Models for predicting hydrogen attack

High temperature hydrogen damage


  • Pressure parts, fabricated from low-alloy steels, operating in hydrogen service at elevated temperatures


  • The susceptibility of such components to hydrogen attack is currently determined by reference to the 'Nelson curves' (API RP541)
  • These material-specific curves provide a go / no-go criterion based on temperature and hydrogen partial pressure
  • For most materials they do not contain any time-dependence, and are thus subject to periodic revision based on accumulating service experience in the industry


  • To provide a secure basis for determining the susceptibility to hydrogen attack


  • Analysis of the current Nelson curves, and the underlying data where available, in terms of accepted models of solubility and diffusion
  • Calculation of theoretical curves based on solubility and diffusion models


  • A significant step towards understanding the basis of the Nelson curves
  • A provisional theoretical form of the Nelson curves with time dependence included
  • A reasonable correlation between the model and recent service experience of failures under conditions below the current published limits
Nelson curves for 1CrMo steel


Brear, J.M. and Church, J.M.
“Technical basis for API Publication RP941 (Nelson Curves)”
Third Int Conf ‘Engineering Structural Integrity Assessment’, Cambridge, September 1996

Plant Integrity