Effects of Scale Porosity, Second-Phase Oxides, and Doping in the High-Temperature Oxidation of Cobalt and Dilute Cobalt-Chromium Alloys

Abstract

The oxidation mechanisms of high‐purity cobalt and dilute Co‐Cr alloys have been investigated over a 950°–1350°C temperature range in oxygen atmospheres of 10–760 Torr. The 99.999% cobalt exhibited parabolic oxidation behavior for weight gains of up to 30 mg oxygen/cm² rectangular specimen surface. Parabolic kinetics were interrupted when fissures developed in the oxide scale as a result of mechanical stresses at the metal/scale interface or through anisotropic decomposition along grain boundaries. Molecular oxygen then short‐circuited to the porous inner scaling layer in accelerated attack of fresh Co surfaces. Diffusion of Co²⁺ through the scale is considered to be the primary rate determining process during periods of steady‐ state oxidation. The effect of porosity at the metal/scale interface prior to perforation of the outer scaling layer is slight, but is believed to result in a small reduction of the oxidation rate due to a reduction in the contact surface through which cations enter the oxide. Co alloys containing 0.5, 3, 7, and 10 w/o (weight per cent) Cr also obey parabolic kinetics during formation of thinner scaling layers. Porosity and localized loss of scale adherence are aggravated by the presence of and particles in the inner scaling layer to the extent that scale rupture mechanisms quickly obscure parabolic kinetics for chromium contents greater than 3 w/o at temperatures of 1150°C and above in 100 Torr oxygen. Under conditions where initial periods of steady‐state parabolic oxidation were discernible on thermobalance curves, maximum reaction kinetics are associated with 1–2 w/o Cr additions (which approach the Cr³⁺ solubility limit in ) as rationalized by Wagner's semiconductor valence theories.