3 minute read
WA Branch Technical Meeting - 8 November 2021
Source: Dr Gilles Dour (Principal Integrity Engineer, Advisan)
The Western Australian Branch recently held a technical evening focused on hydrogen embrittlement, transport and storage. The evening included an engaging presentation by Dr Gilles Dour, a multidisciplinary engineer with a research background in materials and process engineering, materials science and mechanical engineering. Hydrogen is currently a hot greenenergy topic. While most of the focus has been on its generation and storage, intermediate transport will be required to link these two operations. One potential avenue for transport will be existing pipelines. With this as background, Dr Dour shared the outcomes of the literature review undertaken by Advisian’s asset advisory team on hydrogen embrittlement in steels for pipelines used for hydrogen transport. The two main modes of hydrogen transport are low-pressure (~10 MPa, 1bar) reticulation of hydrogenenriched natural gas or LPG (typically less than 10% hydrogen) and high pressure (>60 MPa) transmission of hydrogen at high concentration. The main concern in transporting hydrogen is loss of containment, as mixtures of hydrogen and air are explosive over a very wide range of concentrations. Loss of containment may occur by leakage through joints, diffusion through the pipe wall, cracks resulting from hydrogen embrittlement of pipeline materials, and rupture through catastrophic crack propagation. In this context, Dr Dour summarised the factors that need to be considered in determining whether a pipeline is safe for hydrogen transport. Adsorption of hydrogen from dry gas required catalysis. With steels, this is provided through mobile surface dislocation, grain boundaries and existing cracks and notches. It can be slowed by thin (PVD or CVD) coatings of metals, such as tungsten or cobalt, in which the diffusivity of hydrogen is low. However, polymer coatings are ineffective because hydrogen can diffuse through them. Surface preparation (such as chemical polishing) can reduce adsorption, as can traces of oxygen and water vapour; hydrogen sulphide promotes adsorption. There are several laboratory methods of measuring adsorbed or nearsurface hydrogen, but only micro laser-induced breakdown spectroscopy is approaching practical use outside specialised facilities. Once adsorbed, hydrogen can diffuse through the steel. Diffusivity in ferrite is quite high; hydrogen can diffuse through a pipe wall in hours. In contrast, diffusivity in austenite is six orders of magnitude lower than in ferrite; it would take thousands of years to diffuse through the wall of an austenitic stainless steel pipe. Hydrogen embrittlement in a dry gas with high external load is different from the hydrogen cracking experienced with, for example, wet hydrogen sulphide. The hydrogen content is low (less than 0.1 ppm by weight) and embrittlement occurs through many varying mechanisms. This is because hydrogen is attracted to grain boundaries, precipitates, phase boundaries, voids and dislocations. Thus, hydrogen embrittlement is ultimately dependent on microstructure. This has a critical implication in specifying steels for hydrogen pipelines. It is not safe to specify a steel based on strength (such as API 5L X70); it is necessary also to specify the microstructure. Testing, using the dynamic punch test, shows no effect hydrogen partial pressures less than around 10-2 MPa (0.1 bar). The effects of hydrogen on steel properties increase with higher partial pressures up to around 10 MPa (100 bar), beyond which there is little further change. Dry hydrogen does not have much effect on yield or tensile strength, but has strong effects on elongation and notch sensitivity. To minimise the effects of hydrogen ferrite-pearlite and lower bainitic structures should be avoided, along with inclusion-formers such as sulphur and oxygen. Vanadium reduces susceptibility to hydrogen sensitivity. In light of this summary, it is evident why it is relatively straightforward to reticulate low-pressure gas containing less than 10% hydrogen in steel pipes. The main issue is ensuring that the network does not contain plastic piping, through which hydrogen readily diffuses. Regarding the design of pipelines for transport of hydrogen, Dr Dour identified several factors that are likely to be of greater concern, when compared to conventional pipeline design, for both onshore and offshore pipelines. Leak-before-break is a more critical failure criterion (as with pressure vessels); protection from dents and falling objects is also likely to be more critical. Full-pressurecycle fatigue, external corrosion and pressure containment are also likely to be of somewhat more concern.
