Permeation studies of hydrogen in 304 and 316 stainless steel: the influence of surface oxides

A time varying harmonically modulated flow method, analysed with a two rate constant surface reaction model, provides a series of permeation, Pm, diffusion, D, and surface rate coefficients, k1, k2, for hydrogen passing through 304 and 316 stainless steel. The method developed here allows the evaluation of a consistent set of bulk metal values of D and Pm, in the temperature range 502 < T/K < 963, despite wide ranging surface conditions, and are described by: D304=(1.22±0.06)x10-8exp{[-(6.596±0.049)x103/T]/K-1} m2s-1 Pm304=(4.82±0.21)x10-7exp{[-(7.990±0.044)x103/T]/K-1} molm 1s-1Pa-1/2 D316=(7.28±0.94)x10-7exp{[-(6.296±0.109)x103/T]/K-1} m2s-1 Pm316=(8.09±0.70)x10-7exp{[-(8.189±0.076)x103/T]/K-1} molm-1s-1Pa-1/2 When surfaces are treated to form a stable oxide layer, the surface reaction rate constants are found to be interpretable in terms of molecular flow of hydrogen through the oxide. The permeation coefficients, Pmox, describing this flow are as follows: Pmox3o4=(1.43±0.29)x10-11exp([-7.661±0.081)x103/T)/K-1) molm-1s-1Pa-1 Pmox3ls=(5.36±2.68)x10-11exp{[-6.967±0.228)x103/T)/K-1) molm-1s-1Pa-1