Hydrogen traps in the outgassing model of a stainless steel vacuum chamber

Abstract

This article describes a model for hydrogen outgassing into a stainless steel vacuum chamber. It accounts for the geometry of the chamber components, the hydrogen dissolved in those components, and the processes of diffusion, recombination, and trapping. Strongly bound or "trapped" hydrogen, which occurs at heterogeneities such as dislocations and grain boundaries, can hold most of the dissolved hydrogen even though those locations comprise fewer than 0.1% of all lattice sites. Four simplifications allowed practical use of the model: (1) Each component was described as a one-dimensional object. (2) The hydrogen initially dissolved in each component was described as a uniform concentration. (3) Accurate, consistent values were used to describe diffusion and recombination in stainless steel types 304 and 316 [Grant et al., J. Nucl. Mater. 149, 180 (1987); 152, 139 (1988)]. (4) Only one type of hydrogen trap was considered, and trapping was ignored in components made from vacuum remelted stainless steel. The simple model was developed and validated by comparing it to outgassing measurements. Traps were required to describe the outgassing from a component made of drawn stainless steel 304. The initial hydrogen concentration in that component was comparable to concentrations found elsewhere by thermal desorption and almost 100 times larger than in the components made of vacuum remelted 316 stainless steel. The model's usefulness was illustrated by using it to predict the outgassing of a vacuum chamber made of type 304 stainless steel. [

Investigations of medium-temperature heat treatments to achieve low outgassing rates in stainless steel ultrahigh vacuum chambers

“…Hydrogen atoms must recombine to desorb from the surface, and for large values of F 0 , the outgassing reduction may be limited by recombination and the presence of hydrogen traps. 16,28 The outgassing flux for our medium temperature bake is also about 1 order of magnitude larger than that achieved by Park et al . for a similar heat treatment.…”

Apparatus To Measure the Vapor Pressure of Slowly Decomposing Compounds from 1 Pa to 105 Pa

“…Figure 9 shows subsequent measurements of vapor pressure at 99° C. Following an evacuation and the short recovery from evaporative cooling, the pressure increased at less than 0.02 Pa h −1 , which was similar to the expected rate of hydrogen outgassing [21]. …”

“…A model based on the manifold materials and geometry( P calc )indicated that the initial concentration of atomic hydrogen in the sample tube was 22 mol m -3 . In contrast, the initial concentration in the remainder of the manifold, which was made of type 316 vacuum remelted stainless steel, was 100 times smaller[ 21]. …”

“…The pressure was measured while the manifold was cyclically evacuated for 12 hours and then closed for 12 hours to allow hydrogen to accumulate in the manifold. The calculated pressure P calc is discussed elsewhere [ 21]. …”

Thermal outgassing rates of low-carbon steels

“…The expected reduction suggests that H 2 outgassing from the low-carbon steels tested in this study might have been governed by a mechanism other than bulk diffusion, such as hydrogen "deep traps." 26,[28][29][30] Therefore, the very low H 2 outgassing rates of untreated low-carbon steels may occur because the bulk material contained very little dissolved hydrogen, i.e., mobile hydrogen atoms were fully degassed during manufacturing of the steels, which probably included a vacuum degassing step, such as the RH process.…”