Models for predicting hydrogen attack

High temperature hydrogen damage

Component:

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

Situation:

  • 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

Objective:

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

Approach:

  • 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

Benefits:

  • 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

Publication:

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