Page 84 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

an organic sulfur compound that could be repeatedly reduced to a mercaptan and then oxidized
back to a disulfide. This oxidation-reduction cycle stimulates the corrosion of the brass.
Work at the British Non-Ferrous Metals Research Association (Leidheiser 1979) suggests
that the abnormal pitting of copper condenser tubes is associated with bacteria that produce
black or brown pigmentation. Melanin, which was identified as one of these pigments, can be
produced by the action of the enzyme tyrosinase on tyrosine; bacteria producing tyrosinase
accelerated corrosion in direct relationship to the amount of tyrosine present. Other oxygen-
deficient reactions that occur in marine burial include the activities of the sulfate-reducing
bacteria and the possible reactions of hydrogen sulfide with copper to form an array of other
sulfides—from covellite, CuS, to chalcocite, Cu 2 S.
I RATES OF CORROSION I N VARIOUS MARINE ENVIRONMENTS
TABLE 1.7 shows the different aspects of the marine environment divided into seven zones:
atmospheric, splash, tidal, shallow water, continental shelf, deep ocean, and mud. Copper alloys
can be found in any one of these environmental niches, but in the marine regions, most would
be in shallow water, continental shelf, deep ocean, or underwater mud. Based on weight-loss
measurements, the corrosion rates of copper alloys over twenty years were found to be from
0.01 to 0.17 mils of penetration per year (mpy) (0.25- 4.3 μιη/year ), with rates of attack higher
in tropical regions. In the splash and tidal zones, the behavior of copper alloys is analogous to
their performance in the marine atmosphere rather than in immersed conditions (Schumacher
1979). Part of the reason copper alloys do not corrode quickly when immersed in the sea is that
a cuprite film tends to develop over the surface. In research on the corrosion of copper con­
denser tubes, it was found that especially resistant films are often produced on the surface f
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small amounts of iron are present in the alloy. Similarly, Schumacher mentions that a patent
was issued for improving the corrosion resistance of an aluminum brass by the addition of small
amounts of iron to the alloy. Some laboratory studies suggest that the ferrous ions are oxidized
to lepidocrocite by the dissolved oxygen in water. During the experiments, the lepidocrocite
formed a colloidal film that was deposited electrophoretically at the cathode and thus acted as a
cathodic inhibitor by polarizing the reduction of oxygen.
In general, the oxygen content of a marine environment is a significant factor in the cor­
rosion of copper, since the depolarization of the cathode by oxygen, the oxidation of Cu (I) to
Cu (II), and the film-forming properties of copper are all dependent on it. In polluted seawater,
which may be oxygen deficient, the generation of copper sulfides occurs readily since hydro­
gen sulfide is commonly available as a product of plant decay. The copper sulfide film formed
on copper in polluted seawater is more cathodic than the normal corrosion crust formed in
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clean seawater; f there are any breaks in the film, local attack is greatly stimulated by the large
area of the cathode. Some alloys are more resistant to this type of attack: copper-nickel alloys
and copper-aluminum alloys, for example, are less affected than copper or a binary copper-zinc
brass. f copper is submerged in seawater, the rate of corrosion can range from 0.5 to 2 mpy.
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