Study of tin corrosion: the influence of alloying elements

Abstract

This paper focuses on the corrosion behaviour of tin objects stored in museums. A set of authentic objects was investigated using optical microscopy (OM) and scanning electron microscopy with energy dispersive X-ray detection (SEM-EDX). The goal existed in acquiring information on the appearance of the corroded surfaces and the chemical composition of the alloys. The analyses made it possible to obtain an overview of typical corrosion forms seen on ancient tin objects. In order to study the influence of the alloying elements and corrosive agents on the corrosion behaviour, a simulation study was set up in which five ancient alike tin alloys were produced and artificially corroded by using different corrosive agents. The corroded surfaces were analysed using OM, SEM-EDX and Fourier transform infrared spectroscopy (FTIR) and the results were compared with those obtained from the authentic samples. © 2004 Elsevier SAS. All rights reserved.Keywords: Ancient tin alloys; Corrosion; OM; SEM-EDX; FTIR Research aimsThe objective of this paper is to contribute to a better understanding of the degradation of tin objects present in museum collections. Specific emphasis has been laid on the study of factors which may influence the corrosion, such as the alloying elements and the corrosive agents. IntroductionSeveral publications mention tin to be corrosion resistant. Results have indeed shown that under natural conditions water and atmospheric oxygen have no harmful effects on metallic tin, although tarnishing of tin is sometimes observed in indoor atmospheric conditions [1][2][3]. Nevertheless, tin may also suffer from corrosion by mineral acids and organic acids in the presence of air [2]. This general idea has in the past led to believe that tin objects stored under normal museum conditions are little affected by atmospheric corrosion. As a result tin pest was often considered as the major deterioration cause of tin objects stored in museums. Tin pest is a physical phenomenon, i.e. an allotropic transformation of metallic white tin (b-tin) to grey (a-tin) which occurs at low temperatures, namely below 13.2°C. Metallic white tin is very ductile, while grey tin is brittle. Both have different crystallinity and density-white tin (7.29 g cm -3 ) and grey tin (5.77 g cm -3)-which result in the transformation of the objects into powder when tin pest occurs [4]. Once an object is harmed by tin pest it will be completely destroyed after a period of time. No treatment is possible in this case. The corrosion of tin on the other hand is (electro)chemical by nature and can be treated in many cases. The most typical and most stable corrosion product of tin is tin(IV)oxide, SnO 2 [4].The general belief that tin is not affected by corrosion, and on the other hand the general assumption that tin pest is the main degradation form of tin objects, has led to the fact that today many of the tin objects which are present in museums are in a certain state of decay. Detailed examination of several museum objects has, however, shown that...

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