Ouyang Xin 1, Cheng Lei 1, Peng Shidiao 1, Yuan Chunming 2, Ma Zhiyuan 2, Wang Jinhua 3, Guo Dagang 2

（1. National Oil and Gas Pipeline Network Group Co., Ltd. Science and Technology Research General Institute Branch, Tianjin 300457; 2. School of Materials, Key Laboratory of Metal Material Strength, Xi'an Jiaotong University, Xi'an, Shaanxi 710049; 3. National Key Laboratory of Green Hydrogen and Electricity, Xi'an Jiaotong University, Xi'an, Shaanxi ７１００４９）

［Abstract］：This paper summarizes the main research progress of materials for key components of centrifugal compressors, such as baffles, baffler devices, seals, impellers, and bearings, as well as the cutting-edge achievements in hydrogen compatibility studies. It systematically reviews the hydrogen compatibility testing methods and the three hydrogen embrittlement mechanisms (HEDE, HELP, and AIDE) and their synergistic effects. Finally, it looks forward to the future development trends.

［Key words］：Hydrogen-enriched natural gas; Materials for centrifugal compressors; Hydrogen compatibility Hydrogen embrittlement mechanism

Chinese Library Classification Number：ＴＨ４５２ Document Code：Ｂ

Article Number：１００６－２９７１（２０２６）０１－００５９－０６

1 Introduction

In recent years, the global energy structure has been accelerating its transition towards a low-carbon model. Hydrogen, due to its zero-carbon emission characteristic, has become a key energy carrier for achieving the goal of carbon neutrality. The International Energy Agency (IEA) predicts that by 2050, hydrogen will account for 12% to 15% of global terminal energy consumption [1]. China's "Medium and Long-Term Development Plan for Hydrogen Energy Industry (2021-2035)" clearly states that it is necessary to focus on breaking through the technical bottlenecks of hydrogen storage and transportation, and promote demonstration projects of hydrogen blending in natural gas pipelines [2]. Against this backdrop, using the existing natural gas pipelines for hydrogen blending transportation has become an important path to reduce the cost of hydrogen infrastructure construction. Studies have shown that when the hydrogen blending ratio is controlled at 10% to 20%, the compatibility of existing pipeline materials and equipment is better, and the renovation investment can be reduced by 50% to 70% [3]. However, the high diffusivity and permeability of hydrogen pose severe challenges to the material properties of the core equipment in the hydrogen blending natural gas transportation system - centrifugal compressors. As the key equipment in the hydrogen blending natural gas transportation system, centrifugal compressors undertake core functions such as gas pressurization, energy conversion, and stable transportation. This equipment is mainly composed of stators and rotors. The stators include cylinders, baffles, seals, bearings, etc., while the rotors are mainly composed of impellers, main shafts, thrust discs, and couplings, etc. Each key component is subjected to complex dynamic loads under high-temperature, high-speed, and high-pressure conditions, requiring the materials used to not only possess traditional high strength, good wear resistance, and corrosion resistance [4], but also exhibit good hydrogen compatibility in a hydrogen-containing environment to inhibit hydrogen intrusion and hydrogen embrittlement [5]. Due to the strong diffusivity of hydrogen, its infiltration into metals may lead to weakened local grain boundaries, increased dislocation activity, and the generation of microcracks, which poses severe challenges to the long-term stable operation of the equipment. Therefore, the selection and optimization of materials for each key component of the centrifugal compressor have become an important research direction for addressing the safety issues of hydrogen blending transportation.

In a hydrogen-enriched environment, metal materials often experience performance degradation due to hydrogen intrusion [6]. This is mainly manifested in local hydrogen-induced brittle fracture, accelerated fatigue crack propagation, and reduced fracture toughness. A large number of studies have shown that even if the static mechanical properties of the material do not significantly decrease under high hydrogen partial pressure or hydrogen doping conditions, its fatigue resistance and ductility will significantly decline [7]. Traditional pre-hydrogening tests are difficult to accurately reflect the hydrogen damage behavior under actual working conditions due to the hydrogen escape effect. Therefore, how to accurately evaluate and predict the hydrogen compatibility of metal materials in a hydrogen-enriched environment has become an important issue that needs to be urgently addressed at present. To this end, scholars at home and abroad have continuously explored various test techniques, such as online high-pressure hydrogen testing, slow strain rate tensile test (SSRT), fatigue crack propagation test, and low-cycle fatigue test, aiming to establish a set of testing methods that can truly reflect the performance of materials under hydrogen conditions. At the same time, by establishing multi-scale numerical models and in-situ characterization techniques, the diffusion and aggregation behavior of hydrogen in metal materials is deeply analyzed, providing theoretical basis for the design of new anti-hydrogen brittle materials [8]. In addition, regarding the mechanism of hydrogen embrittlement, currently, there are mainly theories such as hydrogen reducing cohesion (HEDE), hydrogen-induced local plastic deformation (HELP), and adsorbed hydrogen-induced dislocation emission (AIDE). These mechanisms have their own characteristics and often exhibit coupling effects in practical applications, making it difficult to explain the hydrogen erosion resistance behavior of materials with a single theory completely.

