Influence of Ultrasonic Rolling Extrusion Static Pressure on Corrosion Resistance of GCr15 Bearing Steel

Abstract

The influence of static pressure during focused ultrasonic rolling extrusion on the corrosion resistance of GCr15 bearing steel was investigated. Quenched GCr15 bearing steel served as the subject of this study, wherein ultrasonic rolling extrusion was performed using a CNC lathe. Static pressure levels of 200 N, 400 N, and 500 N were applied during the experiments. Following the preparation of samples, which included grinding and cleaning, electrochemical assessments were conducted utilizing an electrochemical workstation. These assessments encompassed measurements of open-circuit potential, Tafel polarization, and electrochemical impedance spectroscopy, employing a three-electrode configuration. Additionally, the microstructural characteristics of the samples were examined using scanning electron microscopy, optical microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The findings indicate that an increase in static pressure results in a forward shift of the open-circuit potential and a reduction in corrosion susceptibility. Tafel analysis revealed an increase in linear polarization resistance, a decrease in corrosion current, and a positive shift in corrosion potential. The impedance spectroscopy results demonstrated that both the modulus of low-frequency impedance and charge transfer resistance increased with elevated static pressure. Microstructural analysis indicated that higher static pressure contributes to a smoother and more compact surface, with a reduction in defects. The primary corrosion products identified were iron oxides and hydroxides. In conclusion, the corrosion resistance of GCr15 bearing steel subjected to ultrasonic rolling extrusion is enhanced as static pressure increases.

1. Introduction

GCr15 bearing steel is a critical material extensively utilized in mechanical engineering, particularly in the manufacturing of bearings and various industrial applications, due to its remarkable characteristics, including high strength, excellent wear resistance, and substantial hardenability [1]. Nonetheless, under actual operational conditions, GCr15 bearing steel is frequently subjected to corrosive environments, which can adversely affect its surface performance, thereby significantly diminishing the service life and reliability of the bearings. This deterioration may ultimately result in mechanical failures, leading to economic losses and safety risks [2,3].

Currently, prevalent technologies for enhancing bearing surface strength encompass surface coating, surface modification, and surface mechanical strengthening techniques. Research conducted by Parveez B [4] and colleagues indicates that the application of materials such as graphite, ZrO2, ceramics, molybdenum disulfide, and nano-boron nitride onto the bearing substrate can significantly improve certain mechanical properties. Nonetheless, traditional surface coating methods for high-strength materials often struggle to simultaneously optimize surface strength and lubricity, and further investigation is required to validate the performance of multiphase composite coatings under substantial loads. Additionally, challenges such as inadequate adhesion of the coating, variability in coating thickness, and high equipment costs must also be addressed. In the realm of surface modification technologies, commonly employed techniques include vapor deposition, chemical heat treatment, and laser surface modification. Notably, laser surface modification, particularly laser hardening, involves rapidly heating the material’s surface to its phase change temperature using a high-energy laser beam, thereby facilitating phase change hardening [5]. However, this method is constrained by equipment costs and the heat-affected zone, which can result in thermal deformation of the substrate.

