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The economic loss of corrosion is estimated to exceed 3% of the world’s annual GDP. Thus, the prevention of corrosion, namely of alloys such as steel, via passivation layers is of great importance. Dealloying in electrolytic solution (leaching) occurs if a binary alloy of elements with sufficiently different Nernst potentials is exposed to an aqueous electrolyte in which no stable bulk oxide is formed. This process has wellknown detrimental consequences, e.g., in perilous stress corrosion cracking or for the reactivity of the exposed atomic skin of binary metal catalysts, in use for fuel cells. However, it is also one of the oldest metallurgical processes for refining surfaces, employed in industry for producing the Raney-Nickel catalyst, and also proposed for other applications utilizing nanoporous metal structures.
Some of the more reactive element will dissolve when the freshly exposed alloys surface is polarized to a potential above the its equilibrium potential Eeq. Above a so-called critical potential Ec, and with a sufficiently high content of the more reactive component in the alloy, the entire alloy can be transformed into a nanoporous network of the noble metal. For potentials between Eeq and Ec, passivity is caused by an initially formed metallic layer, which is not well understood as of yet.
We investigated the corrosion of an alloy in sulphuric acid from the very first stage, using a Cu3Au(111) single crystal as a model system. We employed hard x-radiation at the ESRF to in situ characterize the structure and chemical composition of the electrolyte/alloy interface during the development of the passivation layer. At potentials slightly positive of the Cu dissolution potential Eeq we observe the formation of a 2-3 monolayer thick, Au enriched single-crystalline alloy layer with an inverted stacking sequence [1]. At more positive potentials, this Au-rich CuAu passivation layer vanishes, in favor of thicker, pure Au islands of the same lateral size. Similar obesrvations are made for the (001) surface and a CuPd alloy. The influence of minor additions of chlorine ions and further structural transitions close to Ec will be discussed.
[1] Frank Renner et al., Nature 439, 707 (2006). |