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Salt-Fog Corrosion Behavior of GCr15 Steels Treated by Ultrasonic Strengthening Grinding Process

Salt-Fog Corrosion Behavior of GCr15 Steels Treated by Ultrasonic Strengthening Grinding Process applied sciences Article Salt-Fog Corrosion Behavior of GCr15 Steels Treated by Ultrasonic Strengthening Grinding Process 1 , 2 , 3 1 , 2 4 3 3 3 , Xincheng Xie , Zhongning Guo , Zhuan Zhao , Zhongwei Liang , Jun Wu , Xiaochu Liu * 3 , and Jinrui Xiao * State Key Laboratory of Precision Electronic Manufacturing Technology and Equipment, Guangdong University of Technology, Guangzhou 510006, China; cxixie@163.com (X.X.); znguo@gdut.edu.cn (Z.G.) Guangzhou Key Laboratory of Nontraditional Machining and Equipment, Guangzhou 510006, China Guangdong Engineering Research Centre for Strengthen Grinding and Micro/Nano High-Performance Machining, Guangzhou University, Guangzhou 510006, China; liangzhongwei@gzhu.edu.cn (Z.L.); wujunair2021@163.com (J.W.) School of Physics and Materials Science, Guangzhou University, Guangzhou 510006, China; zhaozhuan2008@gmail.com * Correspondence: xiaochuliu64@163.com (X.L.); meexiaojinrui@gzhu.edu.cn (J.X.) Abstract: In this paper, the corrosion resistance of four GCr15 steel samples has been investigated. Three samples were initially surface-treated by ultrasonic shot peening (USP), a dry ultrasonic strengthening grinding process (DUSGP), and a wet ultrasonic strengthening grinding process (WUSGP), respectively, while another one was only polished. Then, all the processed samples were subjected to salt spray corrosion. Sample properties, such as capacitance arc, self-corrosion potential (E ), self-corrosion current density (I ), corrosion morphology, and corrosion products were corr corr analyzed. Results show that the sample treated by WUSGP has the best corrosion resistance, which can be attributed to the high dislocation density, small grain size, needle-like and lath-like shape of Citation: Xie, X.; Guo, Z.; Zhao, Z.; martensite content formed in the treatment, which hinders Cl from eroding the matrix. The obtained Liang, Z.; Wu, J.; Liu, X.; Xiao, J. corrosion pits and cracks in Sample WUSGP show a width of approximately 1.4 m and 2.1 m, Salt-Fog Corrosion Behavior of respectively, degrading 78% and 75% compared to polishing. The possible corrosion mechanism GCr15 Steels Treated by Ultrasonic of the samples has been discussed. The findings denote that the treatment fused ceramic balls, Strengthening Grinding Process. strengthened liquid, and corundum in USP could be an efficient method to improve the corrosion Appl. Sci. 2022, 12, 7360. https:// resistance of some mechanical parts. doi.org/10.3390/app12157360 Academic Editor: Ioannis Keywords: GCr15 steel; ultrasonic strengthening grinding process; salt spray corrosion; corrosion Kartsonakis resistance Received: 6 July 2022 Accepted: 16 July 2022 Published: 22 July 2022 1. Introduction Publisher’s Note: MDPI stays neutral GCr15 steels are widely used to manufacture mechanical components, such as with regard to jurisdictional claims in bearings [1,2], ball screws [3], and steel balls [4], mainly due to their high hardness and published maps and institutional affil- wear resistance [3]. These mechanical components have played a significant role in many iations. industrial fields, such as automotive, aviation, mining, and marine equipment [3,5]. Met- als are easily corroded when working in a marine environment, which causes economic and material loss for marine equipment [6]. The corrosion of metal components can be at- Copyright: © 2022 by the authors. tributed to multiple factors, including temperature [7,8], the PH value of the seawater [9,10], Licensee MDPI, Basel, Switzerland. micro-organisms [11], etc. Consequently, a severe degradation occurs in the mechanical This article is an open access article properties of the metals and finally leads to fatigue failure. distributed under the terms and To address this issue, many methods have been proposed for the long-term pro- conditions of the Creative Commons tection of steel components, such as surface coating [12–15] and surface chemical heat Attribution (CC BY) license (https:// treatment [16–18]. In particular, surface coating attracts widespread research interests, creativecommons.org/licenses/by/ and great efforts for developing high-performance protective coating have been studied. 4.0/). Appl. Sci. 2022, 12, 7360. https://doi.org/10.3390/app12157360 https://www.mdpi.com/journal/applsci Appl. Sci. 2022, 12, x FOR PEER REVIEW  2  of  14  Appl. Sci. 2022, 12, 7360 2 of 13 example, Al coating developed by the thermal‐sprayed method can provide efficient pro‐ For example, Al coating developed by the thermal-sprayed method can provide efficient tection for steels due to the existence of passivating film [19,20]. Nevertheless, a serious  protection for steels due to the existence of passivating film [19,20]. Nevertheless, a serious pitting problem occurs once the passivating film has been damaged [21,22]. Moreover, the  pitting problem occurs once the passivating film has been damaged [21,22]. Moreover, Al coatings formed by thermal‐spraying are easy to peel off under heavy loads [23]. Zn‐ the Al coatings formed by thermal-spraying are easy to peel off under heavy loads [23]. Al [12,24], Zr‐Ti [13,25], and NiCr‐Cr3C2 [14,15] coatings have also been studied broadly.  Zn-Al [12,24], Zr-Ti [13,25], and NiCr-Cr C [14,15] coatings have also been studied broadly. 3 2 An example of corrosion behavior of a Zn‐Al composite coating in a salt spray corrosive  An example of corrosion behavior of a Zn-Al composite coating in a salt spray corrosive environment, which exhibits the excellent anti‐corrosion properties by absorbing the ben‐ environment, which exhibits the excellent anti-corrosion properties by absorbing the ben- efits of both Zn and Al, has been investigated by Zhao et al. [12]. However, the good cor‐ efits of both Zn and Al, has been investigated by Zhao et al. [12]. However, the good rosion resistance of Zn‐Al coatings is only suitable for low‐carbon steel substrates [12].  corrosion resistance of Zn-Al coatings is only suitable for low-carbon steel substrates [12]. With the rapid development of the mechanical industry, new techniques to improve the  With the rapid development of the mechanical industry, new techniques to improve the anti‐corrosion properties of steel are urgently needed.   anti-corrosion properties of steel are urgently needed. As an alternative, mechanical strengthening has emerged as a promising solution to  As an alternative, mechanical strengthening has emerged as a promising solution to ob- obtain a good corrosion resistance for metals, mainly due to the chilled layer and com‐ tain a good corrosion resistance for metals, mainly due to the chilled layer and compressive pressive residual stress microstructures formed on metal surfaces after surface treatment  residual stress microstructures formed on metal surfaces after surface treatment [26–28], [26–28], which results in the improvement of fatigue life. So far, mechanical strengthening  which results in the improvement of fatigue life. So far, mechanical strengthening tech- techniques, such as shot peening (SP) [29,30] and rolling finishing [31,32] have been pro‐ niques, such as shot peening (SP) [29,30] and rolling finishing [31,32] have been proposed posed extensively for corrosion resistance investigation of metals. However, traditional  extensively for corrosion resistance investigation of metals. However, traditional shoot shoot peening typically produces a high roughness on the metal surface, and the protec‐ peening typically produces a high roughness on the metal surface, and the protection ability tion ability is still far from the practical demands [33]. On the other hand, the hardened  is still far from the practical demands [33]. On the other hand, the hardened layer produced layer produced by rolling and the internal material have an obvious delamination phe‐ by rolling and the internal material have an obvious delamination phenomenon [34], which nomenon [34], which easily causes the surface layer to fall off. In contrast, ultrasonic shot  easily causes the surface layer to fall off. In contrast, ultrasonic shot peening has the charac- peening has the characteristics of low cost, large shape correction range, small footprint,  teristics of low cost, large shape correction range, small footprint, good controllability and good controllability and repeatability. Surprisingly, the use of ultrasonic shot peening for  repeatability. Surprisingly, the use of ultrasonic shot peening for improving the corrosion improving the corrosion resistance of GCr15 steels is poorly investigated.  resistance of GCr15 steels is poorly investigated. In this work, the corrosion behavior of four GCr15 steel samples has been investi‐ In this work, the corrosion behavior of four GCr15 steel samples has been investigated gated in a salt spray corrosion environment. Samples are surface‐treated with ultrasonic  in a salt spray corrosion environment. Samples are surface-treated with ultrasonic shot shot peening (USP), a dry ultrasonic strengthening grinding process (DUSGP), a wet ul‐ peening (USP), a dry ultrasonic strengthening grinding process (DUSGP), a wet ultrasonic trasonic strengthening grinding process (WUSGP), and polishing, respectively. The ca‐ strengthening grinding process (WUSGP), and polishing, respectively. The capacitance arc pacitance arc and polarization curves of these samples were studied. The corrosion mor‐ and polarization curves of these samples were studied. The corrosion morphology and phology and products were further analyzed to reveal possible corrosion mechanisms.  products were further analyzed to reveal possible corrosion mechanisms. 2. Materials and Methods  2. Materials and Methods 2.1. Samples Preparation  2.1. Samples Preparation Four GCr15 steel samples with geometrical parameters of 100 mm × 75 mm × 10 mm  Four GCr15 steel samples with geometrical parameters of 100 mm  75 mm  10 mm (See Figure 1a) were employed in this study. Their main chemical composition is shown  (See Figure 1a) were employed in this study. Their main chemical composition is shown in in Table 1, which was analyzed by Energy Dispersive Spectroscopy (EDS). The heat treat‐ Table 1, which was analyzed by Energy Dispersive Spectroscopy (EDS). The heat treatment ment procedure is shown Figure 1b. The samples were initially quenched at a temperature  procedure is shown Figure 1b. The samples were initially quenched at a temperature of of 855 °C to obtain a good hardness, followed by an oil cooling process. Then, the samples  855 C to obtain a good hardness, followed by an oil cooling process. Then, the samples were treated with a tempering process of 120 min at 160 °C to reduce the brittleness of the  were treated with a tempering process of 120 min at 160 C to reduce the brittleness of the quenched workpiece. Finally, the sample surfaces were polished with 200# sandpaper to  quenched workpiece. Finally, the sample surfaces were polished with 200# sandpaper to obtain a roughness < 0.2 μm and cleaned with alcohol through an ultrasonic bath.  obtain a roughness < 0.2 m and cleaned with alcohol through an ultrasonic bath. (a)  (b)  Figure 1. (a) The geometric parameters of samples; (b) the heating process.  Figure 1. (a) The geometric parameters of samples; (b) the heating process.   Appl. Sci. 2022, 12, 7360 3 of 13 Appl. Sci. 2022, 12, x FOR PEER REVIEW  3  of  14  Table 1. Chemical element composition of GCr15 steels. Table 1. Chemical element composition of GCr15 steels.  