To address these limitations, ultrasonic rolling extrusion technology has emerged as an innovative surface enhancement technique in the field of metal material surface modification in recent years. This method integrates ultrasonic vibrations with a rolling extrusion process to exert periodic mechanical forces on the surface of a metal material. The process facilitates the annihilation and generation of dislocations, resulting in grain refinement to the nanometer scale. This refinement enhances the blockage and entanglement of dislocations, thereby increasing intergranular slip resistance and the material’s overall deformation resistance [6,7,8]. Current research has made notable advancements in understanding the impact of ultrasonic rolling extrusion on the surface properties of metal materials. Numerous scholars have developed finite element models to simulate the ultrasonic rolling extrusion process, analyze the distribution and trends of the residual stress field, and utilize numerical simulation outcomes to inform experimental designs [9,10,11,12,13]. Common experimental methodologies employed to investigate the influence of process parameters on the strengthening effects include single-factor tests [14,15] and orthogonal tests [16]. For instance, Su Chong [17] examined the effects of process parameters such as static pressure, feed rate, and rotational speed on the surface roughness and hardness of GCr15 bearing rollers through finite element simulations and ultrasonic rolling extrusion experiments, subsequently establishing predictive models for roughness and hardness. Similarly, Xiameng Xue [18] conducted ultrasonic roll extrusion on GCr15 steel, creating a three-dimensional simulation model via the finite element method and designing orthogonal tests to explore the mechanisms by which process parameters affect surface microhardness, microstructure, and wear resistance. Furthermore, Xiquan Ma [19] provided a detailed analysis of the effects of varying rolling times on the surface roughness, micro-hardness, residual stress, wear, and corrosion properties of GCr15 steel through both experimental and finite element simulation approaches. Haojie Wang [20] conducted a theoretical analysis and machining experiments on ultrasonic surface rolling process (USRP), examining the relationship between process parameters and surface roughness. Their findings indicate that surface roughness initially decreases and subsequently increases with rising amplitude and static pressure, while it consistently increases with higher feed rates and speeds. These results serve as a significant reference for the optimization of USRP processing technology.

Jiahui Cong [21] employed ultrasonic rolling equipment to perform multiple USRP treatments on TC4 titanium alloy joints, revealing that USRP markedly enhances the fatigue limit and lifespan of welded joints. Specifically, at room temperature, the fatigue strength of the welded components improved by 2.04% to 4.58%, and the corrosion fatigue life increased by a factor of 1.72 to 2.88. Jinyou Kang [22] modified the radial residual stress distribution of circular saw blades through ultrasonic rolling extrusion technology, resulting in an enhancement of dynamic performance by over 10%. Hao Zhan [23] applied ultrasonic rolling extrusion strengthening to GCr15 spherical bearings, which led to a significant reduction in surface roughness and an increase in hardness, thereby enhancing the stability of the bearing material. Pengcheng Wang [24] utilized ultrasonic roll compression technology to induce beneficial residual compressive stress on the surface of Ti-6Al-4V alloy, which substantially improved surface microhardness and wear resistance. Kaikuai Zheng [25] investigated the effects of USRP on the surface quality, friction and wear properties, corrosion resistance, and wear mechanisms of TC11 titanium alloy under varying temperatures and corrosive environments. Their results demonstrate that USRP effectively refines surface grain, stabilizes surface hardness under harsh conditions, reduces the friction coefficient of titanium alloy at elevated temperatures, and enhances wear resistance in low-temperature and corrosive environments. Luo Yan [26] fabricated graded structure (GS) shaft samples from EA4T alloy steel using the ultrasonic surface rolling process (USRP), significantly improving the fatigue and corrosion fatigue strength of the shaft steel by introducing compressive residual stress (CRS).

Nevertheless, systematic research on the impact of ultrasonic rolling static pressure on the corrosion resistance of GCr15 bearing steel remains limited. A comprehensive investigation into the internal relationship between ultrasonic rolling extrusion static pressure and the corrosion resistance of GCr15 bearing steel, along with its underlying mechanisms, is essential. Such exploration would not only broaden the application scope of ultrasonic rolling extrusion technology in the bearing steel sector but also provide a robust theoretical foundation and technical support for optimizing the surface treatment processes of bearing steel, thereby enhancing its performance in corrosive environments. Therefore, this study focuses on the influence of ultrasonic rolling static pressure on the corrosion resistance of GCr15 bearing steel and reveals its inherent law through systematic experimental research and theoretical analysis, which has important scientific significance and engineering application value.

2. Materials and Methods

2.1. Materials

GCr15 bearing steel was chosen as the subject of this research and subsequently subjected to a quenching process. The primary chemical composition of the steel is detailed in

Table 1. Main chemical composition of GCr15 bearing steel (mass fraction %) [23].

