Impact of Wet Etching on the Tribological Performance of 304 Stainless Steel in Hydrogen Compressor Applications

Abstract

Hydrogen has emerged as an eco-friendly and sustainable alternative to fossil fuels. However, the utilization of hydrogen requires high-pressure compression, storage, and transportation, which poses challenges to the durability of compressor components, particularly the diaphragm. This study aims to improve the durability of 304 stainless steel diaphragms in hydrogen compressors by optimizing their surface roughness and corrosion resistance through wet etching. The specimens were prepared by immersing 304 stainless steel in a mixture of sulfuric acid and hydrogen peroxide, followed by etching in hydrochloric acid for various durations. The surface morphology, roughness, and wettability of the etched specimens were characterized using optical microscopy, surface profilometry, and water contact angle measurements. The friction and wear characteristics were evaluated using reciprocating sliding tests. The results showed that increasing the etching time led to the development of micro/nanostructures on the surface, thereby increasing surface roughness and hydrophilicity. The friction coefficient initially decreased with increasing surface roughness owing to the reduced contact area but increased during long-term wear owing to the destruction and delamination of surface protrusions. HCl-30M exhibited the lowest average friction coefficient and a balance between the surface roughness and oxide film formation, resulting in improved wear resistance. These findings highlight the importance of controlling the surface roughness and oxide film formation through etching optimization to obtain a uniform and wear-resistant surface for the enhanced durability of 304 stainless steel diaphragms in hydrogen compressors.

1. Introduction

Hydrogen energy has garnered attention as an ecofriendly and sustainable energy source to replace fossil fuels. Hydrogen combustion produces only water and does not emit greenhouse gases; it can be produced from various energy sources, making it advantageous in terms of energy security [1]. Owing to these advantages, hydrogen energy is being utilized in diverse fields, such as the automotive industry, power generation, and residential fuel cells. In particular, with the accelerating commercialization of hydrogen fuel cell vehicles, the establishment of a hydrogen infrastructure is actively underway [2].

To utilize hydrogen, it is essential to compress and store it at a high pressure and transport it. There are several methods of hydrogen compression, including diaphragm compressors, scroll compressors, and piston compressors. Diaphragm compressors operate by separating the compression chamber from the fluid and applying hydraulic or mechanical forces to move the diaphragm [3]. During this process, as the compressor moved toward the compression chamber, hydrogen gas was compressed. Diaphragm compressors operate in an oilfree manner, enabling them to maintain high hydrogen purity and are widely used owing to their excellent durability owing to their structural characteristics [4].

The hydrogen gas inside the compressor is condensed under high pressures ranging from 35 to 70 MPa, during which fatigue wear occurs owing to the reciprocating motion of the cylinder [5]. Repeated pressure changes in the compressor induce mechanical stress inside the cylinder, causing fatigue and cracks in the material. This can lead to corrosion problems and significantly affect the lifespan of diaphragm components exposed to hydrogen environments. Notably, hydrogen can diffuse into the metal interior, causing hydrogen embrittlement, which makes material selection and surface treatment crucial [6].

The scratches and wear on the diaphragm were significantly influenced by the surface roughness. As the surface roughness increases, the friction coefficient increases, accelerating wear, and cracks are more likely to occur owing to the stress concentration effects [7]. Conversely, appropriate control of the surface roughness can increase the contact area, disperse stress, enhance lubrication effects, and reduce the friction coefficient [8]. Therefore, maintaining optimal surface roughness is essential for improving the durability of diaphragm compressors.

Various technologies exist for controlling the surface roughness, such as chemical etching, plasma etching, and laser etching, which can polish the surface and obtain uniform roughness [9-11]. Among these methods, wet etching selectively removes the material surface using liquid-state chemicals. It is uniform, low-cost, and has a fast processing speed, making it applicable to various materials, such as metals, glass, and polymers [12]. However, to obtain a uniform surface, optimization of etching conditions and precise process control are necessary.

