Page 28 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

THE ANATOMY OF C O R R O S I O N
It is not possible to adequately discuss aeruginous (copper rust) corrosion products without
introducing several principles of corrosion and descriptions of minerals. For these, see useful
textbooks by Evans (i960); Jones (1992); Dean and Rhea (1982); Schumacher (1979); Ρ alache,
Berman, and Frondel (1951); Tite (1972); Roberts, Rapp, and Weber (1974); and Brown and
colleagues (1977). Dispatching the subject of corrosion is very much like dispatching a multi-
headed hydra: f one head is eliminated, there are always others that remain a source of con­
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sternation or bewilderment. Microbially influenced corrosion (MIc), for example, has recently
begun to make an appearance in standard textbooks as a very important aspect of metallic cor­
rosion, even in the corrosion of submerged copper alloys, yet the influence of microbes was once
thought to be so minor that the subject was not even mentioned in older texts. This represents
another hydra head in the process of being slain, and there will undoubtedly be others to come.
The behavior of the common alloys of copper with tin, zinc, arsenic, antimony, or lead is
important, but it is not always known or properly understood. These alloying elements may pro­
duce subtle or major changes to the behavior of copper in different environments. In the most
commonly encountered patina to form on buried bronzes, for example, there is an enrichment
of tin in the outer corrosion layers due to the formation of cassiterite, Sn0 2 , or hydrated tin
oxides with variable degrees of crystallinity. These tin minerals afford enhanced protection of
the metal under the developed patina and are largely responsible for the smooth, lustrous sur­
face on ancient bronzes that is known as "water patina." With Chinese bronzes, for example,
these patinas occur quite often and are much admired and usually carefully preserved by col­
lectors or museums.
Brasses with substantial amounts of zinc, however, may become destabilized compared
with a bronze. They can lose zinc through a process called dezincification, which is discussed at
length later in this chapter. Dezincification results in plugs or pits in the alloy where the object
has been greatly weakened by the process. Protective patinas are usually not very well developed
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on these brasses, f present at all, since compared with destannification, or loss of tin, the pref­
erential corrosion of the zinc does not result in a uniform deposition of zinc oxides. An inade­
quate amount of work has been carried out on the corrosion of arsenical copper alloys, which
appear to survive quite well in burial environments, compared with the relatively large amount
of information available for tin bronzes. Current understanding of corrosion is therefore lim­
ited and based on certain models that may or may not be applicable to the particular corrosion
event being studied. An understanding of the major concepts of pH, Eh, Pourbaix diagrams
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(discussed later in this chapter), and kinetic and thermodynamic principles, as well as posses­
sion of the requisite chemical background knowledge, is essential for a more detailed under­
standing of the subject. Pertinent basic information can be found in Pourbaix (1973), Moore

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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