The influence of sulphur segregation on the oxidation of industrial FeCrMo steel

Fe-9Cr-1Mo steel is used as a corrosion resistant alloy in pressurised water reactors. The high steam temperature and pressure inside the reactors result in erosion corrosion. The steel's hardness and resistance to various forms of corrosion are determined by the composition of the oxide layer formed on the alloy at elevated temperatures. Samples were taken from pieces of pipe removed after failure and that have an operation history in excess of 20 years in a power generation facility. Auger electron spectroscopy was used to monitor the changes on the sample surface during annealing between 200 degrees C and 809 degrees C in UHV. Cr, P and N first segregate to the surface, followed by S and Sn. Mo and Sn as well as Cr, P and N display co-segregation kinetics. No C segregation was measured. At 809 degrees C the segregation of S dominates. The same sample was oxidised at oxygen partial pressures of 1 x 10(-7) and 5 x 10(-8) Torr. The amount of O on the surface decreased with an increase in temperature and the O Auger peak-to-peak height changed from a linear-parabolic curve to a more S-like curve at higher temperatures for both pressures. Auger spectra confirmed that this is due to the segregation of S to the surface. The segregation of S at high temperatures not only leads to site competition with O-atoms during chemisorption, but also forms a diffusion barrier restricting the movement of ions between the oxide and the metal. This restricts the growth in thickness of the oxide layer. (C) 2000 Elsevier Science Ltd. All rights reserved.