Page 48 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

in solutions containing excess copper (II) ions (Jones 1992). Selective dissolution of zinc from
alpha brass, which requires solid-state diffusion of zinc, is too slow to account for the usual cor­
rosion penetration of brass alloys. The redeposition of copper seems to be important in the cor­
rosion of most of these brass alloys.
Brass alloys are also particularly susceptible to stress-corrosion cracking in which small
amounts of corroding material may create cracking of the alloy in the presence of any applied
stress; the penetration may be intragranular or intergranular. Brass instruments or highly
stressed sheet brass are two possible types or groups of objects that may be susceptible to stress-
corrosion cracking. Some of the copper-ammonia complexes are implicated in this process,
including Cu(NH 3 ) 5 H 2 0] , which is the highest ammonia complex that can exist in aque­
[
2 +
ous solution. These cupric complexes can react at the surface of the brass alloy to form cuprous
complex ions as shown in the following:
[Cu(NH 3 ) 4 (H 2 0) 2 ] 2 + + Cu = 2[Cu(NH 3 ) 2 H 2 0] + 1.13

These cuprous ions are then unstable in solutions containing oxygen and ammonia so that

2[Cu(NH 3 ) 2 H 2 0] + = [Cu 2 (NH 3 ) 4 (H 2 0) 2 ] 2 + 1.14

The process will be autocatalytic and has been demonstrated in ammonium hydroxide for
unstressed sheet specimens of copper and of brass with a composition of 70% copper and 30%
zinc. The copper content of this ammoniacal solution increases with immersion time at an
Z
increasing rate. Zinc can also enter into solution as the stable complex n(NH 3 ) 4 . After about
2 +
one hundred hours in the ammonia solution, the dissolution rate drops because a film of corro­
sion products develops; the film is primarily cuprite. The tarnish film that forms on brass alloys
in many different tarnishing solutions is due to the deposition of an epitaxial film. While this
film is growing, a range of interference colors, which depend on the thickness of the film, may
be visible. The tarnish consists of small platelets that are about 500 A in diameter and 100 Â
thick. These platelets are similarly orientated and grow epitaxially from the brass surface.
Cuprite exists in this tarnish film, which penetrates into the metal. When unstressed 70-30
brasses are immersed for long periods in tarnishing solutions, they become extremely brittle.
Intercrystalline fracture, due to penetration of the corrosion along the grain boundaries, is seen
in all cases. The fact that thick corrosion films do not form on pure copper under the same con­
ditions shows that the zinc content is important. The presence of zinc in the solid phase is the
most salient factor.
Stress-corrosion cracking of brass alloys is often observed in moist, contaminated air,
where shallow layers of condensed water can form on the metal surface in the presence of oxy­
gen and ammonia. There are probably undocumented occurrences of such deterioration in
musty, antiquated museum collections, particularly of musical instruments or other stressed
objects. The small water volume in moist-air deposition implies a high concentration of

C O R R O S I O N AN D E N V I R O N M E N T
31