IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Vacuum Science & Technology Wednesday Sessions
       Session VST-WeM

Paper VST-WeM2
Hydrogen Equilibrium Relation for Stainless Steel

Wednesday, October 31, 2001, 8:40 am, Room 125

Session: Gas Sorption Phenomena I
Presenter: B. Zajec, ITPO, Institute of Surface Engineering and Optoelectronics, Slovenia
Authors: B. Zajec, ITPO, Institute of Surface Engineering and Optoelectronics, Slovenia
V. Nemanic, ITPO, Institute of Surface Engineering and Optoelectronics, Slovenia
Correspondent: Click to Email
The thermodynamic equilibrium relation for hydrogen concentration in metal is usually described by the solubility, (Sieverts law), determined by fast and inaccurate methods. These data and the diffusivity were usually applied as the starting point for calculations of hydrogen outgassing kinetics (q@sub out@) during thermal treatment of stainless steel, proceeded to achieve as low as possible q@sub out@ at room temperature. This, in turn, sets the requirement for a large pumping speed when the ultimate pressure in the chamber should be in the extreme high vacuum (EXV) range. In general, such calculations do not match well with the observed data. In the present study, hydrogen sorption and desorption kinetics close to equilibrium was investigated by the sensitive gas accumulation method. The pressure was monitored by means of a spinning rotor gauge just before the pinch-off and for the following next months at two stabilised temperatures: 25 ° C and 55 ° C. Prior to the measurement, the stainless steel test cell of uniform wall thickness 0.15 mm, volume 125 cm @super 3@ and inner surface 460 cm@super 2@ was pumped during bake-out at 200 ° C for several days. The quantity of released hydrogen during the whole procedure equalled the concentration change @DELTA@ C=8.8 x10@super 16@ atoms H/cm @super 3@. After the pinch-off at room temperature at 4x10@super -4@ mbar, the hydrogen pressure slowly declined with an initial rate dp/dt (328K) = 6.2x10@super -11@ mbar s@super -1@ and then afterwards straightened to a state, which could be termed as an equilibrium. In similar reported experiments, where measurements started in ultra high vacuum (UHV), the dp/dt was always positive and almost constant over several orders of magnitude. A sudden change of temperature of 30 K was applied a few times to investigate the stability of the equilibrium and the change in the reaction rate. The results are compared with: new experimental data on hydrogen content in stainless steel and the equilibrium relation, particularly with present models of hydrogen outgassing in relation to UHV and EXV.