Multiscale Hydrogen Embrittlement Assessment for Subsea Conditions

While Norwegian continental shelf calls for development of new material solutions to handle harsher conditions, or development of subsea gas processing systems, the material integrity in the intended conditions must be secured - including the effects of hydrogen exposure. Hydrogen embrittlement is a long-standing challenge for the subsea industry and a critical failure mode that must be considered in life extension strategy. The current guideline for assessing hydrogen embrittlement of subsea components is limited to duplex stainless steels. A practical guideline to quantitatively address hydrogen embrittlement of other subsea material groups is missing. Although significant efforts have been made to understand hydrogen embrittlement, todays theories still offer a qualitative and binary treatment of hydrogen embrittlement. For industrial applications, a quantitative prediction is needed. In the period from 2019 to 2020, the PhD students in the project have been mainly focusing on finishing the compulsory course trainings and doing literature studies. Efforts have been spent on understanding the limitations and advantages of the existing methods for measuring the hydrogen diffusion. During the period from 2020 to 2021, significant progresses have been made in the three work packages. The hydrogen diffusion behaviour of nickel alloy 625 has been studied. A fit for purpose electrochemical hydrogen charging cell with industry relevant charging conditions was designed and a special permeation cell for studying hydrogen properties in Alloy 625 was built. An important relationship between hydrogen diffusivity and temperature as well the de-trapping activation energies have been obtained. The hydrogen induced mechanical property degradation of the Alloy 625 has been studied by using the multiscale experiments, including in situ electrochemical nanoindentation and macroscale tensile tests. The tensile test result shows that alloy 625 is highly susceptible to hydrogen embrittlement. Atomistic simulations have advanced the understanding of the hydrogen embrittlement mechanisms. Molecular dynamics simulations show that when the hydrogen concentration is low, the fracture mode is transgranular. When the hydrogen concentration increases, vacancies will accumulate at the grain boundary. The accumulated vacancies will nucleate nanovoids which eventually will lead to grain boundary de-cohesion and intergranular fracture. The last year of the project is a harvesting year. The main outcome of the project in this year can be summarized as follows: we have provided a hydrogen diffusion equation and gained deep understanding regarding the effect of various parameters including temperature, cathodic potential, current density on the hydrogen diffusion in the alloy 625. The proposed hydrogen diffusion equation can directly be used to estimate the hydrogen penetration depth of Alloy 625 under a certain temperature. It was experimentally shown that the hydrogen embrittlement index (reduction in elongation) increases with increase in pre-strain value. Both the experimental and atomistic simulation studies have confirmed that hydrogen-induced vacancy formation at the grain boundary is a precursor for intergranular fracture. A unique void-based model was proposed to assess the hydrogen-induced ductile to intergranular transition. The proposed hydrogen diffusion model, new vacancy-void based mechanism as well as the proposed predictive model provide an excellent starting point for a physics-based hydrogen embrittlement assessment framework. In order to disseminate the scientific results, an international symposium focusing on hydrogen embrittlement in nickel based super alloys was organized in December 2022. There were 50 participants in total and representatives from ten Norwegian companies in the oil and gas industry attended the symposium. The main conclusion from the symposium is that all nickel alloys are susceptible to hydrogen embrittlement. The microstructure and grain boundary precipitates play a chief role in hydrogen embrittlement of nickel alloys. The project had advanced our understanding regarding the hydrogen diffusion and mechanisms as well as predictive models for alloy 625. However, a reliable testing method which can provide the needed model inputs is missing. Further research should be carried out to increase our knowledge about the brittle microstructures and build a direct link in the predictive model between the microstructures and the mechanisms in order to establish a practical guideline for the industry. To improve the understanding of the existing precipitation hardened nickel alloys and to search for and study new alternative alloys which have better resistance to hydrogen was identified as the future research tasks.

The project was focused on the nickel alloy 625 used for subsea application. By the end of the project period, we have achieved the following: - A world-leading voiding mechanism-based predictive framework for hydrogen embrittlement assessment - A tailor-made hydrogen coupled complete Gurson model for nickel alloys used in subsea - Accurate description of the temperature and plastic strain dependent hydrogen diffusion which provides the essential data for the life extension of the subsea components made of the alloy - A predictive framework developed can be readily applied to the renewable energy systems - Together a reliable testing method which generates the input data for the models, the predictive framework can be used to optimize the material solutions.

Hydrogen embrittlement is a long standing challenge for the subsea industry and a critical failure mode that must be considered in life extension strategy. The current framework for assessing hydrogen embrittlement of subsea components is limited to duplex stainless steels. A practical framework to quantitatively address hydrogen embrittlement of other subsea material groups is missing. This project aims to enable prediction of hydrogen embrittlement when designing subsea components and to evaluate hydrogen embrittlement sensitivity of materials. This will be accomplished by developing and implementing a new multiscale assessment framework for hydrogen embrittlement. The proposed solution represents an efficient and accurate methodology to address hydrogen embrittlement. It provides quantitative methods to reduce and manage uncertainty and thereby risk. With reduced risk, the safety factors can be optimized to allow more cost effective solutions through savings in material use, project execution and product development. Significant efforts have been made to understand hydrogen embrittlement. Current theories offer a qualitative and binary treatment of hydrogen embrittlement. However, for industrial applications, a quantitative prediction is needed. Hence a gap exists which restricts direct application of scientific understandings in practical engineering. The proposed project aims to bridge said gap for hydrogen embrittlement through competence-building research. While Norwegian continental shelf calls for development of new material solutions to handle harsher conditions, or development of subsea gas processing systems, the material integrity in the intended conditions must be secured - including the effects of hydrogen exposure. The proposed solution can be extended beyond the oil and gas industry to the renewable energy sector, e.g. offshore windmills, to the fuel cell industry and others