Sulfur-induced corrosion at metal and oxide surfaces and interfaces

Sulfur adsorbed on metallic and oxide surfaces, whether originating from gaseous environments or segregating as an impurity to metallic interfaces, is linked to the deterioration of alloy performance. This research dealt with investigations on the interactions between sulfur and iron or iron alloy metallic and oxide surfaces under ultra-high vacuum conditions. Sulfur was either intentionally dosed from a H{dollar}sb2{dollar}S source on an atomically clean metal surface, or segregated out as an impurity from the bulk to the metal surface by annealing at elevated temperatures.; It was found, based on temperature programmed desorption (TPD) and Auger electron spectroscopy (AES) measurements, that sulfur located at the iron-oxide interface reacts with the oxide to form SO{dollar}sb2{dollar} destroying interfacial bonds. This interfacial reaction suggests a microscopic mechanism for the instability of the oxide. X-ray photoelectron spectroscopy (XPS) measurements showed that on a Fe-Cr-Ni surface, the presence of interfacial sulfur retards the growth of an oxide but does not prevent the segregation of chromium at elevated temperatures. Moreover, while interfacial sulfur adversely affects the thermal stability of the iron oxide, the chromium oxide phase of the alloy remains stable. Ultra-high vacuum scanning tunneling microscopy (UHV-STM) and low energy electron diffraction (LEED) studies on Fe(111) revealed that the segregation of varying coverages of sulfur causes faceting transformations and reactivity changes on the Fe(111) surface. The Fe(111) surface was found to exhibit an unusual triangular pitting upon the segregation of a high coverage of S. This high-S coverage surface is extremely inert towards oxidation near 300 K. In addition, the effects of N segregation on the reactivity of a Fe-Cr(100) surface was examined. It was found that co-segregation of Cr and N results in the formation of a CrN overlayer which retards oxidation of the surface at 300 K.; This research has potential implications to industrial processes such as electric power generation, petrochemistry and coal gasification, where alloy components are exposed to chemically aggressive environments at elevated temperatures.