P S Ni Cu Chemical El‐ P  S  Ni  Cu  Chemical Element C Si Mn Cr C  Si  Mn  Cr  ement  ≤  Content  Content (wt%) 0.95~1.05 0.15~0.35 0.20~0.40 1.30~1.60 0.027 0.02 0.30 0.25 0.95~1.05 0.15~0.35  0.20~0.40  1.30~1.60  0.027  0.02  0.30  0.25  (wt%)  2.2. Surface Treatment of the Samples 2.2. Surface Treatment of the Samples  Three samples were surface-treated by ultrasonic processing equipment, while another Three samples were surface‐treated by ultrasonic processing equipment, while an‐ sample was without surface treatment for experimental comparison. The working principle other sample was without surface treatment for experimental comparison. The working  of the ultrasonic equipment is reported in Figure 2. First, ultrasonic waves were produced principle of the ultrasonic equipment is reported in Figure 2. First, ultrasonic waves were  by an electro-mechanical ultrasonic transducer and then applied to a workpiece. An produced by an electro‐mechanical ultrasonic transducer and then applied to a workpiece.  ultrasonic transducer was utilized to energize an acoustically tuned resonator bar, causing An ultrasonic  transducer was utilized to energize an acoustically tuned resonator bar,  it to vibrate. Finally, the energy generated from these impulses was imparted to treat the causing it to vibrate. Finally, the energy generated from these impulses was imparted to  surface through the ceramic balls. The main difference between the surface treatment of treat the surface through the ceramic balls. The main difference between the surface treat‐ the ment thr of ee thsamples e three sa was mple the s wa employment s the employof ment dif fer of ent differ trent eatment  treatm materials, ent material ass,depicted  as de‐ in Figur picted e in 2b–d.  Figure The  2b–d. working  The working parameters  paramet of e ultrasonic rs of ultraspr onocessing ic processising pr is esented  presentin ed Tin able   2. One Tablesample  2. One was samptr leeated  was treat withedceramic  with cera balls, mic ball while s, while the other  the ottwo her two samples  samplwer es wer e treeated   with treatemixed d with mixed materials.  materia The ls. surface The surfa trceatment e treatment that tha only t only consider  consider ededceramic  ceramic balls balls  was was named USP, and mixed white corundum and the ceramic balls was named DUSGP.  named USP, and mixed white corundum and the ceramic balls was named DUSGP. The The other one that mixed white corundum and ceramic ball, as well as strengthening liq‐ other one that mixed white corundum and ceramic ball, as well as strengthening liquid uid was named WUSGP [35]. The strengthening liquid was composed of extrusion addi‐ was named WUSGP [35]. The strengthening liquid was composed of extrusion additive tive (i.e., C10H14N2Na2O8∙2H2O), triethanolamine (C6H15NO3) and water. The samples sur‐ (i.e., C H N Na O 2H O), triethanolamine (C H NO ) and water. The samples surface- 10 14 2 2 8 2 6 15 3 face‐treated by USP, DUSGP, WUSGP, and polishing are named Sample USP, DUSGP,  treated by USP, DUSGP, WUSGP, and polishing are named Sample USP, DUSGP, WUSGP, WUSGP, and polished, respectively.  and polished, respectively. Figure 2. (a) Schematic diagram of ultrasonic processing equipment; (b) ultrasonic shoot peening  Figure 2. (a) Schematic diagram of ultrasonic processing equipment; (b) ultrasonic shoot peening treatment; (c) dry ultrasonic strengthening grinding process treatment; (d) wet ultrasonic strength‐ treatment; (c) dry ultrasonic strengthening grinding process treatment; (d) wet ultrasonic strengthen- ening grinding process treatment.  ing grinding process treatment.    2.3. Salt Spray Process of the Samples Each processed sample was cut into three plates with a diameter of 10 mm to calculate the mean values of experimental data. The salt frog test was performed using HT-YW-60 salt spray test machine and based on the standard ASTM B117. The working temperature was 35 (2) C, the precipitation rate of the salt spray was 1.5 mL/(cm h) with an air pressure of 96 kpa. PH value was set in the range of 6.5–7.2, and the total spray time was 96 h. Appl. Sci. 2022, 12, 7360 4 of 13 Table 2. Working parameters of ultrasonic processing. Parameters Value Vibration frequency (kHz) 20 Peening distance (mm) 30 Processing time (min) 3 Diameter of ceramic balls (mm) 1 Grain size of brown corundum (m) 15 2.4. Characterization The cut samples were mounted with epoxy and then used to make electrodes by welding a wire on their surface. The electrochemical impedance spectroscopic (EIS) mea- surement has been carried out using Solartron 1260 + 1287 electrochemical workstation. The potentiodynamic polarization test was performed at a scan rate of 1.5 mV/s in the range of 0.5 V–5 V. An X-ray diffractometer (XRD, Rigaku + UltimaIV, Rigaku Corp, Akishima, Tokyo, Japan) was used to analyze the phase and corrosion components of the samples. The scan- ning electron microscope (SEM, ZEISS Gemini 300, Carl Zeiss AG, Oberkochen, Germany) coupled with an energy dispersive spectrometer (EDS, Oxford XMax 50, Oxford, UK) was employed to observe the surface micromorphology. Autofocus Microscope (GP420H, Kunshan gaopin precision instrument co. LTD, Kunshan, China) was utilized to observe the macromorphology. 3. Results and Discussion Figure 3 reports the Nyquist diagram of all the samples immersed in 3.5% NaCl solution made in the laboratory for 96 h. The capacitive loop was utilized to characterize the charge transfer resistance at the interface of corrosion products. It can be seen that the capacitive loop of Sample polished is much smaller than the other samples, which can be attributed to the accumulation of corrosion products. The capacitive loops of the samples Appl. Sci. 2022, 12, x FOR PEER REVIEWwith   surface treatment are significantly improved. In particular, the biggest capacitive 5  ofar  14 c  radius was obtained in Sample WUSGP. The magnitudes of the charge transfer resistance of all the samples are ranked as follows: WUSGP > DUSGP > USP > Polished. Figure 3. Nyquist plots of the samples immersed in seawater for 96 h. Figure 3. Nyquist plots of the samples immersed in seawater for 96 h.  The corrosion potential (E ) and the corrosion current density (I ) are critical corr corr The corrosion potential (Ecorr) and the corrosion current density (Icorr) are critical pa‐ parameters to evaluate the performance of corrosion resistance, which are calculated rameters  to  evaluate  the  performance  of  corrosion  resistance,  which  are  calculated  through Tafel extrapolation [36]. The polarization curves of the samples and the obtained through Tafel extrapolation [36]. The polarization curves of the samples and the obtained  results are reported in Figure 4 and Table 3. It can be found that the Sample WUSGP has the results are reported in Figure 4 and Table 3. It can be found that the Sample WUSGP has  largest corrosion potential (i.e., 0.3999), while the corrosion potentials of Sample DUSGP the  largest  corrosion  potential  (i.e., −0.3999),  while  the  corrosion  potentials  of  Sample  and USP are relatively close, realizing a value of 0.5926 V and 0.5122 V, respectively. DUSGP and USP are relatively close, realizing a value of −0.5926 V and −0.5122 V, respec‐ tively. This can be by explained considering the effect of the strengthening liquid in sur‐ face treatment, which degrades the corrosion potential of Sample WUSGP. Generally, the  lower the corrosion potential, the worse the corrosion resistance. Furthermore, the corro‐ sion current of sample WUSGP is two times lower than sample Polished, exhibiting a  value of 9.435 mA/cm . A smaller corrosion current means a lower corrosion rate. There‐ fore, sample WUSGP possesses the best properties of corrosion resistance.  Figure 4. The polarization curves of all the samples.  Table 3. Corrosion potential (Ecorr) and the corrosion current density (Icorr) obtained in electrochem‐ ical corrosion test for each sample.  −2 Serial Number  Ecorr/V  Icorr/(mA·cm )  Polished −0.7309  18.15  USP −0.5926  15.28  DUSGP −0.5122  12.79  WUSGP −0.3999  9.435  Appl. Sci. 2022, 12, x FOR PEER REVIEW  5  of  14  Figure 3. Nyquist plots of the samples immersed in seawater for 96 h.  The corrosion potential (Ecorr) and the corrosion current density (Icorr) are critical pa‐ rameters  to  evaluate  the  performance  of  corrosion  resistance,  which  are  calculated  through Tafel extrapolation [36]. The polarization curves of the samples and the obtained  results are reported in Figure 4 and Table 3. It can be found that the Sample WUSGP has  the  largest  corrosion  potential  (i.e., −0.3999),  while  the  corrosion  potentials  of  Sample  DUSGP and USP are relatively close, realizing a value of −0.5926 V and −0.5122 V, respec‐ Appl. Sci. 2022, 12, 7360 5 of 13 tively. This can be by explained considering the effect of the strengthening liquid in sur‐ face treatment, which degrades the corrosion potential of Sample WUSGP. Generally, the  This can be by explained considering the effect of the strengthening liquid in surface lower the corrosion potential, the worse the corrosion resistance. Furthermore, the corro‐ treatment, which degrades the corrosion potential of Sample WUSGP. Generally, the lower sion current of sample WUSGP is two times lower than sample Polished, exhibiting a  the corrosion potential, the worse the corrosion resistance. Furthermore, the corrosion value of 9.435 mA/cm . A smaller corrosion current means a lower corrosion rate. There‐ current of sample WUSGP is two times lower than sample Polished, exhibiting a value fore, of sa 9.435 mpl mA/cm e WUSGP . A smaller  posses corr sesosion  the best current  properties means a lower  of corrosion corrosion rate. resist Ther anc efor e.  e, sample WUSGP possesses the best properties of corrosion resistance. Figure 4. The polarization curves of all the samples. Figure 4. The polarization curves of all the samples.  Table 3. Corrosion potential (E ) and the corrosion current density (I ) obtained in electrochemi- corr corr Table 3. Corrosion potential (Ecorr) and the corrosion current density (Icorr) obtained in electrochem‐ cal corrosion test for each sample. ical corrosion test for each sample.  Serial Number E /V I /(mAcm ) corr corr −2 Polished 0.7309 18.15 Serial Number  Ecorr/V  Icorr/(mA·cm )  USP 0.5926 15.28 Polished −0.7309  18.15  DUSGP 0.5122 12.79 USP −0.5926  15.28  WUSGP 0.3999 9.435 DUSGP −0.5122  12.79  WUSGP −0.3999  9.435  The X-ray diffraction patterns of all the samples are shown in Figure 5a. It is noted that the new diffraction peaks occurred in the surface-treated samples, such as (200). The new diffraction peak is particularly obvious in Sample WUSGP. To further understand the possible mechanism, the grain size and dislocation density of these samples have been calculated (See Figure 5b) through Williamson–Hall equations [37]. Sample polished has the maximum grain size and the minimal dislocation density, realizing a value of 28.7 m 16 2 and 0.3658  10 m , respectively. After surface treatment, the grain size decreases, and the dislocation density increases. In sample WUSGP, the grain size is refined to 6.45 m, 16 2 and the dislocation density increased to 2.639  10 m , which is mainly due to the plastic deformation of the surface tissue after intensive grinding and the phase transforms, resulting in an increase in the martensite content (See Figure 5a). The martensite structures typically show a shape of needle and lath, making the surface structure denser. Previous studies demonstrated that the dense structure of high dislocation density and small grain size can significantly improve the corrosion resistance of the material [38]. This evidence further illustrates the benefits that USGP may bring. Appl. Sci. 2022, 12, x FOR PEER REVIEW  6  of  14  The X‐ray diffraction patterns of all the samples are shown in Figure 5a. It is noted  that the new diffraction peaks occurred in the surface‐treated samples, such as (200). The  new diffraction peak is particularly obvious in Sample WUSGP. To further understand  the possible mechanism, the grain size and dislocation density of these samples have been  calculated (See Figure 5b) through Williamson–Hall equations [37]. Sample polished has  the maximum grain size and the minimal dislocation density, realizing a value of 28.7 μm  16 −2 and 0.3658 × 10  m , respectively. After surface treatment, the grain size decreases, and  the dislocation density increases. In sample WUSGP, the grain size is refined to 6.45 μm,  16 −2 and the dislocation density increased to 2.639 × 10  m , which is mainly due to the plastic  deformation of the surface tissue after intensive grinding and the phase transforms, re‐ sulting in an increase in the martensite content (See Figure 5a). The martensite structures  typically show a shape of needle and lath, making the surface structure denser. Previous  studies demonstrated that the dense structure of high dislocation density and small grain  Appl. Sci. 2022, 12, 7360 6 of 13 size can significantly improve the corrosion resistance of the material [38]. This evidence  further illustrates the benefits that USGP may bring.  