Page 87 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

metallic surface kinetically favors the formation of one of the copper (II) trihydroxychlorides
(atacamite), although this cannot be shown on the Pourbaix diagram, which exists independ­
ently of, and is blind to, kinetic factors. This may invoke a guarded response to predictions based
on Pourbaix diagrams in the absence of any empirical data, but f the empirical data are avail­
i
able, as well as the Pourbaix diagram, the combination is a powerful tool for evaluating the cor­
rosion process.
In marine environments, the principal cathodic reaction in the corrosion of copper is the
reduction of oxygen:

+ 2 H 2 0 + 4e" = 4 0 H " 1.17
0 2
The available oxygen in seawater is temperature dependent so that at 0 °C there is 8.1 ml/1
of oxygen, and at 25 °C there is 5.4 ml/1 of oxygen. In some sites, biological activity may result
in the sulfate-reducing bacteria becoming more important than dissolved oxygen in the cor­
rosion of copper. Corrosion rates in seawater are generally much higher than in freshwater
because of the presence of so many different cations and anions. The Eh of the water may fall
below the hydrogen evolution potential when oxygen is depleted, in which case the principal
cathodic reaction would become

2H + + 2e" = H 2 1.I 8

Copper alloys are common nonferrous metals found at shipwreck sites. Copper- or brass-
sheet cladding was used to protect wooden ships from the teredo worm and from fouling
by marine organisms. The corrosion products in oxygenated seawater range from cuprite to
cuprous chloride and include the isomers of the copper trihydroxychlorides and the copper
sulfides; relatively uncommon are the carbonates and sulfates. The value of Eh in normal sea­
water is 0.691 V, while the metal E corr may vary from 0.04 V to 0.09 V. Some of the basic
copper chlorides identified by MacLeod (i987a) from Western Australian maritime sites proved
to be more closely related to the X-ray diffraction patterns for synthetic paratacamite and ata-
camite. Synthetic atacamite, ICDD 25-269, is even assigned a slightly more complex formula
than the natural mineral, namely Cu 7 Cl 4 (OH) 10 -H 2 O, although how viable this stoichiometry
actually is remains to be established by further research. The occurrence of both malachite and
atacamite in the sea burial of copper has been reported by Mor and Beccaria (1972). Evidence
for postexcavation changes of marine patinas was reported by MacLeod (1991), who noticed
that the red-brown cuprite patina on some copper sheathing from a Western Australian marine
wreck turned a deep blue green after transportation to the conservation laboratory. This blue-
green formation was characterized as a mixture of synthetic malachite and a basic copper sul­
fate hydrate, Cu 3 (S0 4 ) 2 (OH) 2 -4H 2 0 (ICDD 2-107). The precipitation of cupric ions as a basic
chloride or basic carbonate is dependent on the salinity of the seawater, temperature, pH, and

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