This article reviews the research progress on the materials of key components of centrifugal compressors and their hydrogen compatibility. It focuses on discussing the materials of each key component of the centrifugal compressor, as well as the main research progress, hydrogen compatibility testing methods, factors influencing hydrogen compatibility, and the hydrogen-induced multi-mechanism coupling damage theory. The aim is to provide theoretical references for improving the reliability of hydrogen-infused transportation equipment.

2 Research Progress on Materials for Centrifugal Compressors

The materials required for different components of the centrifugal compressor vary according to their functions and the stress environments they are subjected to: The diaphragms need to have excellent casting properties, wear resistance, and impact resistance, and are often made of gray cast iron or ductile iron; The returners need to have good machinability and uniform stress distribution, and are mostly made of low-carbon steel or stainless steel; The sealing components not only need to achieve good airtightness but also must have wear resistance and corrosion resistance during high-speed operation. In recent years, PEEK composite materials have attracted increasing attention due to their high performance; The impellers, as the most directly involved components in energy conversion, not only require high strength and corrosion resistance but also have strict requirements for fatigue resistance. Commonly used materials are specific stainless steel or low-alloy high-strength steel; The bearings focus on the overall fatigue life and wear resistance, and often use materials with special structural designs such as tin-based alloys. The commonly used materials and research progress of each component of the centrifugal compressor are shown in Table 1.

The diaphragm is a gas passage that forms a fixed component in a centrifugal compression machine [23]. It is mostly made of cast iron or ductile iron. Among them, QT400-18 ductile iron and 16Mn are more suitable as diaphragm materials due to their excellent comprehensive mechanical properties, welding performance, and wear resistance, and they have promising application prospects.

The returner is a uniform pressure-raising channel, and it is equipped with a certain number of blades inside to improve the gas flow condition and guide the airflow smoothly into the next stage impeller. It is composed of two partitions and the blades placed between the partitions (24). The material of the returner is usually Q235-A and 0Cr18Ni9. Compared with Q235-A, 0Cr18Ni9 steel has better comprehensive mechanical properties and corrosion resistance, and is more suitable as the material for the returner.

The function of the sealing piece is to reduce the air leakage between the rotor of the compressor and the fixed components. The PEEK material specifically for centrifugal compressors has superior sealing performance, mechanical properties and corrosion resistance compared to ZL102, ZL104, and ZL105. Moreover, the integrated design of metal and plastic significantly reduces material costs and has a very broad application prospect in the field of sealing materials.

The impellers are divided into open type, semi-open type and closed type. The commonly used materials include X12Cr13 stainless steel, FV520B, KMN steel, etc. Compared with other materials, KMN and FV520B have better strength, toughness and corrosion resistance, and are often used as impeller materials by compressor manufacturers.

Bearings are components that bear significant forces and experience complex and variable loads during the operation of compressors. Therefore, the bearing materials should possess high compressive strength, fatigue strength, sufficient plasticity and toughness, as well as high vibration resistance and other comprehensive properties [25]. Commonly used bearing materials include ZSnSb11Cu6, ZChSnSb11-6, QT500-7, etc. [26].

3 Research Progress on Hydrogen Compatibility of Materials for Centrifugal Compressors

In the hydrogen-enriched natural gas transmission system, the centrifugal compressor serves as the core equipment, and the hydrogen compatibility issue faced by its materials during service has become increasingly prominent. Hydrogen seeping into the interior of the metal often leads to hydrogen embrittlement of the material, reducing its ductility, fatigue life, and fracture toughness, which may result in equipment failure or even accidents. Therefore, scholars at home and abroad have conducted a large number of experiments and theoretical studies on the hydrogen compatibility of centrifugal compressor materials, and have successively proposed pre-charging hydrogen tests, online high-pressure hydrogen tests, and slow strain rate tensile tests.