Figure 5. (a) The X‐ray diffraction pattern characteristics on sample surface; (b) dislocation density  Figure 5. (a) The X-ray diffraction pattern characteristics on sample surface; (b) dislocation density and grain size of samples.  and grain size of samples. Figure 6 shows the surface macro‐morphology of the samples. It is found that most  Figure 6 shows the surface macro-morphology of the samples. It is found that most regions on the matrix surface of Sample polished are corroded (See Figure 6a), and the  regions on the matrix surface of Sample polished are corroded (See Figure 6a), and the width of corrosion cracks is larger than in other samples. The corroded area of Sample  width of corrosion cracks is larger than in other samples. The corroded area of Sample USP, USP, DUSGP, and WUSGP is relatively smaller (See Figure 6b–d). Especially in Sample  DUSGP, and WUSGP is relatively smaller (See Figure 6b–d). Especially in Sample WUSGP, WUSGP, the uncorroded area occupied approximately half of the whole matrix. The white  the uncorroded area occupied approximately half of the whole matrix. The white substance substance is CaCO3 formed in the corrosion process because the OH  produced by ca‐ − 2− is thodic CaCO  reacts formed  with HCO in the 3  an corr d gen osion eratpr es ocess the CO because 3 , which the  furt OH her rea prcoduced ted with by the cathodic Ca on  reacts the sample surface [39]. Corrosion products were further analyzed by XRD, as depicted  with HCO and generates the CO , which further reacted with the Ca on the sample 3 3 in  Figure  7.  In  Sample  polished,  the  corrosion  products  mainly  consist  of  Fe,  Fe2O3,  surface [39]. Corrosion products were further analyzed by XRD, as depicted in Figure 7. In Fe(OH)3, and Fe3O4 (See Figure 7a), but in other samples, FeOOH appeared (See Figure  Sample polished, the corrosion products mainly consist of Fe, Fe O , Fe(OH) , and Fe O 2 3 3 3 4 7b–d). This could be attributed to the chemical reaction between Fe(OH)2 and O2. In the  (See Figure 7a), but in other samples, FeOOH appeared (See Figure 7b–d). This could initial phase of corrosion, the passive film Fe(OH)2 on the sample surface will undergo a  be attributed to the chemical reaction between Fe(OH) and O . In the initial phase of 2 2 redox reaction with O2 to form FeOOH. Over time, Fe(OH)2 will absorb O2 to produce red‐ corrosion, the passive film Fe(OH) on the sample surface will undergo a redox reaction dish‐brown Fe(OH)3. At the same time, Cl  will intrude into the vulnerable regions of the  with O to form FeOOH. Over time, Fe(OH) will absorb O to produce reddish-brown 2 2 2 matrix, resulting in the further oxygen absorption reaction of Fe(OH)3 to form the black  Fe(OH) At the same time, Cl will intrude into the vulnerable regions of the matrix, Fe3O4. It is also noted that the diffraction peak intensity of Fe is the strongest in Sample  resulting in the further oxygen absorption reaction of Fe(OH) to form the black Fe O It WUSGP, indicating that there are fewer corrosion areas, and the content 3  of the formed  3 4. is also noted that the diffraction peak intensity of Fe is the strongest in Sample WUSGP, FeOOH and Fe3O4 is relatively low.   Appl. Sci. 2022, 12, x FOR PEER REVIEW  7  of  14  indicating that there are fewer corrosion areas, and the content of the formed FeOOH and Fe O is relatively low. 3 4 Figure 6. Macroscopic corrosion morphology of each sample: (a) the corrosion macro‐morphology  Figure 6. Macroscopic corrosion morphology of each sample: (a) the corrosion macro-morphology of Sample polished; (b) the corrosion macro‐morphology of Sample USP; (c) the corrosion macro‐ of Sample polished; (b) the corrosion macro-morphology of Sample USP; (c) the corrosion macro- morphology of Sample DUSGP; (d); the corrosion macro‐morphology of Sample WUSGP.  morphology of Sample DUSGP; (d) the corrosion macro-morphology of Sample WUSGP. Figure 7. XRD patterns of corrosion products for each sample: (a) the corrosion macro‐morphology  of Sample polished; (b) the corrosion macro‐morphology of Sample USP; (c) the corrosion macro‐ morphology of Sample DUSGP; (d) the corrosion macro‐morphology of Sample WUSGP.  The corrosion micromorphology of Sample polished, USP, DUSGP, and WUSGP are  shown in Figure 8a–d, respectively. Corrosion holes in Sample polished have a diameter  of approximately 6.4 μm, while sample WUSGP has a diameter of only approximately 1.4  um, which degrades  78%. This  can be explained by  the surface plastic deformation of  Appl. Sci. 2022, 12, x FOR PEER REVIEW  7  of  14  Figure 6. Macroscopic corrosion morphology of each sample: (a) the corrosion macro‐morphology  Appl. Sci. 2022, 12, 7360 7 of 13 of Sample polished; (b) the corrosion macro‐morphology of Sample USP; (c) the corrosion macro‐ morphology of Sample DUSGP; (d); the corrosion macro‐morphology of Sample WUSGP.  Figure 7. XRD patterns of corrosion products for each sample: (a) the corrosion macro‐morphology  Figure 7. XRD patterns of corrosion products for each sample: (a) the corrosion macro-morphology of Sample polished; (b) the corrosion macro‐morphology of Sample USP; (c) the corrosion macro‐ of Sample polished; (b) the corrosion macro-morphology of Sample USP; (c) the corrosion macro- morphology of Sample DUSGP; (d) the corrosion macro‐morphology of Sample WUSGP.  morphology of Sample DUSGP; (d) the corrosion macro-morphology of Sample WUSGP. The corrosion micromorphology of Sample polished, USP, DUSGP, and WUSGP are  The corrosion micromorphology of Sample polished, USP, DUSGP, and WUSGP are shown in Figure 8a–d, respectively. Corrosion holes in Sample polished have a diameter  shown in Figure 8a–d, respectively. Corrosion holes in Sample polished have a diameter of approximately 6.4 μm, while sample WUSGP has a diameter of only approximately 1.4  of approximately 6.4 m, while sample WUSGP has a diameter of only approximately um, which degrades  78%. This  can be explained by  the surface plastic deformation of  1.4 um, which degrades 78%. This can be explained by the surface plastic deformation of WUSGP-processed samples, resulting in grain refinement and a denser surface layer. Corrosion holes in these samples exhibit different shapes, such as island, cotton, and lath- like. This is because Cl gradually erodes the matrix’s weak regions during the corrosion process, forming countless corrosion micro-batteries and reacting with the corrosion so- lution. The internal stress of the matrix material and the accumulation of reaction energy push corrosion micro-bubbles to rupture, form “island-like” and “cotton-like” structures, and then further form pitting holes. As the corrosion process intensifies, the initial local pitting gradually develops into corrosion holes, which then connect with each other and finally form the cracks. Figure 9a–d report the micromorphology of corrosion cracks for Sample polished, USP, DUSGP, and WUSGP, respectively. Different widths of corrosion cracks are observed. The polished sample shows a width of approximately 8.4 m, while samples that experienced surface treatment have a relatively narrow width. For instance, Sample WUSGP realizes a crack width of only 2.1 m, which degrades 75% compared to the polishing. The energy spectrum of the corrosion crack area has been analyzed using EDS. The element distribution and atomic percentage of the samples are shown in Figure 10 and Table 4, respectively. It can be found that the Fe atomic percentage gradually decreases after surface treatment, and a lowest percentage was obtained in Sample WUSGP. This is mainly due to the peeling of the rust layer as the thickness increases. The higher the Fe atomic percentage, the more intense the oxidation reaction and the wider the corrosion crack, which is consistent with the corrosion crack micromorphology analysis. In addition, there are fewer Cl, Ca, and other elements in the corrosion products, which are mainly produced in the deposition process of salt spraying. O atomic percentage increases after surface treatment, owing to the oxidation during the corrosion process. Comparing the content Appl. Sci. 2022, 12, x FOR PEER REVIEW  8  of  14  WUSGP‐processed samples, resulting in grain refinement and a denser surface layer. Cor‐ rosion holes in these samples exhibit different shapes, such as island, cotton, and lath‐like.  −  This is because Cl gradually erodes the matrix’s weak regions during the corrosion pro‐ cess, forming countless corrosion micro‐batteries and reacting with the corrosion solution.  The internal stress of the matrix material and the accumulation of reaction energy push  corrosion micro‐bubbles to rupture, form “island‐like” and “cotton‐like” structures, and  then further form pitting holes. As the corrosion process intensifies, the initial local pitting  Appl. Sci. 2022, 12, 7360 8 of 13 gradually develops into corrosion holes, which then connect with each other and finally  form the cracks. Figure 9a–d report the micromorphology of corrosion cracks for Sample  polished, USP, DUSGP, and WUSGP, respectively. Different widths of corrosion cracks  of Cl elements, it can be seen that the percentage of Cl in Sample WUSGP is much higher are observed. The polished sample shows a width of approximately 8.4 μm, while samples  than in other samples, indicating that the corrosion rust layer in this sample has strong that experienced surface treatment have a relatively narrow width. For instance, Sample  adsorption on Cl . This behavior makes it difficult for Cl to react with other substances. WUSGP realizes a crack width of only 2.1 μm, which degrades 75% compared to the pol‐ Thus, the corrosion resistance of Sample WUSGP has been significantly improved. ishing.  Figure 8. Micro‐morphology of corrosion holes for each sample; (a) the micromorphology of corro‐ Figure 8. Micro-morphology of corrosion holes for each sample; (a) the micromorphology of corrosion sion holes for Sample polished; (b) the micromorphology of corrosion holes for Sample USP; (c) the  holes for Sample polished; (b) the micromorphology of corrosion holes for Sample USP; (c) the micromorphology  of  corrosion holes for Sample DUSGP; (d) the  micromorphology  of  corrosion  Appl. Sci. 2022, 12, x FOR PEER REVIEW  9  of  14  micromorphology of corrosion holes for Sample DUSGP; (d) the micromorphology of corrosion holes holes for Sample WUSGP.  