alloys. Selected properties of
the PGM oxides are shown in Table 6.
Volatilisation of platinum and its alloys at high operating temperatures may be substantially
reduced by avoiding contact with air or oxygen, especially if the environment is flowing, for
example due to convection currents. This may be achieved by completely embedding the
metal in high-purity alumina refractory; flame-sprayed coatings, for example, are effective in
preventing free circulation of air over the metal. Only alumina that is largely free from silica
and other oxides that are more easily reduced, can be used, otherwise contamination and
embrittlement of the platinum may result from partial reduction of such oxides.
Grain-growth of platinum and its alloys when operating continuously at high temperatures is
often responsible for failure of the metal, resulting from weaknesses developed by large inter-
crystal boundaries. This defect may be largely eliminated by the use of sintered metal
produced by powder-metallurgical techniques, or by the incorporation of a small amount of a
refractory oxide, carbide or nitride in powder form in the body of the metal, such as zirconia-
dispersed platinum-rhodium alloys [6].
6 Selected Applications
6.1 Chemical process equipment
6.1.1 Linings
Traditionally, noble metal linings were used in chemical and pharmaceutical production as
linings for steel and copper equipment and occasionally as vessels, condensers and other
equipment. However, such uses have generally been superseded either by more cost effective
coating materials (e.g. tantalum, glass lining), or alternative construction materials (e.g.
stainless steel, graphite).
Linings may be of two types: either loose or bonded. Loose linings provide good contact
between vessel and lining - adequate for good heat transfer - but are not suitable for reduced
pressure use as the gap between vessel and lining will expand causing the lining to fail.
Traditionally silver linings for chemical plant were used particularly for highly alkaline
environments in which silver has excellent performance; where used, silver linings are
generally 0.5 mm to 1 mm thick. Solid silver construction, 1 to 2.5 mm thick, may be
employed for condenser coils, distillation heads, etc. Traditional bonded silver linings for
mild steel or copper vessels are generally soldered in-situ onto the walls of the vessel using a
tin-silver solder. Such soft-soldered linings should not exceed 200°C for their maximum
continuous operating temperature. However, bonded linings are suitable for operation under
vacuum conditions, and provide excellent heat-transfer characteristics.
Platinum and rhodium-platinum and iridium-platinum alloys are employed to line and sheath
autoclaves [54], reaction vessels and tubes, calorimeters [55] and a range of other laboratory
and commercial equipment [56]. Linings are generally 0.13 mm to 0.38 mm thick, and for
certain applications co-extruded platinum-lined nickel-based alloys (Inconel 625) or other
metal reactor or cooling tubes are fabricated. In such cases the platinum is bonded to the base
metal, but in all other instances platinum linings are of the ‘loose’ type.
6.1.2 Bursting discs
Bursting discs are a simple and effective (fail-safe) protection against over-pressurisation in a
closed system. The protection of pressure vessels containing corrosive materials presents a
special problem for the selection of bursting discs since bursting discs should not corrode (and
hence weaken) until they are required to fracture under an overpressure. For this reason,
corrosion resistant, high reliability bursting discs have been traditionally fabricated from
noble metals [57], although other cost effective alternatives now exist (e.g. tantalum or
niobium). The recommended maximum temperatures for continuous use are 80°C for gold,
150°C for silver, 300°C for palladium and 450°C for platinum.
6.1.3 Spinnerets
The spinning of viscose rayon for the production of yarn, tyre cord and staple fibre involves
the extrusion of an alkaline solution of cellulose into an acid bath. The orifices through which
individual fibres are extruded are often extremely small - down to 30 µm or less in diameter,
and their dimensional accuracy must be maintained to a very high degree for long periods
while operating in two highly corrosive media simultaneously. Platinum-gold alloys are the
traditional material of construction for rayon spinnerets, in particular Au-30Pt + 0.5Rh as a
grain-refining additive. This alloy has greater hardness that permits a high polish to be
produced in a scratch-resistant exit face while small grain size ensures that the holes produced
have a high grade of uniform circularity. Other noble metal alloys for spinneret construction
include rhodium-platinum, iridium-platinum, iridium-rhodium-platinum, ruthenium-platinum
and ruthenium-palladium and platinum-palladium. This application is being superseded by the
introduction of tantalum, which has comparable corrosion resistance at lower cost [58].
