A Study on the Influence Regulation of Surface Integrity on the Corrosion Resistance of Hydrogen Production Reactor Material

Abstract

Corrosion can hurt the quality and service life of a workpiece, which could create potential safety hazards. However, improving the workpiece’s surface integrity through surface treatment could improve the effect of corrosion resistance. To study the effect of surface integrity on the corrosion resistance of low-alloy steel for hydrogen reactors, electrolytic corrosion experiments were carried out on specimens that were processed using different grit sandpapers or using different shot peening pressures using a self-built electrolytic platform. The influence regulation of initial surface roughness and surface residual stress on the corrosion rate of the low-alloy steel for a hydrogen reactor under different lengths of corrosion time, surface roughness after corrosion, and the tensile property degradation of the specimens after 4 h of corrosion were analyzed, respectively. In this paper, based on experimental research, we obtain the influence regulation of the processing parameters on the corrosion resistance of low-alloy steel for hydrogen reactors and provide processing parameters that could improve the corrosion resistance of low-alloy steel, which guides the corrosion resistance processing of hydrogen production reactors. It was found through experiments that with an increase in the initial surface roughness of the specimens, the corrosion rate of the specimen tends to decrease with the increase in corrosion duration; the surface roughness of the specimen after corrosion first increases and then decreases with the increase in corrosion time; and the tensile strength of specimen with the initial surface roughness of Ra 0.168 μm is relative good after 4 h of corrosion. With the increase in residual compressive stress on the surface of the specimen, the corrosion rate of the specimen decreases with the increase in corrosion time; the surface roughness of the specimen after corrosion first increases and then decreases with the increase in corrosion time; and the tensile strength of specimen with the surface residual stress of −335.64 MPa is relative good after 4 h of corrosion.

1. Introduction

Hydrogen has the advantages of a high calorific value and no pollution, which makes it widely used in aerospace, automobile manufacturing, energy, and other fields [1,2]. Medium- and low-carbon steel and low-alloy steel have the advantages of good machinability, mechanical strength, and low manufacturing costs, making them widely used in producing hydrogen production reactors, hydrogen storage vessels, and prohydrogen chemical equipment. The hydrogen reactor produces hydrogen, water, and other products during operation, which produces corrosive media inside the reactor, and the corrosive media can cause corrosion on the inner walls of containers and equipment surfaces, causing materials to be subjected to both hydrogen permeation and corrosion, resulting in adverse effects, such as material removal and stress corrosion cracking, reducing the mechanical properties and service life of the materials used. The most serious situation would lead to sudden failure of the hydrogen reactor under the working conditions due to the bad influence of corrosion, which would cause serious safety accidents; therefore, it is important to improve the corrosion resistance of the hydrogen reactor. Currently, the microstructure of the manufactured material can be changed by adjusting the ratio of the elements used or optimizing the heat treatment process, which improves the material’s corrosion resistance [3,4]. The surface integrity of materials can also be improved by reducing the material surface roughness, improving the surface topography, or adjusting the surface residual stress distribution, which can improve the corrosion resistance of the material. Ultimately, avoiding a reduction in the material mechanical properties and service life caused by corrosion achieves the goal of improving equipment reliability.

Currently, many scholars take the influence of material surface integrity on corrosion resistance as the research object. Furthermore, corrosion experiments were carried out to study different materials’ surface topography, roughness, and surface residual stress to study the influence of the regulation of surface integrity on the corrosion resistance of different materials. Šolić et al. [5] analyzed the influence of the regulation of surface roughness on the rate of corrosion penetration in steel and found that an increase in material surface roughness accelerates corrosion development and increases the rate of penetration into the interior of the material, so the appropriate surface protection techniques need to be selected to prevent unnecessary wear of the steel. Li et al. [6] analyzed the effect of surface roughness on the corrosion resistance of 7075 aluminum alloy after multiple ultrasonic rolling; the results showed that the combined effect of surface nanosizing and the roughness reduction in the processed specimens improved the corrosion resistance of 7075 aluminum alloy. Paknahad et al. [7] studied the corrosion resistance of iron aluminide coatings with different surface roughness, and the experiment results showed that the surface roughness would significantly affect the corrosion rate: the lower the surface roughness, the lower the corrosion rate of the substrate and aluminide-coated specimens. Balusamy et al. [8] processed 304 stainless steel (SMAT) with different methods and found that increased surface roughness could hurt the corrosion resistance of the processed specimen surface. Latifi et al. [9] studied the surface properties of 316 L stainless steel and found that the density of the corrosion current on a stainless steel surface is positively correlated with its surface roughness; when the material surface is smoother and the corrosion current density is smaller, the corrosion resistance of stainless steel surface can be stronger. Filho et al. [10] carried out lathe experiments on a Ti-6Al-4V alloy and found that corrosion resistance is related to the surface roughness; by reducing the surface roughness Rz and Ra by 11.9% and 5.5%, respectively, the corrosion resistance improved by 18.8%. Niu et al. [11] carried out experiments to compare the corrosion resistance of an Al-Li 2A97 alloy after milling and polishing. The experimental results showed that the surface roughness caused by the milling process was unfavorable to the corrosion resistance of the alloy; still, the surface hardening caused by the milling process made the corrosion resistance of the Al-Li 2A97 alloy after the milling process better than that caused by polishing. Gumpel et al. [12] used different processing methods to process stainless steel workpieces and studied the effective regulation of surface topography on the corrosion resistance after processing. The results showed that not only does the workpiece’s surface roughness affect the workpiece’s corrosion behavior, but the shape of the peaks and valleys formed on the workpiece surface also affects the corrosion resistance of the workpiece. Nguyen et al. [13] studied the relationship between the surface roughness and corrosion rate of pure Mg and chose pure Mg samples with different initial roughness values to carry out corrosion experiments; the experimental results showed that the surface roughness had a significant effect on the corrosion rate of pure Mg, and the corrosion rate increased with the increase in surface roughness. Meanwhile, pitting corrosion was not observed, which indicates that surface roughness does not affect the pitting potential of Mg. Chen et al. [14] carried out corrosion experiments on Ti specimens with different surface roughness, and the results showed that Ti specimens with different surface roughness have good corrosion resistance. However, corrosion resistance still decreased with the increase in surface roughness compared with a smooth surface, and the microrough surface has better corrosion resistance. Yan et al. [15] compared the surface properties of the coated materials prepared using different additive processes, and the experiment results showed that due to the lower surface roughness of the coated material prepared using selective beam melting (SEBM), the prepared material has better corrosion resistance. The scholars mentioned above carried out corrosion experiments on different materials with different surface roughness and found that the corrosion resistance of the specimen increases as the surface roughness decreases. However, the corrosion resistance of the specimen decreases when the surface roughness is below a specific value.

