Abstract
Entering the 21st century against a background of rapid technological innovation and the growing importance of pollution control, industry must optimise control of its resources and use technology to its best advantage. Thus, process mechanisms must be known so that consideration may be given, both to the degradation in service or during manufacture, and to the fate of the by-products of the manufacturing process. Many corrosion problems are associated with the presence of impurities which are able to bring about the destruction of the passive layer on a metal surface. The principal elements associated with such corrosion are chlorine and sulphur. The conditions under which a chlorine concentration may result in damage to passive films, by pitting on stainless steels for example, have been well researched over the years. The important role of sulphur as a corrodent has only recently come to light. Sulphur is increasingly present as a pollutant, in coal, oil and gas, as leaner mines and wells are used, and is also found in many metal-bearing ores. From these sources, as well as natural ones such as volcanoes and the oceans, sulphur may enter the atmosphere, the waters and the material goods of the earth by many different routes. The evidence to date on the role of sulphur in corrosion suggests that it is active at much lower concentrations than chlorine, often at monolayer concentrations. With current materials developments emphasising the minaturisation of devices, the use of thin films and the control of the interface in composite materials, the importance of research into the action of sulphur cannot be denied. The work reported in this thesis examines the information and experimental methods that are available to the corrosion scientist interested in the action of sulphur on metal surfaces in very small quantities - of the order of a monolayer. The work is divided into three main parts : i) a survey of the literature concerning the role of sulphur in corrosion and the many materials which are subject to accelerated corrosion under the action of sulphur at high temperature, in air, from micro-organisms and in solutions. The survey of the incidence of sulphur-induced corrosion is deliberately wide to emphasise the ubiquitous nature of sulphur's activities in materials degradation. Attention is also drawn to the properties of a monolayer of sulphur adsorbed onto a metal surface and the subsequent consequences for surface passivation and dissolution. These observations lead into the second half of this chapter which is concerned with the use of experimental analytical techniques for surface characterisation, principally X-ray photoelectron spectroscopy (XPS). The development of methods for estimating the character of nearest neighbours (to the element under study) from photoelectron line shifts is covered : from Koopman's approximation for electron orbital energy to the incorporation of both inter- and extra-atomic relaxation energy terms. The results from studies on silicon and chlorine are reviewed. ii) Chapter 2 examines in more detail the experimental results in the literature pertaining to the surface analysis of sulphur, and the characterisation of sulphur in the solid state. The experimental work reported covers the surface analysis of a range of sulphides and sulphates and compares the data with those for sulphur monolayers on CuNi and gaseous and organic sulphur compounds. The correlation of extra-atomic relaxation energy with alternative measures of electron orbital polarizability is investigated. iii) The third chapter is a study of the mechanism of stimulation of corrosion of CuNi(90,10) in deaerated sea-water. Two contrasting theories of this alloy's corrosion are considered : 1) the acceleration of the cathodic reaction due to the formation of a thicker and more porous oxide film in sulphur polluted waters. 2) the catalytic stimulation of the cathodic reaction by adsorption of sulphur. The behaviour of the alloy is investigated in solution by introducing H2S and by using samples with previously adsorbed monolayers of sulphur. Electrochemical characterisation is followed by surface analysis. The results are compared to the data in Chapter 2. iv) Chapter 4 is a summary of the main findings of this work and includes recommendations for further work.