6.2 High temperature materials
6.2.1 Molten glasses and salts
A principal application for PGM metals is in the production of fine and optical glasses and
glass fibres, where platinum alloy crucibles and spinnerets are generally resistant to molten
glasses at temperatures from 1,200-1,500°C, frequently with continuous operation at 1,400°C.
Traditionally, Pt-Rh alloys were used for this purpose with typically 10% Rh added to
improve high temperature creep-rupture performance and to reduce evaporation loss. Useful
gains in creep-rupture properties can be made using zirconia dispersed materials, either of
pure platinum or of Pt-10Rh alloy [6]. Materials may be utilised used as sheet for fabricated
metal crucibles, as thin foil liners for covering ceramic (i.e. alumina) crucibles and as flame-
sprayed coatings [59] on a ceramic substrate. Also, flame sprayed or plasma evaporated
alumina coatings can be used to protect PGM materials from evaporation losses at high
operation temperatures [60].
Platinum is generally highly resistant to air-saturated oxide glasses due to the formation of a
thin stable layer of oxide that passivates the metal [61].Stressing during immersion in molten
glass (and hence cracking the passive oxide) greatly increases the dissolution rate of the metal
in the glass [62]. Under reducing conditions, for example non-oxide glasses and/or with an
inert gas atmosphere, platinum is much less resistant to molten glass with increased
dissolution and additional reaction to form embrittling intermetallic compounds [63]. Where
platinum cannot be used, or where even higher temperatures are required (1500-2000°C) then
iridium crucibles can be utilised [64].
Platinum crucible materials are also generally suitable for containing molten salts over similar
temperature ranges as for molten glass. However, salts that can easily form complex ions with
platinum will cause its corrosion; also coupling to other metals can result in enhanced
dissolution if it encourages the formation of the complex ionic species in the melt [65].
Platinum and iridium crucibles and equipment are extensively used in single crystal growth
production from molten precursors.
6.2.2 Metal joining
Solders and brazes (the distinction is not clear although solders are useful for “lower”
temperatures while brazes are suitable for “higher” temperatures) are widely used materials
for metal joining. Their key characteristics are that:
1. Their melting point is lower than the parent material to be joined.
2. A metallurgical bond, usually by intermetallic development or interdiffusion, is
formed between the solder and the parent material.
One common formulation of a tin-based lead-free solder contains several percent silver and is
more corrosion resistant than the material it replaces. For higher temperature use, silver-based
brazing alloys are widely used. In both cases, galvanic corrosion of the parent (joined)
material should be considered as the joining alloy is commonly significantly more noble.
Thus, where silver brazing alloys are used to join stainless steel, a narrow zone of corrosion
on the stainless steel often occurs on subsequent exposure in tap water [66].
PGM brazing alloys, commonly containing palladium with alloying additions including gold,
silver, nickel, copper are also utilised. They have significant advantages including high-
temperature strength and stability as well as corrosion resistance and have been used to join
stainless steels [67], nickel-based superalloys [68] and other refractory materials with
minimal corrosion problems. Their low vapour pressure gives them advantages in high
vacuum systems and their low toxicity (if nickel-free) gives them advantages as a dental
brazing alloy [69].
6.2.3 Furnace windings
Rhodium-platinum alloys containing up to 40% Rh have been traditionally used in the form of
wire or ribbon in electrical resistance windings for furnaces to operate continuously at
temperatures up to 1,750°C. Such windings are usually completely embedded in a layer of
high-grade alumina cement or flame-sprayed alumina to prevent volatilisation losses from the
metal due to the free circulation of air over its surface. However, these types of furnace
windings have been largely superseded by molybdenum disilicide materials that are resistant
in air to a similar temperature. For reducing (hydrogen) atmospheres, Pt-Rh alloy windings
are still useful although molybdenum metal windings may also be used at considerably lower
cost.
