Hydrogen Trapping in X70 Structural Pipeline Steel and Weldments

Sezgin, Jean-Gabriel (AIST-Kyushu University Hydrogen Materials Laboratory (HydroMate)) | Fjær, Hallvard G. (Institute for Energy Technology) | Matsunaga, Hisao (Kyushu University) | Yamabe, Junichiro (AIST-Kyushu University Hydrogen Materials Laboratory (HydroMate), Kyushu University) | Olden, Vigdis (SINTEF)

OnePetro 

ABSTRACT

Diffusion measurements and thermal desorption measurements have been performed on a X70 pipeline steel, in as received and normalized and quenched condition, after being hydrogen charged for 200 h at 100 MPa hydrogen gas pressure and 85°C. Numerical simulations based on the assumption of thermodynamic equilibrium were performed, aiming to compare the trapping energies when fitting to the TDS spectra. TDS experiments revealed a reversible trap site with activation energy of 26.2 kJ.mol−1. Irreversible trap sites with an activation energy of >100 kJ.mol−1 were observed from both as-received and heat-treated condition. In contrast, a reversible trap site with an activation energy of 49.2 kJ.mol−1 was observed only from the heat-treated condition. The numerical modelling based on the assumption of equilibrium between hydrogen in traps and hydrogen in lattice is seen to provide a good fit to the experimental data.

INTRODUCTION

Subsea oil and gas structural steel pipelines are exposed to hydrogen at the steel surface due to cathodic protection towards corrosion. Hydrogen reduces the fracture toughness in the base metal as well as in welded joints, which may be critical for the structural integrity of the pipeline. As atomic hydrogen enters the steel it occupies lattice sites and traps, as dislocations, grain boundaries and precipitates, often categorized as reversible and irreversible traps according to their trapping energy.

To be able to build predictive models for the fracture susceptibility of pipelines under operation conditions, knowledge of the amount of diffusible hydrogen and trapped hydrogen that may contribute to fracture, are vital information. Thus, knowledge of the trapping energies is essential.

In the present work, results from Thermal Desorption Spectrometry (TDS) measurements of X70 structural steel will be presented. As-received steel and heat-treated (normalized and quenched) steel, representative of the coarse grain heat affected zone of a welded joint, are investigated. Finally, the measured trapping energies, diffusivity and hydrogen concentration are discussed and compared to a numerical model, where the trapping energies are assessed.