Corrosion fatigue of high-strength steel in low-pressure hydrogen gas

Fatigue tests on 835M30 steel in hydrogen gas at atmospheric pressure have confirmed that two distinct forms of corrosion fatigue crack growth can occur. The dominant mechanism of corrosion fatigue is governed by whether the cyclic stress intensity level is above or below the threshold stress intensity value for hydrogen-induced crack growth under static load, and by the stress ratio and cyclic frequency employed during testing. It has been shown that existing mechanistic models are unable to account quantitatively for the observed corrosion fatigue behaviour at stress intensity levels below the static load threshold. However, a model designated the ‘Process Competition Model’ can be used for quantitative predictions of corrosion fatigue behaviour at stress intensity levels above the static load threshold. An extended version of the model is also described in which the effects of cyclic waveform are incorporated. The translation of crack propagation rates to cyclic lifetimes is discussed and the effects of corrosion fatigue on S/N curves is emphasized. The direction of future research on quantitative models of corrosion fatigue crack growth is discussed.