Meanwhile, many scholars have also studied the influence of specimens’ surface residual stress on the corrosion resistance of different materials. Lai et al. [16] measured and analyzed the residual tensile stress on the electrochemical corrosion behavior of 304 stainless steel and found that the higher the residual tensile stress in the workpiece, the higher the dislocation density, the higher the corrosion current, and the more severe the corrosion. Lin et al. [17] studied the influence regulation of surface residual stress on the corrosion behavior of Al-Zn-Mg-Cu alloy plates under different predeformation conditions and found that the corrosion rate was accelerated when residual tensile stresses existed on the plate surface. Through experiments, Bai et al. [18] found that under the effect of welding residual compressive stress, the surface atomic density of the metal was increased, offsetting the promoted effect of surface residual stress and increasing the corrosion resistance of the welding structure. Ishibashi et al. [19] designed experiments that evaluated the effect of welding residual stress on corrosion fatigue properties and discussed the effect of the regulation of welding residual stress on the corrosion pit growth. Hou et al. [20] explored the effect of residual stress on a workpiece’s surface after grinding on the corrosion resistance of the workpiece. The results showed that the type of corrosion on the workpiece surface depends on the influence degree between the surface topography and surface residual stress; when the residual stress is stronger, corrosion cracking is the main form of corrosion. Xiong et al. [21] simulated the corrosion behavior of an X80 UOE pipe in the natural environment. The simulation experiment found that residual stress accelerated the generation of stress corrosion cracks on the surface of the X80 UOE pipe; meanwhile, the specimens with higher surface residual tensile stress are prone to local corrosion. Fang et al. [22] studied the effective regulation of the rolling process on the corrosion resistance of seamless automotive steel plates and found that with the increase in residual stress generated by rolling, the corrosion resistance of steel plates could be improved. Bai et al. [23] studied the effect of residual stress on the corrosion behavior of welded structures and found that residual tensile stress can decrease the activation energy and surface atomic density of the welded construction, which reduces the corrosion resistance of the welded construction. Residual compress stress minimizes the need for activation energy to convert metal atoms into metal ions and increases the surface atomic density of the welded structure, which promotes corrosion resistance. Wan et al. [24] observed the corrosion process of high-strength aluminum alloys after milling and found that the combination of residual stress and microcracks has an important effect on the corrosion resistance of processed aluminum alloy workpieces. Anita et al. [25] studied the connection between stress and corrosion. Surface residual compressive stress has the opposite effect of residual tensile stress on surface corrosion resistance, and the residual tensile stress shows a negative correlation trend with corrosion resistance; with an increase in surface residual tensile stress, corrosion resistance decreases. Xiong et al. [26] compared the corrosion behavior of AZ80 magnesium with different surface residual stress, and the results show that the higher residual compress stress can improve the corrosion resistance of AZ80 magnesium. The scholars mentioned above analyzed the influence regulation of surface residual stress generated using different processing methods on the corrosion resistance of workpieces, which is that the residual tensile stress caused by processing can weaken the corrosion resistance of materials. In contrast, the residual compressive stresses can enhance the corrosion resistance of materials.

After years of research, many scholars have studied the corrosion resistance effect regulations of different materials on the aspects of surface topography, surface roughness, and the type of surface residual stress. The research found that the effect of surface integrity indexes on corrosion resistance is relatively complex, and the effective regulations of different materials are different. To study the effective regulation of surface integrity on the corrosion resistance of low-alloy steel for hydrogen production reactors, this paper chose surface roughness and surface residual stress as the main surface integrity evaluation indexes and carried out corrosion experiments on hydrogen production reactor steel. The effect regulations of surface integrity on the corrosion resistance of the hydrogen production reactor were analyzed by comparing the surface topography, corrosion rate, surface roughness, and tensile mechanical property changes before and after electrolytic corrosion. Based on the experimental research, in this paper, we obtain the influence regulation of the processing parameters on the corrosion resistance of the low-alloy steel for hydrogen reactors and propose processing parameters that could improve the corrosion resistance of the material based on the obtained regulations, which provides a certain guide for the corrosion-resistant fabrication of hydrogen production reactors.

