Page 31 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

In reconstructive events, the original corrosion product is completely dissolved or chemi­
cally altered to form a new product that is entirely different from the original. For example,
cuprite may be dissolved by chemical solutions of low pH and low partial pressures of oxygen,
resulting in the deposition of metallic copper in place of cuprite. This metallic copper may have
a twinned crystal structure (Scott 1991); it grows as cuprite dissolution proceeds (Chase 1994).
Alternatively, we could view the reactions involved in corrosion product formation as either

1. chemical—involving a transfer of electric charge that takes place only
locally between atoms; or
2. electrochemical—involving a nonlocal transfer of electric charge through
a conductor that connects the anodic and cathodic areas.
Chemical corrosion reactions are those in which chemical species alone are predominant
in terms of the reaction or series of reactions. 3 Electrochemical corrosion reactions are those
controlled by a potential difference in the reaction process that leads to a flow of ionic species
from one region or surface to another. Most corrosion processes are primarily electrochemical
because both anodic and cathodic sites are usually present. The utility of thinking of them in
terms of either chemical or electrochemical categories is therefore rather limited.
More important perhaps, corrosion reactions can also be viewed as dominated by either
1. thermodynamic principles, or
2. kinetic principles.

In thermodynamic terms, all natural, spontaneous processes must proceed with a negative
Gibbs free energy of reaction. An example is the reaction of cupric oxide with moisture and
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sulfuric acid to form antlerite given by the following equation:

3CuO + H 2 S 0 4 + H 2 0 = CuS0 4 -2Cu(OH) 2 1.1

The relevant thermodynamic data are readily available from a number of sources. The calcula­
tion for the reaction given by equation 1.1, based on the data of Weast (i984), shows a Gibbs free
energy of formation of-19.5 kcal/mol. This reaction proceeds from left to right, with the dis­
solution of cuprite and the formation of antlerite. This is observed in the corrosion of outdoor
bronzes exposed to rain of low pH, which is associated with industrialized regions of the world.
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Kinetics investigates how fast a chemical reaction will occur. f a particular reaction or step
in a series of reactions is very slow, thermodynamic equilibrium may never be attained. In cor­
rosion processes, for example, polarization effects may limit the ability of hydrogen atoms to be
discharged as hydrogen gas. The result of this delimitation is that the corrosion process involved
may come to a standstill because the hydrogen cannot be discharged. Hence, kinetic factors may
be as important as thermodynamic ones. For corrosion processes that take place over hundreds
or thousands of years, however, the kinetic factors are not easy to model.

C H A P T E R O N E
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