Mechanical properties of superplastically formed titanium and aluminium alloys: Partridge, P.G., Mcdarmaid, D.S., Bottomley, I. and Common, D.

Abstract

Electrochemical methods were investigated to assess the tarnish resistance of typical gold alloys used mainly for decorative purposes. Three mechanically clad alloys (14 and 18 carats (ct) Au-Ag-Cu and 14 ct Au-Ag-Cu-Zn) and three electroplated coatings (18 ct Au-Cu-Cd, 22 ct Au-Ni and 22 ct Au-Ni plus a flash layer for colour matching) were studied. Potentiostatic, cyclic constant-current chronopotentiometric and coulometric measurements were carried out in 1 N NaOH containing between 10-5 and 0.26 M Na2S. The electrochemical data were compared with the results obtained from liquid-based and vapour-phase chemical tests by means of colorimetric measurements and the data were expressed in the Commission Internationale de l'Eclairage (CIE) 1976 L∗a∗b∗ colour space. The electrochemical measurements showed that the sulphidation of non-noble metals of the gold alloys occurred in parallel with the oxidation of sulphide ions. Only the first reactions contributed to tarnish by producing insoluble, adherent and coloured sulphide films, while the oxidation of sulphide led to mainly colourless and non-adherent species. The current density measured at a properly chosen potential of the potentiostatic scan correlated with the non-noble metal content of the alloy. The cathodic charge determined by chronopotentiometry provided an excellent indication of the extent of sulphidation of a given alloy. The results of these electrochemical techniques were in good agreement with the data obtained from chemical tests. The thickness of the tarnished films as determined by cathodic reduction in 1 N NaOH was smaller than 100 Å assuming a silver sulphide coverage of unity. The potential value and length of the arrest in the potential vs. time curve gave interesting qualitative and quantitative information on the chemical composition of the tarnished films.