The oxidation of cobalt in air from room temperature to 467�C

Abstract

The oxidation of cobalt in air is investigated for times ranging over three orders of magnitude up to 1000 hr and for temperatures ranging from room temperature up to 467°C. Three different growth processes are observed. When a clean surface is exposed to air at room temperature, an 8–10 Afilm of Co(OH)2forms in seconds or less. For temperatures of 50–100°C, very little additional film forms for times up to 1000 hr. For temperatures of 100–225°C, a film of CoO grows in a manner which can be described empirically with a fourth root rate law with an activated rate constant. Between 225 and 325°C there is a transition to a quadratic rate law. At 425°C the film appears to be a mixture of CoO and Co3O4.