Microstructure, Mechanical Properties and Corrosion Behavior of the Aluminum Alloy Components Repaired by Cold Spray with Al-Based Powders

Aluminum alloy components of high-speed trains have a great risk of being corroded by various corrosive medium due to extremely complex atmospheric environments. This will bring out huge losses and reduce the safety and stability of trains. In order to solve the problem, cold spray process was used for repairing the damage of the aluminum alloy components with Al-based powders. Microstructure, mechanical properties and corrosion behavior were studied. The results indicated that there were very few pores and cracks in the repaired areas after repairing. The average microhardness of the repaired areas was 54.5 HV ± 3.4 HV, and the tensile strength of the repaired samples was 160.4 MPa. After neutral salt spray tests for 1000 h, the rate of mass loss of the samples repaired by cold spray was lower than that of 6A01 aluminum alloy. The electrochemical test results showed that the repaired areas had a higher open circuit potential than 6A01 aluminum alloy. As a result, the repaired areas such as the anode protected its nearby substrate. The samples repaired by cold spray exhibited better corrosion than 6A01 aluminum alloy. Cold spray process and Al-based powders are applicable for repairing the aluminum alloy components of high-speed trains.


Introduction
The high-speed railway in China has had rapid developments during these years. Meanwhile, there has been a rapid change in high-speed trains. In order to realize the goal of light weight and a higher speed, aluminum alloy materials are used as car body of highspeed trains, such as 6XXX aluminum alloy and 7XXX aluminum alloy. High-speed trains are running in extremely complex atmospheric environments which include industrial atmosphere, marine atmosphere and rural atmosphere [1,2]. Hence, the components of the car body may be corroded by various corrosive medium. For instance, many corrosion products were observed on the surface of the gutter at the bottom of the train door in the overhaul of trains. The reason is that rainwater, cleaning agents and disinfectants flow on the surface of the gutter, and in this case the gutters often comprise humid and corrosive environments. In addition, general corrosion and the pitting corrosion occur in the floor under dustbins, and galvanic corrosion occurs in the position where aluminum alloys are connected with stainless steel bolts. Therefore, these corrosion problems will cause damage in the aluminum alloy components of high-speed trains. As time passes, the damages will decrease the service life of trains and induce painful financial loss. What is even more serious is the reduction in safety and stability while the train is in motion.
Many technologies can be used for repairing the damage of the components. For instance, metal-inert gas welding (MIG), thermal spray and laser cladding have been  The raw materials for cold spray consisted of two powders: pure Al powders (74 wt.%) and Al 2 O 3 powders (26 wt.%). The SEM morphologies of the powders are shown in Figure 1. As shown, pure Al powders (D50 23.73 µm) are spherical, and the particle size is within the range of 5-40 µm. Al 2 O 3 powders (D50 43.94 µm) in the shape of irregular polygons are within the range of 20-60 µm. Due to its low density and high interparticle friction, pure Al powders have a poor flowability. The addition of the irregular Al 2 O 3 powders can accelerate the flowability of the powders [13,14]. Hence, the powder feeder will not be blocked, and a supply can be guaranteed.
The raw materials for cold spray consisted of two powders: pure Al pow wt.%) and Al2O3 powders (26 wt.%). The SEM morphologies of the powders are s Figure 1. As shown, pure Al powders (D50 23.73 μm) are spherical, and the part is within the range of 5-40 μm. Al2O3 powders (D50 43.94 μm) in the shape of i polygons are within the range of 20-60 μm. Due to its low density and high inte friction, pure Al powders have a poor flowability. The addition of the irregul powders can accelerate the flowability of the powders [13,14]. Hence, the powde will not be blocked, and a supply can be guaranteed.