for Sample WUSGP. Figure 9. Corrosion crack morphology of each sample; (a) the corrosion crack morphology of Sample  Figure 9. Corrosion crack morphology of each sample; (a) the corrosion crack morphology of Sample polished; (b) the corrosion crack morphology of Sample USP; (c) the corrosion crack morphology of  polished; (b) the corrosion crack morphology of Sample USP; (c) the corrosion crack morphology of Sample DUSGP; (d) the corrosion crack morphology of Sample WUSGP.  Sample DUSGP; (d) the corrosion crack morphology of Sample WUSGP. The energy spectrum of the corrosion crack area has been analyzed using EDS. The  element distribution and atomic percentage of the samples are shown in Figure 10 and  Table 4, respectively. It can be found that the Fe atomic percentage gradually decreases  after surface treatment, and a lowest percentage was obtained in Sample WUSGP. This is  mainly due to the peeling of the rust layer as the thickness increases. The higher the Fe  atomic percentage, the more intense the oxidation reaction and the wider the corrosion  crack, which is consistent with the corrosion crack micromorphology analysis. In addition,  there are fewer Cl, Ca, and other elements in the corrosion products, which are mainly  produced in the deposition process of salt spraying. O atomic percentage increases after  surface treatment, owing to the oxidation during the corrosion process. Comparing the  content of Cl elements, it can be seen that the percentage of Cl in Sample WUSGP is much  higher than in other samples, indicating that the corrosion rust layer in this sample has  − − strong adsorption on Cl . This behavior makes it difficult for Cl  to react with other sub‐ stances.  Thus,  the  corrosion  resistance  of  Sample  WUSGP  has  been  significantly  im‐ proved.   Appl. Sci. 2022, 12, x FOR PEER REVIEW  10  of  14  Appl. Sci. 2022, 12, 7360 9 of 13 Figure 10. (a1,a2) are the surface element distribution and content of Sample polished, respectively; Figure 10. (a1,a2) are the surface element distribution and content of Sample polished, respectively;  (b1,b2) are the surface element distribution and content of Sample USP, respectively; (c1,c2) are the (b1,b2) are the surface element distribution and content of Sample USP, respectively; (c1,c2) are the  surface element distribution and content of Sample DUSGP, respectively; (d1,d2) are the surface surface element distribution and content of Sample DUSGP, respectively; (d1,d2) are the surface  element distribution and content of Sample WUSGP, respectively. element distribution and content of Sample WUSGP, respectively.  Table 4. Element atomic fraction of corrosion product in crack area of samples.  Experimental  Element/wt%  Group  C  O  Fe  Cr  Cl  Ca  Polished  12.60  22.11  63.92  0.80  0.57 ‐  USP  13.93  25.23  58.40  0.92  1.52 ‐  DUSGP  12.71  32.62  51.37  0.58  1.78  0.94  Appl. Sci. 2022, 12, 7360 10 of 13 Table 4. Element atomic fraction of corrosion product in crack area of samples. Appl. Sci. 2022, 12, x FOR PEER REVIEW  11  of  14  Element/wt% Experimental Group C O Fe Cr Cl Ca WU Polished SGP  12.8712.60   32.50 22.11 48.25 63.92 1.03 0.80 4.57 0.57   0.78-  USP 13.93 25.23 58.40 0.92 1.52 - The main corrosion mechanisms are described in Figure 11, and the details are de‐ DUSGP 12.71 32.62 51.37 0.58 1.78 0.94 + −  picted by the following chemical reaction equations (See Equation (1)). First, Na  and Cl WUSGP 12.87 32.50 48.25 1.03 4.57 0.78 are gradually accumulated on the sample surface, ferrite and cementite form numerous  corrosion micro‐batteries, and ferrite in the anode area preferentially dissolves to form  2+ The main corrosion mechanisms − are described in Figure 11, and the details are de- Fe , which then reacts with OH  to form Fe(OH)2. However, Fe(OH)2 is unstable in the  picted by the following chemical reaction equations (See Equation (1)). First, Na and Cl initial stage of corrosion, which will be decomposed into FeO or oxidized in the test cham‐ are gradually accumulated on the sample surface, ferrite and cementite form numerous ber, resulting in a redox reaction to form FeOOH [40,41]. FeOOH is an effective oxidant  2+ corrosion micro-batteries, and ferrite in the anode area preferentially dissolves to form Fe , (except oxygen), α‐FeOOH has a relative high stability, while γ‐FeOOH has a low stabil‐ which then reacts with OH to form Fe(OH) . However, Fe(OH) is unstable in the initial 2 2 ity, which results in the transfer of electrons during the corrosion process, pushing it to  stage of corr2+ osion, which will be decomposed into FeO or oxidized in the test chamber, react with Fe  to generate Fe3O4.  resulting in a redox reaction to form FeOOH [40,41]. FeOOH is an effective oxidant (except   Ο  2Η Ο 4e  4ΟH oxygen), -FeOOH has a relative high stability, while -FeOOH has a low stability, which (1) 2 2 results in the transfer of electrons during the corrosion process, pushing it to react with 2+ 2  (2) Fe to generate Fe O . Fe Fe  2e   3 4 O + 2H O + 4e ! 4OH (1) 2 2 2  Fe  2Cl  4H O FeCl  4H O (3) 2 2 2 2+ Fe ! Fe + 2e (2)   FeCl  4H O  Fe（OH）  2Cl  2H  2H O 2+ (4) 2 2 2 Fe + 2Cl + 4H O ! FeCl + 4H O (3) 2 2 2 2  + FeCl + 4H O ! Fe Fe(OH 2OH)  + Fe 2Cl (OH+ ) 2H + 2H O (5(4) ) 2 2 2 2+ Fe + 2OH ! Fe(OH) (5) 4Fe(OH)  O  4FeOOH 2H O (6) 2 2 4Fe(OH) + O ! 4FeOOH + 2H O (6) 2 2   (7) 3γ FeOOH H  e  Fe O  2H O  3 4 2 3g FeOOH + H + e ! Fe O + 2H O (7) 3 4 2 2  2+   + Fe  2γ FeOOH Fe O  2H (8) Fe + 2g FeOOH ! Fe O + 2H (8) 3 3 4 6Fe 6F(OH e（OH ) ） + O O!2Fe 2Fe O O + 66H H O O (9) 2 3 4 2 (9) 2 2 3 4 2 Figure 11. Schematic diagram of salt spray corrosion reaction of samples for initial stage (a) and  Figure 11. Schematic diagram of salt spray corrosion reaction of samples for initial stage (a) and reaction stage (b).  reaction stage (b). Due to the presence of H  in the corrosion solution, the chemical reaction between  Due to the presence of H in the corrosion solution, the chemical reaction between the the dissolution of the corrosion product film Fe(OH)2 and the metal matrix material will  dissolution of the corrosion product film Fe(OH) and the metal matrix material will be be accelerated. However, the corrosion product film will also be eroded in redox reactions  accelerated. However, the corrosion product film will also be eroded in redox reactions because Cl  is extremely corrosive, penetrating the corrosion product film to further ac‐ because Cl is extremely corrosive, penetrating the corrosion product film to further celerate corrosion, and the corrosion product gradually accumulates and forms a layer on  accelerate corrosion, and the corrosion product gradually accumulates and forms a layer on the sample surface. The thickness of the corrosion layer becomes thicker over time, which  reduces the corrosion rate of Cl  to the matrix material in the later stage of corrosion. In  Appl. Sci. 2022, 12, 7360 11 of 13 the sample surface. The thickness of the corrosion layer becomes thicker over time, which reduces the corrosion rate of Cl to the matrix material in the later stage of corrosion. In Sample USGP, a dense strengthening layer and film are formed on the surface after surface treatment. Thus, the corrosion rate of Cl was reduced in the initial stage of corrosion. The corrosion mainly occurs in the weak regions of the substrate and in the active anode area will trigger a redox reaction to produce Fe(OH) film. With the continuous erosion of Cl , the migration rate of Cl deoxygenation increases, which destroys the corrosion layer. Then, the electrolyte slowly flows into the matrix to corrode at the damaged regions, but the corrosion rate is greatly reduced due to the effect of the dense strengthening layer and the passivation film. In WUSGP, an obvious better performance of corrosion resistance was obtained and can be interpreted as follows: the surface matrix undergoes plastic deformation after surface treatment, resulting in an increase in the martensite content with different shapes, such as needle-like and lath-like, which makes the surface structure denser and hinders Cl from eroding the matrix. Moreover, the high dislocation density and small grain size can significantly improve corrosion resistance. 4. Conclusions This paper investigated the corrosion behavior of four GCr15 steel samples in a salt-frog environment. These samples were treated by ultrasonic shot peening (USP), a dry ultrasonic strengthening grinding process (DUSGP), a wet ultrasonic strengthening grinding process (WUSGP), and polishing. These process samples were named Sample polished, USP, DUSGP, and WUSGP, respectively. The capacitance arc, polarization curves, corrosion morphology, and corrosion products were analyzed. The sample that experienced a surface treatment showed significantly better perfor- mance in terms of corrosion resistance, especially the Sample WUSGP, which has the largest capacitive loops. The corrosion potential and corrosion current values are 0.3999 V and 9.435 mA/cm , respectively. Corrosion morphology revealed that the Sample WUSGP has the smallest corrosion holes and cracks, exhibiting a diameter of approximately 1.4 m and a width of approximately 2.1 m, respectively, degrading 78% and 75% compared with the polished sample. Corrosion products analysis showed that the corrosion mainly occurs in the weak regions of the substrate and in the active anode area will produce Fe(OH) film through a redox reaction. The continuous erosion of Cl destroyed the corrosion layer. Then, the electrolyte slowly flows into the matrix to corrode at the damaged regions, but the existence of a dense strengthening layer and passivation film greatly reduces the corrosion rate. The best corrosion resistance obtained in sample WUGP can be interpreted as the high dislocation density, small grain size, needle-like, and lath-like shape of martensite formed in the treatment, which hinders Cl from eroding the matrix. This research demonstrated that traditional ultrasonic shot peening fused the strengthened liquid and corundum, and ceramic balls for treating materials can significantly improve the corrosion resistance of steels. Author Contributions: Formal analysis and writing original draft, X.X.; resources and writing, Z.Z.; conceptualization, review, and editing, X.L. and J.X.; data curation and validation, Z.G. and Z.L.; investigation, visualization, and project administration, J.W.; All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (52075109, 51975136), the Science and Technology Plan of Guangzhou (202102080225), the Science and technology Special Fund program of Guangdong Province (2019B020404), the Industry-University-Research Cooperation Key Project of Guangzhou Higher Educational Universities (202235139), and Guangzhou University Research Project (YJ2021002). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Appl. Sci. 2022, 12, 7360 12 of 13 Data Availability Statement: The data presented in this study are available in the article. Acknowledgments: We thank the Guangzhou Key Laboratory of High-Performance Metal Grinding Processing of Guangzhou University. Conflicts of Interest: The authors declare no conflict of interest. References 1. Bhadeshia, H. Steels for bearings. Prog. Mater. Sci. 2012, 57, 268–435. [CrossRef] 2. Li, K.; Chen, Z.X.; Liu, P.P.; Li, G.L.; Ding, M.R.; Li, Z.X. Characterization and performance analysis of 3D reconstruction of oil-lubricated Si3N4-GCr15/GCr15-GCr15 friction and wear surface. J. Therm. Anal. Calorim. 2021, 144, 2127–2143. [CrossRef] 3. Cao, Y.J.; Sun, J.Q.; Ma, F.; Chen, Y.Y.; Cheng, X.Z.; Gao, X.; Xie, K. Effect of the microstructure and residual stress on tribological behavior of induction hardened GCr15 steel. Tribol. Int. 2017, 115, 108–115. 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Salt-Fog Corrosion Behavior of GCr15 Steels Treated by Ultrasonic Strengthening Grinding Process