（ＳＳＲＴ）、Fatigue crack propagation tests, as well as various low-cycle fatigue tests, are among the hydrogen compatibility testing methods. At the same time, through in-situ characterization techniques and multi-scale numerical simulation methods, the microscopic processes such as hydrogen diffusion, adsorption, segregation, and crack propagation were deeply explored. Currently, the mainstream viewpoints on the hydrogen embrittlement mechanism mainly include hydrogen reducing cohesion (HEDE), hydrogen-induced local plastic deformation (HELP), and adsorbed hydrogen-induced dislocation emission (AIDE), while the latest research shows that in actual working conditions, these three mechanisms often act in a coupled manner, jointly determining the hydrogen embrittlement sensitivity of the material.

３.１ Hydrogen compatibility testing method

The commonly used hydrogen compatibility testing methods are shown in Table 2. In the field of material hydrogen embrittlement research, the traditional pre-charging hydrogen method has obvious limitations. This method pre-charges hydrogen through electrochemical or gas environment before conducting mechanical tests, but it cannot accurately simulate the synergistic effect of hydrogen and stress under actual service conditions. In contrast, the high-pressure hydrogen environment test achieves triple similarity in the three dimensions of environment, stress field, and hydrogen concentration field to the actual working conditions. Such tests usually adopt direct slow strain rate tensile, fatigue crack propagation, and low-cycle fatigue tests under high-pressure hydrogen environments, combined with scanning electron microscope fracture analysis, to reveal the damage mechanism of hydrogen on centrifugal compressor materials at the macro and micro levels. This dynamic coupling test method can simultaneously observe the interaction between hydrogen adsorption, diffusion, aggregation behavior and the deformation process of the material, providing more reliable data support for engineering applications.

３.２ Factors Affecting Hydrophilicity

３.２.１ Atmospheric pressure

Under high-pressure hydrogen environment, in some areas of the metal material, the hydrogen concentration reaches the critical value, resulting in a significant reduction in toughness and the occurrence of hydrogen-induced delayed fracture phenomenon [28]. Moreover, the solubility of hydrogen atoms in metals such as iron is proportional to the square root of hydrogen pressure [29]. Additionally, studies have shown that within a certain range, increasing the environmental pressure will promote the further dissolution of hydrogen atoms in the metal [30], especially increasing the enrichment of hydrogen atoms at the crack tip of the metal, which promotes the occurrence of embrittlement [31].

In recent years, the research on hydrogen embrittlement of metal materials has attracted significant attention from scholars in the field of materials across various countries. ANDREW et al. [32] studied the fatigue crack propagation behavior of two pipeline steel alloys, X52 and X100, under hydrogen pressure ranging from 1.7 to 48 MPa. They found that these two materials exhibited significantly different fatigue crack propagation rates under hydrogen and air environments, with the former being 1 to 2 times higher than the latter.

On a scale, and within a certain intensity factor range, this degradation effect intensifies with the increase of hydrogen pressure. AMARO et al. [33] further studied the tensile and fatigue crack propagation properties of these two types of pipeline steel alloys in a hydrogen environment.

Studies have shown that the elongation rates of the two steel specimens under tensile tests in a 13.8 MPa high-pressure hydrogen environment are significantly lower than those in an air environment, and the toughness loss of the former is significantly greater than that of the latter. Within a certain range of stress intensity factors, the fatigue crack propagation rates of the two types of steel increase with the increase in hydrogen pressure. When the hydrogen pressure increases from 1.72 MPa to 20.68 MPa, the fatigue crack propagation rate of X100 steel increases by 2 to 10 times.

It is worth noting that when the hydrogen pressure reaches a certain threshold, the deterioration degree of the material's performance tends to stabilize. This phenomenon has been further verified in the study of the X70 steel's heat-affected zone [34]. Currently, there is still a need for further systematic and in-depth research on the critical threshold of hydrogen pressure for hydrogen embrittlement in various engineering pipeline steel materials and the mechanism of hydrogen embrittlement.

３.２.２ Hydrogen content ratio

Hydrogen gas transportation through natural gas pipelines with hydrogen blending is an effective method for large-scale hydrogen delivery [35-36]. Especially, hydrogen blending transportation through the natural gas pipeline system for providing gas to residents and businesses is currently recognized as an extremely efficient and feasible strategy. However, in order to minimize the significant decline in the mechanical properties (such as fracture toughness and fatigue performance) of pipeline metal materials caused by hydrogen blending, it is necessary to first determine the appropriate range of hydrogen blending ratio.

