Page 46 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Langenegger and Callaghan (1972) found that the rate of dezincification could be correlated
with the oxidizing power of the solution. The addition of tin to the alpha+beta brasses helped
to inhibit attack on the alpha phase and gave the alloys reasonably good resistance to dezinci­
fication in marine environments. The single-phased admiralty brass (71Cu28ZnlSn) resisted
dezincification f small amounts of phosphorus, arsenic, or antimony were present. Since most
i
ancient brass alloys contain small amounts of impurity elements, or have some tin content, their
behavior under long-term corrosion conditions is generally far superior to modern binary
copper-zinc alloys.
Dezincification can occur by two possible reaction sequences: (1) zinc may simply be selec­
tively leached out of the brass, leaving a weak assembly of copper grains; or (2) both copper and
zinc may dissolve in solution, and the more noble copper may be redeposited. Extensive
research indicates that both processes can occur in separate but overlapping potential regimes.
Selective leaching of zinc requires solid-state diffusion of the zinc atoms, and this appears to
be a rather slow process. In modern alloys, dezincification can occur by simultaneous dissolu­
tion and redeposition, with selective dissolution also occurring but in a subsidiary fashion. In
ancient alloys, there may be an accumulation of basic zinc salts, which represent the corrosion
of the more zinc-rich regions of cast brasses. General dezincification may occur in worked
brasses without the formation of plugs of dezincified copper alloy.
FIGURE 1.4 shows some of the potential regions for the corrosion of alpha brass alloys in
0.1 M chloride solution. The thermodynamic half-cell electrode potentials are also shown in the
figure. Below 0.0 V on the Standard Hydrogen Electrode scale, copper metal is stable and will
not dissolve. Above -0.90 V, zinc can dissolve by selective dissolution, but the reaction is slow.
Above 0.0 V, copper begins to dissolve, and the rate of dissolution increases as the potential
increases while the rate of loss of zinc also increases. Between 0.0 V and +0.2 V, copper (II) ions
that have accumulated in solution can redeposit on the surface as copper metal. At higher poten­
tials, copper and zinc dissolve at equivalent rates, but no dezincification occurs. The following
individual reactions, although somewhat difficult to follow, are of importance in the corrosion:

Zn = Z n 2 + + 2e~ 1.7
Cu + 2C1" = CuCl 2 " + e" 1.8

CuCl 2 " = C u 2 + + 2G1" + e" 1.9

Cu = C u 2 + + 2e~ M O

CuCl = C u 2 + + CI" + e" 1.11

Cu + CI" = CuCl + e" 1.12

C O R R O S I O N AN D E N V I R O N M E N T
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