The Influence on Hydrogen Permeation through Steel of Surface Oxide Layers and Their Characterisation Using Nuclear Reactions

The rate of hydrogen permeation through metals is important in many technological applications, not least in the nuclear industry. Permeation measurements at elevated temperature, made under conditions likely to be found in industry, have shown that oxide layers which form on the surface of steels can sometimes reduce permeation drastically. Measurements have been made on a range of steels, including AISI 321, AISI 316L, Fecralloy and 9% Cr/1% Mo, at temperatures from 600°C up to 1100°C and the oxide layers produced have been investigated using a range of nuclear reactions. Oxygen content has been measured using the 160(d,p) reaction and, in the case of Fecralloy, the thickness of Al2O3 measured using the 27Al(p,γ) reaction. Rutherford backscattering of alpha particles has also been used to investigate the behaviour of the surface layers, particularly for those alloys containing additives. Results show that at the lower temperatures investigated the oxide layers are generally mixed, while at higher temperatures they are largely Cr2O3, or Al2O3 in the case of Fecralloy. The addition of small amounts of rare earth tends to stabilise the oxide and prevent cracking and spalling. This higher degree of stability is found to decrease hydrogen permeation by up to four orders of magnitude.