Improvement of Tribological Properties and Corrosion Resistance of AISI 4340M Steel by Shot Peening and Plating Technologies

Abstract

In this study, shot peening (SP), quenching and tempering (QT) heat treatment, baking heat treatment and Cr-plating technologies were used to enhance the hardness, tribological properties and corrosion resistance of AISI 4340M steel. The purpose of this study is to develop repair process technology for an overhaul of landing gear applied to the MRO process and establish a quality assurance system. The effects of SP, QT heat treatment, Cr-plating, stripping after Cr-plating and Cr-plating after stripping and re-SP on the tribological properties and corrosion resistance of AISI 4340M steel were investigated, and the obtained results were compared with the base AISI 4340M steel. One of the reasons for stripping after Cr-plating is to find out how many times stripping can be done after Cr-plating. Moreover, it is important to investigate the effect of re-SP after Cr-plating on the tribological properties and corrosion resistance. The tribological properties of the specimens were investigated using a ball-on-disk tribometer at room temperature against AISI 52100 steel for 60 min under dry conditions. The corrosion resistance was investigated using a potentiodynamic polarization test in NaCl 3.5% solution. The results showed that the application of SP, QT heat treatment, Cr-plating, stripping after Cr-plating and Cr-plating after stripping and re-SP had a significant effect on the tribological properties and corrosion resistance of AISI 4340M steel. The effects of SP and Cr-plating post-treatment technologies on the wear and corrosion enhancement and mechanisms were discussed based on the microstructural and surface morphology of worn and corroded surfaces.

1. Introduction

AISI 4340M steel is an excellent choice for landing gear applications due to its high strength, toughness, and other properties [1], making it well-suited for applications in the aerospace industry, where high tensile strength and hardness are required [2]. Additionally, AISI 4340M is often used in parts that require good fatigue resistance, where its properties make it a reliable and durable material for such critical applications [3]. The landing gear is an important component that bears the whole weight of the aircraft during landing impact. Therefore, it is of interest to improve its durability and understand degradation mechanisms.

Although AISI 4340M steel offers many advantages to be used in aerospace applications, it has still some disadvantages: it can be more expensive than some other types of steel due to its specific alloying elements and heat treatment requirements; it can be more challenging to machine compared to some lower-alloy steels, requiring appropriate tooling and machining techniques; it requires proper heat treatment for optimal mechanical properties, and incorrect heat treatment can result in reduced performance or potential issues such as cracking and, finally, it has good strength and toughness, but it is not as wear and corrosion-resistant, and proper surface treatment applications may be required to protect it from wear and corrosion. Understanding these limitations and taking appropriate precautions can help mitigate any disadvantages when using this AISI 4340M steel in landing gear applications, as it requires high strength and anti-corrosion, anti-wear and anti-fatigue properties due to extreme impact.

In this regard, AISI 4340M steel needs to be treated by surface treatment techniques such as shot peening (SP) [4], ion nitriding surface hardening [5], ultrasonic nanocrystal surface modification (UNSM) [6], etc., to increase the strength, wear performance, corrosion resistance and fatigue strength. Clarke et al. discussed the perspective of quenching and tempering (QT) the heat treatment of AISI 4340 steel to better understand the microstructural and chemical evolution [7]. It was found that QT heat treatment is essential to modify the microstructure features such as carbon redistribution, transition carbide and cementite formation, and retained austenite decomposition that occur during QT. More importantly, it controls the amount and stability of retained austenite. In addition, some beneficial effects of various surface technologies, such as electroplating and thermal spray coating, on the fatigue strength of AISI 4340 steel have been investigated [8]. It was reported that hard chromium electroplating showed better fatigue performance because of lower microcrack density, while thermal spray WC coating introduced a compressive residual stress within the metal substrate to enhance the fatigue properties of AISI 4340 steel.

In our previous study, the effects of baking heat treatment at various temperatures on the fatigue performance of AISI 4340M steel subjected to SP were investigated. It was found that the fatigue performance of SPed AISI 4340M steel was improved, where a temperature of 246 °C was found to be optimum [9]. Tabieva et al. pointed out that plasma hardening can be used to create a hardened surface layer to increase the wear and corrosion performance of steel by increasing the surface hardness [10]. The effect of individual and combined SP and UNSM treatment on the fatigue behavior of AISI 4340 steel has been previously investigated [11]. It was reported that the combination of these two technologies increased the fatigue strength more than that of the individual technologies. Overall, SP, surface modification and coating technologies are widely used methods in various industries to improve the mechanical properties and performance of metal components subjected to cyclic loading, wear or corrosion degradation. It is an effective way to enhance component durability and reliability. However, the influence of re-SP on the heat-treatment, SPed and Cr-plating has not been investigated yet.