Ingredient	C	Mn	Si	S	P	Cr
Proportion (wt%)	0.95~1.05	0.20~0.40	0.15~0.35	≤0.02	≤0.027	1.30~1.65

[23]. The ultrasonic rolling extrusion process for GCr15 bearing steel was conducted in its entirety on a CNC lathe. The ultrasonic rolling extrusion enhancement apparatus utilized the ultrasonic shock stress relief technology developed by Shanghai Xuanbang Metal New Material Technology Co., Ltd. (Shanghai, China). The ultrasonic frequency maintained during the processing was a constant 20 kHz. The parameter configurations and the number of each experimental group are presented in

Table 2. Ultrasonic rolling extrusion processing parameter setting.

Number	Static Pressure (N)	Speed (r/min)	Feed Speed (mm/min)	Amplitude (μm)
1	200	200	30	10
2	400	200	30	10
3	500	200	30	10

. (See Figure 1).

2.2. Electrochemical Test

For the electrochemical corrosion testing, the Shanghai Chenhua CHI760E electrochemical workstation (Shanghai, China) was employed. A three-electrode system was utilized for the experiment, comprising a saturated calomel electrode (SCE) as the reference electrode, a platinum sheet as the counter electrode, and the experimental sample serving as the working electrode. To mitigate electrochemical reactions in the nonexposed regions of the sample, the surface of the cut sample was sealed with epoxy resin, exposing only approximately 0.8 cm² of the testing area. The test medium consisted of a 3.5 wt% NaCl solution, and the experiments were conducted at room temperature.

Initially, the sealed sample underwent an open-circuit potential (OCPT) test for a duration of 4 h using the electrochemical workstation, with a sampling interval of 0.1 s and a potential sweep range from −1 V to +1 V. This was followed by a Tafel polarization test conducted at a scan rate of 10 mV/s and a potential range of −2 V to +3 V. Finally, electrochemical impedance spectroscopy (EIS) testing was performed, covering a frequency range of 0.01 to 10 kHz, which was segmented into six frequency bands, with 50 data points collected for each band. The initial voltage for this test was set to the open-circuit voltage, and the signal amplitude was established at 5 mV.

2.3. Microstructure Analysis

To examine the impact of electrochemical corrosion on the surface characteristics of the sample, scanning electron microscopy (SEM, INSPECT F50, FEI, Hillsboro, OR, USA) was employed to analyze the surface morphology following corrosion under varying static pressure conditions (200 N, 400 N, and 500 N). Concurrently, optical images were captured using an optical microscope (BX51M, Olympus, Tokyo, Japan) at magnifications of 500 μm and 50 μm, respectively. To further investigate the electrochemical corrosion products generated during the tests, the phase composition of the surfaces of all samples post-corrosion was assessed using X-ray diffraction (XRD, Smart Lab, Rigaku, Japan). The analysis utilized Cu Kα radiation, with an angular range set from 15° to 90° and a scanning speed of 5°/min. Additionally, X-ray photoelectron spectroscopy (XPS, AXIS UltraDLD spectrometer, Kratos Analytical—a Shimadzu group company, equipped with a path-heated gas cell and charge neutralizer, Japan) was utilized, employing Al Kα radiation at an energy of 1486.6 eV as the excitation source to characterize both untreated samples and those subjected to a static pressure of 500 N. The passing energy was maintained at 50 eV with a step size of 0.1 eV. Upon completion of the tests, the resulting spectra were analyzed and fitted using the least squares method with Avantage software (Version: 5.948).

3. Results

3.1. Analysis of Electrochemical Test Results

3.1.1. OCPT

Figure 2 presents the results of open-circuit potential tests conducted on the sample under varying static pressure conditions. The data indicate that all curves exhibit a rapid decline during the initial phase, followed by a gradual stabilization of potential over time. The slope of the curves diminishes, suggesting that the material surface approaches a relatively stable state. Notably, the curve corresponding to a static pressure of 500 N displays minor potential fluctuations during the stabilization phase, which may be attributed to environmental factors at the testing site, localized corrosion on the material’s surface, or the formation and subsequent breakdown of a passivation film.

