Selective dissolution and corrosion of some binary noble metal alloys

• Main Author: Durkin, Peter
• Published: University of Warwick 1982
• Subjects: 669; QD Chemistry
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.345008

Tho aims of the project are to investigate the fundamental processes involved when one component selectively dissolves from a binary metal alloy. This is done with reference to noble metal alloys, particularly silver-gold alloys dissolved in nitric acid. These are prepared as electron transparent single-crystal samples and therefore have a simple initial microstructure which can be viewed directly using transmission electron microscopy. After corrosion very detailed microstructural information can be resolved and this is used to elucidate the processes operating during corrosion itself. A well defined and characteristic morphology is seen to develop on silver-gold alloys which is dependent on the exact extent of corrosion. This can be accounted for by a model involving selective dissolution of silver and re-arrangement of the residual gold in a manner analogous to that occurring during the vapour deposition of thin films on crystal substrates. A corrosion phase develops when silver-gold alloys dissolve in nitric acid and careful examination of its structure and epitaxy using electron diffraction and dark field electron microscopy reveals that it is probably gold I oxide. This indicates that gold is directly afflicted by the corrosion reaction and therefore provides an indication of the mechanisms by which these alloys dissolve. Evidence of a very' aggressive attack on the whole fabric of the alloy during corrosion is given by the observation of dislocations and dislocation networks in all of the systems studied and of mixing of components during corrosion in the copper-gold system. The observations presented lead to the idea that corrosion produces widespread attack of the alloy structure involving the noble component as well as the actively dissolving one. The work demonstrates the importance of surface diffusion processes in the selective dissolution corrosion of the systems under investigation.