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applied sciences Article Salt-Fog Corrosion Behavior of GCr15 Steels Treated by Ultrasonic Strengthening Grinding Process 1 , 2 , 3 1 , 2 4 3 3 3 , Xincheng Xie , Zhongning Guo , Zhuan Zhao , Zhongwei Liang , Jun Wu , Xiaochu Liu * 3 , and Jinrui Xiao * State Key Laboratory of Precision Electronic Manufacturing Technology and Equipment, Guangdong University of Technology, Guangzhou 510006, China; cxixie@163.com (X.X.); znguo@gdut.edu.cn (Z.G.) Guangzhou Key Laboratory of Nontraditional Machining and Equipment, Guangzhou 510006, China Guangdong Engineering Research Centre for Strengthen Grinding and Micro/Nano High-Performance Machining, Guangzhou University, Guangzhou 510006, China; liangzhongwei@gzhu.edu.cn (Z.L.); wujunair2021@163.com (J.W.) School of Physics and Materials Science, Guangzhou University, Guangzhou 510006, China; zhaozhuan2008@gmail.com * Correspondence: xiaochuliu64@163.com (X.L.); meexiaojinrui@gzhu.edu.cn (J.X.) Abstract: In this paper, the corrosion resistance of four GCr15 steel samples has been investigated. Three samples were initially surface-treated by ultrasonic shot peening (USP), a dry ultrasonic strengthening grinding process (DUSGP), and a wet ultrasonic strengthening grinding process (WUSGP), respectively, while another one was only polished. Then, all the processed samples were subjected to salt spray corrosion. Sample properties, such as capacitance arc, self-corrosion potential (E ), self-corrosion current density (I ), corrosion morphology, and corrosion products were corr corr analyzed. Results show that the sample treated by WUSGP has the best corrosion resistance, which can be attributed to the high dislocation density, small grain size, needle-like and lath-like shape of Citation: Xie, X.; Guo, Z.; Zhao, Z.; martensite content formed in the treatment, which hinders Cl from eroding the matrix. The obtained Liang, Z.; Wu, J.; Liu, X.; Xiao, J. corrosion pits and cracks in Sample WUSGP show a width of approximately 1.4 m and 2.1 m, Salt-Fog Corrosion Behavior of respectively, degrading 78% and 75% compared to polishing. The possible corrosion mechanism GCr15 Steels Treated by Ultrasonic of the samples has been discussed. The findings denote that the treatment fused ceramic balls, Strengthening Grinding Process. strengthened liquid, and corundum in USP could be an efficient method to improve the corrosion Appl. Sci. 2022, 12, 7360. https:// resistance of some mechanical parts. doi.org/10.3390/app12157360 Academic Editor: Ioannis Keywords: GCr15 steel; ultrasonic strengthening grinding process; salt spray corrosion; corrosion Kartsonakis resistance Received: 6 July 2022 Accepted: 16 July 2022 Published: 22 July 2022 1. Introduction Publisher’s Note: MDPI stays neutral GCr15 steels are widely used to manufacture mechanical components, such as with regard to jurisdictional claims in bearings [1,2], ball screws [3], and steel balls [4], mainly due to their high hardness and published maps and institutional affil- wear resistance [3]. These mechanical components have played a significant role in many iations. industrial fields, such as automotive, aviation, mining, and marine equipment [3,5]. Met- als are easily corroded when working in a marine environment, which causes economic and material loss for marine equipment [6]. The corrosion of metal components can be at- Copyright: © 2022 by the authors. tributed to multiple factors, including temperature [7,8], the PH value of the seawater [9,10], Licensee MDPI, Basel, Switzerland. micro-organisms [11], etc. Consequently, a severe degradation occurs in the mechanical This article is an open access article properties of the metals and finally leads to fatigue failure. distributed under the terms and To address this issue, many methods have been proposed for the long-term pro- conditions of the Creative Commons tection of steel components, such as surface coating [12–15] and surface chemical heat Attribution (CC BY) license (https:// treatment [16–18]. In particular, surface coating attracts widespread research interests, creativecommons.org/licenses/by/ and great efforts for developing high-performance protective coating have been studied. 4.0/). Appl. Sci. 2022, 12, 7360. https://doi.org/10.3390/app12157360 https://www.mdpi.com/journal/applsci Appl. Sci. 2022, 12, x FOR PEER REVIEW  2  of  14  Appl. Sci. 2022, 12, 7360 2 of 13 example, Al coating developed by the thermal‐sprayed method can provide efficient pro‐ For example, Al coating developed by the thermal-sprayed method can provide efficient tection for steels due to the existence of passivating film [19,20]. Nevertheless, a serious  protection for steels due to the existence of passivating film [19,20]. Nevertheless, a serious pitting problem occurs once the passivating film has been damaged [21,22]. Moreover, the  pitting problem occurs once the passivating film has been damaged [21,22]. Moreover, Al coatings formed by thermal‐spraying are easy to peel off under heavy loads [23]. Zn‐ the Al coatings formed by thermal-spraying are easy to peel off under heavy loads [23]. Al [12,24], Zr‐Ti [13,25], and NiCr‐Cr3C2 [14,15] coatings have also been studied broadly.  Zn-Al [12,24], Zr-Ti [13,25], and NiCr-Cr C [14,15] coatings have also been studied broadly. 3 2 An example of corrosion behavior of a Zn‐Al composite coating in a salt spray corrosive  An example of corrosion behavior of a Zn-Al composite coating in a salt spray corrosive environment, which exhibits the excellent anti‐corrosion properties by absorbing the ben‐ environment, which exhibits the excellent anti-corrosion properties by absorbing the ben- efits of both Zn and Al, has been investigated by Zhao et al. [12]. However, the good cor‐ efits of both Zn and Al, has been investigated by Zhao et al. [12]. However, the good rosion resistance of Zn‐Al coatings is only suitable for low‐carbon steel substrates [12].  corrosion resistance of Zn-Al coatings is only suitable for low-carbon steel substrates [12]. With the rapid development of the mechanical industry, new techniques to improve the  With the rapid development of the mechanical industry, new techniques to improve the anti‐corrosion properties of steel are urgently needed.   anti-corrosion properties of steel are urgently needed. As an alternative, mechanical strengthening has emerged as a promising solution to  As an alternative, mechanical strengthening has emerged as a promising solution to ob- obtain a good corrosion resistance for metals, mainly due to the chilled layer and com‐ tain a good corrosion resistance for metals, mainly due to the chilled layer and compressive pressive residual stress microstructures formed on metal surfaces after surface treatment  residual stress microstructures formed on metal surfaces after surface treatment [26–28], [26–28], which results in the improvement of fatigue life. So far, mechanical strengthening  which results in the improvement of fatigue life. So far, mechanical strengthening tech- techniques, such as shot peening (SP) [29,30] and rolling finishing [31,32] have been pro‐ niques, such as shot peening (SP) [29,30] and rolling finishing [31,32] have been proposed posed extensively for corrosion resistance investigation of metals. However, traditional  extensively for corrosion resistance investigation of metals. However, traditional shoot shoot peening typically produces a high roughness on the metal surface, and the protec‐ peening typically produces a high roughness on the metal surface, and the protection ability tion ability is still far from the practical demands [33]. On the other hand, the hardened  is still far from the practical demands [33]. On the other hand, the hardened layer produced layer produced by rolling and the internal material have an obvious delamination phe‐ by rolling and the internal material have an obvious delamination phenomenon [34], which nomenon [34], which easily causes the surface layer to fall off. In contrast, ultrasonic shot  easily causes the surface layer to fall off. In contrast, ultrasonic shot peening has the charac- peening has the characteristics of low cost, large shape correction range, small footprint,  teristics of low cost, large shape correction range, small footprint, good controllability and good controllability and repeatability. Surprisingly, the use of ultrasonic shot peening for  repeatability. Surprisingly, the use of ultrasonic shot peening for improving the corrosion improving the corrosion resistance of GCr15 steels is poorly investigated.  resistance of GCr15 steels is poorly investigated. In this work, the corrosion behavior of four GCr15 steel samples has been investi‐ In this work, the corrosion behavior of four GCr15 steel samples has been investigated gated in a salt spray corrosion environment. Samples are surface‐treated with ultrasonic  in a salt spray corrosion environment. Samples are surface-treated with ultrasonic shot shot peening (USP), a dry ultrasonic strengthening grinding process (DUSGP), a wet ul‐ peening (USP), a dry ultrasonic strengthening grinding process (DUSGP), a wet ultrasonic trasonic strengthening grinding process (WUSGP), and polishing, respectively. The ca‐ strengthening grinding process (WUSGP), and polishing, respectively. The capacitance arc pacitance arc and polarization curves of these samples were studied. The corrosion mor‐ and polarization curves of these samples were studied. The corrosion morphology and phology and products were further analyzed to reveal possible corrosion mechanisms.  products were further analyzed to reveal possible corrosion mechanisms. 2. Materials and Methods  2. Materials and Methods 2.1. Samples Preparation  2.1. Samples Preparation Four GCr15 steel samples with geometrical parameters of 100 mm × 75 mm × 10 mm  Four GCr15 steel samples with geometrical parameters of 100 mm  75 mm  10 mm (See Figure 1a) were employed in this study. Their main chemical composition is shown  (See Figure 1a) were employed in this study. Their main chemical composition is shown in in Table 1, which was analyzed by Energy Dispersive Spectroscopy (EDS). The heat treat‐ Table 1, which was analyzed by Energy Dispersive Spectroscopy (EDS). The heat treatment ment procedure is shown Figure 1b. The samples were initially quenched at a temperature  procedure is shown Figure 1b. The samples were initially quenched at a temperature of of 855 °C to obtain a good hardness, followed by an oil cooling process. Then, the samples  855 C to obtain a good hardness, followed by an oil cooling process. Then, the samples were treated with a tempering process of 120 min at 160 °C to reduce the brittleness of the  were treated with a tempering process of 120 min at 160 C to reduce the brittleness of the quenched workpiece. Finally, the sample surfaces were polished with 200# sandpaper to  quenched workpiece. Finally, the sample surfaces were polished with 200# sandpaper to obtain a roughness < 0.2 μm and cleaned with alcohol through an ultrasonic bath.  obtain a roughness < 0.2 m and cleaned with alcohol through an ultrasonic bath. (a)  (b)  Figure 1. (a) The geometric parameters of samples; (b) the heating process.  Figure 1. (a) The geometric parameters of samples; (b) the heating process.   Appl. Sci. 2022, 12, 7360 3 of 13 Appl. Sci. 2022, 12, x FOR PEER REVIEW  3  of  14  Table 1. Chemical element composition of GCr15 steels. Table 1. Chemical element composition of GCr15 steels.  