The corrosion properties of alloys are of enormous practical importance: modern life would be very different without stainless steels. Alloy
corrosion is also an intriguing field of scientific study that combines electrochemical kinetics with fashionable aspects of the morphological
evolution of surfaces, and even a dash of ancient history, via the studies of Forty1
on "depletion gilding" practiced by Early
Andeans during pre-Columbian times in South America.
The basic alloy corrosion process, as used by the metalsmiths to gold-coat artifacts, is de-alloying. This is defined as the selective
electrolytic dissolution of one or more components from a metallic solid solution. For this to happen, there must be a significant difference in the
equilibrium metal/metal-ion electrode potentials for the two metals, taking into account any complex ions that might be formed in the
electrolyte. For example, we can expect de-alloying in Au-Cu alloys, but not in Au-Pt alloys.
De-alloying shows sharp parting limits, expressed as critical atom percentages of the more reactive component above which that component
can be removed from the alloy by electrochemical dissolution in an oxidizing environment such as nitric acid. Parting limits range from about 20
to 60 atomic percent. This concept is still used in noble metal technology to separate noble from base metals. For example, an alloy of 55 at.%
gold and 45 at.% silver does not de-alloy, but if it is re-melted with additional silver so that the atom fraction of Ag is greater than 60 percent,
the gold can be separated almost completely by nitric acid immersion.
De-alloying was actively used as an investigatory tool in the study of physical metallurgy around 1920, notably by Tammann3
. Tammann proposed a model based on patterns formed by the elements in an ordered alloy, and misinterpreted the existence of sharp