The proposed project addresses the call topic area of ‘low-carbon energy carrier roles in accelerating decarbonisaton pathways’ by characterising the condition of legacy gas (methane) pipeline steels after service exposure. Existing hydrogen embrittlement relationships do not properly consider the effect of steel pedigree on degradation. This research will use the results of detailed characterisation to fully document steel microstructure and rigorously track hydrogen interaction with specific features. The results will be directly relevant to assessing the suitability of the current network for hydrogen gas transport. Moreover, the learnings will inform the design and manufacture of future systems. Support involving the supply of suitable legacy specimens has been obtained from National Grid and EPRI will provide review and input on methods used and aid dissemination of findings.
Benefits
Detailed assessment of the materials microstructure to better inform our decisions on hydrogen injections into the NTS
Learnings
Outcomes
RESEARCH OVERVIEW
Electrochemical hydrogen charging studies, using Devanathan-Stachurski-based cells were carried out to determine hydrogen diffusivity, permeation, sub-surface concentration and trap site number densities. Membranes were extracted from different regions of baseplate and girth welds for two ex-service X60 methane pipes (1971 vintage base and girth weld; 1998 vintage base) and a 2006 X80 pipe and girth weld (not placed in service). Detailed microstructural quantification (grains, second phases and inclusions) were used to identify regions with a single dominant microstructure for charging studies in three orientations (through-thickness, longitudinal and hoop). These involved testing of smaller charging solution-exposed areas than is usually carried out.
KEY FINDINGS
Capillary effects allow qualitative but not quantitative trends between samples to be determined.
The legacy X60 grades (from 1971 and 1998) show higher non-metallic inclusion (mostly Sulphur rich MnS and CaS) contents that are aligned parallel to the pipe axis, whose interfaces with the ferritic matrix act as reversible traps and fast diffusion paths for hydrogen (Section 5).
Alignment of inclusions and high angle grain boundaries (HAGBs) parallel to the pipe axis causes transport of hydrogen in the longitudinal direction, invalidating the assumptions behind the derivation of most quantitative measures (diffusivity, permeation, sub-surface concentration and trap density) as transport is only 1D for membranes normal to the pipe axis, which are not commonly studied (Figure 13).
Orientation effects are strongest in the 1971 X60 (MnS stringers present) then the 1998 X60 (aligned, rounded inclusions and aligned HAGBs); directionality is still seen in X80 (weaker than in the X60 samples) due to aligned HAGBs (Tables 5 – 8).
Bainite in the X80 acts as a more irreversible trap in both baseplate and girth welds, whereas the inclusions in the 1971 X60 re-form in the girth weldmetal giving similar behaviour for basemetal and weldmetal (Section 6).
RESEARCH OVERVIEW
Micro-XRF derived chemical maps were used to identify composition variations through the baseplates, seam welds and girth welds of the three pipeline specimens. Large area EBSD supplemented this with spatial grain size and texture variations to identify microstructural regions to be quantified by optical and scanning electron microscopy, including EDS and automated inclusion analysis.
KEY FINDINGS
Variation in grain boundary length per unit area as a function of specimen, location and orientation
Variation in inclusion type, perimeter and size as a function of specimen, location and orientation.
Second phase type, size and perimeter as a function of specimen, location and orientation.
Likely effects on hydrogen interaction summarized.
Lessons Learnt
The following lessons learnt have been captured within this project:
- Availability of required resource (e.g. permeation testing capabilities) to be confirmed prior to project start. For example, the design, build and commissioning of bespoke hydrogen test facilities is a high-risk activity from a project delivery standpoint.
- Strong relationships are required at all stakeholder levels across projects that involve multiple parties – working level and management levels. This includes regular communication and governance protocols where risks and issues are clearly discussed, and solutions are developed together.
- Offers of support need to be communicated within these chains and escalated when needed.
- Industry experts to be utilised to confirm and inform decision making on test development.
- Decisions regarding cancellation of projects might want to be considered earlier within project delivery if it is clear that supplier might not be able to deliver the scope.
- Consideration might also be given to the number and proportional value of payment milestones. This could potential enable greater leverage over suppliers to deliver as per agreed scope.
