Nicola Faraone, an International Welding Engineer for voestalpine Böhler Welding with a focus on welding consumables for the power and process industries, introduces the company’s recently developed welding solutions for hydrogen storage and transportation.
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In a world where energy consumption is projected to grow, there is an urgent need to reduce CO2 emissions drastically, as renewable energies take over to meet energy targets. Wind and solar energy have been shown to be reliable options for producing CO2- free energy. The main drawback of these is that wind and sunshine are not constantly available. This in turn causes issues during peak times when demand on the grid is high. There is also a risk of wasting energy when production exceeds demand.
Hydrogen offers options for stabilising energy production for renewables. The gas is well known as a fuel and a feedstock, but it is also projected to become the most popular energy carrier in an integrated cycle connected to the energy produced from renewable sources.
We will therefore need pipelines and tankers for hydrogen transportation, as well as tanks for storage. Hydrogen can be transported and stored in gaseous or liquid state, bringing different challenges for the choice of materials used. voestalpine Böhler Welding is developing and proving a suitable range of welding consumables to meet the fabrication requirements of this future challenge.
Hydrogen production
Hydrogen is the most abundant element and the smallest molecule in the universe. It can be utilised as a fuel in tur- bines or in fuel cells, and is a fundamental feedstock for the petrochemical and urea industries. The main technologies to produce hydrogen include: Methane reforming; coal gasification; and the electrolysis of water.
Depending on the production technology and source, the hydrogen produced is labelled using different colours. Grey hydrogen is not climate-neutral, since coal and natural gas are used as the raw materials for production, which results in significant CO2 emissions. Blue hydrogen is a more CO2-neutral way of producing grey hydrogen, because the CO2 generated during production is separately captured and permanently stored; while green hydrogen is produced from renewable energy using electrolysis, so is 100% CO2-neutral with respect to the fuel used and the process. Of these, green hydrogen is the one pushing the development of renewable energy use and electrolysers, while blue hydrogen is considered to be an important part of the transition to 100% green hydrogen.
Arc welding processes for the hydrogen transportation and storage components
The main welded components for hydrogen transportation and storage are pipelines, storage tanks, carriers, trailers, vessels, etc. These components are generally well-known, with the welding processes involved being the same as those we already see used in industries such as Oil & Gas and petrochemicals.
In particular, the following processes are mainly used:
- GTAW, mainly for root passes and filling passes for low thickness components.
- GMAW for filling passes.
- SMAW, most notably, for the pipelines.
- FCAW for high productivity and out-of-position welding.
- SAW for heavy-walled vessels for CGH2 (compressed gas hydrogen) storage and for the manufacturing of longitudinal seams of welded pipe.
In the welding industry – apart from some small additions to the shielding gases for GTAW and GMAW processes and in the rutile and cellulosic flux covered electrodes – hydrogen is rarely used. The reason is that hydrogen is often detrimental to weld joints, creating serious defects, most notably, cracks.
Hydrogen can enter a weld pool through the residual humidity in the coverings of electrodes or fluxes, but there are also potential risks – especially in the O&G industry – associated with the presence of hydrogen in process streams. Sulphide stress corrosion cracking (SSCC) and high temperature hydrogen attack (HTHA), for example, are known causes of failures in the Oil & Gas industry.
Defects caused by the hydrogen in the weld joints include worm holes, porosity, fish eyes, hydrogen-assisted cold cracking (HACC), hydrogen-assisted cracking (HAC), and hydrogen induced cracking (HIC). These are all undesirable phenomena associated with residual quantities of hydrogen present in the weld metal. The future challenge for construction in the hydrogen industry is to guarantee safe service conditions in 100 % hydrogen environments, including some potential residual detrimental elements, such as the electrolytes. In particular, the main task for the materials and welding engineers will be the assessment of the potential for hydrogen embrittlement on steels being used.
Hydrogen molecules can attack the surface of the steel by absorption, separate into atomic hydrogen by dissociation, and migrate as hydrogen atoms into the steel. There, the hydrogen atoms may react with metallic materials resulting in specific issues:
1. Hydrogen embrittlement: the absorption of the hydrogen atoms into the steel with the direct consequence of reducing the ductility and toughness of the steel. In general, the susceptibility to hydrogen embrittlement increases as the material strength increases.
2. Property changes at low temperatures: tensile properties of austenitic stainless steels increase at sub-zero temperatures, while elongation and impact properties reduce.
The role of the ammonia in the hydrogen economy and related welding challenges
Hydrogen is an important feedstock in the fertiliser industry and hydrogen used for ammonia production is often defined as green ammonia (GNH3). Green ammonia can play an important role in the hydrogen industry, not only as a feedstock for the chemical industry, but also as the energy carrier for hydrogen transportation.
In green ammonia projects, the core welded components, in addition to the ones for the process equipment for the production of ammonium carbamate – [NH4+][H2NCO2-] – are the pressure vessels used to store the H2 produced by electrolysis and renewable energy (wind or solar).
