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 950~176 temperature range in oxygen atmospheres of 10-760 Torr. The 99.999% cobalt exhibited parabotic oxlaation behavior for weight gains of up to 30 mg oxygen/em 2 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 CoO grain boundaries. Molecular oxygen then short-circuited to the porous inner scaling layer in accelerated attack of fresh Co surfaces. Diffusion of Co 2+ through the CoO scale is considered to be the primary rate determining process during periods of steadystate 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 CoO/Co 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 CoCr204 and Cr203 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~ 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 Cr 3+ solubility limit in CoO) as rationalized by Wagner's semiconductor valence theories.Previous oxidation studies of high-purity cobalt have been reported in the literature (1-8). Linear kinetics are observed during the growth of very thin oxide films, with a subsequent transition to parabolic growth kinetics for thicker oxide layers. At elevated temperatures CoO is a metal deficient, p-type semiconductor (9, 10) and the electrical conductivity of CoO at near atmospheric pressures, where singly charged cation vacancies supposedly predominate (9), is proportional to Po21/4 (9, 10). The self-diffusion of Co in CoO is reported to be proportional to Po2 lln, where n varies from values of ~-.4 at 10-8 atm to --3 at 1 atm oxygen over a 950~176 temperature range (11). The parabolic rate constant for the high-temperature oxidation of cobalt has also been reported to obey a Po21/n dependence where 4 > n > 3 (1-4). Parabolic oxidation of cobalt is rationalized according to the Wagner theory (12). Cobalt cations diffuse from the metal/oxide interface via cation vacancies in the oxide lattice to combine with oxygen at the oxide/gas interface and cation diffusion through the oxide is considered to be the rate-controlling process.Incremental additions of chromium have been reported to increase the Co-Cr alloy oxidation rate to a maximum value at approximately 10 w/o (weight pe...

Application of Wagner's Theory to the Parabolic Growth of Oxides Containing Different Kinds of Defects: I . Pure Oxides

“…The parabolic rate constant for the oxidation of cobalt at 1000~ and 1 atm 02 ranges between 3.2 • 10 -s and 1.57 • 10 -s g2 cm-4 sec-1 (9)(10)(11)(12)(39)(40)(41)(42)(43)(44)(45)(46), the lowest value being that measured by Mrowec et aL (46), which has been obtained by taking into account appropriate correction factors to the experimental value. Conversion of the latter value to the rate constant expressed in terms of scale thickness as used here yields at 1000~ /~ --1.07 X 10 -s cm 2 sec -1…”

“…[46] into Eq. [43] ] " [49] since usually [Vr'] " >> [Vr'] '. The parabolic rate constant kx can be put in a different form frequently used by means of the relationship (37)…”

Generalized diffuse interface model for determination of kinetic parameters in high-temperature internal corrosion reactions (gas/solid system)

“…The experimental time is obtained from the corrosion rate data given for the oxidation of cobalt [7], iron [14], and nickel [15] by oxgen at elevated temperatures and one atmosphere total pressure. The following oxidation reactions are assumed to be predominant: Experimental fractional conversion (X) can be calcu- …”

“…The experimental data [7] for cobalt oxidation by pure oxygen at elevated temperatures compared with those predicted by DIM.…”

“…Table II shows the values obtained for the experimental data of the oxidation of cobalt [7] with oxygen at a total pressure of one atmosphere and 950 ЊC.…”