2. The Design of the Electrolytic Corrosion Experiment

2.1. The Design of the Experiment

2.1.1. The Preparation and Pretreatment of Specimens

The specimens used in electrolytic corrosion experiments were divided into block corrosion and tensile specimens, and the material used for specimens is low-alloy steel used for hydrogen production reactors. The size of block corrosion specimens is 20 mm × 20 mm × 10 mm, and the size and shape of the tensile specimens were determined according to the test method required by ISO 6892-1:2019 [27], Metallic materials—Tensile testing, Part 1, method of test at room temperature. The shape and the size of tensile specimens are shown in Figure 1c, and the dimensions of the specimens shown in Figure 1c are in mm. The experiments were divided into two parts, different surface roughness corrosion and different surface residual compressive stress corrosion, and four sets of experiments were designed for each part. Meanwhile, three specimens were used for each set of experiments to reduce the contingency of experiments. The number of block specimens used in corrosion experiments carried out in the paper is 24, and the number of tensile specimens is 8; before corrosion, all specimens were annealed and stored in a dry room temperature environment.

Before conducting experiments on the effect of surface roughness on corrosion resistance, four different surface processing methods were used to machine the specimens: filing, grinding with 80# sandpaper, grinding with 1200# sandpaper, and polishing to obtain different surface roughness on one side of the block specimen and both sides of the tensile specimens, respectively. Before conducting experiments on the effect of surface residual stress on corrosion resistance, different residual stresses were generated using shot peening on one side surface of the block specimen and on both side surfaces of the tensile specimen, respectively. The shot peening process parameters are as follows: the diameter of the steel pellet is 0.5 mm, the shot peening time is 2 min, and the shot peening pressures are 0 MPa, 0.2 MPa, 0.3 MPa, and 0.4 Mpa, respectively. Using shot peening processing, the surface roughness might be inconsistent. Therefore, the surfaces of specimens were slightly ground with 80# sandpaper after shot peening to avoid inconsistencies in surface roughness. During the grinding process, the surface roughness was checked several times to ensure that the surface roughness of the ground specimens was similar. The surface cleaning for complete rust removal and pretreatment of specimens was carried out uniformly using anhydrous ethanol. The surface topography, surface roughness, surface residual stress, surface microstructure, and initial quality of specimens were tested after surface cleaning.

To ensure the effectiveness of the experiment and avoid the corrosion of noninspection areas, the noninspection area of the specimens was wrapped in special anticorrosion tape. The four sides and the bottom of block specimens and both ends of the tensile specimens were wrapped, exposing the treatment surface of block specimens and the corroding area of tensile specimens; this operation benefits the control of current density. The wrapped specimens are shown in Figure 1a,b. After wrapping, the leakage area of the block specimen is 400 mm², and the leakage area of the tensile specimen is approximately 1560 mm² on each side.

2.1.2. Electrolytic Corrosion Experiment Set-Up

The electrolytic corrosion experiments were carried out using self-designed synchronous electrolysis equipment. The equipment mainly consisted of an eTM-L linear programmable DC power source, electrolytic tank, electrolytic solution, and electrodes. During the electrolytic corrosion process, multigroup specimens were connected in series to avoid errors caused by currency fluctuations. The stainless steel plate was connected to the cathode of the power source, the test piece was connected to the anode, and different groups of electrolyzers were in series connection; the specific connection scheme is shown in Figure 2. Dilute hydrochloric acid with a mass fraction of 10% was used as the electrolyte, and the electrolytic current density was set as 10 mA/cm². The corrosion temperature was room temperature, and the lengths of corrosion time were 1 h, 2 h, 3 h, and 4 h, respectively. The corrosion process ensures that the specimens are fully submerged in the electrolytic solution. After the corrosion experiment, the specimens were cleaned and dried, the block specimens were inspected for quality and surface topography, and the tensile specimens were tested for tensile strength.

2.2. Testing Index and Testing Theory

To analyze the effectiveness of electrolytic corrosion, the corrosion rate, surface topography, surface roughness, and mechanical properties were chosen as evaluating indicators during the experiments. The IFMG5 3D surface topography measurement was used to measure the surface topography and surface roughness, and the TM4104X universal testing machine was used to test the mechanical properties. The strain rate was selected as 0.00025 s⁻¹ according to the standard ISO 6892-1:2009 [27], and the corresponding tensile rate was calculated to be 1.17 mm/min. The tensile rate was set to 1 mm/min according to the parameters selected for the tensile experiment of similar materials [28], and the pulling force and deformation data were recorded to plot the stress–strain curve of the tested specimens. The corrosion rate was calculated according to Formula (1) provided by the standard ISO 18069:2015 [29]. The corrosion rate was calculated according to the mass loss per unit of time and unit area of the experiment. The mass of the specimen before and after corrosion was obtained using digital electronic balance with an accuracy of 0.001 g:

$$R=\frac{8760\times (M-M_t)}{STD}$$

(1)

where R is corrosion rate (mm/a); M is the mass of the specimen before corrosion (g); M_t is the mass of the specimen after corrosion (g); S is the total area of the specimen (mm²⁾; T is the corrosion time (h); and D is the density of the material (g/mm³).

2.3. The Results of Pretreatment Testing

2.3.1. Surface Roughness and Surface Topography

After pretreatment in different ways, the surface roughness and surface topography of specimens are shown in Figure 3 and Figure 4. It can be seen from Figure 4 that with the improvement in precision of the surface treatment process, the original pits and other defects gradually disappear, surface quality improves significantly and gradually tends to mirror, and the measurement error reduces. As the surface topography of the specimen is gradually bright and tends to mirror, the difference in surface roughness between different analytical areas on the specimen surface reduces due to the uneven processing, which reduces the measurement error as the surface roughness decreases.