Stainless steel is widely used as a material for diaphragm compressors because of its excellent corrosion resistance and mechanical properties. In particular, the surface of 304 stainless steel is covered with a chromium oxide layer, enhancing its corrosion resistance. This oxide layer is formed by reaction with oxygen in air and possesses self-healing characteristics [13]. However, the uniformity and density of the oxide layer can vary depending on the surface roughness, which makes it important to determine optimal etching conditions.

This study aims to improve the durability of diaphragm compressors used in hydrogen environments by treating the surface of 304 stainless steel using the wet etching method to obtain optimal surface roughness. A mixture of sulfuric acid and hydrogen peroxide and hydrochloric acid were used as the etching solution and the surface roughness and wettability were evaluated by varying the etching time. In addition, the friction and wear characteristics of the etched specimens were evaluated to verify their suitability as hydrogen compressor components. This is expected to contribute to improving the performance and lifespan of compressors used in the hydrogen energy industry.

2. Materials and Experiments

In this study, 304 stainless steel, which is widely used in diaphragm components, was selected as the experimental material. The specimen preparation process is illustrated in Fig. 1. First, the stainless steel specimens were immersed in DI water and ultrasonically cleaned for 10 min, followed by drying in an oven at 70°C for 30 min. Next, to form a microstructure on the specimen surface, 35 wt.% H₂SO₄ solution and 35 wt.% H₂O₂ solution were mixed in a 7:3 ratio and stirred for 1 hour at a speed of 300 rpm or higher. The prepared specimens were immersed in the solution to allow the formation of a microstructure on the stainless-steel surface. Subsequently, wet etching was performed using hydrochloric acid for up to 40 min to form a micro/nanostructure on the surface. The etched specimens were ultrasonically cleaned twice in DI water for 10 min and then dried in an oven at 70º C for 30 min to preparing a total of 6 specimens. The untreated specimen was named Bare, the specimen etched with sulfuric acid/hydrogen peroxide was named HCl-N, and the specimens etched with hydrochloric acid were named HCl-10M (10 min), HCl-20M (20 min), HCl-30M (30 min), and HCl-40M (40 min), according to the etching time.

Fig. 1. Schematic diagram of the specimen preparation process.

An optical microscope was used to observe the surface morphology of the prepared specimens. Additionally, the surface roughness was analyzed using a surface profiler, and the water droplet contact angle was measured by dropping distilled water onto the specimen surface. A reciprocating sliding friction test was performed to evaluate the friction and wear characteristics. A steel ball with a diameter of 1 mm was used as the counter tip and a load of 100 mN was applied while reciprocating at a distance of 2 mm at a speed of 16 mm/s. The test was conducted for 5,000 cycles, and the average value of the friction coefficient measured during this process was used. After the friction test, the wear track was observed using an optical microscope to analyze the influence of the surface structure on the wear behavior.

This experiment aimed to elucidate the correlation between the microstructural changes in the 304 stainless steel surface according to the wet etching conditions and the resulting friction and wear characteristics. The results obtained through systematic experimental procedures and various analytical methods can be utilized for the development of surface treatment technologies to improve the durability of diaphragm components used in hydrogen environments.

3. Results and Discussion

This study aims to investigate the optimal conditions for achieving the best surface roughness, corrosion resistance, and chemical resistance by applying the wet etching method to the surface of 304 stainless steel to enhance the durability of hydrogen compressor diaphragms.

Fig. 2 shows optical microscope images of the 304 stainless steel specimen surfaces according to the etching time. In the case of the bare specimen, it was confirmed that grains with sizes ranging from a few micrometers to several tens of micrometers were nonuniformly distributed because of grain coarsening during the stainless steel manufacturing process. For the HCl-N specimen, the grain boundaries were clearly revealed through acid cleaning, which was attributed to the removal of surface impurities and natural oxide film [14]. Starting from the HCl-10M specimen, the existing crystal structure began to be destroyed by the strongly acidic solution, and a new dense structure started to form. As the etching time increased, the development of micro/nanostructures was observed.

