The surface behaviour of zinc, tin and their binary solid mixtures towards some tarnishing sulphur compounds.

The subject of this thesis involves .the studies of the surface behaviour of zinc, tin and of their binary solid mixtures towards certain sulphur compounds. The general pattern and the experimental techniques used could be classified into four different aspects as follows: 1‐ The first arpect deals with the studies of surface reactions of certain sulphur compounds,namely H2S, SO2, and CS2 with clean and oxidized films of zinc and tin over the temperature range 193 to 473 K under gas pressures ranging from 10 to 10 Nm. The work involved of adsorption of the sulphur compound on the metal surface and of the subsequent incorporation process. Kinetics of the interaction was followed with the aim of understanding the mechanism of the initial reactions on these metals. The effect of surface oxygen was also determined when the interaction of the sulphur compounds was followed with films of zinc and tin which had been oxidized by molecular oxygen. The object of the work of this aspect was to derive information regarding the reactivity of the surface atoms on zinc and tin films, as well as the reactivity of the underlying layers, towards the three sulphur compounds which are known to be highly aggressive towards metals and alloys. Zinc and tin are widely used in industry as protective coatings particularly on iron. Both metals and their alloys have excellent resistance to corrosion in dry and humid atmospheres. There is a dearth of fundamental knowledge, such as that performed in the present work, to support such information. The second aspect of this research deals with the studies of the surface behaviour of a number of mixed metal films of zinc and tin. One, and the most active, of the three surface compounds (which was H2S) was selected for this investigation. The mixed metal films were bimetal films of zinc with tin and also bimetal films of zinc with iron and of tin with iron. Iron is known to form a number of alloys with zinc and tin and such alloys were widely used as catalysts and in building and construction materials as well in numerous industrial products. This aspect of the research is complementary to that described in the previous aspect but involves more sophisticated techniques. Valuable information could be obtained regarding the behaviour of such binary solid systems towards tarnishing by sulphur compounds. These informations, which may be gained from such experimental technique, are very rare in the available literature and are highly needed. The third aspect of the research deals with the electrical properties and morphology of single and bimetal films which have been dealt with in the previous aspects of this research. Measurements and examinations were made en cloan surfaces, oxidized films as well as on films which had adsorbed H0S or SO‐. Electrical measurements could also be done throughout the interaction of the sulphur compound with the films. Two techniques have been adopted in this work, the first involved measurement of the electrical conductivity and the second concerned with the examination of specimens by scanning electron microscope (SEM). The first technique enabled exploration of the electron mobility, migration or transfer throughout the interaction while the second technique assisted in observing the microstructure of the films as well as of the identification of the general or local attacks taking place as a consequence of the interaction.4‐ The fourth and final aspect of the research concerned with the investigation of the polarization behaviour of a zinc specimen, as a model, in aqueous H2S or SO2 solutions using potentiostatic technique. Thus the cathodic and anodic behaviour of the metal could be followed in environment containing both the sulphur compound and water molecules. The research also covered the effect of chloride ions that may be present in such environment on the intensity of attack on zinc. Kinetics and thermodynamic aspects of the interaction have also been investigated. This part of the research was necessary and useful for complete elucidation of the factors controlling the effect and intensity of attacks on metals by gaseous sulphur compounds. The aims that have been satisfied in the complete programme of this research may be condensed into the following: a‐ The action of the sulphur compounds used on zinc or tin, as well as on the mixed films of these metals without or with iron, w? s intense under the prevailing experimental conditions. In most cases, extensive incorporation was possible following the completion of the surface layer on these films. US showed a greater activity than the other two sulphur compounds for reaction with the films. Oxidation of the films before interaction with the sulphur compound reduced the extent of the subsequent attack on the films. b‐ Deposition of zinc or tin on top of iron films reduced the reactivity of the films towards sulphur compounds, but the coating layers of zinc and tin did not prevent the ultimate diffusion of iron atoms through the surface coatings of zinc and tin and of the consequent exposure to the reacting sulphur compound. Films prepared by simultaneous evaporation on glass substrates showed a greater activity towards the sulphur compound than films prepared by successive evaporation. Films of zinc or tin which were deposited on iron substrate sintered less readily than films deposited directly on glass. c‐ Electrical conductivity measurements revealed the occurrence of mobile adsorption on the films, and enabled the distinction to be made between physical adsorption (ca. of Kr) and chemisorption (i.e., H2S) on the films. Sintering of the films could be detected by conductivity measurements. SEM observations assisted in differentiating between general and local attacks on metal surfaces and to identify the positions of local attacks as well as of sulphide and hydrosulphide growth on the surface subsequent to the interaction of the sulphur compound with metal atoms. A clear evidence could be derived from SEK examination for the diffusion of iron into zinc or tin layers throughout the interaction of bimetal films of zinc with iron and of tin with iron with the sulphur compounds, d‐ The potentiostatic studies defined the role of water molecules in the H2S attacks on zinc, the attack of the sulphur compound on the metal was found to be feasible on thermodynamic grounds,but kinetic investigation revealed a higher energy barrier for the corrosion of zinc in the presence of water than when water was absent. The presence of chloride ions in the corrosion environment enhanced the rate of
metal corrosion considerably.