In this study, SP, which is a mechanical surface treatment process used to enhance wear, corrosion performance and strength, is applied to AISI 4340M steel. In the SP process, small spherical shots impacted the surface of the AISI 4340M steel using compressed air. This study aims to evaluate the effect of baking heat treatment on the tribological properties and corrosion resistance of SPed AISI 4340M steel. In addition, it is also essential to develop repair process technology for an overhaul applied to the maintenance, repair and overhaul (MRO) process of aircraft landing gear and establish a quality assurance system. The effects of QT heat treatment, SP, Cr-plating and stripping after Cr-plating and re-SP on the tribological properties and corrosion resistance of AISI 4340M steel were investigated, and the obtained results were compared with the base AISI 4340M steel and the mechanisms comprehensively discussed.

2. Experiments

2.1. Specimen Preparation, Heat Treatment and Shot Peening

In this study, AISI 4340M steel, which is notable for containing a comparatively high amount of Si, was used as a specimen. The chemical composition of AISI 4340M steel investigated by EDX is listed in

Table 1. Chemical composition of AISI 4340M steel (in wt.%).

C	Mn	Si	Cr	Ni	Mo	V	N	Nb
0.43	0.83	1.62	0.81	1.82	0.39	0.07	0.002	0.01
P	S	Cu	B	Ti	Al	W	Co	Ca
0.006	0.001	0.12	0.0002	0.006	0.07	<0.05	<0.005	<0.001

[9]. QT heat treatment was quenched at 850 °C for 2 h and then followed by double tempering for 3 h at 300 °C. The following specimens with designated letters were prepared for this study as listed in

Table 2. Designated letter of the specimens.

Control	A
QT heat treatment	B
Shot peening before baking heat treatment	C
Cr-plating and baking heat treatment	D
Stripping after Cr-plating and baking heat treatment	E
Re-shot-peening before Cr-plating	F

. SP was applied to the specimens according to the standard overhaul practices as listed in

Table 3. Shot peening parameters (adapted from Ref. [9]).

Cast steel shot	Shot diameter [mm]	Nozzle diameter [mm]	Shot flow [kg/min]	Angle of impingement [degree]
ASH 230 with HRC 55-60	0.7	7.9	3	90
Air pressure [bar]	Peening time [min/place]	Working distance [mm]	Arc height [mmA]	Coverage [%]
3	2	50 ± 5	0.36	200

[9] at an impingement angle of 90°. The purpose of the baking heat treatment is to release the stress after SP. Baking heat treatment was performed at a temperature of 246 °C for 4 h in air cooling conditions. Cr-plating was performed at a temperature of 60 °C at a current density of 46 A/dm², while stripping was performed at ambient temperature in a sodium hydroxide (NaOH) solution at a voltage of 6 V. The Cr-plated specimens were baked, as hydrogen embrittlement will occur if the Cr-plated part is not baked. SP followed by Cr-plating was performed without baking to remove hydrogen. It also needs to be mentioned here that the control and QT heat treatment specimens were polished using 2000-grit silicon carbide (SiC) sandpaper at 250 rpm in an automatic polishing machine.

2.2. Surface Roughness and Hardness Measurements

The surface roughness of the specimens was measured using a Mitutoyo SurfTest SJ-210 portable 2D profilometer (Kawasaki, Japan) with a scanning speed of 0.5 mm/s for 4.8 mm at a measuring force of 0.75 mN. The radius of the diamond stylus tip is 2 µm. Surface roughness measurements were made in accordance with the ISO standard, and the presented definition refers to the DIN standard.

The hardness of the specimens was measured using a Mitutoyo HM-200 hardness testing machine at a load of 300 gf for 10 s of dwell time. Both surface roughness and hardness measurements were repeated three times to obtain repeatable and durable measurement data.