6.2.4 Temperature measurement
Until 1968, platinum-rhodium thermocouples (type S) were used as the standard interpolating
thermometer in IPTS-68 (International Practical Temperature Scale of 1968) because of their
good thermal stability. However, the present temperature scale ITS-90 (International
Temperature Scale of 1990) now uses a Standard Platinum Resistance Thermometer [70]. The
particular advantages of PGM materials in temperature measurement are, of course, their
excellent stability to oxidation and corrosion. Thus, platinum resistance thermometers offer a
more reliable temperature measurement than traditional thermocouples up to around 600-
650°C and are unaffected by the environment. At higher temperatures, Pt/Pt-10Rh (type S) or
Pt/Pt-13Rh (type R) thermocouples may be used for accurate temperature measurement in
essentially all atmospheres up to 1450°C. Both types of thermocouple have a short-term
stability typically better than 0.5°C with a long-term reproducibility of better than 2.0°C. For
higher temperatures to 1800°C then Pt-6Rh/Pt-30Rh (type B) thermocouples can be used.
6.2.5 Gas turbine applications
Improvements to gas turbine operating efficiency require an increase in the gas temperature of
the turbine and combustion chamber. Since most gas turbine materials are now optimised
more for their high temperature strength and creep resistance, their hot corrosion and
oxidation resistance has become relatively poorer. Platinum aluminide coatings have been
developed that, in combination with other coating types, offer improved performance for
nickel-based superalloys [71]. Platinum, by substituting for nickel in the aluminide,
eliminates chromium-rich precipitates from the outer coating layer and limits the diffusion of
refractory transition elements such as molybdenum, vanadium and tungsten into the outer
coating layer [72].
6.3 Dental and medical applications
6.3.1 Dental restorations
Generally the noble metals, particularly gold, have excellent biocompatibility, and the use of
gold and gold-palladium alloys in dental restorations is very common. However, they are too
soft to be used on their own and require alloying (typically with copper, silver and palladium,
plus other minor components) and this increases corrosion susceptibility. There are a number
of classification schemes for these alloys. Firstly, based on their strength and secondly on
their metal content. There are numerous alloys, many proprietary and many designed for
specific aspects of the dental restoration task however, the main classes are listed in Table 7.
Yield stress (MPa)
Elongation (%)
Type I
Type II
Type III
Type IV
< 140
140 200
200 340
> 340
18
18
12
10
Typical composition
High noble
Noble
Base
Au > 40% + total noble metals > 60%
Total noble metals > 25%
Total noble metals < 25%
Table 7: Classification for dental alloys [73].
The oral environment is corrosive and materials for use must be correspondingly corrosion
resistant; thus the general corrosion of dental alloys is of interest partly with a view to
increasing the service life of implants and partly for biocompatibility. The main issues are:
1. Tarnishing and discolouration, usually caused by foods and drinks that contain sulphur
[74].
2. Galvanic corrosion between adjoining oral alloys of differing composition [75].
3. Galvanic corrosion caused by micro-segregation of alloy components at grain
boundaries or in the interdendritic regions in cast materials.
Failure modes exhibited include: generation of expansive corrosion products causing
disbonding of the implant, general thinning of the material and cracking.
Conventional mercury amalgam restorations are typically anodic to noble and high noble
alloys and will corrode preferentially [76] and can lead to cracking. Care has therefore to be
taken in mixing restorative alloy materials. For example, cracking of gold crowns has been
observed as a consequence of intergranular/interdendritic corrosion in association with
corrosion of an underlying mercury-based amalgam alloy [77]. Electrochemical and corrosion
tests in ammonium and sodium sulphide solutions, as a simulation of the oral environment has
demonstrated that the tarnishing is particularly related to the silver and copper contents of the
alloys [78]. Generally, as might be expected, the higher the noble metal content of the alloy
used, the more corrosion resistant [79]; hence, selection of the correct restorative alloy is a
balance between mechanical and other required properties and corrosion resistance [80].