2.3.2. Surface Residual Stress

The Empyrean X-ray diffractometer measurement was used to measure the residual stress on the surface of specimens under different pretreatment processes and different shot peening strengths, and the specific test results are shown in Figure 5. It can be seen from Figure 5 that as the precision of the treatment process increases, the residual compressive stress on the surface of the specimens gradually decreases; as the shot peening strength increases, the residual compressive stress on the surface of the specimens gradually increases, the residual stress measurement error of different surface roughness specimens gradually decreases with the increase in processing accuracy, and the residual stress measurement error of surface residual stress specimens increases with the increase in shot peening intensity. As the surface of specimens is processed using different methods, the processed surface can reflect X-ray better, so the detection measure error reduces. With the increase in shot peening intensity, the surface topography of the specimen is gradually complicated, and the scatter effect of the surface on X-ray is enhanced, so the measured error increases.

2.3.3. Surface Microstructure Testing after Shot Peening

Figure 6 shows the microstructure of the surface layer of the specimens after shot peening with different strengths. It can be seen from Figure 6 that with the increase in shot peening pressure, the surface microstructure of the material gradually appears as a more obvious plastic deformation layer. The depth of the plastic deformation layer of the material increases, and the phase boundary between the microstructures gradually blurs. The lamellar pearlite and ferrite near the surface of the specimen gradually change to an elongated pike shape. Moreover, the size of grains significantly reduces and the axiality of the grains parallel to the specimen surface. However, this phenomenon disappears deep below the surface layer of specimens.

3. The Effect of Surface Roughness on Material Corrosion

3.1. The Effect of Surface Roughness on the Corrosion Rate of Material

Figure 7 shows the corrosion rate of specimens with different initial surface roughness at different corrosion moments. It can be seen from Figure 7 that with the increase in corrosion time, the corrosion rate of specimens with different initial surface roughness shows a decreasing trend and stabilizes after 3 h of corrosion. With the specimen surface roughness decreasing, the corrosion rate decreases, while the change rate of the corrosion rate is first reduced and then increased. Among them, the specimen with an initial surface roughness of Ra 3.012 μm shows the largest corrosion rate during the entire corrosion process. During the corrosion stage of 1–2 h, the corrosion rate of the Ra 3.012 μm specimen decreases significantly, which reduces by 29.8%; after 3 h of corrosion, the corrosion rate is close to the corrosion rate of the specimens with a surface roughness of Ra 0.052 μm and Ra 1.208 μm. In addition, the four measured corrosion rates of Ra 3.012 μm are 251.1 mm/a, 175.8 mm/a, 148.8 mm/a, and 138.8 mm/a, and the corrosion rate decreases by 45.3% after 4 h of corrosion. The specimen with an initial surface roughness of Ra 0.168 μm shows the lowest corrosion rate during the entire corrosion process. The four measured corrosion rates are 145.1 mm/a, 135.3 mm/a, 120.9 mm/a, and 119.3 mm/a. The corrosion rate declines steadily during 1–3 h; the corrosion rate becomes stable after 3–4 h of corrosion; and after 4 h of corrosion, the corrosion rate decreased by 17.8%. However, for the specimens with the initial surface roughness of Ra 1.208 μm and Ra 0.052 μm, the corrosion rate and the rate of change in the corrosion rate are approximately consistent.

The reason for this phenomenon is that during the initial corrosion stage, the reaction between the specimen surface and corrosion medium is unstable due to the inhibition of substances, such as impurities and oxide films. As the corrosive medium continues to corrode the specimen material, the reaction between the corrosive medium and specimen material gradually stabilizes, which, in turn, leads to the reduction and stabilization of the corrosion rate. Furthermore, when the specimen surface topography is relatively large, the electrochemical activity at the surface peak of the specimen is large, making the specimen surface material react with the electrolyte for easier corrosion. With the increase in specimen surface roughness, the difference in the electron escape function increases at the peaks and valleys of the specimen surface, resulting in a decrease in the overall surface electron work and an increase in the fluctuation, which represents that the corrosion rate increases with the increase in surface roughness. Meanwhile, the surface roughness of the specimen is relatively large at the beginning of corrosion. The contact area of the corrosive medium is also relatively large, which is conducive to a corrosion reaction. Therefore, the corrosion rate and its change rate are relatively large at the beginning of corrosion.

The specimen with an initial surface of Ra 0.052 μm shows an increase in the corrosion rate when the specimen surface roughness is too low. The specimen surface oxide film appears uneven or missing, resulting in different parts of the material due to different oxygen potential and potential oxygen differences. When the surface roughness is lower than a specific value, the oxygen potential at the peak and trough of the material is lower, resulting in a thin oxide film or no oxide film formation. The oxide film only forms on the peaks of the specimen surface, which constitutes microcurrent corrosion and contributes to corrosion [30]. However, the specimen with an initial surface roughness of Ra 0.168 μm does not have a thin oxide film or fails to form one due to low-oxygen potential at the peaks and valleys while having a lower roughness, reflecting a better corrosion resistance. When the surface roughness is large, the surface can form an oxide film. However, the oxide film is easy to break due to the tip effect, which can also contribute to corrosion. Meanwhile, the large rough surface also increases the contact area between the material and electrolyte, so the Ra 3.012 μm specimen shows the largest corrosion rate during the whole corrosion stage. After 2 h of corrosion, the surface roughness of all specimens decreases and stabilizes, making the oxide film form more easily on the surface of the specimen; corrosion resistance is improved, which makes the corrosion rate change at a slower rate. The specimen with an initial surface roughness of Ra 0.168 μm shows a lower corrosion rate during whole corrosion, reflecting better corrosion resistance. While with the specimen with an initial surface roughness of Ra 3.012 μm, the corrosion rate decreases significantly, and the corrosion rate is similar to that of Ra 1.208 μm and Ra 0.052 μm specimens in the subsequent stage of corrosion, the corrosion rate is relatively high during the entire corrosion stage, which indicates poor corrosion resistance.

