Page 33 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

corrosion and lead corrosion may result in mixed copper-lead mineralization on the surface.
Similar events may occur with arsenic and antimonial alloys of copper, although here the ele­
i
ments form solid solutions of various types. Segregation is still possible, especially f due to cast­
ing, which may result in dendritic segregation and coring of the intradendritic regions. The
compositional gradients between these regions may result in an electrochemical potential, pro­
ducing enhanced corrosion of one phase or component of the alloy. Thus arsenical compounds
may be present in the corrosion of copper-arsenic alloys and antimonial ones with copper-
antimony alloys. The literature on such compounds in the archaeological context is sparse, and
further research is needed to identify all of the arsenates and other salts that are undoubtedly
present as corrosion products.
When a metal is placed in seawater, a number of different oxidation and reduction reac­
tions can occur on the surface, and the metal assumes a potential (E) that is dependent on the
combined effects of all these possible reactions. The defined potential for an environment is its
corrosion potential, E corÎ. All reactions with Ε more positive than £ c o r r will participate in
reduction and form cathodic sites on the metal, while all reactions with Ε more negative than
will be oxidative and form anodic sites. For an inert electrode, the measured potential will
£ c o r r
depend solely on reactions between solution species; this potential is referred to as the Eh seen
in Pourbaix diagrams, discussed later in this chapter. In seawater, for example, the alloy known
as admiralty brass, of composition 71Cu28ZnlSn, behaves in a more noble way than a yellow
brass of composition 65Cu35Zn, and both of these are more cathodic than a low-carbon steel.
Some relationships are shown in Hack (i987). These reactions are important since the galvanic
coupling of different metals is well known to cause severe corrosion to the anodic component.
Such effects were already exploited in the nineteenth century for the protection of copper hulls
by galvanic coupling with zinc or magnesium. The zinc or magnesium would corrode prefer­
entially, thereby protecting the copper.

S O M E H I S T O R I C A L A S P E C T S O F C O P P E R A N D C O R R O S I O N

The corrosion chemistry of copper was exploited in the past, sometimes unwittingly, to create
new technologies; some of those discoveries are briefly described here. For further information
on a particular topic, see the literature cited.

Primitive wet-cell batteries? A crude wet-cell battery can be made by pushing iron and
copper rods into a lemon and then touching the rods with the
tongue, which elicits the sensation of an electric tingle. This simple act, or a similar one, could
have lead to the accidental discovery of electrochemical phenomena more than fifteen hundred
years ago, as certain mysterious artifacts seem to suggest. These puzzling objects date to the Sas-
(
sanid or Persian dynasties 226-641) of ancient Iraq; some date to as early as the first century.

C H A P T E R O N E
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