The paper of Prof.Brian P. Somerday, Prof.Petros Sofronis, Prof.Reiner Kirchheim (Hydrogen Materials Compatibility Research Division, I2CNER), K.A. Nibur, C. San Marchi was published in  “Acta Materialia“.

Elucidating the variables affecting accelerated fatigue crack growth of steels in hydrogen gas with low oxygen concentrations

Summary

Fatigue is a degradation mechanism demonstrated by crack propagation when materials are subjected to cyclic loading.  In the presence of gaseous hydrogen, this crack propagation is accelerated for reasons we still do not understand.  In this work we found out that by adding a few molecules of oxygen in the hydrogen gas, we can suppress the hydrogen induced acceleration of fatigue.  Most importantly we devised a compact formula governing the mitigation of the hydrogen effect in terms of the material, environmental, and loading parameters.

■Background information

The transition to a hydrogen-based economy faces many challenges in terms of production, delivery to end-user stations, storage, and energy generation  Hydrogen embrittlement is a severe environmental type of failure that can cause a sudden and catastrophic failure of materials in contact with hydrogen under normally safe working loads.  In the case of hydrogen-accelerated fatigue failure, we do not understand how relatively low pressures of hydrogen can markedly degrade the material resistance to cyclic loading.  In general, assessing component lifetime, developing mitigation or remediation strategies, or designing smart structural materials for employment in a hydrogen environment is one of the main goals of I2CNER.

■Content

In the present work, an international team of I2CNER researchers from California, Germany, and Illinois found out that a few molecules of oxygen per million molecules of hydrogen can eliminate the hydrogen degradation effect altogether.  To reach this conclusion the team applied and fused experimental and analytical techniques from the disciplines of materials science, materials physics, and applied mechanics.  The reason for this mitigation effect is that oxygen is preferentially adsorbed on the surface of a microstructural defect (crack) and as a result, the created surface oxide prevents hydrogen from entering the material and in turn degrade the material resistance to cyclic loading.

■Benefit or positive impact

A very important outcome of this work is the development of a simple formula that describes the amount of oxygen needed to mitigate the hydrogen effect under give hydrogen pressure, frequency and magnitude of loading, and material strength.  In fact, it is for the first time that engineers are given a single formula that describes the hydrogen effect on fatigue acceleration in terms of all parameters that govern this complex phenomenon of fatigue crack growth.

■Future plan

To examine the effect of other unsaturated chemical compounds such as CO on crack propagation and their mitigation potency.  In addition, we plan to use first principles calculations to understand why hydrogen cannot enter the material in the presence of an oxygen.