Role of hydrogen in operational degradation of pipeline steel

Abstract

Long-term operation of oil and gas pipeline steels causes a significant worsening of their mechanical properties, especially characteristics of brittle fracture resistance. General regularities of in-service degradation of pipeline steels are analysed in the paper. Basing on these, two stages of pipeline steel degradation are distinguished: deformation aging and in-bulk steel dissipated microdamaging. Hydrogenation intensifies the damaging process due to hydrogen induced microcracking of steel and, in this way, facilitates its operational degradation. In-service damaging intensified by hydrogen can cause untypical mechanical behaviour of a metal: a) a decrease in strength and hardness is accompanied by reducing brittle fracture resistance; b) an increase in elongation simultaneously with a decrease in reduction in area. Some examples of the destructive role of hydrogen in-bulk material degradation are considered, when transported hydrocarbons serve as a source of hydrogen. It is shown that investigations of hydrogen behaviour in steels by hydrogen extraction and permeation methods provide valuable information for assessment of in-service dissipated damaging degree. In particular, a decrease of brittle fracture resistance is in a good agreement with hydrogen concentration in metal. Special attention is paid to the role of hydrogen in damaging rolled steels with texture during their operation. The main mechanism of microdamaging for textured steels is suggested to be the accumulation of hydrogen at the interface “elongated non-metallic inclusion–matrix” creating high pressure in defects with the following microdelamination along the rolling direction. This leads also to anisotropy of characteristics of brittle fracture resistance of rolled steels, which becomes more pronounced with operation time.