In recent years, scholars both at home and abroad have conducted research on the performance of steel under different hydrogen-diluted natural gas environments. NGUYEN et al. [37-38] conducted a study on the hydrogen embrittlement behavior of X70 pipeline steel in hydrogen-diluted natural gas. Their results showed that in hydrogen-diluted natural gas with a total pressure of 5 to 10 MPa, the hydrogen embrittlement sensitivity of this material significantly increased with the increase in hydrogen dilution ratio. When the hydrogen volume fraction reached a critical value, the fracture mode of the material changed from ductile fracture to brittle fracture. AN et al. [39] conducted a study on X80 pipeline steel in a hydrogen-diluted environment with a total pressure of 12 MPa and found that with the increase in hydrogen dilution ratio, the fatigue cycle number of the notched specimens decreased rapidly (by 20% to 90%), while the crack propagation rate of the compact tensile specimens increased sharply (by 7 to 14 times). This study further pointed out that the increase in crack propagation rate was the main reason for the reduction in the fatigue life of X80 steel. MENG et al. [40] conducted a study on the influence of hydrogen volume fraction of 0 to 50% in natural gas and hydrogen mixture on the mechanical properties of X80 pipeline steel under a pressure of 12 MPa, and also reached similar conclusions.

At present, although significant progress has been made in the research on the influence of hydrogen content ratio in a hydrogen-enriched environment on the hydrogen embrittlement sensitivity of steel, new understandings have been established among scholars in the field of materials. The influence patterns and degrees have also generally been recognized. However, there is currently no unified conclusion on the operating pressure and hydrogen content ratio range for safe operation of various pipeline steel materials in a hydrogen-enriched environment. Further in-depth and systematic research is still needed, which will be of crucial significance for large-scale hydrogen transportation strategies.

３.３Hydrogen embrittlement mechanism

When hydrogen is introduced into natural gas pipelines, it may cause risks such as hydrogen embrittlement, hydrogen blistering, decarburization and hydrogen corrosion in the pipeline materials. Among these, hydrogen embrittlement and hydrogen corrosion pose the greatest risks and cause the most severe damage. Hydrogen embrittlement and hydrogen corrosion of pipelines are complex processes involving the formation of solid solutions, hydrogen compounds, molecular hydrogen and gas products when hydrogen reacts with the pipeline metal or the additives in the metal. This can weaken the bonding force at the metal grain boundaries, resulting in decreased ductility of the pipe material, causing brittle fracture or microcracks or pitting corrosion [41-43]. Factors influencing hydrogen embrittlement include not only pressure and hydrogen content but also environmental temperature, the strength level of the pipeline material, deformation rate and microstructure.

In fact, the state of hydrogen in the pipeline also affects the way it interacts with the metal, thereby resulting in different types of hydrogen embrittlement. According to the differences in the behavior of hydrogen atom aggregation, hydrogen embrittlement can be classified into three types: internal hydrogen embrittlement (IHE), environmental hydrogen embrittlement (EHE), and hydrogen reaction hydrogen embrittlement (HRE) [44]. Among them, internal hydrogen embrittlement is related to the lattice distortion, dislocation pinning, and crack nucleation caused by the entry of hydrogen atoms into the metal lattice; environmental hydrogen embrittlement is related to the formation of hydrogen molecule aggregation zones at the crack tip under high-pressure hydrogen conditions, the promotion of hydrogen adsorption by local stress, or the decrease in surface energy; and hydrogen reaction hydrogen embrittlement is related to the irreversible reactions with specific elements in the material (such as carbon, alloy elements, etc.). Moreover, the mechanism of hydrogen embrittlement is extremely complex. Based on the microscopic mechanism of hydrogen-induced damage, there are currently three main mainstream theories: hydrogen reduction of cohesion theory (HEDE), hydrogen-induced local plastic deformation theory (HELP), and adsorption hydrogen-induced dislocation emission theory (AIDE).

