The Transport of Chromium in Cr2 O 3 Scales in Sulfidizing Environments

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Abstract
The diffusivity of chromium in scales formed during the oxidation of chromium at 900°–1000°C appears to increase by several orders of magnitude when an initially present oxide scale is exposed to sulfur‐bearing atmospheres at low oxygen partial pressure. As a result chromium sulfide scales grow on both sides of the preexisting oxide layer. The transition from oxidation to sulfidation corresponds to a change from parabolic to linear weight gain, indicating that the sulfide growth is controlled by a constant rate of supply of chromium through the oxide to the outer oxide‐sulfide interface, under fixed experimental conditions. The sulfidation rate increases with sulfur partial pressure, indicating that chromium transport in the oxide is affected by changes in the defect structure of induced by the presence of sulfur. A model is proposed in which sulfur enters the oxide as an electron donor, thereby increasing the concentration of chromium vacancies and interstitials.

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