Turnbull, A; Wright, L (2014) Hydrogen permeation modelling with generalised boundary conditions at the charging surface. NPL Report. MAT 69

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Abstract

Diffusion and trapping of hydrogen in a low alloy steel has been modelled with the distinctive feature of generalised boundary conditions at the charging surface to replace the constant concentration or constant flux boundary conditions commonly adopted. Rates of hydrogen charging, chemical and electrochemical recombination of adsorbed hydrogen atoms, and absorption and desorption are incorporated. The adoption of such boundary conditions allows theoretical demonstration of the effect of specimen thickness on hydrogen permeation flux and sub-surface hydrogen concentration, and an assessment of the extent to which changing the charging current density can alter the balance between diffusion control and surface reaction control of hydrogen diffusion. As expected conceptually, a lower sub-surface hydrogen concentration is predicted for thin specimens compared with thick specimens and the suggestion that data from the former is more relevant to sulphide stress corrosion cracking is questioned. Partial surface reaction control of transport manifests itself in a reduced slope of the permeation flux-time curve relative to the prediction for thick specimens. For testing where the thickness of the specimen is limited, increasing the charging current density would be more likely to establish diffusion control transport.

The steady-state sub-surface hydrogen concentration is shown experimentally for steel to obey a square root dependence on charging current density but to attain a limiting value at higher current density. A theoretical explanation is proposed based on electrochemical recombination becoming the dominant recombination reaction as the applied current increases. There is some indication for controlled cathodic charging experiments that the effect on hydrogen uptake of poisons such as H2S and arsenic may relate to their influence on the electrochemical recombination step rather than the chemical recombination step.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MAT 69
Keywords:	hydrogen, diffusion, metals
Subjects:	Advanced Materials Advanced Materials > Corrosion
Last Modified:	02 Feb 2018 13:13
URI:	http://eprintspublications.npl.co.uk/id/eprint/6253

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