P S Ni Cu Chemical El‐ P  S  Ni  Cu  Chemical Element C Si Mn Cr C  Si  Mn  Cr  ement  ≤  Content  Content (wt%) 0.95~1.05 0.15~0.35 0.20~0.40 1.30~1.60 0.027 0.02 0.30 0.25 0.95~1.05 0.15~0.35  0.20~0.40  1.30~1.60  0.027  0.02  0.30  0.25  (wt%)  2.2. Surface Treatment of the Samples 2.2. Surface Treatment of the Samples  Three samples were surface-treated by ultrasonic processing equipment, while another Three samples were surface‐treated by ultrasonic processing equipment, while an‐ sample was without surface treatment for experimental comparison. The working principle other sample was without surface treatment for experimental comparison. The working  of the ultrasonic equipment is reported in Figure 2. First, ultrasonic waves were produced principle of the ultrasonic equipment is reported in Figure 2. First, ultrasonic waves were  by an electro-mechanical ultrasonic transducer and then applied to a workpiece. An produced by an electro‐mechanical ultrasonic transducer and then applied to a workpiece.  ultrasonic transducer was utilized to energize an acoustically tuned resonator bar, causing An ultrasonic  transducer was utilized to energize an acoustically tuned resonator bar,  it to vibrate. Finally, the energy generated from these impulses was imparted to treat the causing it to vibrate. Finally, the energy generated from these impulses was imparted to  surface through the ceramic balls. The main difference between the surface treatment of treat the surface through the ceramic balls. The main difference between the surface treat‐ the ment thr of ee thsamples e three sa was mple the s wa employment s the employof ment dif fer of ent differ trent eatment  treatm materials, ent material ass,depicted  as de‐ in Figur picted e in 2b–d.  Figure The  2b–d. working  The working parameters  paramet of e ultrasonic rs of ultraspr onocessing ic processising pr is esented  presentin ed Tin able   2. One Tablesample  2. One was samptr leeated  was treat withedceramic  with cera balls, mic ball while s, while the other  the ottwo her two samples  samplwer es wer e treeated   with treatemixed d with mixed materials.  materia The ls. surface The surfa trceatment e treatment that tha only t only consider  consider ededceramic  ceramic balls balls  was was named USP, and mixed white corundum and the ceramic balls was named DUSGP.  named USP, and mixed white corundum and the ceramic balls was named DUSGP. The The other one that mixed white corundum and ceramic ball, as well as strengthening liq‐ other one that mixed white corundum and ceramic ball, as well as strengthening liquid uid was named WUSGP [35]. The strengthening liquid was composed of extrusion addi‐ was named WUSGP [35]. The strengthening liquid was composed of extrusion additive tive (i.e., C10H14N2Na2O8∙2H2O), triethanolamine (C6H15NO3) and water. The samples sur‐ (i.e., C H N Na O 2H O), triethanolamine (C H NO ) and water. The samples surface- 10 14 2 2 8 2 6 15 3 face‐treated by USP, DUSGP, WUSGP, and polishing are named Sample USP, DUSGP,  treated by USP, DUSGP, WUSGP, and polishing are named Sample USP, DUSGP, WUSGP, WUSGP, and polished, respectively.  and polished, respectively. Figure 2. (a) Schematic diagram of ultrasonic processing equipment; (b) ultrasonic shoot peening  Figure 2. (a) Schematic diagram of ultrasonic processing equipment; (b) ultrasonic shoot peening treatment; (c) dry ultrasonic strengthening grinding process treatment; (d) wet ultrasonic strength‐ treatment; (c) dry ultrasonic strengthening grinding process treatment; (d) wet ultrasonic strengthen- ening grinding process treatment.  ing grinding process treatment.    2.3. Salt Spray Process of the Samples Each processed sample was cut into three plates with a diameter of 10 mm to calculate the mean values of experimental data. The salt frog test was performed using HT-YW-60 salt spray test machine and based on the standard ASTM B117. The working temperature was 35 (2) C, the precipitation rate of the salt spray was 1.5 mL/(cm h) with an air pressure of 96 kpa. PH value was set in the range of 6.5–7.2, and the total spray time was 96 h. Appl. Sci. 2022, 12, 7360 4 of 13 Table 2. Working parameters of ultrasonic processing. Parameters Value Vibration frequency (kHz) 20 Peening distance (mm) 30 Processing time (min) 3 Diameter of ceramic balls (mm) 1 Grain size of brown corundum (m) 15 2.4. Characterization The cut samples were mounted with epoxy and then used to make electrodes by welding a wire on their surface. The electrochemical impedance spectroscopic (EIS) mea- surement has been carried out using Solartron 1260 + 1287 electrochemical workstation. The potentiodynamic polarization test was performed at a scan rate of 1.5 mV/s in the range of 0.5 V–5 V. An X-ray diffractometer (XRD, Rigaku + UltimaIV, Rigaku Corp, Akishima, Tokyo, Japan) was used to analyze the phase and corrosion components of the samples. The scan- ning electron microscope (SEM, ZEISS Gemini 300, Carl Zeiss AG, Oberkochen, Germany) coupled with an energy dispersive spectrometer (EDS, Oxford XMax 50, Oxford, UK) was employed to observe the surface micromorphology. Autofocus Microscope (GP420H, Kunshan gaopin precision instrument co. LTD, Kunshan, China) was utilized to observe the macromorphology. 3. Results and Discussion Figure 3 reports the Nyquist diagram of all the samples immersed in 3.5% NaCl solution made in the laboratory for 96 h. The capacitive loop was utilized to characterize the charge transfer resistance at the interface of corrosion products. It can be seen that the capacitive loop of Sample polished is much smaller than the other samples, which can be attributed to the accumulation of corrosion products. The capacitive loops of the samples Appl. Sci. 2022, 12, x FOR PEER REVIEWwith   surface treatment are significantly improved. In particular, the biggest capacitive 5  ofar  14 c  radius was obtained in Sample WUSGP. The magnitudes of the charge transfer resistance of all the samples are ranked as follows: WUSGP > DUSGP > USP > Polished. Figure 3. Nyquist plots of the samples immersed in seawater for 96 h. Figure 3. Nyquist plots of the samples immersed in seawater for 96 h.  The corrosion potential (E ) and the corrosion current density (I ) are critical corr corr The corrosion potential (Ecorr) and the corrosion current density (Icorr) are critical pa‐ parameters to evaluate the performance of corrosion resistance, which are calculated rameters  to  evaluate  the  performance  of  corrosion  resistance,  which  are  calculated  through Tafel extrapolation [36]. The polarization curves of the samples and the obtained through Tafel extrapolation [36]. The polarization curves of the samples and the obtained  results are reported in Figure 4 and Table 3. It can be found that the Sample WUSGP has the results are reported in Figure 4 and Table 3. It can be found that the Sample WUSGP has  largest corrosion potential (i.e., 0.3999), while the corrosion potentials of Sample DUSGP the  largest  corrosion  potential  (i.e., −0.3999),  while  the  corrosion  potentials  of  Sample  and USP are relatively close, realizing a value of 0.5926 V and 0.5122 V, respectively. DUSGP and USP are relatively close, realizing a value of −0.5926 V and −0.5122 V, respec‐ tively. This can be by explained considering the effect of the strengthening liquid in sur‐ face treatment, which degrades the corrosion potential of Sample WUSGP. Generally, the  lower the corrosion potential, the worse the corrosion resistance. Furthermore, the corro‐ sion current of sample WUSGP is two times lower than sample Polished, exhibiting a  value of 9.435 mA/cm . A smaller corrosion current means a lower corrosion rate. There‐ fore, sample WUSGP possesses the best properties of corrosion resistance.  Figure 4. The polarization curves of all the samples.  Table 3. Corrosion potential (Ecorr) and the corrosion current density (Icorr) obtained in electrochem‐ ical corrosion test for each sample.  −2 Serial Number  Ecorr/V  Icorr/(mA·cm )  Polished −0.7309  18.15  USP −0.5926  15.28  DUSGP −0.5122  12.79  WUSGP −0.3999  9.435  Appl. Sci. 2022, 12, x FOR PEER REVIEW  5  of  14  Figure 3. Nyquist plots of the samples immersed in seawater for 96 h.  The corrosion potential (Ecorr) and the corrosion current density (Icorr) are critical pa‐ rameters  to  evaluate  the  performance  of  corrosion  resistance,  which  are  calculated  through Tafel extrapolation [36]. The polarization curves of the samples and the obtained  results are reported in Figure 4 and Table 3. It can be found that the Sample WUSGP has  the  largest  corrosion  potential  (i.e., −0.3999),  while  the  corrosion  potentials  of  Sample  DUSGP and USP are relatively close, realizing a value of −0.5926 V and −0.5122 V, respec‐ Appl. Sci. 2022, 12, 7360 5 of 13 tively. This can be by explained considering the effect of the strengthening liquid in sur‐ face treatment, which degrades the corrosion potential of Sample WUSGP. Generally, the  This can be by explained considering the effect of the strengthening liquid in surface lower the corrosion potential, the worse the corrosion resistance. Furthermore, the corro‐ treatment, which degrades the corrosion potential of Sample WUSGP. Generally, the lower sion current of sample WUSGP is two times lower than sample Polished, exhibiting a  the corrosion potential, the worse the corrosion resistance. Furthermore, the corrosion value of 9.435 mA/cm . A smaller corrosion current means a lower corrosion rate. There‐ current of sample WUSGP is two times lower than sample Polished, exhibiting a value fore, of sa 9.435 mpl mA/cm e WUSGP . A smaller  posses corr sesosion  the best current  properties means a lower  of corrosion corrosion rate. resist Ther anc efor e.  e, sample WUSGP possesses the best properties of corrosion resistance. Figure 4. The polarization curves of all the samples. Figure 4. The polarization curves of all the samples.  Table 3. Corrosion potential (E ) and the corrosion current density (I ) obtained in electrochemi- corr corr Table 3. Corrosion potential (Ecorr) and the corrosion current density (Icorr) obtained in electrochem‐ cal corrosion test for each sample. ical corrosion test for each sample.  Serial Number E /V I /(mAcm ) corr corr −2 Polished 0.7309 18.15 Serial Number  Ecorr/V  Icorr/(mA·cm )  USP 0.5926 15.28 Polished −0.7309  18.15  DUSGP 0.5122 12.79 USP −0.5926  15.28  WUSGP 0.3999 9.435 DUSGP −0.5122  12.79  WUSGP −0.3999  9.435  The X-ray diffraction patterns of all the samples are shown in Figure 5a. It is noted that the new diffraction peaks occurred in the surface-treated samples, such as (200). The new diffraction peak is particularly obvious in Sample WUSGP. To further understand the possible mechanism, the grain size and dislocation density of these samples have been calculated (See Figure 5b) through Williamson–Hall equations [37]. Sample polished has the maximum grain size and the minimal dislocation density, realizing a value of 28.7 m 16 2 and 0.3658  10 m , respectively. After surface treatment, the grain size decreases, and the dislocation density increases. In sample WUSGP, the grain size is refined to 6.45 m, 16 2 and the dislocation density increased to 2.639  10 m , which is mainly due to the plastic deformation of the surface tissue after intensive grinding and the phase transforms, resulting in an increase in the martensite content (See Figure 5a). The martensite structures typically show a shape of needle and lath, making the surface structure denser. Previous studies demonstrated that the dense structure of high dislocation density and small grain size can significantly improve the corrosion resistance of the material [38]. This evidence further illustrates the benefits that USGP may bring. Appl. Sci. 2022, 12, x FOR PEER REVIEW  6  of  14  The X‐ray diffraction patterns of all the samples are shown in Figure 5a. It is noted  that the new diffraction peaks occurred in the surface‐treated samples, such as (200). The  new diffraction peak is particularly obvious in Sample WUSGP. To further understand  the possible mechanism, the grain size and dislocation density of these samples have been  calculated (See Figure 5b) through Williamson–Hall equations [37]. Sample polished has  the maximum grain size and the minimal dislocation density, realizing a value of 28.7 μm  16 −2 and 0.3658 × 10  m , respectively. After surface treatment, the grain size decreases, and  the dislocation density increases. In sample WUSGP, the grain size is refined to 6.45 μm,  16 −2 and the dislocation density increased to 2.639 × 10  m , which is mainly due to the plastic  deformation of the surface tissue after intensive grinding and the phase transforms, re‐ sulting in an increase in the martensite content (See Figure 5a). The martensite structures  typically show a shape of needle and lath, making the surface structure denser. Previous  studies demonstrated that the dense structure of high dislocation density and small grain  Appl. Sci. 2022, 12, 7360 6 of 13 size can significantly improve the corrosion resistance of the material [38]. This evidence  further illustrates the benefits that USGP may bring.  