Page 149 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Pitting corrosion The investigation of pitting corrosion as a result of industrial
failure of commercial copper objects, such as pipes and boilers,
provides one of the few scientific insights into the role and formation of cuprous chloride in cor­
rosion of this type. May (1953) observed that when a clean copper surface was exposed to cor­
rosive waters containing dissolved oxygen, there was a brief period of rapid formation of soluble
copper corrosion products and the conversion of some of these products to insoluble basic salts,
which appeared as a cloudy precipitate in the layer of liquid on the copper surface. Later, a
visible film formed on the metal, which reduced the rate of attack. A layer of cuprous oxide can
appear under the initial film and may replace it, becoming comparatively thick and often non­
uniform. This layer may be more cathodic and therefore favorable to pitting at local defects. The
copper inside this defect zone acts as an anode due to depletion of oxygen, and chloride ions are
drawn inward, producing porous crystalline cuprous chloride deposits next to the metal.
Lucey (1972) shows that these pits function as electrochemical cells. The cuprite that forms
over the cuprous chloride acts as a diffusion barrier, which reduces the loss of dissolved copper
ions into the outer zone. The cuprite also behaves as a bipolar electrode, with an anodic reac­
tion taking place on the inner surface of the cuprite and a cathodic reaction occurring on the
outer surface. Cuprous ions diffuse through the cuprite and can become oxidized by oxygen in
water to form cupric ions, some of which can be lost into the soil groundwater, some precipi­
tated as basic salts, and some reduced back to the cuprous state at the outer membrane surface.
The corresponding anodic reaction is less well understood, and Lucey suggests that the
cuprous ions inside the pits are oxidized to cupric ions. This increase in cupric ion concentra­
tion disturbs the equilibrium between metallic copper and the cuprous and cupric ions. Copper
can then dissolve to maintain equilibrium.
The following equations describe the reactions in aqueous conditions contiguous with a
copper surface:
Cu + Cu 2 + = 2Cu + 4.8

2Cu + + H 2 0 = Cu 2 0 + 2 H + 4.9

4Cu + + 0 2 + 2 H 2 0 = 4Cu 2 + + 4(OH) _ 4.10

Lucey proposes that the balance among these three equations is responsible for the precipi­
tation of cuprous chloride. Under these conditions, the rate of formation of the cuprous ions
exceeds the conversion into cuprite or cupric compounds, and a layer of cuprous chloride can
then form. He also presents four essential equations describing pit corrosion processes. The first
reaction occurs within the mound of corrosion above a pit and can vary depending on whether
carbonate ions, chloride ions, or other ionic species are available to interact with the copper:

C H A P T E R F O U R
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