The electrochemical corrosion behaviour of quaternary gold alloys when exposed to 3.5 % NaCl solution

L P Ward,A P O’Mullane,D Chen

doi:10.1007/s13404-012-0079-0

Abstract

Lower carat gold alloys, specifically 9 carat gold alloys, containing less than 40 % gold, and alloying additions of silver, copper and zinc, are commonly used in many jewellery applications, to offset high costs and poor mechanical properties associated with pure gold. While gold is considered to be chemically inert, the presence of active alloying additions raises concerns about certain forms of corrosion, particularly selective dissolution of these alloys. The purpose of this study was to systematically study the corrosion behaviour of a series of quaternary gold–silver–copper–zinc alloys using dc potentiodynamic scanning in saline (3.5 % NaCl) environment. Full anodic/cathodic scans were conducted to determine the overall corrosion characteristics of the alloy, followed by selective anodic scans and subsequent morphological and compositional analysis of the alloy surface and corroding media to determine the extent of selective dissolution. Varying degrees of selective dissolution and associated corrosion rates were observed after anodic polarisation in 3.5 % NaCl, depending on the alloy composition. The corrosion behaviour of the alloys was determined by the extent of anodic reactions which induce (1) formation of oxide scales on the alloy surface and or (2) dissolution of Zn and Cu species. In general, the improved corrosion characteristics of alloy #3 was attributed to the composition of Zn/Cu in the alloy and thus favourable microstructure promoting the formation of protective oxide/chloride scales and reducing the extent of Cu and Zn dissolution.

Highlights

The inherent physical properties of gold, including electrical conductivity and chemical inertness combined with attractive appearance, have established its use in a number of areas, such as the electronics, dental and jewellery industries
Attention has focussed on the development of a range of lower carat gold alloys, 9 carat gold alloys, where compositions vary from ternary gold–silver–copper alloys to quaternary alloys where zinc is added primarily to improve workability through contraction of the two-phase field of the phase diagram [1] and to a lesser extent counteract the red colour induced by copper [2]
Further potentiodynamic scanning from the free corroding potential to +1,000 mV for all the gold alloys revealed the presence of a discontinuous curve in the anodic region, suggesting the presence of a number of competing anodic reactions, possibly associated with the selective dissolution of the alloying elements

Summary

Introduction

The inherent physical properties of gold, including electrical conductivity and chemical inertness combined with attractive appearance, have established its use in a number of areas, such as the electronics, dental and jewellery industries. Pure (24 carat) gold is expensive and mechanically weak (soft) and has limited applications even in jewellery applications. In order to function satisfactorily in jewellery applications, certain requirements have to be met with respect to mechanical, chemical and biological properties, where prolonged contact with human skin is evident. Gold itself is considered chemically inert and biologically compatible with the body. There is concern with respect to the corrosive nature of these gold alloys, the lower carat, quaternary alloys where the presence of non-noble metals in the alloy are likely to strongly influence the corrosion resistance

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