Figure 5. (a) The X‐ray diffraction pattern characteristics on sample surface; (b) dislocation density  Figure 5. (a) The X-ray diffraction pattern characteristics on sample surface; (b) dislocation density and grain size of samples.  and grain size of samples. Figure 6 shows the surface macro‐morphology of the samples. It is found that most  Figure 6 shows the surface macro-morphology of the samples. It is found that most regions on the matrix surface of Sample polished are corroded (See Figure 6a), and the  regions on the matrix surface of Sample polished are corroded (See Figure 6a), and the width of corrosion cracks is larger than in other samples. The corroded area of Sample  width of corrosion cracks is larger than in other samples. The corroded area of Sample USP, USP, DUSGP, and WUSGP is relatively smaller (See Figure 6b–d). Especially in Sample  DUSGP, and WUSGP is relatively smaller (See Figure 6b–d). Especially in Sample WUSGP, WUSGP, the uncorroded area occupied approximately half of the whole matrix. The white  the uncorroded area occupied approximately half of the whole matrix. The white substance substance is CaCO3 formed in the corrosion process because the OH  produced by ca‐ − 2− is thodic CaCO  reacts formed  with HCO in the 3  an corr d gen osion eratpr es ocess the CO because 3 , which the  furt OH her rea prcoduced ted with by the cathodic Ca on  reacts the sample surface [39]. Corrosion products were further analyzed by XRD, as depicted  with HCO and generates the CO , which further reacted with the Ca on the sample 3 3 in  Figure  7.  In  Sample  polished,  the  corrosion  products  mainly  consist  of  Fe,  Fe2O3,  surface [39]. Corrosion products were further analyzed by XRD, as depicted in Figure 7. In Fe(OH)3, and Fe3O4 (See Figure 7a), but in other samples, FeOOH appeared (See Figure  Sample polished, the corrosion products mainly consist of Fe, Fe O , Fe(OH) , and Fe O 2 3 3 3 4 7b–d). This could be attributed to the chemical reaction between Fe(OH)2 and O2. In the  (See Figure 7a), but in other samples, FeOOH appeared (See Figure 7b–d). This could initial phase of corrosion, the passive film Fe(OH)2 on the sample surface will undergo a  be attributed to the chemical reaction between Fe(OH) and O . In the initial phase of 2 2 redox reaction with O2 to form FeOOH. Over time, Fe(OH)2 will absorb O2 to produce red‐ corrosion, the passive film Fe(OH) on the sample surface will undergo a redox reaction dish‐brown Fe(OH)3. At the same time, Cl  will intrude into the vulnerable regions of the  with O to form FeOOH. Over time, Fe(OH) will absorb O to produce reddish-brown 2 2 2 matrix, resulting in the further oxygen absorption reaction of Fe(OH)3 to form the black  Fe(OH) At the same time, Cl will intrude into the vulnerable regions of the matrix, Fe3O4. It is also noted that the diffraction peak intensity of Fe is the strongest in Sample  resulting in the further oxygen absorption reaction of Fe(OH) to form the black Fe O It WUSGP, indicating that there are fewer corrosion areas, and the content 3  of the formed  3 4. is also noted that the diffraction peak intensity of Fe is the strongest in Sample WUSGP, FeOOH and Fe3O4 is relatively low.   Appl. Sci. 2022, 12, x FOR PEER REVIEW  7  of  14  indicating that there are fewer corrosion areas, and the content of the formed FeOOH and Fe O is relatively low. 3 4 Figure 6. Macroscopic corrosion morphology of each sample: (a) the corrosion macro‐morphology  Figure 6. Macroscopic corrosion morphology of each sample: (a) the corrosion macro-morphology of Sample polished; (b) the corrosion macro‐morphology of Sample USP; (c) the corrosion macro‐ of Sample polished; (b) the corrosion macro-morphology of Sample USP; (c) the corrosion macro- morphology of Sample DUSGP; (d); the corrosion macro‐morphology of Sample WUSGP.  morphology of Sample DUSGP; (d) the corrosion macro-morphology of Sample WUSGP. Figure 7. XRD patterns of corrosion products for each sample: (a) the corrosion macro‐morphology  of Sample polished; (b) the corrosion macro‐morphology of Sample USP; (c) the corrosion macro‐ morphology of Sample DUSGP; (d) the corrosion macro‐morphology of Sample WUSGP.  The corrosion micromorphology of Sample polished, USP, DUSGP, and WUSGP are  shown in Figure 8a–d, respectively. Corrosion holes in Sample polished have a diameter  of approximately 6.4 μm, while sample WUSGP has a diameter of only approximately 1.4  um, which degrades  78%. This  can be explained by  the surface plastic deformation of  Appl. Sci. 2022, 12, x FOR PEER REVIEW  7  of  14  Figure 6. Macroscopic corrosion morphology of each sample: (a) the corrosion macro‐morphology  Appl. Sci. 2022, 12, 7360 7 of 13 of Sample polished; (b) the corrosion macro‐morphology of Sample USP; (c) the corrosion macro‐ morphology of Sample DUSGP; (d); the corrosion macro‐morphology of Sample WUSGP.  Figure 7. XRD patterns of corrosion products for each sample: (a) the corrosion macro‐morphology  Figure 7. XRD patterns of corrosion products for each sample: (a) the corrosion macro-morphology of Sample polished; (b) the corrosion macro‐morphology of Sample USP; (c) the corrosion macro‐ of Sample polished; (b) the corrosion macro-morphology of Sample USP; (c) the corrosion macro- morphology of Sample DUSGP; (d) the corrosion macro‐morphology of Sample WUSGP.  morphology of Sample DUSGP; (d) the corrosion macro-morphology of Sample WUSGP. The corrosion micromorphology of Sample polished, USP, DUSGP, and WUSGP are  The corrosion micromorphology of Sample polished, USP, DUSGP, and WUSGP are shown in Figure 8a–d, respectively. Corrosion holes in Sample polished have a diameter  shown in Figure 8a–d, respectively. Corrosion holes in Sample polished have a diameter of approximately 6.4 μm, while sample WUSGP has a diameter of only approximately 1.4  of approximately 6.4 m, while sample WUSGP has a diameter of only approximately um, which degrades  78%. This  can be explained by  the surface plastic deformation of  1.4 um, which degrades 78%. This can be explained by the surface plastic deformation of WUSGP-processed samples, resulting in grain refinement and a denser surface layer. Corrosion holes in these samples exhibit different shapes, such as island, cotton, and lath- like. This is because Cl gradually erodes the matrix’s weak regions during the corrosion process, forming countless corrosion micro-batteries and reacting with the corrosion so- lution. The internal stress of the matrix material and the accumulation of reaction energy push corrosion micro-bubbles to rupture, form “island-like” and “cotton-like” structures, and then further form pitting holes. As the corrosion process intensifies, the initial local pitting gradually develops into corrosion holes, which then connect with each other and finally form the cracks. Figure 9a–d report the micromorphology of corrosion cracks for Sample polished, USP, DUSGP, and WUSGP, respectively. Different widths of corrosion cracks are observed. The polished sample shows a width of approximately 8.4 m, while samples that experienced surface treatment have a relatively narrow width. For instance, Sample WUSGP realizes a crack width of only 2.1 m, which degrades 75% compared to the polishing. The energy spectrum of the corrosion crack area has been analyzed using EDS. The element distribution and atomic percentage of the samples are shown in Figure 10 and Table 4, respectively. It can be found that the Fe atomic percentage gradually decreases after surface treatment, and a lowest percentage was obtained in Sample WUSGP. This is mainly due to the peeling of the rust layer as the thickness increases. The higher the Fe atomic percentage, the more intense the oxidation reaction and the wider the corrosion crack, which is consistent with the corrosion crack micromorphology analysis. In addition, there are fewer Cl, Ca, and other elements in the corrosion products, which are mainly produced in the deposition process of salt spraying. O atomic percentage increases after surface treatment, owing to the oxidation during the corrosion process. Comparing the content Appl. Sci. 2022, 12, x FOR PEER REVIEW  8  of  14  WUSGP‐processed samples, resulting in grain refinement and a denser surface layer. Cor‐ rosion holes in these samples exhibit different shapes, such as island, cotton, and lath‐like.  −  This is because Cl gradually erodes the matrix’s weak regions during the corrosion pro‐ cess, forming countless corrosion micro‐batteries and reacting with the corrosion solution.  The internal stress of the matrix material and the accumulation of reaction energy push  corrosion micro‐bubbles to rupture, form “island‐like” and “cotton‐like” structures, and  then further form pitting holes. As the corrosion process intensifies, the initial local pitting  Appl. Sci. 2022, 12, 7360 8 of 13 gradually develops into corrosion holes, which then connect with each other and finally  form the cracks. Figure 9a–d report the micromorphology of corrosion cracks for Sample  polished, USP, DUSGP, and WUSGP, respectively. Different widths of corrosion cracks  of Cl elements, it can be seen that the percentage of Cl in Sample WUSGP is much higher are observed. The polished sample shows a width of approximately 8.4 μm, while samples  than in other samples, indicating that the corrosion rust layer in this sample has strong that experienced surface treatment have a relatively narrow width. For instance, Sample  adsorption on Cl . This behavior makes it difficult for Cl to react with other substances. WUSGP realizes a crack width of only 2.1 μm, which degrades 75% compared to the pol‐ Thus, the corrosion resistance of Sample WUSGP has been significantly improved. ishing.  Figure 8. Micro‐morphology of corrosion holes for each sample; (a) the micromorphology of corro‐ Figure 8. Micro-morphology of corrosion holes for each sample; (a) the micromorphology of corrosion sion holes for Sample polished; (b) the micromorphology of corrosion holes for Sample USP; (c) the  holes for Sample polished; (b) the micromorphology of corrosion holes for Sample USP; (c) the micromorphology  of  corrosion holes for Sample DUSGP; (d) the  micromorphology  of  corrosion  Appl. Sci. 2022, 12, x FOR PEER REVIEW  9  of  14  micromorphology of corrosion holes for Sample DUSGP; (d) the micromorphology of corrosion holes holes for Sample WUSGP.  for Sample WUSGP. Figure 9. Corrosion crack morphology of each sample; (a) the corrosion crack morphology of Sample  Figure 9. Corrosion crack morphology of each sample; (a) the corrosion crack morphology of Sample polished; (b) the corrosion crack morphology of Sample USP; (c) the corrosion crack morphology of  polished; (b) the corrosion crack morphology of Sample USP; (c) the corrosion crack morphology of Sample DUSGP; (d) the corrosion crack morphology of Sample WUSGP.  