The influence of sulphur segregation on the oxidation of industrial FeCrMosteel

Fe-9Cr-1Mo steel is used as a corrosion resistant alloy in pressurised wate r reactors. The high steam temperature and pressure inside the reactors res ult in erosion corrosion. The steel's hardness and resistance to various fo rms of corrosion are determined by the composition of the oxide layer forme d 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 us ed to monitor the changes on the sample surface during annealing between 20 0 degrees C and 809 degrees C in UHV. Cr, P and N first segregate to the su rface, followed by S and Sn. Mo and Sn as well as Cr, P and N display co-se gregation kinetics. No C segregation was measured. At 809 degrees C the seg regation of S dominates. The same sample was oxidised at oxygen partial pre ssures of 1 x 10(-7) and 5 x 10(-8) Torr. The amount of O on the surface de creased 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 tem peratures for both pressures. Auger spectra confirmed that this is due to t he segregation of S to the surface. The segregation of S at high temperatur es not only leads to site competition with O-atoms during chemisorption, bu t also forms a diffusion barrier restricting the movement of ions between t he oxide and the metal. This restricts the growth in thickness of the oxide layer. (C) 2000 Elsevier Science Ltd. All rights reserved.