2.3. Wear Resistance Experiment

Tribological studies were investigated using a linearly reciprocating ball-on-flat sliding wear tester (Anton Paar GmbH, Graz, Austria) in accordance with ASTM G133 [12] with sinusoidal reciprocating rotary motion. The testing conditions are listed in

Table 4. Tribological test conditions of AISI 4340M steel against AISI 52100 bearing steel.

Applied load, N	10
Sliding velocity, cm/s	2.51
Total sliding time, min	60
Temperature, C	24
Environment	Dry
Counterface ball material	AISI 52100
Counterface ball diameter, mm	12.7 mm

. The wear resistance, namely the wear rate of the specimens, was evaluated based on the dimensions (depth, width and length) of the wear track obtained by the same portable surface profilometer that was used to measure the surface roughness of the specimens. It is essential to mention that the three cross-sectional profiles were used to calculate the average wear rate of the specimens, considering the ratio of wear volume to the multiplication of the applied normal load and total sliding distance.

2.4. Electrochemical Corrosion Resistance Experiment

The corrosion studies were investigated using a potentiodynamic polarization test, as illustrated in Figure 1. It was used to characterize the corrosion behavior of AISI 4340M steel. It involved measuring the current responses and varying the potential in a specific range. The potential was first set to a starting value, and then the potential was swept at a constant rate in either the positive (anodic) or negative (cathodic) direction. During this potential sweep, the resulting current response was measured. The scan rate was 0.32 mV/s with a scan range of −0.5 V to +0.5 V in a solution of 3.5% NaCl. The obtained current-potential results were used to construct a polarization curve, which typically provides information about the corrosion potential, corrosion current and passivation behavior.

2.5. Surface and Wear Track Characterizations

The top, worn-out and corroded surfaces of the specimens and the worn-out surface of the counterface balls were analyzed by scanning electron microscopy (SEM: JSM-6610LA, JEOL, Akishima, Japan) and energy-dispersive X-ray spectroscopy (EDX: JED2300, JEOL, Akishima, Japan).

3. Results and Discussion

3.1. Surface Morphology, Roughness and Hardness Results

SEM images of the specimens showing the surface morphology are shown in Figure 2. It is obvious that the surface of the control and QT heat-treated specimens is relatively homogenous and smooth after polishing, as shown in Figure 2a,b. Sang et al. achieved superior strength and toughness in AISI 4340 steel by intercritical QT heat treatment [13]. In the case of the SPed specimen (see Figure 2c), the surface was roughened as small spherical shots were randomly blasted onto the surface of the specimen, creating indentations on the surface and leading to a rough surface. This led to plastic deformation of the surface layer, resulting in compressive residual stresses and work-hardening of the specimen. Figure 2d shows that the SEM image of the Cr-plated specimen had a slightly different surface morphology, where it provided a bright and reflective surface finish. It was confirmed that the surface of the substrate made of AISI 4340M was fully covered by the Cr-plating layer, where the cracks were visible and were propagated along the grain boundaries due to the consequence of the introduction of the compressive stress in the chromium deposits. Figure 2e shows the surface morphology of the stripped Cr-plating specimen, where stripping involves removing the Cr-plating layer from substrate surfaces. This process was carried out by chemical stripping agents designed to dissolve the Cr-plated metal layer without damaging the underlying substrate. However, some stripping-induced defects on the surface can be observed. Figure 2f shows the surface morphology of the Cr-plating on the surface, which was subjected to re-SP. It is obvious that Cr-plating provided a reflective surface finish of the SPed substrate. The cracks, along with “cauliflower-like”-shaped nodules, which are very typical for Cr-plating metallic alloys, were observed.

