Environmentally assisted “in-bulk” steel degradation of long term service gas trunkline

Comparison of mechanical (characteristics of strength, reduction of area and elongation, impact strength, hardness, fracture toughness), corrosion-mechanical (stress corrosion cracking in the artificial brine and hydrogen embrittlement) properties and parameters of hydrogen behaviour in the X52 steel in as-received state and after long term service are presented in the paper. The analysis of a change of the mentioned characteristics together with the results of hydrogen permeation and vacuum hydrogen extraction measurements indicate considerable “in-bulk” material degradation of trunk pipeline steel after long term service and essential role of hydrogen in these processes. The measurements of the internal friction have shown the enhancement of grain boundary relaxation, which has been associated with the formation of defects at the grain boundaries. Two stages of the “in-bulk” degradation of steel in the course of its long term exploitation in the pipe lines have been postulated: (1) the deformation aging and (2) the formation and evolution of microdefects.