Page 32 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The electrochemical series The electrochemical series is important in determining the pref­
erential corrosion of metals in an alloy. The reduction potential
for the reaction of copper (II) with two electrons is +0.340 V (on the E° scale where the reduc­
tion of hydrogen is zero), as shown in the following equation:

Cu 2 + + 2e" = Cu 1.2

Corrosion products and corrosion potentials, however, are not related to the equilibrium poten­
tial of the respective elements. The same reaction for zinc occurs with a potential of -0.783 V:
Z n 2 + + 2e" = Zn 1.3

In a brass alloy, therefore, although copper and zinc may be randomly substituted in the
alpha solid solution, there is still a considerable driving force for the corrosion of zinc as the
anodic component while the copper may be retained uncorroded. By comparison, the same
reaction with tin has a much lower negative potential of -0.136 V; tin is still anodic to copper
but substantially less so. The corrosion of tin normally results in the accumulation of the insol­
uble tin oxides; this impedes the further anodic process of destannification, or tin dissolution.
In some circumstances, copper may be corroded while the tin compounds remain behind, and
therefore a prediction made on the basis of the electrochemical series as to which elements will
be most corroded or dissolved away cannot be made, unless the wide range of environmental
and chemical factors that may influence the corrosion can be evaluated.
The general principles of the electrochemical series must be understood, however, since
any argument for exceptions to the order of elements in the series must be based on the initial
behavior predicted. For example, there is a considerable difference between the initial corrosion
outdoors of a 10% tin bronze and that of a 15% zinc brass. The bronze is much more corro­
sion resistant, which is predicted by the electrochemical series for these freshly polished
alloys. Within a year, the brass will have darkened considerably while the bronze may be prac­
tically unchanged. The brass is influenced by the electropotential difference between copper
and zinc, resulting in a more general corrosion of the surface at a much faster rate than that of
the tin bronze.
In addition to zinc and tin, the common alloying elements of lead, arsenic, or antimony may
also behave anodically to copper, at least in theory. Since lead is insoluble in copper at room
temperature, it is usually present in the form of discrete globules as a separate phase from the
copper alloy. This segregation can result in severe corrosion of the lead phase. The globules are
surrounded by a largely cathodic copper region; this can cause the lead to become oxidized to
the basic carbonates or oxides. Organic acids, such as those found in unsuitable storage condi­
tions, may interact preferentially with these lead globules, producing a whitish surface haze to
the bronze, or more severe corrosion excrescences. In some cases, a combination of copper

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