Investigation of hydrogen diffusion and accumulation in X70 pipeline steel

Abstract

Pipelines quickly, safely and economically transport large volumes of oil and gas compared to trains, trucks and ships. However, they occasionally fail leading to environmental pollution, fatalities and financial losses. Pipeline cracking by hydrogen embrittlement is the main cause of pipeline failure. The main focus of this research was on the effect of grain size and misorientation on hydrogen diffusion and accumulation in X70 pipeline steel.

In this investigation, the hydrogen permeation experiment was used to determine the parameters for hydrogen diffusion, and the hydrogen microprint technique was used to visualize the diffusion path in X70 steel. Hydrogen atoms were oxidized in the hydrogen permeation experiment, whereas they were reacted with an emulsion coating in the hydrogen microprint experiment. The samples for studying the effect of grain size on hydrogen diffusion and accumulation were taken from the mid-layer at the segregation zone and the top-layer of the first batch of X70-1 steel. The top-layer of another batch of steel, X70-2, was used to study the effect of misorientation on hydrogen diffusion and accumulation.

Hydrogen permeation experiment, in X70-1 steel, allowed in concluding that the permeability increased for larger grain sizes in both layers of steel. Also, the density of total, reversible, and irreversible hydrogen trapping sites of top-layer decreased for larger grains. However, the irreversible and total trapping sites of mid-layer showed an initial growth and subsequent decay with an increase in grain size. The hydrogen permeation experiment, in X70-2 steel, allowed in concluding that the permeability and effective diffusion coefficient decreased with an increase in grain misorientation. Also, the density of total and irreversible trapping sites increased with an increase in grain misorientation. The conclusions from hydrogen permeation experiments were validated with hydrogen microprint technique results. Grain boundaries, triple junctions and deformed grains in the steel microstructure are considered as reversible trapping sites due to their superior hydrogen diffusion in hydrogen microprint experiments. Also, preferential hydrogen diffusion through steel microstructure increased in the order of non-deformed grains, grain boundaries, inclusion interfaces, triple junctions, cementites and matrix-inclusion interfaces with matrix-inclusion interfaces being the easy path for diffusion. The analysis of hydrogen microprint technique results also allowed in concluding that the presence and absence of a circular pattern of superimposed white silver particles around inclusions and precipitates is a method to distinguish the type of hydrogen traps in materials.