Page 85 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Hummer, Southwell, and Alexander (i968) recorded a rate of attack for copper of 0.20 mpy
after sixteen years of exposure in the shallow waters of the Pacific Ocean off the coast of Pan­
ama. Some pitting corrosion of the copper occurred, with a maximum depth of 57 mils. Com­
paring corrosion rates for copper alloys in shallow water and at a depth of 1828.8 m β ο ο ο ft.)
(
showed that the rates of corrosion decreased in both environments in an almost linear relation­
ship with increasing duration of exposure. The corrosion of copper and of silicon bronzes was
not affected by changes in the concentration of seawater oxygen during one year of exposure.
Further details for the corrosion rates of a variety of copper alloys at different depths are found
in the review by Schumacher (1979).
According to North and MacLeod (i987), the corrosion rates for isolated copper samples
in oxygenated seawater is about 0.02 mm per year, and this could increase by a factor of 2 for
1
every 0 °C rise in water temperature. This rate may decrease in anaerobic water or in sedi­
ments and by galvanic coupling to iron on shipwrecks. There are, however, a number of com­
plex situations that can result in increased corrosion under such circumstances, apart from the
action of sulfate-reducing bacteria. Copper bolts in the hulls of wooden ships, for example, are
often partially covered with wood and partially exposed to the open sea (MacLeod 1987b).
Beneath the wood, oxygen is depleted, and these regions become anodic; the corrosion reaction
may be accelerated by the chemical decay of the wood, which releases acetic acid, ammonia, and
amine compounds. These compounds can complex with the copper ions, thus shifting any equi­
librium toward the dissolution of copper and thereby accelerating corrosion. The value of Eh
from the Pourbaix diagram for copper in oxygenated seawater at 25 °C is 0.691 V. The metal
value of 0.09 V would indicate that cuprite should be the stable phase. Given that the
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usual range of pH n seawater is 6.2-9.2, the formation of cuprous chloride from cuprite is not
favored since the reaction

Cu 2 0 + 2 H + + 2C1" = 2CuCl + H 2 0 1 . I 6

requires a pH below 5.30 at the normal chloride-ion activity of 0.319 molar.
The pH of the bulk of seawater usually remains near 8, but large variations can occur
in restricted environments and under concretions, which means that cuprous chloride can be
found as a corrosion product in seawater when the local pH level is low enough to favor its for­
mation (MacLeod 19 87 a).
Bianchi and Longhi (1973) surveyed the corrosion of copper in seawater based on the rele­
vant Pourbaix diagrams and on stability diagrams, two of which are shown in FIGURE 1.9. Their
calculations showed that there may be a competing series of reactions producing atacamite and
malachite, which have been found together on rare occasions as marine corrosion products,
even though atacamite is usually the much more predominant species. The two minerals may
occur together because, compared to malachite, atacamite can be precipitated at slighdy more
acidic pH and under oxidizing conditions. The ability of chloride ions to migrate toward the

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