Figure 3 shows the surface roughness measurement results of the specimens. It is clear from Figure 3a that the control and QT heat-treated specimens had very similar surface roughness Ra values of 0.042 and 0.039 µm, respectively. It means that both specimens were polished under the same conditions. As expected, the SP hugely increased the surface roughness Ra of the specimen up to 1.86 µm due to the formation of indentations on the surface. The surface roughness of the Cr-plated specimen was slightly reduced after Cr-plating, which is a surface finish technology that smoothens the surface of the substrate. When the Cr-plated layer was removed by stripping, the surface roughness of the substrate was further reduced. However, it was still far from the polished control and heat-treated specimens. Re-SP again increased the surface roughness of the substrate to the same Ra value of 1.91 µm due to the formation of indentations on the surface. Torres et al. investigated the effect of SP on the surface roughness of AISI 4340 steel [14]. It was found that the SP increased the surface roughness, which had a beneficial influence on the fatigue strength of AISI 4340 steel. Figure 3b shows the surface roughness of Rz values. It was noticed that the Rz values followed a similar trend to the Ra values, but the Rz value of the Cr-plated specimen was increased after SP, while the Ra value was decreased. It may be attributed to the thickness of the Cr-plating layer, the depth of the indentations and the height of the formed pileups during the SP process. It can be concluded that the SP increased the surface roughness of the control, Cr-plated and stripped specimens. Overall, the rough surface texture resulting from SP plays a crucial role in enhancing the mechanical properties and performance of metal components, particularly in applications where fatigue strength and resistance to crack propagation are critical.

Figure 4 shows the result of the microhardness measurement of the specimens. The control specimen was found to have the lowest value among other specimens. The QT heat treatment increased the hardness of the control specimen. SP performed on the control specimen increased the surface hardness, which was slightly higher than that of the QT heat-treated specimen. The increase in hardness after SP may be attributed to the refined grains and the introduced compressive residual stress [15]. It is also important to mention that SP increased the dislocation density of the refined structure, which plays a significant role in increasing the strength of materials. As noticed, the Cr-plating led to an increase in surface hardness compared to those of the control, SPed and heat-treated specimens. However, the hardness of the stripped specimen was significantly reduced, and the hardness of the re-SPed specimen increased to the level of the Cr-plating, as shown in Figure 4. The re-SPed specimen showed the highest hardness value at the surface. This is thought to come from grain refinement due to the compressive residual stress effect caused by the surface plastic deformation by SP. It is worth mentioning here that, in our previous study, the hardness results as a function of the depth for the SPed and, with baking heat treatment before Cr-plating at a temperature of 246 °C, specimens were obtained. It was reported that the effective depth of SP and baking heat treatment was found to be approximately less than 400 µm [9].

3.2. Frictional Behavior and Wear Resistance

The results of the variation in the friction coefficient as a function of sliding time for one hour are shown in Figure 5a. The COF has been recorded as the ratio of friction force to applied normal load with a sinusoidal periodicity due to a reciprocating motion. All specimens demonstrated a “running-in zone” in which there was a leveling of high irregularities, adhesion of the specimen and counter body surfaces and wear of the original surface film (see Figure 5b). These changes can both increase and decrease the COF depending on the tribo-contact parameters. The interaction of sliding pairs consisting of the AISI 52100 bearing steel counter body slid against the control specimen had a very stable frictional behavior with a short “running-in zone” at the beginning of the test. The specimen subjected to QT heat treatment had a low friction coefficient at the very beginning of the test, but it increased rapidly to a friction coefficient value of 0.27, and then gradually increased to a maximum friction coefficient value of 0.31. Afterwards, it started decreasing gradually to an average friction coefficient value of about 0.22. An increase in COF may be attributed to the smooth surface, which increases frictional forces between mating surfaces due to close atoms interacting with each other. In this case, the relative smooth surface of the QT heat treatment specimen formed more bonds between surfaces, leading to higher COF, but the formed bonds were destroyed easily, decreasing the COF. In addition, high surface hardness, which increases the sliding resistance between two mating surfaces, could be a good reason for the high COF. Tomaz et al. investigated the sliding friction behavior of AISI 4340 steel against AISI 52100 bearing steel under dry conditions [16]. It was found that the friction coefficient results were in good agreement with the reported results. In the case of the SPed specimen, the frictional behavior was very stable throughout the sliding time for one hour, but the friction coefficient reached a maximum value of 0.2 at the beginning of the test, which may be attributed to the rough surface due to the formation of indentations on the surface of the specimen. The frictional behavior of the Cr-plated specimen was somehow similar to the frictional behavior of the QT heat-treated specimen, where the Cr-plated specimen had a low friction coefficient at the very beginning of the test, but it increased rapidly to a friction coefficient value of 0.23, and then gradually increased to a maximum friction coefficient value of 0.25. However, it started stabilizing only after 30 min of sliding, as can be seen from Figure 5a. The stripped-after Cr-plating specimen demonstrated frictional behavior, where the “running-in transition zone” was relatively short due to the reduced surface roughness after Cr-plating was stripped from the substrate surface. It had a very similar frictional behavior to the control specimen during the steady-state conditions. The re-SPed specimen had the longest “running-in transition zone” of almost 40 min among other specimens due to the high surface roughness. It was confirmed that the re-SP had a detrimental influence on the frictional behavior of the specimens.