The open-circuit potential values for each sample are summarized in

Table 3. Open-circuit potential data of different samples.

Sample	Untreated	200 N	400 N	500 N
Open-Circuit Potential (V)	−0.657	−0.654	−0.650	−0.649

. The sample that did not undergo ultrasonic rolling extrusion treatment ultimately stabilized at an open-circuit potential of −0.657 V. In contrast, the sample subjected to a static pressure of 200 N stabilized at −0.654 V. When the static pressure was increased to 400 N, the open-circuit potential stabilized at −0.650 V and, at 500 N, it further stabilized at −0.649 V. These findings indicate that, although the open-circuit potential exhibits minimal variation with increasing static pressure during ultrasonic rolling extrusion, there is a gradual positive trend, suggesting a reduction in the corrosion tendency and an enhancement in corrosion resistance of the sample.

3.1.2. Tafel

Figure 3 illustrates that all curves exhibit a characteristic Tafel curve shape, wherein the current density remains relatively stable in the low-potential region but increases sharply in the high-potential region. This behavior indicates that the electrochemical corrosion characteristics of GCr15 bearing steel under different static pressure conditions share similar fundamental traits.

Table 4. Corrosion data from Tafel tests on different samples.

Sample	Polarization Resistance (Ω)	Corrosion Current (A)	Corrosion Potential (V)	Corrosion Rate (g/m2·a)
Untreated	169.5	$2.459\times {10}^{-4}$	−1.074	51.90838422
200 N	535.3	$7.679\times {10}^{-5}$	−1.077	15.68546983
400 N	2244.5	$1.628\times {10}^{-5}$	−0.964	4.933485458
500 N	11360.2	$2.947\times {10}^{-6}$	−0.675	1.008556627

delineates the results of Tafel tests conducted on various samples, encompassing measurements of linear polarization resistance, corrosion current, and corrosion potential. In comparison to untreated samples, at a static pressure of 200 N, the linear polarization resistance exhibited a twofold increase, while the corrosion current experienced a reduction of two thirds, with negligible variation in corrosion potential. This phenomenon is attributed to the ultrasonic rolling extrusion process, which modifies the surface structure without significantly altering the distribution of electrochemically active centers, thereby preserving a stable local chemical environment. At a static pressure of 400 N, the polarization resistance increased fourfold, the corrosion current decreased by four fifths, and the corrosion potential exhibited a slight positive shift. Samples subjected to a static pressure of 500 N demonstrated the highest polarization resistance, the lowest corrosion current, and the most positive corrosion potential.

Linear polarization resistance and corrosion current serve as critical metrics for assessing the corrosion rate of materials. Specifically, an increase in resistance and a decrease in current indicate enhanced corrosion resistance, while a more positive corrosion potential correlates with greater resistance to corrosion. The findings indicate that, as static pressure increases, linear polarization resistance rises, corrosion current diminishes, and corrosion potential exhibits a progressive positive shift. Utilizing the Stern–Geary equation in conjunction with Tafel experimental data, the corrosion rates of various samples were determined. The findings indicated that the samples subjected to conventional processing exhibited the highest corrosion rates. In contrast, a significant reduction in corrosion rates was observed following ultrasonic rolling extrusion treatment, with a further decline noted as static pressure increased. Consequently, the corrosion resistance of the material is markedly enhanced with increasing static pressure.

3.1.3. EIS

Figure 4a illustrates the frequency variation curve of the impedance modulus for the GCr15 bearing steel sample and the untreated sample subjected to ultrasonic rolling under varying static pressure conditions. The overall trend reveals that the impedance modulus for all samples decreases progressively with increasing frequency, ultimately stabilizing. In the low-frequency range (below 10 Hz), the untreated sample displays a relatively low impedance modulus. In contrast, the impedance modulus of the sample subjected to 200 N of static pressure is higher than that of the untreated sample, while the 400 N sample exhibits further enhancement. The impedance modulus for the 500 N sample reaches its peak in the low-frequency region.