３.３.１ The Hydrogen Decreased Cohesion Energy (HEDE) Theory降低内聚力 （ＨＥＤＥ）

The HEDE theory posits that hydrogen atoms accumulate at metal grain boundaries, crack tips, or defects, reducing the interatomic bonding forces and causing brittle fracture of the material under low stress. This theory is particularly applicable to explaining the mechanism of hydrogen-induced cracking in high-strength steel under high-pressure hydrogen environments [45]. ORIANI [46] proposed through thermodynamic analysis that the segregation of hydrogen within the metal significantly reduces the grain boundary energy, thereby weakening the strength of the grain boundaries. First-principles calculations further verified this view: JIANG and CARTER [47] discovered that hydrogen adsorption at the α-Fe grain boundaries can reduce the interface binding energy by more than 30%. NAGUMO et al. [48] confirmed through hydrogen thermal analysis (TDS) that the enrichment concentration of hydrogen at the grain boundaries of steel can reach hundreds of times that of the matrix, consistent with the critical hydrogen concentration for brittle fracture. However, the HEDE theory is difficult to explain the local plastic deformation characteristics observed in hydrogen embrittlement. In recent years, multi-scale simulations (such as coupling of molecular dynamics and continuum mechanics) have shown that hydrogen may simultaneously reduce cohesion and promote dislocation motion through a synergistic effect, promoting the integration of the HEDE theory with other theories.［４９］。

３.３.２ Hydrogen-Induced Local Plastic Deformation (HELP) Theory

The HELP theory suggests that hydrogen enhances the mobility of dislocations, leading to the concentration of local plastic deformation, and subsequently triggering crack initiation and propagation. BEACHEM [50] discovered through fracture analysis that there are significant plastic flow traces at the crack tip in the hydrogen environment, and proposed that hydrogen promotes local slip by shielding the interaction between dislocations.

BIRNBAUM and SOFRONIS [51] further proposed the mechanism by which hydrogen reduces the nucleation energy barrier for dislocation formation, and observed through in-situ transmission electron microscopy that the rate of dislocation proliferation significantly increased in the hydrogen environment. ROBERTSON et al. [52] confirmed using environmental transmission electron microscopy (ETEM) that hydrogen can promote the inter-slip and climb of dislocations in stainless steel, leading to local softening. Molecular dynamics simulations show that hydrogen atoms reduce the Peierls stress by approximately 40% to lower the activation energy for dislocation motion [53]. However, the interpretation of the HELP theory for brittle fracture at low hydrogen concentrations remains controversial. Some scholars believe that it is more applicable to high hydrogen concentrations or dynamic loading conditions.［５４］。

３.３.３ Adsorption hydrogen-induced dislocation emission (AIDE) theory

The AIDE mechanism was proposed by LYNCH, which suggests that the adsorption of hydrogen on the crack surface reduces the surface energy required for dislocation emission, facilitating the emission of dislocations from the crack tip, resulting in the crack propagating through alternating dislocation emission and cleavage expansion [55]. XING et al.［５６］The density functional theory (DFT) calculations indicate that hydrogen adsorption can reduce the surface energy at the crack tip of aluminum by 35%, significantly promoting dislocation nucleation. Zhao et al. [57] combined atomic probe tomography (APT) and first-principles calculations to reveal the atomic-scale distribution of hydrogen traps in aluminum alloys and their effects on embrittlement. The AIDE theory successfully explains the formation of mixed (crack pit + cleavage) morphologies in hydrogen embrittlement fractures, but its neglect of the effect of bulk hydrogen has been questioned. Kirchheim [58] proposed that AIDE may cooperate with Hede/Help, for example, hydrogen adsorption promotes dislocation emission (AIDE), while bulk hydrogen further accelerates dislocation movement (HELP).

4 Looking forward

The rapid development of hydrogen-enriched natural gas transportation technology has placed increasingly higher demands on the material properties of centrifugal compressors. Current research has significantly enhanced the mechanical properties and corrosion resistance of key components such as diaphragms and impellers through alloying design, surface modification, and advanced manufacturing technologies. New composite materials and functional coating technologies have demonstrated significant application potential. However, the brittleness problem of materials under hydrogen environment remains prominent. The increase in hydrogen partial pressure and hydrogen enrichment ratio leads to a loss of material toughness and a sharp increase in crack propagation rate, which is fundamentally attributed to the enrichment of hydrogen at grain boundaries and the coupling effect of multiple mechanisms. Future research should focus on developing anti-hydrogen embrittlement materials, analyzing multi-scale damage mechanisms, and establishing an engineering applicability evaluation system. Key efforts should be directed towards exploring new anti-hydrogen embrittlement material systems, using a combination of various characterization techniques to reveal the interaction mechanism of hydrogen-stress-environment, and establishing a material performance database and intelligent prediction model covering complex operating conditions. Through the collaborative innovation of material composition, structure, and properties, the hydrogen-enriched compressor should be developed towards higher pressure and longer lifespan, providing support for the efficient and safe operation of hydrogen infrastructure.