Page 49 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

complex ions; this generates intercrystalline failure in tarnished samples and transcrystalline
cracking in untarnished ones. These events also depend on the ammonia concentration and on
the zinc content of the alloy. In general, most ancient copper alloys have less than 30% zinc con­
tent and are much less susceptible to stress-corrosion cracking than to general corrosion. Gen­
eral corrosion produces a range of basic zinc salts, such as chloride, sulfate, or other products.
This is generally beyond the scope of this book, although some of the mixed compounds con­
taining both copper and zinc are discussed in relevant sections of later chapters.

P O U R B A I X D I A G R A M S AND E N V I R O N M E N T A L E F F E C T S

A Pourbaix diagram can be thought of as a kind of map that shows how oxidizing or reducing,
acidic or alkaline an environment can be. It provides a plot of the redox potential (Eh) of the
system (its ability to act in either an oxidizing or reducing manner) against its acidity or alka­
linity (pH). A Pourbaix diagram could show, for example, that peat bogs are both somewhat
acidic and reducing, suggesting that this environment may be better at preserving different
groups of materials than, say, a brackish stream, which is alkaline and oxidizing. These dia­
grams can also be very useful in relating the environmental conditions to the corrosion prod­
ucts or compounds that are predicted to form on the basis of the thermodynamics of the system,
as indicated by the diagram.
The basis for Pourbaix diagrams is that electrochemical reactions are the result not only of
chemical species but also of electrical charges. One of the important measurements for study­
ing these electrochemical reactions is the electrode potential. This potential is obtained with a
reference electrode of known potential, such as a hydrogen or calomel electrode, that is placed
in an electrolyte solution containing the metal species being studied. This provides a measure of
the oxidizing or reducing power (Eh) of the system for that particular metal and is shown on
one axis of the Pourbaix diagram. The system's pH is shown on the other axis. In this way, a dia­
gram can be built that describes how a particular metal will (or should) behave in a certain type
of chemical environment, which must be precisely specified for each diagram. These diagrams,
combined with information about environmental conditions, such as the concentration of chlo­
ride or sulfate ions, make it possible to plot stability regions for different mineral species on the
same Pourbaix diagram. The concentration of soluble species must be defined for each plot, and
thus many different plots may be necessary for the same system. The number of plots needed
depends on the concentration of species at a given temperature.
Two instructive Pourbaix diagrams are shown in FIGURES 1.5 and 1.6. A series of plots
of natural aqueous environments is shown in FIGURE 1.5, which indicates which equilibrium
region different environments may occupy. FIGURE 1.6 is a plot of the distribution of thousands
of Eh and pH measurements taken from natural aqueous environments. The vast majority of
these measurements show an aqueous pH between 4 and 8, and a range of environments from

C H A P T E R O N E
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