Electrochemical Behavior of Sintered CuAg4 at. pct Alloy
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COPPER has excellent conductivity but poor resistance to softening and low strength at moderate temperatures. For commercial use, both electrical and mechanical properties are of great importance. Recent literature describes efforts devoted to the investigation of copper-based alloys in search of improvements in strength and maintenance of strength at high temperatures.[1–4] The copper-silver alloy is an example of eutectic systems (Figure 1). The eutectic point of silver and copper is at 1052 K (779 °C) when the alloy contains 72 pct silver and 28 pct copper. On both sides of the phase diagram, there is a small solubility of the mentioned metals in each other. The maximum solubility of silver in copper is 4.9 at. pct, and the slope of the solvus line indicates the possibility of age-hardening of certain alloy compositions. A similar phenomenon— the strengthening of cold worked substitutional solid solutions after annealing up to the recrystallization temperature—is termed anneal hardening. The anneal hardening effect had been observed in Cu-Ag alloys in the annealing temperature range of 413 K to 673 K (140 °C to 400 °C), the hardness being increased with the degree of predeformation.[5] The passivity of copper and its alloys is of interest, with respect to basic and applied research, because of its wide application in industry. Silver-copper alloys have been investigated elsewhere from the corrosion point of view[6–8] or as a microelectrode material for use in MIRJANA RAJCˇIC´-VUJASINOVIC´ and SVETLANA NESTOROVIC´, Professors, VESNA GREKULOVIC´ and IVANA MARKOVIC´, Assistants, and ZORAN STEVIC´, Professor, are with the Technical Faculty in Bor, University of Belgrade, Bor 19210, Serbia. Contact e-mail: [email protected] Manuscript submitted January 15, 2010. Article published online July 30 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
voltammetric field apparatus,[9] but the content of silver in all these alloys is more than 15 pct. The electrodes examined in the mentioned articles were prepared by melting and cast. Many papers, both older and recent, discuss investigations of electrochemical oxidation of pure copper[10–22] or its alloys[23–26] in alkaline media. Pure silver is also investigated widely from the electrochemical point of view.[27–40] The presence of adsorbed hydroxyl ions on both copper and silver electrode surface in alkaline solutions, even in so-called doublelayer region, was proved by in situ scanning electron microscopy[10,12] and other surface analytical methods. Thus, the adsorption of hydroxyl ions is considered to be the first reaction step that can take place during the anodic polarization of CuAg4 at. pct alloy: CuAg4 þ OH ¼ CuAg4ðOHÞads þe
½1
As can be observed from the Pourbaix diagram[42] (Figure 2), during the anodic oxidation of copper in alkaline solution, the following reactions may occur: 2Cu þ 2OH ¼ Cu2 O þ H2 O þ 2e
½2
Cu2 O þ 2OH ¼ 2CuO þ H2 O þ 2e
½3
These two reactions are the steps of the following overall reaction: Cu þ 2OH ¼ CuO þ H2 O þ 2e
½4
A
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