Selective Dissolution below the Critical Potential and Back Alloying in Copper-Gold Alloy

Abstract

Below the critical potential, E c , selective dissolution of Cu from Cu-18 atom % Au proceeds via Cu transport through an essentially planar Au-rich product layer, which over time increases to hundreds of atom layers in thickness. Transmission electron micrographs ͑TEM͒ taken after the selective dissolution ͑at E Ͻ E c ͒ and subsequent ͑current-off͒ back alloying processes reveal nearly uniform Moire ´patterns characteristic of the superposition of two layers with similar crystal structures, i.e., the Cu-depleted ͑Au-rich͒ product layer and the parent alloy. The Moire ´fringe spacing decreases with the time of Cu dissolution ͑for ϳ60 s͒, indicating solid-solution Au enrichment in the product layer during dissolution. The Moire ´patterns give no indication of discrete islands of residual Au. The decreasing Cu dissolution rate follows the parabolic law for the first 60 s with an effective diffusivity of Cu in the product layer of ϳ10 Ϫ13 cm 2 s Ϫ1 at 23°C. The TEMs also show a few pits, whose diameter is in the Ͻ10 nm range and whose number increases with time, which may account for the higher rate of Cu dissolution ͑than given by the parabolic law͒ after 1 min.

Volume Change during the Formation of Nanoporous Gold by Dealloying

“…1(b), are extremely rare, consistent with the established notion (see, e.g., Refs. [8,14]) that the crystal lattice orientation is retained during dealloying and with the conservation of the 50-100 m grain size of the master alloy. This grain structure remains visible in FIB images of NPG [ Fig.…”

Environmentally Induced Failure of Gold Jewelry Alloys

“…The sample fractured after 661 h at a constant potential of 143 mV Based upon the half-reaction oxidation potentials, given in Table 5, of each element it is determined that the corrosion rate is controlled initially by the dissolution of Zn and the formation of ZnCl 2 . This is followed by the dissolution of copper, nickel, or palladium below E c (15). There is a gradual enrichment of the Cu, Ni, or Pd-depleted layer with Au producing a concomitant increase in the lattice parameter during dissolution.…”

“…The experiments showed that all three alloys suffered from IGSCC. Since all tests were done at the maximum corrosion rate, below E c , for the system the fracture morphology was primarily intergranular (15). Figure 6 shows an SEM image of the fracture surface of 18K yellow gold after failure in dead weight loading in caustic sodaLiquiChlor.…”

Dealloying

“…In recent years research has concentrated on using dealloying as a means of producing high-surface-area substrates for catalysis and other purposes [3][4][5][6][7][8]. The subject of deliberate dealloying as a manufacturing process has been reviewed in the American Society for Metals (ASM) International Metals Handbook [9] and will not be discussed further in this chapter.…”