Degassing properties of nickel

Abstract

The grades of nickel used in the electronic valve industry, containing magnesium, silicon and carbon among other impurities, have been found to have gas contents in the range 1.5 to 16 ml. torr g-1, the average value being about 5.0 ml. torr g-1. The total quantity of gas evolved is limited by the amount of oxygen present in the form of oxides of the impurities, particularly magnesium. These oxides have been found, both in wire and sheet, to be concentrated near the surface. In the case of sheet material 0.150 mm thick, 80% or more of the total gas content is in a skin 15μ thick. The diffusion constant of the gas desorbed from the nickel (largely carbon monoxide resulting from the reaction of carbon with the impurity oxides) has been measured in the temperature range 850° C to 975° C and, comparing the results with the diffusion of carbon in nickel, it is concluded that the evolution rate of carbon monoxide is largely controlled by the rate of carbon diffusion. In nickel which contains no stable impurity oxides the evolution of carbon monoxide proceeds at a slower rate and is controlled by the diffusion rate of the oxygen in the nickel lattice. The differences in evolution rate of carbon monoxide from the various grades of nickel can be explained in terms of the location of the oxygen in the nickel.