Figure 4b presents the frequency variation curve of the phase angle for the GCr15 bearing steel sample and the untreated sample following ultrasonic rolling under different static pressure conditions. All curves initially increase before decreasing, exhibiting phase peaks within the intermediate frequency range. This behavior reflects the response characteristics of various components within the electrochemical system across different frequencies.

Electrochemical impedance spectroscopy (EIS) is employed to assess the charge transfer resistance of samples subjected to various treatment conditions. Figure 5 illustrates the Nyquist plots for each sample under differing static pressure conditions, revealing an irregular semicircular arc in the high-frequency region and a diagonal line with a defined arc in the low-frequency region. In the context of the Nyquist plot, the diameter of the semicircle is directly proportional to the charge transfer resistance (Rct), indicating that the corrosion process is predominantly governed by charge transfer mechanisms [27]. The untreated sample exhibits the smallest semicircle radius in the low-frequency region, suggesting minimal surface charge transfer resistance and a heightened susceptibility to corrosion. At a static pressure of 200 N, the semicircle radius in the low-frequency region increases, signifying an enhancement in the electrochemical performance of the sample surface following ultrasonic rolling treatment at this pressure, which results in an increase in charge transfer resistance (Rct) and a corresponding inhibitory effect on the corrosion process, albeit within a limited range. As the static pressure escalates to 400 N, the semicircle radius further increases, correlating with an additional rise in charge transfer resistance. Upon reaching a static pressure of 500 N, the semicircle radius attains its maximum value, indicating the highest level of charge transfer resistance and the least likelihood of corrosion occurrence.

3.2. Microstructure and Chemical Characterization

3.2.1. Microstructure

Figure 6 illustrates the surface microstructure of GCr15 bearing steel samples that have undergone strengthening through ultrasonic rolling under varying static pressure conditions, following electrochemical corrosion testing. Panels (a), (b), and (c) depict the surface morphology of samples subjected to static pressures of 200 N, 400 N, and 500 N, respectively, at a magnification of 200 μm. Conversely, panels (d), (e), and (f) present the surface characteristics of the same samples at a higher magnification of 50 μm. The sample treated under a static pressure of 200 N exhibited pronounced surface roughness, characterized by irregular protrusions and indentations, along with visible cracks and areas of spalling. In contrast, the sample subjected to a static pressure of 400 N displayed a markedly smoother surface, with a significant reduction in both crack and spalling areas, resulting in a more uniform topography. Following treatment at 500 N static pressure, the surface roughness of the sample was further improved, with a notable decrease in the size of cracks and spalling areas, yielding a smoother texture. These findings indicate that treatment at 500 N static pressure further optimizes the surface topography of the sample, enhances the density of the material’s surface, and effectively mitigates corrosion resulting from environmental exposure.

Figure 7 presents optical images of various samples, with Figure 7a,e depicting samples subjected to static pressures of 200 N and 500 N, respectively, under conditions devoid of electrochemical corrosion, at a magnification of 50 μm. Figure 7b–d illustrate the optical images of the samples at a scale of 500 μm following electrochemical corrosion at 200 N, 400 N, and 500 N, respectively. Additionally, Figure 7f–h display the optical images of the samples at a scale of 50 μm after undergoing electrochemical corrosion at the same static pressures.