3.2. The Effect of Surface Roughness on Surface Corrosion Topography of the Material

Figure 8 shows the results of surface roughness change after the electrolytic corrosion of specimens with different initial roughness surfaces. It can be seen from Figure 8 that after 4 h of corrosion, the surface roughness of specimens with an initial surface roughness of Ra 0.168 μm and Ra 0.052 μm are higher than the surface roughness before corrosion. The surface roughness increased from Ra 0.168 μm to Ra 1.02 μm and from Ra 0.052 μm to Ra 1.203 μm, respectively. The surface roughness of specimens with the initial surface roughness of Ra 3.012 μm and Ra 1.208 μm all decreased after 4 h of corrosion, and the surface roughness of the specimens decreased from Ra 3.012 μm to Ra 2.81 μm and from Ra 1.208 μm to Ra 1.02 μm, respectively. Meanwhile, in addition to the specimen with the initial surface roughness of Ra 0.168 μm, the surface roughness of the other three different initial surface roughness specimens increases first and decreases obviously during the corrosion process. All three groups of specimens show a significant increase in surface roughness: the surface roughness of the specimen with the initial surface roughness of Ra 3.012 μm increased to Ra 5.01 μm, and the surface roughness of the specimen with the initial surface roughness of Ra 0.168 μm also has an obvious growth, and after 1 h of corrosion, its surface roughness increased to Ra 1.55 μm. After 2 h of corrosion, the surface roughness of the three groups of specimens is reduced compared to the surface roughness after corrosion for 1 h, and the specimen with the initial surface roughness of Ra 3.012 μm reduced from Ra 5.01 μm to 3.71 μm, which decreased significantly in the three sets of specimens. Meanwhile, the surface roughness of the specimens with an initial surface roughness of Ra 1.208 μm and Ra 0.052 μm also reduced to Ra 1.673 μm and Ra 1.312 μm, respectively. After 3 h of corrosion, the surface roughness variation and surface roughness of the specimens tends to be stable, and the surface topography changes tend to be smooth. Among these, the surface roughness of specimens with the initial surface roughness of Ra 0.052 μm and Ra 1.208 μm are relatively close.

The changing trend in Figure 8 is that with the change in specimen surface roughness, the material surface oxide film appears a certain degree uneven or missing, which causes the oxygen potential difference, and the oxygen potential difference between the various parts of the material surface is one of the main reasons for the occurrence of electrochemical corrosion of the surface. When the surface roughness of the specimen is low, the oxygen potential at the peak and valley of the surface is low and causes a thin or missing oxide film, while the rough surface and the tip effect make the surface oxide film easy to rupture. Both situations mentioned above lead to an uneven distribution of the oxide film on the material surface, which contributes to the formation of corrosion current and promotes the electrolytic corrosion of the material surface. Meanwhile, from the initial stage of corrosion, the corrosion rate is affected by surface roughness and the contact area between the material and corrosion medium; after 1 h of corrosion, the surface roughness of specimens with the initial surface roughness of Ra 3.012 μm, Ra 1.208 μm, and Ra 0.052 μm increase due to a large amount of surface material corrosion removal. After 3 h of corrosion, the material corrosion rate also tends to be stable; the part of the specimen surface removed by material corrosion is dominated by the surface roughness peak, which causes the change in surface roughness to tend to be stable. However, the specimens with a surface roughness of Ra 0.168 μm are relatively low, and the oxide film would not be thin or fail to form due to the low oxygen potential at the peaks and valleys on the surface of the specimens. The mentioned reason is eventually shown, and the specimens with an initial surface roughness of Ra 0.168 μm have no significant change in surface roughness during the corrosion process and show better corrosion resistance and stability. At the initial stage of the experiment, there may be significant errors in the test results. During the pretreatment of the specimens, the processing method resulted in uneven processing and significant differences in the surface topography between different surface regions, which may lead to an increase in the corrosion amount of the specimens during the initial stage of corrosion, resulting in significant errors.

3.3. The Effect of Surface Roughness on the Degradation of Corrosion Material Properties

Figure 9 shows the stress–strain curve of specimens with different initial surface roughness after 4 h of electrolytic corrosion. As can be seen from Figure 9, the mechanical properties of each surface roughness sample show different degrees of deterioration after 4 h of corrosion; the specimen with an initial surface roughness of Ra 0.052 μm has the most obvious reduction in tensile strength after electrolytic corrosion, which is 33.86% lower than before electrolytic corrosion. Figure 10 shows the specimens’ tensile strength and extensibility curve with different initial surface roughness after corrosion. It can be seen from Figure 10 that as the initial surface roughness of specimens increases, the tensile strength and elongation of each specimen after corrosion tend to increase first and then decrease. The tensile strength and elongation of the Ra 0.168 μm specimen are the largest, which are 527.84 MPa and 19.8%, respectively. When the initial surface roughness is Ra 3.012 μm, the tensile strength and elongation of the specimen are the smallest, which are 477.3 MPa and 18.12%, respectively.

