Abstract
Pits have been observed to initiate at certain inclusions in stainless steel, which affect the composition and properties of the passive film in that region. In this thesis, the VG MA500 high resolution scanning Auger microscope associated with Link EDX is used to investigate the surface chemistry of inclusion groups in stainless steel 316 relative to the bulk composition, before and after exposure to sodium chloride solution. The work demonstrates that careful control of experimental conditions allows the progress of initiation of pits to be observed systematically. The role of inclusions as initiators of pitting corrosion has been studied from the very inception of attack. Detailed observation have been taken from separate manganese sulphide as well as mixture of manganese sulphide and multi-element oxide at various pH values of solutions to trace the behaviour of these inclusions during exposures. It is suggested from this investigation that corrosive attack starts on MnS inclusion proper. The dissolution of MnS takes place in sodium chloride solution followed by the acidification in the microarea and dissolution of metal in the region adjacent to the inclusion. The mechanism for the crevice corrosion is of importance after the microcrevice forms. The observations showed that sulphur precipitated on the area surrounding the inclusion group in acidic sodium chloride solution. This is in contrast to the finding in prior studies by electron probe or EDX analyses in which sulphur precipitation was only observed on the inclusion. Because pitting normally occurs on surfacesthat are initially exposed to air before being immersed in the corrosive solution, an understanding of the change of surface film associated with the change of medium is very important. XPS was used in this work to study the air formed and aqueous films, the transformation when the specimens were immersed in solutions. The study of the energy-loss structure of photoelectron peaks has lead to a better understanding of these various films. It has been used to study in greater detail the composition and properties of the films formed on Fe-Cr alloys during oxidation, vacuum annealing, and to find the relationship between the depth distribution of the components in the films and energy loss background of photoelectron peaks. Three features are of particular importance: the background loss tail height, the baseline slope and the post-peak loss structure. These features have been related quantitatively to the relative amounts of the element and the surrounding matrix in the layers which contribute the greatest signal at any given take-off angle. The present work illustrates that the additional depth information available from closer inspection of the inelastic loss background feature may aid an understanding of the change from the air-formed film to the aqueous film as well its effect on the initiation of pits in stainless steel 316. The combination of AES and EDX with high spatial resolution gave an opportunity to re-investigate the initiation of pits at the site of inclusion on stainless steel by offering both bulk and surface information. The use of AES and XPS associated with the energy loss structure of photoelectron peaks facilitates a comprehensive understanding for the whole surface of various films and the local areas which are involved in the formation of pits.