The big challenge for the materials and welding engineers is to guarantee safe compressed gas hydrogen storage at high pressure (up to 200 bar). Moreover, in order to reduce the wall thicknesses for the vessels and to increase the operating pressure, high strength steels may be selected, with a higher potential risk of hydrogen embrittlement compared to common steels.
Hydrogen forms explosive mixtures at concentrations of 4 to 74% and the use of ammonia as an intermediate energy vector reduces this potential risk. After transportation, the ammonia can be transformed back into H2 before use, or it can be used directly as feedstock as well as fuel in turbines to produce CO2-free electricity.
Welding engineers also need to find optimal solutions for ammonia storage tanks and carriers, while considering the potential risks of stress corrosion cracking related to condensed ammonia in the anhydrous state. For this reason, materials with ultimate tensile strengths of a maximum of 485 MPa (70KSi) are often selected and need to be carefully welded with appropriately developed filler materials in order to control tensile and hardness properties in the weld joint.
Constant load testing on Böhler Welding filler materials
The susceptibility of welded components such as pipes and tanks to hydrogen embrittlement can be assessed using different tests. These include, amongst others: Constant load tests in accordance with ISO 16573 Part 1; Slow strain tests in accordance with ISO 16573 Part 2; as wells as Fracture mechanics tests; Small punch tests; Permeation tests; and Dynamic tests.
In order to verify the hydrogen embrittlement resistance of filler materials developed for the construction of pressure vessels for gas hydrogen storage, voestalpine Böhler Welding performs constant load test on a selection of welding products.
The constant load test is performed under 100 % hydrogen, which makes it more representative of the operating condition than other tests designed to demonstrate HIC (hydrogen-induced cracking) resistance in H2S service (EN 10229 or NACE TM 02/84). A selection of filler materials for the main processes used in pressure vessel and pipeline construction have been tested in order to verify the resistance to hydrogen embrittlement. The constant load test enables estimates of the maximum diffusible hydrogen content to be determined at which a material does not fail due to hydrogen embrittlement under a constant load.
The GTAW (TIG) process was not included in this testing campaign since it is used mainly for root pass/tack welding under high dilution conditions that do not represent all-weld-metal for the constant load test. In addition, FCAW has been preferred to GMAW because of the better usability, especially for out-of-position welding.
The mechanical properties of the selected filler materials, following usual post-weld heat treatment, match the requirements of the carbon steel that is permitted for selection for this application (P355 NL1, for example. See Table 4). The higher the temperature and the pressure, the tougher the testing condition.
No fractures on any of the all-weld metal specimens were observed under dry or wet conditions. The results confirm the low tendency to hydrogen embrittlement under the H2-gas environment of these Böhler Welding products.
Böhler welding filler materials for liquid hydrogen applications
Similar to natural gas, gaseous hydrogen can be liquefied by cooling at cryogenic temperature. For hydrogen, the liquefaction temperature is -253 °C. In its liquid state, hydrogen can be stored and transported in tanks that require a lower volume compared to the gaseous state. This is a very important property when the hydrogen cannot be transported using pipelines (overseas, for example).
The metallic materials for the liquid hydrogen tank manufacturing must be carefully selected, considering the operating temperature of below -253 °C. A typical choice for this application is stainless steel, due to its good toughness properties given by an austenitic structure at sub-zero temperatures.
voestalpine Böhler Welding has a strong heritage in the production of stainless steel filler materials and, in particular, for critical applications at cryogenic temperatures. A comprehensive portfolio of controlled-ferrite products is available and we are able to guarantee the requested impact properties for liquefied natural gas applications.
In the ASME BPV Code, Section VIII Div.1, part UHA-51 defines the rules for impact testing heat affected zones and base metals, depending on the MDMT (minimum design metal temperatures) for pressure vessels constructed from high alloy steels. The typical requirement for the weld metal is 0.38 mm of lateral expansion at -196 °C.
Also available and well established in the market is a welding consumables portfolio that guarantees outstanding properties at cryogenic temperatures even lower than -196 °C, under the product names Böhler ASN 5 and Thermanit 18/17 E Mn.
When the minimum design metal temperature (MDMT) is colder than –196 °C (-320 °F), as for liquid hydrogen applications where the MDMT is below -253 °C, UHA-51 sets further rules for permissible welding processes (SMAW, FCAW, GMAW, SAW, PAW and GTAW) and for toughness testing – for both PQR qualifications and filler materials pre-use testing. Specific requirements for the ferrite content and impact properties – such as 0.53 mm of lateral expansion at -196 °C, for example – are defined when type 308L and 316L filler metals are welded using GTAW, GMAW and FCAW process, and voestalpine Bohler Welding is further improving this portfolio in order to meet these ASME requirements.
Conclusions
Thanks to multi-purpose use as a fuel, feedstock and energy carrier, hydrogen will play a primary role in the reduction of our carbon footprint, and new investments in hydrogen terminals, pipelines and carriers will be necessary to support this economy.
voestalpine Böhler Welding is actively working to support its partners on this path to the reduction of emissions, investing in new product developments and testing in order to be one step ahead in this emerging market.