A comparative analysis of the optical microscope images reveals that the surfaces of the samples subjected to static pressures of 200 N and 500 N exhibited characteristics indicative of ultrasonic rolling extrusion strengthening in the absence of electrochemical corrosion, with the surface treatment at 500 N (Figure 7e) demonstrating a more compact structure. Following electrochemical corrosion, at the 500 μm scale, Figure 7b (200 N) reveals the presence of dispersed corrosion pits, while Figure 7c (400 N) shows a reduction in the number of corrosion pits, which are more sparsely distributed. Figure 7d (500 N) indicates a further decrease in both the number and size of corrosion pits, suggesting that increased static pressure correlates with enhanced resistance to electrochemical corrosion. At the 50 μm scale, Figure 7f (200 N) displays deep cross-corrosion cracks, with both the depth and width of these cracks diminishing in Figure 7g (400 N). In Figure 7h (500 N), only a minimal accumulation of corrosion products is observed, indicating a less pronounced corrosion characteristic. In conclusion, as the static pressure of ultrasonic rolling extrusion is increased from 200 N to 500 N, the electrochemical corrosion resistance of GCr15 bearing steel is progressively enhanced. The strengthened surface structure resulting from higher static pressures is more effective in impeding the penetration of corrosive media and mitigating the extent of corrosion. This corresponds to the results obtained by SEM above.

Figure 8 presents the X-ray diffraction (XRD) patterns for various samples, with each phase denoted by distinct symbols. The findings indicate that samples not subjected to electrochemical corrosion retain the original phase structure characteristic of GCr15 bearing steel, predominantly exhibiting the α-Fe phase. The intensity of the diffraction peaks varies in response to different static pressures. Following the corrosion of the various samples, a layer of iron oxide is observed on the surface. In conjunction with the electrochemical test results, it is suggested that iron oxides and hydroxides may manifest as films, thereby enhancing the corrosion resistance of the samples. Notably, there is no significant variation in the phase composition of the corrosion products across different power levels.

3.2.2. Chemical Characterization

Figure 9 presents the results of X-ray photoelectron spectroscopy (XPS) analysis for the C1s, O1s, Cr2p, and Fe2p regions of a sample subjected to ultrasonic rolling extrusion under a static pressure of 200 N, followed by electrochemical corrosion treatment. The findings indicate that carbon (C) predominantly exists in the form of C-C bonds on the sample’s surface. As the etching process progresses, the C element transitions from a singular C-C bond configuration to a state where C-C bonds coexist with C-Fe bonds. Concurrently, the intensity of the C-C bond peak diminishes, while the intensity of the C-Fe bond peak increases. Regarding oxygen (O), it primarily exists as metal oxides and metal carbonates on the surface of the sample. As etching depth increases, the peak intensity associated with metal carbonates decreases, whereas that of metal oxides increases. Upon reaching a certain etching depth, oxygen is predominantly found in the form of metal oxides. Chromium (Cr) is primarily present in the chemical state of Cr⁵ on the sample surface, which gradually converts to Cr³ as etching depth increases. Ultimately, with further etching, Cr is found in its elemental form. Iron (Fe) is mainly present as Fe³⁺ (FeO(OH)) and Fe³⁺ (Fe₂O₃) on the surface. As etching continues, the chemical state of Fe evolves, ultimately resulting in the presence of elemental Fe.

Figure 10 illustrates the X-ray photoelectron spectroscopy (XPS) results for the C1s, O1s, Cr2p, and Fe2p regions of samples treated with electrochemical corrosion at a static pressure of 500 N through ultrasonic rolling extrusion. The analysis reveals that carbon (C) primarily exists as C-C bonds on the sample surface, with a slight decrease in the peak intensity corresponding to C-C bonds as etching progresses. For oxygen (O), it is predominantly found in the form of metal oxides and metal carbonates, reflecting the chemical reactions with oxygen occurring on the sample surface during electrochemical corrosion. The formation of metal oxides and carbonates may be influenced by the sample’s composition and the surrounding corrosion environment. Iron (Fe) is primarily present on the surface in the forms of Fe³⁺ (FeO(OH)), Fe²⁺, and Fe³⁺ (Fe₃O₄). As etching depth increases, the chemical state of Fe transitions to Fe³ (Fe₂O₃), Fe²⁺, and Fe³⁺ (Fe₃O₄), indicating changes in the chemical stability and reactivity of Fe at varying depths. The absence of a distinct X-ray photoelectron spectroscopy (XPS) peak for chromium (Cr) may be attributed to the masking effect of iron oxides, which obscure the Cr signal. The formation of an iron oxide coating on the sample’s surface likely inhibits effective X-ray interaction with Cr, thereby complicating the detection of its signal.

