Page 129 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

McNeil and Mor (1992) discuss the application of Pourbaix diagrams to copper, where the
chloride ion concentration is representative of seawater, and the carbonate ion is in equilibrium
with air. Under these conditions, minerals such as azurite and georgeite do not appear on the
stability diagrams, since there are no stability constants for georgeite, and azurite is not stable
unless the carbon dioxide concentration is raised to levels above those in equilibrium with
the air. Regions for the formation of cuprite, malachite, and paratacamite occur, together with
regions of the dissolved phases CuCl 2 ~ (aq) and Cu 2 + (aq), at pH levels below 5. From pH 8
to 11, malachite is stable in oxidizing environments; there is a small region of cuprite stability
toward intermediate values of Eh. Since the normal range of pH variation in seawater is about
7- 8, it is theoretically possible that some malachite could form in marine burial environments.
Such an occurrence must be the exception rather than the rule since most marine corrosion
products on copper, apart from cuprite, are the copper trihydroxychlorides.
MacLeod (i987a) found only one example of malachite formation in the Western Australian
marine sites he studied, but the mineral may have a postexcavation origin. Mor and Beccaria
(1972) reported the occurrence of both malachite and atacamite from a marine burial environ­
ment, but this juxtaposition is far from common, and no further reports of both minerals being
found together have been published to date.
MacLeod (i987a) explains that the following competitive precipitation reaction occurs
between malachite and copper trihydroxychlorides, such as atacamite, in the well-oxygenated
seawaters off the Australian coast:

Cu 2 (OH) 3 Cl + C 0 3 " = CuC0 3 -Cu(OH) 2 + Cl" + OH" 3.1
2
In normal seawater, which has a carbonate activity of 2.4 X 10" M at pH 8, the formation of
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malachite is favored, at least in theory. A site's local conditions will affect the relative concen­
trations of carbonate and bicarbonate ions; this, in turn, will strongly influence the nature of the
corrosion products formed.
The Pourbaix diagrams that illustrate four Eh-pH charts for the C u - C 0 2 - H 2 0 system at
25 °C (see CHAPTER 2, FIGURE 2.i) can be used to investigate why azurite rather than malachite
forms under these conditions. Although the diagrams neglect various kinetic factors, they do
provide useful information concerning conditions for the formation of the two minerals.
For water at pH 5.8 with 440 ppm of dissolved C 0 2 (see FIGURE 2.1 B), the Pourbaix dia­
gram shows that a mixed product should result; that is, a patina on copper would consist of
malachite with some tenorite, CuO. As discussed previously, however, it is doubtful that tenorite
will form at all. In natural corrosion crusts, cuprite—not tenorite—is found contiguous with the
copper metal.
Malachite is stable in water of approximately pH 8 containing 44 ppm of dissolved C 0 2
(see FIGURE 2.IA). This means that such water will line a copper pipe with malachite. As the
amount of dissolved carbon dioxide increases, however, the stable minerals that form can

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