6.3.2 Medical sensing and electrodes
Unalloyed noble metal (or high noble metal content) body implants are used for a number of
purposes with generally few or no corrosion issues arising. Reference above has been made to
the production of nanoporous gold electrodes of exceptionally high surface area. These have a
number of potential applications in medical sensing; for example, insulin determination.
Noble metals, particularly platinum and platinum alloys, find use as electrode materials for in-
vivo nerve stimulation (e.g. heart, neural, auditory electrode implants, etc). The applied
potentials used are generally transient in nature (i.e. a pulse waveform) and significant
corrosion of electrode materials can be observed. For example, Pt, Au, Rh, Ir, Pt-10Ir and Pt-
10Rh electrodes were severely corroded under a bipolar current pulse of 1 A cm-2 while Rh
was moderately attacked. At 0.1 A cm-2, Au, Rh and Ir were resistant to corrosion [81].
However, although examination of heart pacemaker electrodes has shown evidence of
corrosion in all cases, in no case had this resulted in the failure of the implant [82].
6.4 Electrical contact materials
High reliability electrical contacts are frequently plated with noble metals (Ag, Au, Pd, etc) in
order to limit any corrosion and consequent increase in contact resistance with time. This is
because uncoated copper contact materials are particularly susceptible to corrosion and
contact failure mechanisms involving vibration (e.g. in motor vehicles) giving rise to wear-
fretting effects in addition to corrosion.
Silver coatings are liable to grow intrusive whiskers in sulphidising, and to a lesser extent in
chloridising environments, although much less so than copper or tin. The main environmental
driver is the presence of hydrogen sulphide at or above 200 parts per billion, which can be
reached, for example, in pulp and paper processing [83] where significant failures have
occurred.
Gold alloys and gold coatings are also extensively used for electrical contacts, especially in
the electronics industry. Corrosion failure mechanisms are associated with the growth of gold
shorts from cathodic conductors especially where chloride ions are present. On anodically
biased conductors a voluminous reaction product of Au(OH)3 is produced by anodising
[84,85].
6.5 Anodes
6.5.1 Dimensionally stable anodes
Non-consumable anode materials are used extensively in electroplating, electrowinning,
chlorine production and water electrolysis (oxygen evolution) and have traditionally variously
utilised silver, lead, magnetite, graphite and noble metals. Of these, chlorine production by
electrolysis of brine is amongst the most energy intensive and industrially important.
For chlorine evolution, platinum generally shows very low rates of attack, and platinum
anodes can be used for both cathodic protection in sea-water and for chlorine production at
low overvoltage [86]. This attribute is take advantage of using platinum coated titanium or
niobium substrate materials; the substrates are stable to the conditions of electrolysis while
the electrode reaction (e.g. chlorine evolution) occurs on the platinum surface. At low current
densities, the platinum can be corroded, especially at low pH; however, at high current
densities, passivation of the platinum occurs and corrosion reduces considerably. However,
the corrosion rate of platinum can be much higher if an alternating current component is
present as this tends to thicken the passive film. Rhodium and iridium are as resistant to
anodic corrosion as platinum but are more resistant to the influence of alternating currents.
The single most important advance in anodes for chlorine evolution is in the replacement of
noble metal coatings on stable substrates with oxide coatings using ruthenium modified by
iridium (“mixed metal oxides”) [87]. These provide efficient production of chlorine at low
overvoltage with no corrosion provided the overpotential is kept below a critical value [88].
Such anodes are called “dimensionally stable” because, in principle, they do not need
replacement throughout the life of the plant provided they are operated within the correct
parameters [89]. They are also responsible for a reduction of about 20% in the energy usage
per unit of chlorine production.
Given the success of noble metal oxide-doped anodes in chlorine evolution, they are being
trialled in a number of other applications that would benefit from improved electrocatalysis.
6.5.2 Cathodic protection
There are a wide range of possible anode systems for impressed current cathodic protection
however, noble metal coated titanium mesh anodes have been used for many years in a wide
variety of applications including: protection of reinforcement steel in concrete and cathodic
protection systems in seawater (e.g. for vessels, etc). In severe environments, or where the
system requires a high driving voltage, platinum-coated niobium anodes are used because of
the increased stability of niobium (compared with titanium) to chlorides. More recently, the
use of noble metal oxide-coated electrodes has also become increasingly important with their
advantages of higher current density and increased lifetime under polarisation [90].
