705.    DOEHLEMANN, E.  [Rates of Decarburization and of Carburization of Iron, Nickel, and Cobalt in Carbon Monoxide-Carbon Dioxide Mixtures.]  Ztsch. Elektrochem., vol. 42, 1936, pp. 561-569; Chem. Abs., vol. 30, 1936, p. 8120.

     Velocities of carburization and decarburization of Fe, Co, and Ni foils in stagnant CO-CO₂ mixtures at 920°-970° were determined by following the change in electrical resistance with a Kelvin bridge.  Under the metallic surface is the rate-determining factor.  Diffusion within the metal proceeds rapidly.  Data from the experimental curves show that the decarburization velocity is independent of the C content of the metal and increases linearly with the total pressure for a constant ratio of the partial pressures P_CO2/P_CO.  Carburization or decarburization velocities can be expressed as –dc/at=[K₁P_CO2+K₂(P_CO2/P_CO)] [1-(c_t/c], where K₁ and K₂ are graphically determined velocity constants while c_t and c are the C contents in weight % in the metal at equilibrium and at time t.  From the 1^st bracketed expression it can be seen that the reaction CO₂ (gas)=CO+O (adsorbed) determines the velocity of the process, while the reaction O (adsorbed) +C=CO is essentially in equilibrium.  Values of 0.078, 0.155, and 0.308 for the CO₂:CO ratio were employed.  These corresponded to solubilities of 0.42, 0.20, and 0.087 gm. C:100 gm. Fe.  Carbides do not precipitate at these temperatures until a solubility of C in Fe of 1 gm.:100 gm. is obtained.  As the solubilities of C in Co and Ni at these temperatures are smaller than for Fe, CO₂:CO ratios of 0.036, 0.082, and 0.108 were used for these metals.  Plots of dc:dt vs. reaction pressure for Co and Ni were similar to those for Fe, and the curves for all 3 metals can be similarly expressed.  Different carburization rates may be found for 2 samples of Fe otherwise considered to be identical.