Green Transition Affects Materials Selection and Maintenance of Process Equipment
The green transition challenges the status quo in many societal and industrial activities. Materials are a key enabler of new carbon-neutral technologies. The results of the MASCOT project will provide a basis for materials selection and maintenance activities for the emerging industrial processes.
The goal of the transition to a sustainable carbon-neutral society is underpinned by the hydrogen and circular economy in which new technologies are a cross-cutting theme. The structural materials of investments, such as process equipment, pipelines and raw material storage facilities, are enablers of new technologies and must operate reliably and predictably under new operating conditions. Therefore, understanding the behaviour of structural materials in new environments is essential and the starting point for the right material choices. The maintenance of investments must also be planned based on an understanding of the phenomena.
Hydrogen poses a wide range of demands
Hydrogen is a key piece of the energy puzzle. It can be used for energy storage, for example, to smooth price fluctuations, as fuel for as an energy source, e.g., fuel, and as a raw material for the chemical industry. Hence, it is related to the entire value chain: hydrogen production, storage and transport, and end-use applications.
From a materials selection perspective, there are important issues related to hydrogen storage and transport. The form in which hydrogen is stored and transported (gaseous, liquid, derivative or other chemical) affects the operating conditions that will be needed, such as temperature (liquid hydrogen has a boiling point of -253°C), pressure and possible corrosion resistance requirements.
It should also be noted that not only the materials of the tanks, pressure vessels and piping, but also other components, such as valves and seals, must perform reliably. Since hydrogen affects different materials in different ways, materials selection is an essential part of the hydrogen transfer process.
In the circular economy, the recovery rate is key
In the circular economy, the aim is to close raw material loops and increase value. Circular economy principles include the use of side streams as raw materials, extending product life, reuse, repair and finally recycling of the raw material.
From a circular economy process perspective, the recovery rate is a key parameter, so process conditions are optimised to allow efficient recovery of raw materials. This can mean more aggressive conditions, which in turn require materials used in the process equipment to resist harsh corrosive conditions or even the combined contribution by corrosion and wear.
From a process equipment perspective, circular economy may mean increasing variation in the used feedstock: the use of new types of raw materials, variations in the quality of material flows, and the potential for contaminants to enrich in the cycles. Processes must also operate reliably within these constraints, which must be taken into account when choosing equipment materials.
Assessing suitability requires a new set of capabilities
Hydrogen exists at room temperature as a gas with a very small molecular size. As a result, hydrogen is prone to leakage at the joints of structures. Hydrogen also penetrates easily into materials, especially if there are stress concentrations in the structure. It migrates by diffusion and in some cases significantly degrades the mechanical properties of the material; this is known as hydrogen embrittlement. The potential hydrogen embrittlement susceptibility of materials used in load-bearing structures and applications should therefore be carefully investigated before final materials selection.
Research on surfaces and interfaces supports the green transition.
As hydrogen is a highly flammable and highly combustible gas, safety considerations must always be taken into account when handling hydrogen. From an experimental research perspective, these properties of hydrogen require the test equipment to be tailored to the hydrogen environment, typically using pressure vessels allowing elevated gas pressures. The test procedures are planned to meet the requirement to perform in-situ measurements in hydrogen. Research must also emphasise a safety-first approach. Capabilities to analyse hydrogen, the smallest known atom and molecule, are also needed.
The MASCOT project is working together to find solutions
Figure 1: Fatigue testing equipment assembled and ready for operation. The load ring remains inside the autoclave.
VTT has a long history of research into the environmentally-assisted cracking of materials. Decades of phenomenological expertise combined with the capabilities of modern research equipment are prerequisites for generating new knowledge, evaluating material solutions catalysing and generating new innovations. This combination of capabilities has been successfully applied in the MAterialS for CO2-neutral processes in resource-intensive industries (MASCOT) project, funded by Business Finland and Andritz Oy, Exote Oy, Neste Oyj, Nordic Tank Oy, Metso Oyj and Wärtsilä Finland Oy. This project, coordinated by VTT, also involves the University of Oulu as a research partner.
Process operating conditions are becoming more challenging and complex.