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... Another relevant factor that plays a major role in metal corrosion is the acidity or alkalinity of the applied solution (i.e., pH) [121][122][123]. Copper is highly resistant to most alkaline and acid substances. ...
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Conference Paper
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Silver and silver alloys tarnish readily by the formation of tarnish on the surface when exposed to a wide range of sulphur-containing atmospheres. This tarnish is unsightly and undesirable and can be removed either with a silver polish or a chemical dip. Polishes are, however, not suitable for irregular and embossed surfaces such as on coins, medallions and jewellery items, because of uneven removal of the tarnish layer and excessive wear. Successfully tarnish protection can be achieved by either storing the items away from the aggressive atmosphere or by coating/ treating the exposed surfaces with a protective layer. Storage is feasible for collections but is not suitable for items that are handled or worn regularly, such as jewellery. A number of commercial coatings and treatments have been developed, but to date there has not been a systematic evaluation of tarnish protection. This paper details the performance of a range of generic coatings in an accelerated tarnish test. The degree of tarnish was evaluated by means of loss in reflectivity and change in colour. Four coatings/ treatments improved the tarnish resistance of silver by more than tenfold. These include an organic lacquer, rhodium electroplating and an organic self-assembled monolayer (SAM) conversion coating. Research is continuing to optimise these coatings. The major problem of silver and silver alloys is the tendency to form a tarnish layer in the presence of sulphur compound from the atmosphere. A number of coatings and treatments are available that claim to improve the tarnish resistance of silver. This research was initiated to identify coatings that increased, at least tenfold, the time before noticeable tarnish appeared on silver. Sterling silver samples were ground to an ASTM 600 grit (FEPA 1200) surface finish, degreased and coated/ treated. The following coatings/ treatments were evaluated: Rhodium electroplating Palladium electroplating Hexadecanethiol self-assembled monolayer (SAM) conversion coating Commercial air-drying organic lacquer Four commercial inhibited polishes 2. Coatings/ Treatments 2.1. Rhodium Plating Rhodium plating is commonly used to coat white jewellery and other decorative items. It provides a whiter finish on platinum articles, and tarnish protection for silver and white gold[ 1,2]. Rhodium plating is also used extensively for the coating of engineering articles[ 3]. A rhodium layer, about 0.15µm thick, was applied from a rhodium sulphate-sulphuric acid plating bath at a cathode current density of 1A/ dm 2 for one minute under normal rhodium plating conditions. 2.2. Palladium Plating Palladium plating is often used as an alternative to gold plating for electronic applications[ 4] but is not commonly used to protect silver against tarnish. It is less expensive than rhodium and was evaluated as a potential alternative. A palladium layer, about 0.15µm thick, was electroplated from a Pd(NH 3) 2 Cl 2 , ammonium chloride and ammonium sulphate bath[ 1,5,6] at 30°C. The cathode current density was 1A/ dm 2 for one minute using a platinum anode. 2.3. Hexadecanethiol (SAM) Conversion Coating The tarnish resistance of a number of chromate-free conversion coatings on silver was evaluated by Burleigh et al.[ 7,8]. These were applied by immersing the silver in the solution to form a self-assembled monolayer (SAM). Hexadecanethiol gave the best tarnish protection of the coatings that were evaluated.
Chapter
The rarer and noble metals dealt with in this chapter are situated within or adjacent to the second and third long series of the transition elements as shown in Fig 1. The transition metals occupy positions in the Periodic Table where an incomplete group of eight electrons expands systematically into one of eighteen by the acquisition of ‘d’ band electrons. The resultant tendency towards higher valencies reflects itself in the high strength and high melting point of metals towards the centre of these two long series and explains the importance of these metals for structural and technical applications. Bond strengths diminish as we move to the right of tungsten and molybdenum. The melting points of gold and silver are well below those of the platinum metals and the melting points of tin and indium are close to room temperature.
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