Figure 6 shows the cross-sectional wear track profiles of the specimens obtained after tribological tests. The dashed line in red is a reference line, which is the actual surface of the specimen. The wear resistance of the control specimen was increased by QT heat treatment, as can be seen from Figure 6a,b, where the QT heat-treated specimen showed the highest wear resistance due to increased surface hardness (see Figure 4). Although the SPed specimen had a similar hardness compared to that of the QT heat-treated specimen, the wear resistance was found to be lower than the QT heat-treated specimen, as shown in Figure 6c. Tomaz et al. investigated the effect of residual stress induced by SP on the sliding wear behavior of AISI 4340 steel against AISI 52100 bearing steel under dry conditions [14]. It was found that the induced compressive residual stress by SP enhanced the wear resistance of AISI 4340 steel. Figure 6d showed that the wear resistance of the Cr-plating specimen was lowered compared to the heat-treated and SPed specimens, which may be attributed to the high surface roughness, which is almost at the same level as the SPed specimen, and to the thickness of the Cr-plating layer. In this case, it had a relatively higher surface hardness than the QT heat-treated and SPed specimens. However, high hardness was not a dominant factor in controlling the wear resistance of the Cr-plated specimen. As shown in Figure 6e, the stripping after the Cr-plating specimen demonstrated the lowest wear resistance compared to other specimens due to the lower surface hardness than that of the QT heat-treated, SPed and Cr-plated specimens. As confirmed in Figure 6f, although it had the highest surface hardness, the re-SPed specimen had almost similar wear resistance compared to that of the stripping after the Cr-plating specimen. It was confirmed again that the re-SP had a detrimental influence on the wear resistance behavior of the specimens. Figure 7 shows the wear rate calculation results of the specimens obtained from at least three measurements to obtain reliable measurement data. The error bars shown in Figure 7 are tiny, showing that repeated tests gave good reproducible results due to the excellent set-up of tribological performance. It can be concluded that a direct relationship was observed between the friction coefficient values and the wear track dimensions (depth and width). It is obvious that the specimens with high friction coefficient values, especially in the running-in zone, had increased wear. It can also be observed that lower wear is inherent in the specimens with the presence of a more steady-state condition [17]. After SP, the width of the wear track decreased significantly and reached a minimum value due to the increased surface hardness. The value of the worn surface width correlates with the friction coefficient and wear resistance for all the specimens. Ma et al. investigated the wear behavior of AISI 4340 steel under dry sliding conditions [18]. It was found that the wear behavior of the control specimen had a similar wear behavior.

3.3. Worn-Out Surface Morphology and Wear Mechanisms

Figure 8 shows the SEM images of the wear track of the specimens. It can be seen that the surface roughness and integrity dominated the wear occurrence. It is clear from Figure 8a,b that a relatively smooth surface demonstrated a high resistance to wear compared to that of the relatively rough surfaces. A partially worn-out surface was observed due to the rough surface after SP, as shown in Figure 8c. The correlation of wear resistance with the hardness of the specimens is clearly shown, but it also depends on the surface roughness. A lower value of wear resistance was obtained for the specimens after SP with high surface roughness (see Figure 3) and high surface hardness (see Figure 4). Figure 8d shows a more partially worn-out surface due to the high R_z value, which is the average value of the absolute values of the heights of the highest-profile peaks and the depths. Despite the lower surface roughness of the stripped after Cr-plating layer specimen, the wear resistance lowered (see Figure 8e), which may be attributed to the lowest surface hardness. Figure 8f, which is the re-SPed specimen, showed a similar wear behavior compared to that of the SPed specimen, as shown in Figure 8c.

