There are aspects of hydrogen in steels that are crystal clear. For example, that hydrogen in solid solution embrittles, does not require further work. The mechanism by which it embrittles is far from clear, and there is no generic theory capable of rationalising the vast quantities of data available in the published literature. When combined with the fact that practical alloys have complex mixtures of phases and defects, the confusion can be said to be complete.
However, all is not lost because measures can in principle be taken to limit the potency of hydrogen. I will begin with an introductory review of the subject, mention specific examples where significant performance enhancements have been observed, and deal with the theory and new experimental data on the passage of hydrogen through complicated mixtures of phases.