The project has investigated, among other things, the fatigue behaviour of materials potentially suitable for hydrogen transport and storage in high-pressure hydrogen, using a globally unique double bellows loading device operating in pressurised gas, Figure 1. The fatigue life assessment of materials provides a basis for determining, for example, maintenance cycles. In addition to fatigue tests, the project has developed practices for, among other things, the examination and even automation of fracture surfaces (Figure 2a) and sampling to analyse their hydrogen content.
Research on surfaces and interfaces supports the green transition
As process conditions become more challenging and complex, research on surfaces and interfaces lays the foundations for understanding the interactions between materials and their environments.
Image 2 a) Scanning electron micrograph. Fatigue fracture surface of austenitic stainless steel in high pressure (100 bar) gaseous hydrogen. Image by Aloshious Lambai.
In recent years, VTT has invested heavily in up-to-date, high-quality research equipment to support this theme. For example, in 2023, VTT received a new plasma-FIB (focussed ion beam) FE-SEM (field emission scanning electron microscope), which enables more efficient and reliable examination of interfaces in materials and structures.
VTT is also involved in the Academy of Finland-funded research infrastructure H2MIRI (Hub for Hydrogen-Materials Interaction Research Infrastructures), which strengthens research capabilities in areas such as time-of-flight secondary ion mass spectroscopy (TOF-SIMS) and hydrogen tribology. Tribology refers to the wear, friction and lubrication between surfaces, and hydrogen tribometer, i.e., tribometer operating in the hydrogen atmosphere, will provide new insights into these surface phenomena in the presence of hydrogen.
Contaminants may be enriched in the process environment
Image 2 b) Scanning electron microscope image. Pitting in duplex stainless steel (pH 4.0, [Cl-] 5000 mg/L, T=90°C). Image by Andressa Trentin.
Surface and interface phenomena have also been a cross-cutting theme in the MASCOT project. In circular economy processes, contaminants, such as halides, can be enriched in the process environment and affect introduce corrosion of structural materials.
Figure 2b shows a cross-sectional FE-SEM image of chloride-induced pitting corrosion in a steel, in which attack has progressed beneath a lacy cover under polarization. The lacy cover prevents solution exchange within the pit, resulting in very aggressive local conditions that maintain the damage to the material. It is therefore important to understand the conditions under which pitting corrosion of different alloys occurs and to try to avoid these situations through materials selection or environmental adjustment.
Figure 3 a) Experimental set-up for high-temperature corrosion experiments under green transition conditions.
Similarly, in ammonia-rich conditions at elevated temperatures (Figure 3a), for example in engines, the mechanisms of surface layer formation on material surfaces need to be understood and how they can potentially be influenced. The MASCOT project has also sought to provide a knowledge base on this topic.
In addition to solid materials, new operating environments, such as ammonia and hydrogen, also have a potential impact on the lubricants used between contacting surfaces. In the MASCOT project, these changes in the tribological behaviour and frictional properties of aged lubricants have been investigated in a controlled manner using a Mini Traction Machine (MTM) to measure friction between lubricated surfaces (Figure 3b).
Figure 3 b) MTM apparatus for determining the friction between lubricated surfaces. Photo by Artur Korostavyi.
One of the objectives of the MASCOT project has therefore been to increase the material knowledge necessary to enable a sustainable and safe green transition.
Materials research provides a framework for the maintenance
The purpose of maintenance is to ensure the reliability, efficiency and long life of machinery, equipment and investments, and to guarantee the quality of products.
Materials science provides an excellent framework for predictive maintenance through understanding the mechanisms and kinetics of ageing and deterioration of materials and through prevention strategies. Indeed, the best way to achieve carbon neutrality goals is to combine the design of maintenance activities for green transition technologies with the knowledge of science.
Left to right Vilma Ratia-Hanby, Research Team Leader VTT, Elina Huttunen-Saarivirta, Research Professor VTT, Pekka Pohjanne, Lead VTT
Text: Elina Huttunen-Saarivirta, Vilma Ratia-Hanby, Pekka Pohjanne,
VTT Technical Research Centre of Finland Ltd.
Photos: VTT, Shutterstock