This phenomenon is because the specimens in the electrolytic corrosion process have surface defects, such as erosion, damage, material dissolution, and other surface defects on the material surface. The mentioned surface defects can trigger a certain degree of stress concentration, which leads to the decline of mechanical properties. The different electrochemical activities of different parts of surface material can also cause pitting corrosion. The dimple pits caused by pitting corrosion have the characteristics of small diameter and deep crater depth, which would produce serious stress concentration under the action of external tensile force, resulting in a substantial decrease in the material’s mechanical properties. The incompleteness of the oxide film on the surface of the specimens changes with the change in surface roughness. When the surface roughness is small, the oxide film on the surface of the specimen is thinner due to the lower oxygen potential at the wave crest and trough, and only the peak of the oxygen potential produces the oxide film. When the surface roughness is larger, although the surface layer can form the oxide film, the tip effect leads to the rupture of the oxide film, and the different integrity of the oxide film on the surface of the specimen directly leads to the different corrosion resistance of the material. Different corrosion resistance would also lead to pitting corrosion on the surface of the workpiece adding to the difficulty and distribution of different situations; that is, when the oxide film is more complete and there is small initial surface roughness, pitting corrosion is more likely to occur.

Meanwhile, it can be seen that the corrosion rate obtained by measuring the corrosion rate of the specimens with an initial surface roughness of Ra 0.052 μm, Ra 1.208 μm, and Ra 3.012 μm is relatively large. A large corrosion rate means the removed specimen surface material due to corrosion at the same time is also relatively large; this would have some effect on the strength of the material. The surface roughness of the three specimens after 4 h of corrosion is also relatively large, and the surface topography is relatively more complex than that of the Ra 0.168 μm specimen. The complex surface topography would somewhat promote the stress concentration phenomenon during the tensile process, reducing the specimens’ tensile strength. Therefore, with the increase in initial surface roughness, the degree of corrosion reduces and then increases, showing that the tensile strength and elongation first increase and then decrease. In comparison, we can see that the specimen with a surface roughness of Ra 0.168 μm has the best surface corrosion resistance. Meanwhile, to obtain better corrosion resistance, the surface roughness should not be too low when processing the surface of the workpiece. It should be noticed that when pretreated, both sides of the specimen are polished, although the roughness of both sides of the specimen can be measured and corrected by polishing to ensure consistent roughness, but different polished areas due to uneven processing may lead to uneven corrosion of the specimen, which, in turn, produces stress concentration during the tensile process, thus affecting the accuracy of the test results and causing test errors.

4. The Influence of Surface Residual Stress on Corrosion Resistance

4.1. The Influence of Residual Stress on Material Corrosion Rate

Figure 11 shows the curve of surface residual stress’s effect on the specimens’ electrolytic corrosion rate. It can be seen from Figure 11 that with the increase in corrosion time, the corrosion rate of specimens with different surface residual stress values has a decreasing trend, and the corrosion rate stabilizes after 3 h of corrosion. After 1 h of corrosion, the corrosion rate increases with the increase of surface residual stress, of which the specimens with a surface residual stress of −335.64 MPa, the corrosion rate of 245.1 mm/a, the corrosion rate is the largest. After 2 h of corrosion, the corrosion rate of different specimens changes, which appears the trend of the corrosion rate decreasing with the increase of surface residual stress. At this moment, the corrosion rate of the specimen with a surface residual stress of −335.6 MPa decreased to 139.5 mm/a, which decreases from the maximum to the minimum, compared to the corrosion rate measured after 1 h of corrosion decreased by 43.1%.

Moreover, the corrosion rate of the specimen with surface residual stress of −120.2 MPa increases from the minimum to the maximum; at this time, the corrosion rate, which decreases to 170.3 mm/a compared to the corrosion rate measured after 1 h of corrosion, decreased by 19.1%. When the length of corrosion time is more than 2 h, the order of the corrosion rate of specimens with different residual stress remains the same, and the corrosion rate values remain stable. During the entire corrosion experiment process, the corrosion rate of specimens with surface residual stress of −335.6 MPa decreases by 49.9%, which is the largest decrease among the four specimens with different surface residual stress, and the stabilized corrosion rate is 122.752 mm/a. The corrosion rate of specimens with surface residual stress of −120.2 MPa decreased by 33.1%, the smallest decrease among the four specimens, and the stabilized corrosion rate is 140.881 mm/a.

The reasons for the mentioned phenomenon are as follows: the increase in surface residual stress is due to the surface microstructure dislocation and slip caused by the increase in shot peening strength. In contrast, the electrochemical activity at the boundaries between displacement and slip is relatively high, where the corrosion reaction’s location occurs preferentially [31,32]. Meanwhile, with the increase in surface tensile stress and the decrease in displacement density, the internal crystal structure is damaged by internal residual tensile stress, which reduces the content of active spots at grain boundaries where pitting corrosion occurs. However, the residual compress stress could effectively reduce the oxidation susceptibility of grain boundaries, preventing stress corrosion from occurring via contact and ultimately reducing the corrosion rate as the surface residual compress stress increases. Shot peening is used to introduce residual compress stress during the experiment; although this method can effectively introduce residual compress stress, the surface roughness of the specimen will increase significantly. The surface roughness of the specimens could be close to that of the shot peening process through sandpaper polishing. However, the polished surface topography is still relatively complex. The destructive effect of sandpaper polishing can damage the surface oxide film on the surface of specimens. At this time, the contribution effect on corrosion produced by surface topography is stronger than the inhibition effect on corrosion produced by surface compress stress, which promotes corrosion of the surface material of the specimen. Moreover, the shot peening process also causes plastic deformation and increases the dislocation density of the material on the surface of specimens, and the changes mentioned above on the surface quantity could all increase the corrosion rate [33]. Therefore, during the period from the experiment beginning to 1 h of corrosion, the corrosion rate of the specimen surface material increases as the surface compress stress increases.

