Evolution of hydrogen from AISI 200 stainless steel: A study to determine whether it is a diffusion or recombination limited process and experimental evidence for strongly bound hydrogen

The hydrogen outgassing rate from three equivalent UHV chambers made of 0.5-mm-thick AISI 204 stainless steel was measured in situ over several 10-h cycles at 150 °C, 200 °C and 250 °C. After each cycle, the specific outgassing rate at room temperature was determined using a spinning rotor gauge. A parallel analysis based on thermal desorption spectroscopy, extraction melting and numerical simulation of the recombination limited process reveals a more consistent picture of the outgassing process compared with models given in old vacuum textbooks. At the applied moderate temperatures, the majority of the diffusible hydrogen is released within less than 100 h. Consequently, the specific outgassing rate at room temperature was reduced to the level of q ∼ 1 × 10−13 mbar L/cm2s. At temperatures above 400 °C, the residual hydrogen is activated and is depleted very slowly. It seems that it represents an inner source of hydrogen that is manifested by a stable outgassing rate at moderate temperatures and on the long-term scale at room temperature.