Sample DUSGP; (d) the corrosion crack morphology of Sample WUSGP. The energy spectrum of the corrosion crack area has been analyzed using EDS. The  element distribution and atomic percentage of the samples are shown in Figure 10 and  Table 4, respectively. It can be found that the Fe atomic percentage gradually decreases  after surface treatment, and a lowest percentage was obtained in Sample WUSGP. This is  mainly due to the peeling of the rust layer as the thickness increases. The higher the Fe  atomic percentage, the more intense the oxidation reaction and the wider the corrosion  crack, which is consistent with the corrosion crack micromorphology analysis. In addition,  there are fewer Cl, Ca, and other elements in the corrosion products, which are mainly  produced in the deposition process of salt spraying. O atomic percentage increases after  surface treatment, owing to the oxidation during the corrosion process. Comparing the  content of Cl elements, it can be seen that the percentage of Cl in Sample WUSGP is much  higher than in other samples, indicating that the corrosion rust layer in this sample has  − − strong adsorption on Cl . This behavior makes it difficult for Cl  to react with other sub‐ stances.  Thus,  the  corrosion  resistance  of  Sample  WUSGP  has  been  significantly  im‐ proved.   Appl. Sci. 2022, 12, x FOR PEER REVIEW  10  of  14  Appl. Sci. 2022, 12, 7360 9 of 13 Figure 10. (a1,a2) are the surface element distribution and content of Sample polished, respectively; Figure 10. (a1,a2) are the surface element distribution and content of Sample polished, respectively;  (b1,b2) are the surface element distribution and content of Sample USP, respectively; (c1,c2) are the (b1,b2) are the surface element distribution and content of Sample USP, respectively; (c1,c2) are the  surface element distribution and content of Sample DUSGP, respectively; (d1,d2) are the surface surface element distribution and content of Sample DUSGP, respectively; (d1,d2) are the surface  element distribution and content of Sample WUSGP, respectively. element distribution and content of Sample WUSGP, respectively.  Table 4. Element atomic fraction of corrosion product in crack area of samples.  Experimental  Element/wt%  Group  C  O  Fe  Cr  Cl  Ca  Polished  12.60  22.11  63.92  0.80  0.57 ‐  USP  13.93  25.23  58.40  0.92  1.52 ‐  DUSGP  12.71  32.62  51.37  0.58  1.78  0.94  Appl. Sci. 2022, 12, 7360 10 of 13 Table 4. Element atomic fraction of corrosion product in crack area of samples. Appl. Sci. 2022, 12, x FOR PEER REVIEW  11  of  14  Element/wt% Experimental Group C O Fe Cr Cl Ca WU Polished SGP  12.8712.60   32.50 22.11 48.25 63.92 1.03 0.80 4.57 0.57   0.78-  USP 13.93 25.23 58.40 0.92 1.52 - The main corrosion mechanisms are described in Figure 11, and the details are de‐ DUSGP 12.71 32.62 51.37 0.58 1.78 0.94 + −  picted by the following chemical reaction equations (See Equation (1)). First, Na  and Cl WUSGP 12.87 32.50 48.25 1.03 4.57 0.78 are gradually accumulated on the sample surface, ferrite and cementite form numerous  corrosion micro‐batteries, and ferrite in the anode area preferentially dissolves to form  2+ The main corrosion mechanisms − are described in Figure 11, and the details are de- Fe , which then reacts with OH  to form Fe(OH)2. However, Fe(OH)2 is unstable in the  picted by the following chemical reaction equations (See Equation (1)). First, Na and Cl initial stage of corrosion, which will be decomposed into FeO or oxidized in the test cham‐ are gradually accumulated on the sample surface, ferrite and cementite form numerous ber, resulting in a redox reaction to form FeOOH [40,41]. FeOOH is an effective oxidant  2+ corrosion micro-batteries, and ferrite in the anode area preferentially dissolves to form Fe , (except oxygen), α‐FeOOH has a relative high stability, while γ‐FeOOH has a low stabil‐ which then reacts with OH to form Fe(OH) . However, Fe(OH) is unstable in the initial 2 2 ity, which results in the transfer of electrons during the corrosion process, pushing it to  stage of corr2+ osion, which will be decomposed into FeO or oxidized in the test chamber, react with Fe  to generate Fe3O4.  resulting in a redox reaction to form FeOOH [40,41]. FeOOH is an effective oxidant (except   Ο  2Η Ο 4e  4ΟH oxygen), -FeOOH has a relative high stability, while -FeOOH has a low stability, which (1) 2 2 results in the transfer of electrons during the corrosion process, pushing it to react with 2+ 2  (2) Fe to generate Fe O . Fe Fe  2e   3 4 O + 2H O + 4e ! 4OH (1) 2 2 2  Fe  2Cl  4H O FeCl  4H O (3) 2 2 2 2+ Fe ! Fe + 2e (2)   FeCl  4H O  Fe（OH）  2Cl  2H  2H O 2+ (4) 2 2 2 Fe + 2Cl + 4H O ! FeCl + 4H O (3) 2 2 2 2  + FeCl + 4H O ! Fe Fe(OH 2OH)  + Fe 2Cl (OH+ ) 2H + 2H O (5(4) ) 2 2 2 2+ Fe + 2OH ! Fe(OH) (5) 4Fe(OH)  O  4FeOOH 2H O (6) 2 2 4Fe(OH) + O ! 4FeOOH + 2H O (6) 2 2   (7) 3γ FeOOH H  e  Fe O  2H O  3 4 2 3g FeOOH + H + e ! Fe O + 2H O (7) 3 4 2 2  2+   + Fe  2γ FeOOH Fe O  2H (8) Fe + 2g FeOOH ! Fe O + 2H (8) 3 3 4 6Fe 6F(OH e（OH ) ） + O O!2Fe 2Fe O O + 66H H O O (9) 2 3 4 2 (9) 2 2 3 4 2 Figure 11. Schematic diagram of salt spray corrosion reaction of samples for initial stage (a) and  Figure 11. Schematic diagram of salt spray corrosion reaction of samples for initial stage (a) and reaction stage (b).  reaction stage (b). Due to the presence of H  in the corrosion solution, the chemical reaction between  Due to the presence of H in the corrosion solution, the chemical reaction between the the dissolution of the corrosion product film Fe(OH)2 and the metal matrix material will  dissolution of the corrosion product film Fe(OH) and the metal matrix material will be be accelerated. However, the corrosion product film will also be eroded in redox reactions  accelerated. However, the corrosion product film will also be eroded in redox reactions because Cl  is extremely corrosive, penetrating the corrosion product film to further ac‐ because Cl is extremely corrosive, penetrating the corrosion product film to further celerate corrosion, and the corrosion product gradually accumulates and forms a layer on  accelerate corrosion, and the corrosion product gradually accumulates and forms a layer on the sample surface. The thickness of the corrosion layer becomes thicker over time, which  reduces the corrosion rate of Cl  to the matrix material in the later stage of corrosion. In  Appl. Sci. 2022, 12, 7360 11 of 13 the sample surface. The thickness of the corrosion layer becomes thicker over time, which reduces the corrosion rate of Cl to the matrix material in the later stage of corrosion. In Sample USGP, a dense strengthening layer and film are formed on the surface after surface treatment. Thus, the corrosion rate of Cl was reduced in the initial stage of corrosion. The corrosion mainly occurs in the weak regions of the substrate and in the active anode area will trigger a redox reaction to produce Fe(OH) film. With the continuous erosion of Cl , the migration rate of Cl deoxygenation increases, which destroys the corrosion layer. Then, the electrolyte slowly flows into the matrix to corrode at the damaged regions, but the corrosion rate is greatly reduced due to the effect of the dense strengthening layer and the passivation film. In WUSGP, an obvious better performance of corrosion resistance was obtained and can be interpreted as follows: the surface matrix undergoes plastic deformation after surface treatment, resulting in an increase in the martensite content with different shapes, such as needle-like and lath-like, which makes the surface structure denser and hinders Cl from eroding the matrix. Moreover, the high dislocation density and small grain size can significantly improve corrosion resistance. 4. Conclusions This paper investigated the corrosion behavior of four GCr15 steel samples in a salt-frog environment. These samples were treated by ultrasonic shot peening (USP), a dry ultrasonic strengthening grinding process (DUSGP), a wet ultrasonic strengthening grinding process (WUSGP), and polishing. These process samples were named Sample polished, USP, DUSGP, and WUSGP, respectively. The capacitance arc, polarization curves, corrosion morphology, and corrosion products were analyzed. The sample that experienced a surface treatment showed significantly better perfor- mance in terms of corrosion resistance, especially the Sample WUSGP, which has the largest capacitive loops. The corrosion potential and corrosion current values are 0.3999 V and 9.435 mA/cm , respectively. Corrosion morphology revealed that the Sample WUSGP has the smallest corrosion holes and cracks, exhibiting a diameter of approximately 1.4 m and a width of approximately 2.1 m, respectively, degrading 78% and 75% compared with the polished sample. Corrosion products analysis showed that the corrosion mainly occurs in the weak regions of the substrate and in the active anode area will produce Fe(OH) film through a redox reaction. The continuous erosion of Cl destroyed the corrosion layer. Then, the electrolyte slowly flows into the matrix to corrode at the damaged regions, but the existence of a dense strengthening layer and passivation film greatly reduces the corrosion rate. The best corrosion resistance obtained in sample WUGP can be interpreted as the high dislocation density, small grain size, needle-like, and lath-like shape of martensite formed in the treatment, which hinders Cl from eroding the matrix. This research demonstrated that traditional ultrasonic shot peening fused the strengthened liquid and corundum, and ceramic balls for treating materials can significantly improve the corrosion resistance of steels. Author Contributions: Formal analysis and writing original draft, X.X.; resources and writing, Z.Z.; conceptualization, review, and editing, X.L. and J.X.; data curation and validation, Z.G. and Z.L.; investigation, visualization, and project administration, J.W.; All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (52075109, 51975136), the Science and Technology Plan of Guangzhou (202102080225), the Science and technology Special Fund program of Guangdong Province (2019B020404), the Industry-University-Research Cooperation Key Project of Guangzhou Higher Educational Universities (202235139), and Guangzhou University Research Project (YJ2021002). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Appl. Sci. 2022, 12, 7360 12 of 13 Data Availability Statement: The data presented in this study are available in the article. Acknowledgments: We thank the Guangzhou Key Laboratory of High-Performance Metal Grinding Processing of Guangzhou University. Conflicts of Interest: The authors declare no conflict of interest. References 1. Bhadeshia, H. Steels for bearings. Prog. Mater. Sci. 2012, 57, 268–435. [CrossRef] 2. Li, K.; Chen, Z.X.; Liu, P.P.; Li, G.L.; Ding, M.R.; Li, Z.X. Characterization and performance analysis of 3D reconstruction of oil-lubricated Si3N4-GCr15/GCr15-GCr15 friction and wear surface. J. Therm. Anal. Calorim. 2021, 144, 2127–2143. [CrossRef] 3. Cao, Y.J.; Sun, J.Q.; Ma, F.; Chen, Y.Y.; Cheng, X.Z.; Gao, X.; Xie, K. Effect of the microstructure and residual stress on tribological behavior of induction hardened GCr15 steel. Tribol. Int. 2017, 115, 108–115. 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Journal

Applied Sciences – Multidisciplinary Digital Publishing Institute

Published: Jul 22, 2022

Keywords: GCr15 steel; ultrasonic strengthening grinding process; salt spray corrosion; corrosion resistance

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