However, after 1–2 h of corrosion, the corrosion rate of all specimens decreases, and with the increase in residual compressive stress on the surface, the corrosion rate shows a trend of gradually decreasing; the corrosion rate of the specimen with a surface residual compress stress of 120.2 MPa is the largest. This phenomenon is due to the electrolysis process, and if there is an uneven and discontinuous anode film, it would lead to pitting on the metal surface, which would lead to more complex surface topography and accelerate the corrosion reaction. Meanwhile, the oxygen precipitated from the anode will also cause a nonuniform, discontinuous anode film, which would also intensify corrosion. However, as corrosion proceeds, the stress concentration point dissolves, the current efficiency of the dissolved metal increases, the defects caused by pitting corrosion are significantly reduced or disappear, and the surface finishing increases, reducing the specimen’s corrosion rate material. Meanwhile, the surface finish of the specimen is improved after being corroded during the initial corrosion period, and the promotion of corrosion by the surface topography is weaker than the inhibition of corrosion by residual compressive stress. Therefore, there is a trend that the corrosion rate gradually decreases as the residual compressive stress on the surface increases. When the length of corrosion time is longer than 3 h, the electrolytic corrosion changes into the state of uniform corrosion, and the corrosion rate of each specimen changes less. It should be noted that during the initial stage of the experiment, the surface roughness of the specimens was tested after processing to ensure the consistency of surface roughness. Surface damage, such as surface microcracks, may be caused during the process, which may cause errors and affect the accuracy of the experimental results.

4.2. The Influence of Residual Stress on Corrosion Appearance

Figure 12 shows the roughness change curves after the specimens’ electrolytic corrosion with different surface residual stresses. It can be seen from Figure 12 that the surface roughness of specimens with different surface residual stress shows a trend of first increasing and then decreasing. Among these, the surface roughness of the specimen with a surface residual stress of −120.2 MPa changes slightly during the corrosion process, and its surface roughness grows to a maximum after 1 h of corrosion and then decreases. For the remaining specimens with surface residual stresses after 1 h of corrosion, the surface roughness increases with the increase in surface residual compress stress, and after 2 h of corrosion, the surface roughness of each specimen decreases with the increase in residual compress stress on the surface. During the variation process, the specimen with a surface residual stress of −335.64 MPa has the most pronounced surface roughness variation trend: after 1 h of corrosion, the surface roughness increased to Ra 6.112 μm, while after 4 h of corrosion, the surface roughness eventually decreased to Ra 4.621 μm. For the specimen with a surface residual stress of −215.47 Mpa, its corrosion surface roughness maximum value appears at the moment of 2 h of corrosion, and when the corrosion time extends, the specimen’s surface roughness decreases significantly. The specimen with a surface residual stress of −293.27 MPa has the same surface roughness after 1 h and 2 h of corrosion, and the surface roughness decreases significantly when the corrosion time extends. It is found that, except for the specimen with a surface residual stress of −120.2 MPa, the surface roughness of the remaining specimens is close to Ra 4.7 μm after the specimens reach the corrosion stabilization stage. Moreover, the variation pattern of the surface roughness of other samples with increasing corrosion time is relatively similar. Due to the similar trend of the specimens, except for the surface residual stress of −120.2 MPa, the surface residual stress −335.64 MPa specimen was selected to observe the variation in surface topography throughout the corrosion stage, and the variation in the electrolytic corrosion topography of the specimen with a surface residual stress of −335.64 MPa is shown in Figure 13.

The reasons for this phenomenon are that during the first hour of corrosion, the uneven and incomplete anode film contributes to the pitting corrosion, increasing the surface defects. Meanwhile, during this stage, the corrosion rate of the specimen surface material is also relatively large, so the surface material seriously corrodes in a relatively short period; both the reasons mentioned above lead to an increase in material surface roughness. As the corrosion reaction proceeds, the oxygen precipitated from the anode decreases, and the current efficiency of the dissolved metal increases, resulting in the previous stage of corrosion produced by pitting defects being reduced or disappearing, and the surface roughness decreases again, which is shown in Figure 13a,b. As the surface residual compress stress increases, the degree of surface damage increases, which will increase the contact area of the corrosion reaction. In addition, the specimen’s surface oxygen precipitation increases, which would further cause the incompleteness of the anodic film, increasing pitting corrosion defects and ultimately leading to the material surface roughness increasing with the increase in surface residual compress stress.

After 2 or 3 h of corrosion, the surface oxide film is incomplete, and the material at the original surface rough peaks of the specimen is preferentially corroded. The defects caused by pitting corrosion also decrease or disappear, which is shown in Figure 13c,d. Meanwhile, with the increase in surface residual compress stress, the larger residual compress stress can effectively inhibit the corrosion reaction of specimen surface material. In addition, the inhibition effect on corrosion produced by residual compress stress is stronger than the promotion effect on corrosion produced by surface topography, the corrosion rate of surface material decreases, and the surface material removal of the specimens is also relatively uniform, so the surface roughness decreases with the increase in the residual compressive stress on the surface, as finally shown in Figure 13e. However, the specimen with a surface residual stress of −120.2 MPa is not processed with shot peening, there are no apparent defects on the surface topography, and the anode film is not missing or uneven due to not being treated by shot peening. The inhibition effect on corrosion produced by surface compress stress is relatively balanced with the contribution effect on corrosion produced by surface topography. Therefore, there is no significant change in surface roughness after corrosion. By comparing the changes in surface roughness, it can be seen that the specimen with a surface residual stress of −120.2 MPa has better corrosion resistance. It should be noted that the selected area may cause errors in surface roughness measurement results, which may have an impact on experimental results.