The wear mechanism under severe dry wear conditions and the refined structure by SP with a higher hardness did not demonstrate better wear resistance. It was also determined that, with the increasing surface hardness of the specimen by QT heat treatment, Cr-plating and re-SP, the amount of wear increased for the specimens with higher surface roughness. Figure 9 shows the EDS mapping, including chemical composition analysis of the wear tracks formed on the surface of the specimens. It can be seen that the oxidative wear mechanism was found to be one of the dominant wear mechanisms. The highest oxidation occurred on the interface between the stripped after the Cr-plating specimen and the counterface ball, where the wear rate was on the same level as the Cr-plated and re-SPed specimens due to the formation of the stable oxide layer, which controlled the frictional behavior. The value of the worn surface width correlates with the friction coefficient and wear rate values for all specimens. The worn surface of the specimens shows clear abrasion marks and smeared fragment layers. Material transfer and spalling occurred during sliding friction between the two contact surfaces. Adhesive wear leads to the formation of a certain number of abrasive particles, which results in abrasive wear. Thus, the wear mechanisms of the specimens were predominantly adhesive wear and abrasive wear of varying degrees. The worn-out counter body had a regular round wear scar with different diameters, as shown in Figure 10. It was confirmed that the diameter of the wear scar followed the same trend as the wear rate results. The wear mechanisms of AISI 4340 steel have been previously investigated, where the worn-out surface showed dominant oxidative and adhesive wear mechanisms, which are very similar to the wear behavior of AISI 4340 steel [17,18,19].

3.4. Corrosion Resistance

AISI 4340M steel is highly resistant to atmospheric corrosion because of its combined alloy elements, such as Cr, Mo, Ni and Mn. The presence of these elements not only improves the strength and hardenability of steel, but also improves corrosion resistance. Figure 11 shows the SEM images of the corroded surface of the specimens. It can be seen that the control specimen showed moderate corrosion resistance, as shown in Figure 11a. The corroded surface of the QT heat-treated specimen, as shown in Figure 11b, looks a bit different, where a result of the chemical reaction can be observed, where the EDX confirmed the absence of a newly formed layer on the surface. Figure 11c shows the corroded surface of the SPed surface, where the corrosion did not occur at all, while general corrosion occurred on the surface of the Cr-plated surface, as shown in Figure 11d. The stripped Cr-plated layer demonstrated the worst corrosion resistance, which can be observed in Figure 11e. The re-SPed specimen had better corrosion resistance compared to that of the QT heat-treated, Cr-plated and stripped specimens. It was found that the SP increased the corrosion resistance, which may be attributed to the surface roughness and other properties, but these results need to be confirmed with a more significant number of specimens to clearly understand the passivation mechanisms. It is worth mentioning that the obtained corrosion potential and current density results from the Tafel fit listed in

Table 5. Corrosion potential and current density results of the specimens.

Specimens	Current Density, 10−5 Amps/cm2	Current Potential, V
A	8.0152	−0.88
B	1.3839	−0.68
C	7.6125	−0.87
D	9.4587	−0.92
E	9.1753	−0.84
F	1.9839	−0.62

are in good agreement with the SEM images of the corroded specimens. However, it has been reported that SP decreased the corrosion resistance of steel despite the formation of a nanostructured surface [20,21].

4. Conclusions

In this study, the effects of QT heat treatment, Cr-plating, SP, stripping and re-SP on the tribological properties and corrosion resistance of AISI 4340M steel were investigated.

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It was found that the surface roughness was changed, where the SP increased the surface roughness of the control, Cr-plated and stripped specimens. The control specimen was found to have the lowest surface roughness among the other specimens.

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The QT heat treatment increased the hardness of the control specimen. SP performed on the control specimen increased the surface hardness, which was a bit higher than that of the QT heat-treated specimen.

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The wear resistance of the control specimen was increased by QT heat treatment, where the QT heat-treated specimen showed the highest wear resistance due to increased surface hardness. Although the SPed specimen had a similar hardness compared to that of the QT heat-treated specimen, the wear resistance was found to be lower than the QT heat-treated specimen.

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The wear mechanism under severe dry wear conditions and the refined structure by SP with a higher hardness did not demonstrate better wear resistance due to highly increased surface roughness.

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SP and re-SP had better corrosion resistance compared to that of the QT heat-treated, Cr-plated and stripped specimens. It was found that the SP increased the corrosion resistance, which may be attributed to the surface roughness and other properties, but these results need to be confirmed with a more significant number of specimens to clearly understand the passivation mechanisms.