Page 150 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

4CuCl + Ca(HCO B ) 2 + 0 2 = CuC0 3 Cu(OH) 2 + CaC0 3 + 2CuCl 2 4.n

The second, a cathodic electrode reaction, occurs on the outer surface of the cuprite:

Cu 2 + +e~ = Cu + 4.12

The third, an anodic electrode reaction, occurs on the inner surface of the oxide membrane:

Cu + - e" = C u 2 + 4.13
The fourth reaction is between the anodic product and the copper within the pit:

Cu + C u 2 + - 2Cu + 4.14

The presence of chloride ions within a pit can cause cuprous chloride, CuCl, to form. With
higher chloride concentrations, the soluble complexes of (CuCl 2 )~ and(CuCl 3 ) " can lead to
2
further corrosive reactions. High concentrations of chloride ions can develop because of isola­
tion of the pit from the surrounding environment by the cuprite layer or by the mound of
corrosion products accreting over the cuprite crust. As previously noted, the increased concen­
tration of chloride ions in the pits is primarily due to the migration of chloride ions from the
groundwater into the pits to maintain neutrality. Because of their higher concentration, chloride
rather than hydroxide ions tend to move into these pits; this migration is enhanced by alloying
elements such as tin and zinc. Soluble cuprous species from inside the pits can diffuse out
through cracks in the cuprite membrane, and these complexes are then oxidized to cupric ions:

CuCl 2 " = CuCl 2 + e" 4.15

While this occurs, the usual oxidant, oxygen, is reduced to hydroxide ions:

+ 2 H + + 4e~ = 2 0 H " 4.16
0 2
This cathodic reaction causes a localized increase in pH that tends to result in the precipi­
tation of the basic copper (II) compounds. Experimentally, MacLeod (i98i) found that about half
the copper that has been corroded remains as cuprite in the pit itself. He calculated the influence
of chloride ion concentration on the leaching of copper metal, and the results of this study for
two sets of anodic and cathodic reactions, which are shown in TABLE 4.2, indicate how the free
energy of the pitting reaction can vary with the chloride ion concentration, assuming equilib­
rium conditions. Since the Gibbs free energy of formation varies significantly with the chloride
ion concentration, it can be inferred that a decrease of the chloride ions will stifle the reaction,
since the change in Gibbs free energy moves into positive values in deionized water. The stan­
dard free energies (from the data of Latimer 1952) in TABLE 4.2 have been calculated with the
assumption that the chloride ion activity is equal to the chloride ion concentration and that the
concentration of the pit solution is based on a concentrated brine.

C H L O R I D E S AN D BASI C C H L O R I D E S
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