4.3. The Influence of Surface Residual Stress on Corrosion Degradation Performance of the Material

Figure 14 shows the stress–strain curves of different surface residual stress specimens after 4 h of electrolytic corrosion. It can be seen in Figure 14 that the mechanical properties of different surface residual stress specimens show different degrees of deterioration after 4 h of corrosion; the specimen with a surface residual stress of −120.2 MPa has the most obvious reduction in tensile strength after electrolytic corrosion, which is 18.88% lower than before electrolytic corrosion.

Figure 15 shows the measured tensile strength and elongation curves of different surface residual stress specimens after 4 h of corrosion. It can be seen from Figure 15 that with the increase in surface residual compress stress on the specimen, the tensile strength of the specimen tends to increase after corrosion. In contrast, the elongation tends to decrease gradually. When the surface residual stress is −335.84 MPa, the specimen has a maximum tensile strength of 548.8 MPa, and the elongation is 17.56%. In comparison, the specimen with the surface residual stress of −120.2 MPa has a minimum tensile strength of 494.44 MPa, and the elongation is 19.26%. With the increase in surface residual compress stress, after corrosion, the tensile strength of the specimen increased by 10.9%, and the elongation decreased by 9.7%. With the increase in surface residual compress stress on the specimen surfaces, the tensile properties of the specimens are relatively good after corrosion. The specimen with a surface residual stress of −335.84 MPa under the experimental conditions described in this paper has a better tensile strength after 4 h of corrosion and has the best corrosion resistance.

The reasons for the above phenomenon are that when the specimen material is subjected to external force, the maximum surface load changes as the residual stress in the surface layer changes. The larger surface residual compress stress can effectively reduce the oxidation susceptibility of grain boundaries, inhibit the destruction of crystal structure, and reduce the occurrence of stress corrosion and pitting corrosion at grain boundaries. Meanwhile, although the specimens with large surface residual compress stress have large surface roughness after corrosion, shot peening refines the surface material structure of the specimen and inhibits surface cracks during stretching. The reasons mentioned above reduce the emergence and extension of specimen material microcracking due to the stress concentration caused by pitting corrosion and the surface topography. In addition, as the residual compress stress inhibits corrosion of the surface material of the specimen, the strength of the specimen with higher residual stress on the surface is less affected by corrosion. Therefore, as the residual compressive stress on the surface layer increases, the tensile strength of the specimen material shows a gradual increase. However, with the increase in surface residual, a certain thickness of a fine crystal layer is formed on the surface of the material, which leads to surface hardening and a certain degree of reduction in the plastic properties of the specimen material. Therefore, elongation tends to decrease with the increase in the residual compress stress on the surface of specimens. Comparing different surface residual stress specimens, the specimen with a surface residual stress of −335.64 MPa has the best corrosion resistance. Meanwhile, to obtain better corrosion resistance, the shot peening intensity could be appropriately increased during the shot peening process. It should be noted that the specimens would produce a certain degree of bending deformation after double-sided shot peening, which changes the corrosion area on both sides and affects the corrosion effect, resulting in errors in the experiment results.

5. Conclusions

In this paper, we carried out an experimental study on the influence of the regulation of surface integrity on the degradation of material properties after electrolytic corrosion. The surface roughness and surface residual stress were chosen as the main surface integrity indexes, and the influence of the two indexes on the corrosion rate of surface material, surface roughness after corrosion, and variation in tensile mechanical properties were analyzed. The optimal mechanical processing parameters to improve the corrosion resistance of the material are proposed based on the regulations obtained from the experiments. The conclusions are as follows:

(1)

For the specimens with different initial surface roughness, the corrosion rate of the specimens decreases, and the change rate of the corrosion rate first decreases and then increases with the decrease in surface roughness. Among them, the corrosion rate of specimens with a surface roughness of Ra 3.012 μm decreases the most, which is about 29.8%. During the corrosion process, the surface roughness of specimens first increases and then decreases; after 3 h of corrosion, the variation in specimen surface roughness and surface topography tends to be stable, and during this process, the surface roughness of specimen with a surface roughness of Ra 3.012 μm changes significantly. After corrosion, the tensile strength of the tensile specimens with different surface roughness decreases, and the tensile strength of specimens first increases and then decreases with surface roughness. The specimen with a surface roughness of Ra 0.168 μm has the largest tensile strength and extensibility, which are 527.84 MPa and 19.8%, respectively, and this surface roughness specimen has the best corrosion resistance. During the production process, the best corrosion resistance could be obtained with the surface roughness of Ra 0.168 μm by grinding;

(2)

The corrosion rate decreases in the specimens with different surface residual stress as the corrosion proceeds. Among them, the specimen with a surface residual stress of −335.64 MPa has the largest corrosion rate, which is 245.1 mm/a; after 4 h of corrosion, the corrosion rate of this specimen decreased by 49.9%, which is the largest of four different specimens with different surface residual stress. As the surface residual stress increases, the surface roughness of specimens with different surface residual stress increases and then decreases as the corrosion proceeds. Among them, the surface roughness of the specimen with a surface residual stress of −120.2 MPa does not change significantly, and the surface roughness of the other three specimens finally becomes stable at Ra 4.7 μm. After corrosion, the tensile strength of specimens increases with the increase in surface residual stress. Meanwhile, the extensibility of specimens decreases with the increase in surface residual stress; the specimen with a surface residual stress of −335.84 MPa has the largest tensile strength of the four specimens, which is 548.8 MPa, and this surface residual compressive stress specimen has the best corrosion resistance. During the production process, the process parameters of 0.5 mm steel pellets and 0.4 MPa shot peening pressure could be used to strengthen the surface of the workpiece to obtain the best corrosion resistance.