The effect of surface sulfur on the oxidation of zirconium

In order to examine the effect of surface sulfur on the oxidation of zirconium, we have tried to make a quantitative analysis of sulfur segregation on Zr and made a study of the oxidation of sulfur segregated Zr. Depth profiles of the S-segregated Zr surface determined after high temperature annealing in vacuum show that the saturated surface concentration of sulfur has an S/Zr atomic ratio of about 1 and the total amount of segregated S is about 1 monolayer. These values do not depend on the annealing time. The S-segregated surface is exposed to an oxygen atmosphere in a pressure range of 10⁻⁶−10⁻⁵ Pa up to a total exposure of 9000 L at RT. Changes of the surface concentration of S, O and Zr are monitored by in situ AES analysis. Depth profiling of O, S and Zr for the oxidized specimen is also carried out. Surface oxidation is clearly delayed by segregated sulfur. This effect, however, is not so significant as for the S---Fe system, and three oxidation stages are clearly distinguished in the same manner as in the oxidation of clean Zr surfaces. This is because S is simultaneously oxidized and removed in the form of a volatile oxide such as SO or SO₂.