Fig. 2. Optical microscope images of stainless steel specimens with different etching times.

Fig. 3 shows the changes in the surface roughness of the specimens before and after etching. The average roughness of the bare specimen was 0.157 µm, whereas that of the HCl-N specimen slightly decreased to 0.128 µm. This can be interpreted as the formation of a uniform passive film by immersing the specimen in a solution of sulfuric acid and hydrogen peroxide. As the hydrochloric acid etching time increased, the surface roughness continuously increased from 0.3 to 0.94 µm, which is because the metal salt generated by the reaction between hydrochloric acid and metal components dissolved the surface, forming micro/nano structures. However, for the HCl-40M specimen, the roughness somewhat decreased to 0.64 µm due to excessive etching.

Fig. 3. Surface roughness of stainless steel specimens before and after etching.

Fig. 4 shows the change in the water droplet contact angle on the 304 stainless-steel surface according to the etching time. The contact angle of the bare specimen was measured to be 77.6°, indicating the inherent hydrophilicity of the stainless steel. For the HCl-N specimen, the contact angle decreased to 69.5°, which could be interpreted as an increase in the oxygen atom concentration on the surface according to the Cassie-Baxter equation. In other words, the hydrophilicity was enhanced owing to the increased contact area between the water droplet and air resulting from the formation of a passive film. As the hydrochloric acid etching time increased, the contact angle continuously decreased to 43° because the contact area between the water droplet and the solid surface increased owing to the development of micro/nanostructures. However, for the HCl-40M specimen, the contact angle increased slightly owing to excessive etching.

Fig. 4. Water contact angles of stainless steel specimens with different etching times.

To evaluate the friction and wear characteristics, a sliding friction test was performed, and the changes in the friction coefficient and shape of the wear track according to the etching time were analyzed. Fig. 5 shows the change in the friction coefficient over 5,000 cycles. In the case of the bare specimen, the initial friction coefficient started at 0.4 and rapidly increased to 0.75 within 200 cycles, after which it stabilized. This can be interpreted as the easy occurrence of adhesive wear due to metal-to-metal contact in the absence of an oxide film despite the relatively low surface roughness of the bare specimen. In the initial stage of wear, the friction coefficient increases as the protrusions on the surface deform and break; however, afterward, the friction coefficient stabilizes owing to the repeated shearing and adhesion of wear debris. The HCl-N specimen had a relatively high initial friction coefficient of 0.6 and showed a gradual increase up to 1,000 cycles. Despite having the lowest surface roughness owing to acid cleaning, the initial friction coefficient was high because the hydrophilic oxide film could not exert a lubricating effect. However, during the long-term wear process, as the oxide film was gradually removed and the ductile metal substrate was exposed, the friction coefficient slowly increased owing to the adhesive wear. As the hydrochloric acid etching time increased, changes in the initial friction coefficient were observed. The HCl-10M specimen showed the lowest initial friction coefficient of 0.17, but increased to 0.75 in the early stages and then maintained a constant value. This is because the microstructure of the surface etched for a short time contributes to reducing the friction coefficient by decreasing the initial contact area. However, it is judged that the friction coefficient rapidly increases as the contact area increases, owing to the rapid wear of the fine protrusions. Subsequently, it seems that the surface oxide film was reinforced by etching delayed wear, maintaining a stable friction coefficient. In the case of the HCl-20M specimen, the friction coefficient increased from 0.23 to 0.8 up to 1,000 cycles. This can be interpreted as the initial friction coefficient appearing low owing to the increase in surface roughness with increasing etching time; however, the friction coefficient gradually increased owing to the destruction of protrusions and adhesive wear. On the other hand, HCl-30M and HCl-40M showed a rapid increase in the friction coefficient to 0.73 within 200 cycles but maintained a relatively low friction coefficient afterward. This seems to be because the initial contact area increased as the surface nonuniformity increased owing to the excessive etching. However, it was determined that the friction coefficient decreased as the rough surface wore out and flattened during the long-term wear process.

