The application of the Snoek damping method to the determination of gas desorption from metals

Abstract

As an example for an investigation on the desorption of gases from metals into a high vacuum, wires made of highest purity iron, containing small amounts (up to .07 per cent by weight) of nitrogen were exposed for several hours to temperatures between 500° and 800°C. The exposure took place either in high vacuum (10 −6 torr) or in an atmosphere of hydrogen at pressures between 10 −4 and 10 torr. The resultant desorption of gas originally dissolved in the metal was determined from the reduction in the nitrogen in the α iron with the help of the Snoek (named Snoek after its discoverer 2) damping method. Whilst the desorption rate at hydrogen pressures ranging from 10 −2 to 10 −6 torr is practically independent of the pressure, the desorption of the nitrogen from the iron is markedly increased of the presence of hydrogen at higher pressures. Experiments were carried out at a pressure of 10 −6 torr and a temperature of 700°C with wires of various diameter and for various periods. It appeared that the rate of reduction of the concentration of of the nitrogen in α-iron is proportional to the square of the corresponding nitrogen concentration in the iron. This relationship allows us to conclude that thegaseous atoms dissolved in the metal recombine to form molecules at the metal surface before the gas can desorb into the vacuum. The activation energy for the desorption process of the nitrogen from the interior of the α-iron into a high vacuum can be determined by varying the temperature of the process. It amounts to 28900 (±1200) cal/mol.