Analysis of Figure 9 and Figure 10 indicates that the static pressure applied during ultrasonic rolling extrusion significantly affects the composition, thickness, structure, and properties of the passivation film on the sample’s surface. At a static pressure of 200 N, the primary constituents of the passivation film, following electrochemical corrosion treatment, were identified as FeO(OH) and Fe₂O₃. XPS analysis, in conjunction with relevant microstructural characterization results, suggests that the passivation film formed under these conditions is relatively thin. This may be due to the limited atomic migration and recombination occurring on the material’s surface at lower static pressures, which results in a slower and less effective growth of the passivation film.

In contrast, when the static pressure was increased to 500 N, a notable alteration in the composition of the passivation films was observed, with the primary components being Fe₂O₃ and Fe₃O₄. A comparative analysis of the microstructure and properties of samples subjected to varying static pressures revealed that the passivation film formed at 500 N was both thicker and denser than that formed at 200 N. This enhancement in film characteristics is likely due to the increased atomic activity and migration facilitated by the higher static pressure, which promotes more efficient accumulation and arrangement of the passivation film during its growth, resulting in a more cohesive and compact structure. Such a thicker and denser passivation film exhibits greater stability and provides enhanced protection for the underlying matrix.

From a corrosion protection perspective, the thickness and density of the passivation film serve as a critical isolation layer that directly influences the extent of contact between external corrosive agents and the base material. A thicker and denser passivation film effectively mitigates the penetration and diffusion of corrosive media, thereby reducing the likelihood of corrosion reactions occurring within the matrix material and significantly improving the sample’s corrosion resistance. In conclusion, appropriately increasing the static pressure during ultrasonic rolling extrusion is beneficial for enhancing the performance of the passivation film on the sample surface and improving the material’s corrosion resistance.

4. Conclusions

This study aims to evaluate the corrosion resistance of GCr15 bearings that have been enhanced through ultrasonic rolling extrusion, utilizing an electrochemical workstation for testing. Additionally, the research includes an analysis of the microstructure and chemical properties of the material. The principal findings are summarized as follows:

(1)

Electrochemical corrosion: an increase in static pressure during ultrasonic rolling extrusion results in a marked enhancement of linear polarization resistance, a substantial reduction in corrosion current, a forward shift in corrosion potential, and a decrease in corrosion susceptibility. Additionally, both the low-frequency impedance modulus and charge transfer resistance exhibit an upward trend with increasing static pressure. Following the application of ultrasonic rolling extrusion treatment, there was a marked reduction in the corrosion rate of the samples. Furthermore, it was observed that, as static pressure increased, the corrosion rate continued to decline progressively.

(2)

Microstructure and surface quality: the enhancement of static pressure during ultrasonic rolling extrusion leads to improved surface flatness of GCr15 bearing steel, significantly reducing defects such as cracks, spalling, and voids, while continuously optimizing the microstructure. At a static pressure of 200 N, the surface exhibits considerable roughness and numerous defects. In contrast, at 400 N, there is a notable improvement in surface quality and, at 500 N, the surface becomes denser and smoother, effectively impeding the penetration of external corrosive agents, thereby augmenting the material’s corrosion resistance.

(3)

Passivation film thickness and densification: X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses reveal that the static pressure applied during ultrasonic rolling extrusion significantly influences the composition, thickness, and structure of the passivation film on the surface of GCr15 bearing steel. As static pressure increases, both the thickness and densification of the passivation films improve markedly. The passivation films, primarily composed of Fe₂O₃ and Fe₃O₄, exhibit enhanced densification and stability at a static pressure of 500 N.

The above results show that the ultrasonic rolling extrusion treatment can effectively improve the electrochemical properties of GCr15 bearing steel, and the higher the static pressure, the stronger the corrosion resistance, among which the sample after 500 N static pressure treatment shows the best corrosion resistance.