Fig. 5. Friction coefficient history of stainless steel specimens during sliding friction test.

Fig. 6 shows the change in the average friction coefficient according to the etching time. The bare specimen exhibited the highest average friction coefficient of 0.78, while the etched specimens generally showed similar values of 0.75-0.77. This suggests that the presence or absence of an oxide film and changes in the surface roughness have a limited impact on the average friction coefficient during the long-term wear process. However, the HCl-30M specimen showed the lowest average friction coefficient of 0.75 means that the wear resistance can be improved by balancing the surface roughness and oxide film under optimized etching conditions.

Fig. 6. Average friction coefficient of stainless steel specimens with different etching times.​​​​​​​

Fig. 7 shows the optical microscope images of the wear tracks formed on each specimen surface after the sliding friction test. In the case of Bare and HCl-N, relatively wide wear widths were observed, and the adhesion of wear particles and rough wear shapes were observed. This is because adhesive wear caused by metal-to-metal contact predominantly occurs owing to the absence or brittleness of the oxide film. Particularly in the case of the bare specimen, deep and non-uniform wear marks were observed owing to abrasive wear and plastic deformation. However, starting from the HCl-10M specimen etched with hydrochloric acid, non-uniform wear shapes were observed owing to the microstructure of the surface. In HCl-10M and HCl20M, fine scratches and partial adhesion marks were observed within the wear track owing to the destruction and delamination of surface protrusions. This suggests that the surface structure formed by etching can suppress wear by reducing the contact area in the initial stage, but in the long-term wear process, it can cause nonuniform wear owing to the destruction and delamination of protrusions. HCl-30M and HCl-40M exhibited severe surface destruction and nonuniform wear shapes. This was attributed to the fact that the contact stress was concentrated as the surface roughness increased significantly owing to excessive etching. In particular, deep grooves and adhesion marks were observed within the wear track owing to the local plastic deformation and fracture of the protrusions. These results suggest that it is important to obtain a uniform surface with excellent wear resistance by optimizing etching conditions.

Fig. 7. Optical microscope images of wear tracks on stainless steel specimens after the sliding friction test.​​​​​​​

In summary, the friction and wear characteristics of 304 stainless steel are significantly influenced by the presence or absence of a surface oxide film and changes in surface roughness depending on the etching conditions. In the absence of an oxide film or when the surface roughness is low, adhesive wear caused by metal-to-metal contact occurs predominantly, resulting in a high friction coefficient and non-uniform wear shape. However, under appropriate etching conditions, the microstructure of the surface can reduce the initial friction coefficient by decreasing the contact area; however, during the long-term wear process, non-uniform wear may occur owing to the destruction and delamination of protrusions. Therefore, to obtain a uniform surface with excellent wear resistance, it is important to control the balance between the surface roughness and the oxide film by optimizing the etching conditions.

4. Conclusion

This study underscores the crucial role of etching condition optimization in enhancing the durability of 304 stainless steel diaphragms for hydrogen compressor applications. The experimental results demonstrated that carefully controlled etching processes can yield surfaces with tailored roughness and oxide film characteristics, which significantly influence the friction and wear behavior of the material.

The optimal etching conditions identified in this research provide a foundation for the development of surface engineering strategies aimed at improving the performance and longevity of stainless steel diaphragms in demanding hydrogen compressor environments. Implementing these findings in the design and manufacturing of diaphragms can lead to enhanced durability, reduced maintenance requirements, and improved overall efficiency of the hydrogen compressors.

Future research should focus on evaluating the long-term performance of etched diaphragms under realistic operating conditions and exploring the potential benefits of combining wet etching with other surface modification techniques. By advancing our understanding of surface engineering approaches for critical components in the hydrogen energy sector, we can contribute to the development of more reliable and sustainable hydrogen infrastructure.