Micromorphological Studies of the Corrosion of Gold Alloys

Gold Bulletin, Mar 1981

Considerable insight into the detailed mechanisms by which metals are corroded can be derived from direct microscopic observations. This article describes those micromorphological changes occurring near the surface that can be observed by transmission electron microscopy after gold alloys have been subjected to anodic dissolution in strong acids. These observations are used to discuss the important problem of corrosion by selective dissolution.

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Micromorphological Studies of the Corrosion of Gold Alloys

0 A. J. Forty Department of Physics, University of Warwiek , Coventry, U.K Considerable insight into the detailed mechanisms by which metals are corroded can be derived from direct microscopic observations. This article describes those micromorphological changes occurring near the surface that can be observed by transmission electron microscopy after gold alloys have been subjected to anodic dissolution in strong acids. These observations are used to discuss the important problem of corrosion by selective dissolution. - Direct observation of the surface of a metal after exposure to a corrosive environment has frequently been used as a basis for deductions concerning the structural processes by which chemical reactions inodify the surface and near-surface layers. Although considerable information can be obtained in this way using simple optical microscopy, as, for example, in the case of the aqueous corrosion of gold-copper alloys studied by Graf (1), Bakish and Robertson (2) and Pickering (3), a much more detailed picture can be obtained with the electron microscope, particularly when this is coupled with the powerful analytical techniques of selected area electron diffraction and X-ray microanalysis. The availability of very high resolution microscopes now makes it possible to follow changes in the interaal microstructure, the surface morphology and the composition of metals and alloys on a scale approaching atomic dimensions. The use of the transmission electron microscope necessitates the study of very thin specimens. Great care must be taken to ensure that the micromorphological changes arising from corrosion can be distinguished from those produced during the preparation of such thin films from bulk samples. For this reason, there may be doubts concerning, the validity of some of the observations by Pickering and Swann (4) and by others who have studied the corrosion of alloys prepared as thin foils by electropolishing techniques. Ion-sputtering methods for thinning alloys from bulk specimens may also be suspect, because different sputtering rates for the various constituents can lead to compositional changes. These difficulties have been largely overcome recently by Durkin and Forty (5) who have developed techniques for preparing thin films by vapour deposition of an alloy from its individual components. It. will be shown later how this has contributed to a very detailed understanding of the corrosion micromorphology in the special case of gold-silver alloys. Selective Dissolution The most widely studied and possibly the most important phenomenon involved in the aqueous corrosion of gld alloys is that of selective dissolution, whereby the less noble element is preferentially removed from the alloy, leaving a gold-rich residue (6). This is the basis of various practical methods for the parting gold from its alloys. It is also thought to be an important step in the stress corrosion of gold alloys since rupture of the gold-rich surface layer by an applied stress can lead to the initiation of localized, deeper corrosion and subsequently of a stress corrosion crack (1, 2, 3). As we shall discuss later, selective dissolution might also be an important precursor of other corrosion reactions, such as oxidation. Fig. 1 Schematic representation on an atomic scale of the surface of an alloy composed of dissolvable A atoms and noble B atoms. K is a kink site on a surface step N is a non-kink site on a step T is a terrace site The fundamental question to be answered, as far as the understanding of selective dissolution is concerned, is why a gold-based alloy should continue to dissolve in this way beyond the stage where the surface should be passivated by the gold residue. Such passivation might be expected to develop at a very early stage, as can be seen from a consideration of the atomic processes that might be occurring on the metal surface during dissolution. These are depicted in their simplest form in Figure 1, where we ignore molecular adsorption, oxidation and complexing effects associated with the electrolyte, and assume that dissolution involves only ionization and solvation of the metal atoms. Dissolution is expected to occur preferentially from kink sites (K) in the surface steps where the atoms are least firmly bound and, at sufficiently low potentials, the dissolution current will involve predominantly A atoms the less noble species. As dissolution proceeds, however, this current will be diminished as more and more kink sites become occupied by more noble B atoms. Thereafter, dissolution can proceed only by the removal of A atoms from non-kink sites (N) on steps or from terrace sites (T), which requires a greater activation energy or overpotential. Eventually, the alloy becomes completely passivated when all the surface sites are occupied by B atoms only. For most alloys, and Cu 3Au in particular, this passivation stage should be reached after the removal of A atoms from only a few atomic l (...truncated)


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A. J. Forty. Micromorphological Studies of the Corrosion of Gold Alloys, Gold Bulletin, 1981, pp. 25-35, Volume 14, Issue 1, DOI: 10.1007/BF03216556