Papers

AVS2001 Session VST-WeM: Gas Sorption Phenomena I

Wednesday, October 31, 2001 8:40 AM in Room 125

Wednesday Morning

Start	Invited?	Item
8:40 AM		VST-WeM-2 Hydrogen Equilibrium Relation for Stainless Steel B. Zajec, V. Nemanic (ITPO, Institute of Surface Engineering and Optoelectronics, Slovenia) 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_out) during thermal treatment of stainless steel, proceeded to achieve as low as possible q_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 ³ and inner surface 460 cm² was pumped during bake-out at 200 ° C for several days. The quantity of released hydrogen during the whole procedure equalled the concentration change Δ C=8.8 x10¹⁶ atoms H/cm ³. After the pinch-off at room temperature at 4x10^-4 mbar, the hydrogen pressure slowly declined with an initial rate dp/dt (328K) = 6.2x10^-11 mbar s^-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.
9:00 AM		VST-WeM-3 New Results on Outgassing of Stainless Steel and Insulators L. Westerberg (Uppsala University, Sweden); N. Hilleret, B. Versolatto (CERN, Switzerland); B. Hjörvarsson (Royal Inst. of Technology, Sweden) A series of outgassing measurements of 316LN stainless steel have been performed at CERN, using a standard Fischer-Mommsen test dome. Very large, 1.5 mm thick, sheets were rolled in spiral form, open in both ends, and was mounted into the test chamber. Total areas of up to 13 m² were used in order to increase the accuracy of the measurements. Four series of bakeouts have been performed: 1. Six 48 h bakeout cycles under vacuum to 300°C; 2. One bakeout under vacuum to 450°C; 3. Five bakeouts in air to 450°C followed by one bakeout to 300°C; 4. A sample vacuum fired at 1100°C was exposed to six bakeouts at 300°C. The outgassing rates are lowered from cylce to cycle and are in the 10^-13 to 10^-16 mbar l s^-1 cm^-2 ranges, depending on the sample. For comparison and for further understanding of the underlying processes in outgassing of stainless steel we have performed a different type of experiment at the Tandem Accelerator in Uppsala, namely hydrogen depth profile measurements like in ref.,¹ where extraction measurements of H from stainless steel at 1000 °C were compared to the average hydrogen concentrations from 50 to 700 nm measured by the ¹H(¹⁵N,α + 4.3 MeV γ)¹²C nuclear resonance reaction at 6.4 MeV. For the present experiment we have performed similar H concentration measurements on small samples which were cut out from each of the four large test sheets after the outgassing measurements were completed. ¹L. Westerberg et al., Vacuum 48(1997)771.
9:40 AM		VST-WeM-5 Simulation of Hydrogen Outgassing in a Fusion Device and a UHV Chamber by Recombination Limited Model K. Akaishi (National Institute for Fusion Science, Japan); M. Nakasuga (Kyoto University, Japan); Y. Funato (Suzuka National College of Technology, Japan) Recently it has been asserted by Moore and Nemanic that hydrogen outgassing from stainless steel should be considered by recombination limited model, and they have proposed to use a thin-walled chamber of stainless steel. So we attempted to calculate the recombination rate of hydrogen at a metal surface by solving numerically one dimensional diffusion equation. The calculation were made for two cases: the redesorption rate of hydrogen implanted into the wall of a fusion device during hydrogen pulse discharge, and the hydrogen outgassing rate of a vacuum chamber during baking and in post-baking. For the second case, it was assumed that the initial hydrogen distribution in the wall before baking is uniform, hydrogen desorption occurs only on the vacuum side, and hydrogen permeation from the air side is negligible. As a result of numerical calculation, we will discuss the effect of thin wall on achieving a low outgassing rate in a short time by baking, the role of surface layer such as oxide layer, and so on.
10:00 AM		VST-WeM-6 Hydrogen Diffusion under Reactor Irradiation G.W. Schwarzinger, M.J. Higatsberger (Vienna University, Austria); R.W. Dobrozemsky (Vienna University of Technology, Austria) Diffusion of Hydrogen is of crucial and still growing importance in a variety of technical applications. In this work, the special case of hydrogen diffusion under fission reactor irradiation in stainless steel tubes of AISI types 304N and 316L is presented. Whilst the basic data had been presented in earlier publications,^1,2 the contents of this work provide new insights resulting from a detailed sophisticated analysis of the complete data set. This revealed a previously hidden influence of the reactor irradiation on the pressure dependence of hydrogen diffusion through the samples. The pressure dependence of hydrogen permeation decreased under reactor irradiation with a half-life of approx. 150 hours. With the reactor shut off, this effect was reversed with approx. the same time constant. Based on the conditions of the experiments, a selection of interpretations for possible underlying mechanisms for the given effects are discussed. ¹ R. Dobrozemsky, G.W.Schwarzinger and C. Stratowa, Proc. of IVC8, 15 (1980) ² G.W. Schwarzinger and R. Dobrozemsky, J Nucl Mater 122 & 123, 1560 (1984) .
10:20 AM		VST-WeM-7 Thermal Desorption Study of Austenitic Stainless Steels J.-P. Bacher, C. Benvenuti, P. Chiggiato, M.-P. Reinert, S. Sgobba (CERN, Switzerland); A.-M. Brass (Université Paris-Sud, France) Residual hydrogen in stainless steel results in a steady outgassing from vacuum chamber walls, hindering the achievement of UHV conditions. The total content, the binding states and the diffusivity of residual hydrogen in austenitic stainless steels, which together define the room temperature hydrogen outgassing rate, have been investigated by Thermal Desorption Spectroscopy (TDS). Eight different steel types have been studied by means of two different TDS systems. The study has been extended to the effects of various post-production treatments, aimed at reducing the hydrogen content and/or outgassing, namely vacuum firing and bakeout both in air and under vacuum. A large variety of hydrogen desorption peaks has been observed, which have been attributed to diffusible hydrogen, hydrogen trapping in the surface oxides or in lattice defects induced by precipitates, and steel recrystallisation. The hydrogen depletion effectiveness of vacuum baking at different temperatures has been quantified, and the consequences of air baking have been clarified, leading to practical guide-lines for technological applications.
10:40 AM		VST-WeM-8 Outgassing Measurements of Stacked Laminations for Use as an Electromagnet Core Y. Saito, Y. Sato, T. Kubo (KEK-High Energy Accelerator Research Organization, Japan) Electromagnets, involving ferrite or lamination cores, are often installed in a vacuum of accelerator system so as to bend and extract the beam. Outgassing from the cores is one of the problems, since it causes a poor vacuum, thus shortening a beam lifetime and also inducing an electrical breakdown. Outgassing measurements by a throughput method were carried out for laminations with a few kinds of insulating coatings; a quasi-inorganic and inorganic materials. It is to be noticed that the outgassing rates for loosely- and closely-stacked lamination cores do not show any significant difference for each coating, when the pumping period is as long as 100 hours, or more. Further, though pre-baking of the laminations is effective to shorten the pumping period, once exposed to atmospheric air with humidity, the outgassing rate again shows a larger value. Probably, the adsorbed/absorbed water molecules on/in the coatings require an activation energy to desorb, while, once desorbing, they can diffuse more easily, even in a narrow gap of several microns or less. This indicates that the pump-down characteristic is dominated by the coating materials, rather than by a gap distance between laminations. Indeed, stainless-steel laminations without any coatings showed much lower outgassing rates, even when closely stacked. Further, a time lag in the pump-down characteristic of the closely stacked laminations (a 100 mm-square sheet) is estimated to be several hours by a Monte-Carlo simulation, being shorter than 100 hours of pumping.