Experiment
In order to simulate the defects caused by corrosion, a pit as depicted in Figu processed in the center of each aluminum alloy sheet with a thickness of 4 mm. T under repair were cleaned by an angle grinder to remove the oxidation film. The rior metal and cold spray technology were used to repair the pit. Superior metal (LOCTITE 3478) is a two-part ferro-silicon filled epoxy resin Resin and hardener were blended together as requested, and the mixtures were filling the pits. After 24 h, the repaired areas experienced complete solidification the cold spray process, the powders were kept in a resistance furnace at 60 °C f order to make sure that there was no water or moisture in them. The cold spray was conducted on a low-pressure cold spray system (RF-LP001, Tyontech Co. LT China). The process parameters were as follows: process gas, air; pressure, 0.65 M heating temperature, 300 °C; powder feed rate, 25 g/min; gas flow rate, 0.35 L/m raised areas were removed in order to obtain a perfectly flat surface after all the alu sheets were repaired by the two methods. The samples repaired by these meth shown in Figure 3.

Experiment
In order to simulate the defects caused by corrosion, a pit as depicted in Figure 2 was processed in the center of each aluminum alloy sheet with a thickness of 4 mm. The areas under repair were cleaned by an angle grinder to remove the oxidation film. Then, superior metal and cold spray technology were used to repair the pit. The raw materials for cold spray consisted of two powders: pure Al powders (74 wt.%) and Al2O3 powders (26 wt.%). The SEM morphologies of the powders are shown in Figure 1. As shown, pure Al powders (D50 23.73 μm) are spherical, and the particle size is within the range of 5-40 μm. Al2O3 powders (D50 43.94 μm) in the shape of irregular polygons are within the range of 20-60 μm. Due to its low density and high interparticle friction, pure Al powders have a poor flowability. The addition of the irregular Al2O3 powders can accelerate the flowability of the powders [13,14]. Hence, the powder feeder will not be blocked, and a supply can be guaranteed.

Experiment
In order to simulate the defects caused by corrosion, a pit as depicted in Figure 2 was processed in the center of each aluminum alloy sheet with a thickness of 4 mm. The areas under repair were cleaned by an angle grinder to remove the oxidation film. Then, superior metal and cold spray technology were used to repair the pit. Superior metal (LOCTITE 3478) is a two-part ferro-silicon filled epoxy resin system. Resin and hardener were blended together as requested, and the mixtures were used for filling the pits. After 24 h, the repaired areas experienced complete solidification. Before the cold spray process, the powders were kept in a resistance furnace at 60 °C for 3 h in order to make sure that there was no water or moisture in them. The cold spray process was conducted on a low-pressure cold spray system (RF-LP001, Tyontech Co. LTd, Xi'an, China). The process parameters were as follows: process gas, air; pressure, 0.65 MPa; preheating temperature, 300 °C; powder feed rate, 25 g/min; gas flow rate, 0.35 L/min. The raised areas were removed in order to obtain a perfectly flat surface after all the aluminum sheets were repaired by the two methods. The samples repaired by these methods are shown in Figure 3. Superior metal (LOCTITE 3478) is a two-part ferro-silicon filled epoxy resin system. Resin and hardener were blended together as requested, and the mixtures were used for filling the pits. After 24 h, the repaired areas experienced complete solidification. Before the cold spray process, the powders were kept in a resistance furnace at 60 • C for 3 h in order to make sure that there was no water or moisture in them. The cold spray process was conducted on a low-pressure cold spray system (RF-LP001, Tyontech Co. LTd, Xi'an, China). The process parameters were as follows: process gas, air; pressure, 0.65 MPa; preheating temperature, 300 • C; powder feed rate, 25 g/min; gas flow rate, 0.35 L/min. The raised areas were removed in order to obtain a perfectly flat surface after all the aluminum sheets were repaired by the two methods. The samples repaired by these methods are shown in Figure 3.
The repaired sheets were cut into small pieces for different tests. The samples for microstructure analysis were sanded down, polished, etched with a liquid mixture (95 mL H 2 O, 2.5 mL HNO 3 , 1.5 mL HCl and 1.0 mL HF) and then investigated by an optical microscope (OM, OLYMPUS, Tokyo, Japan) and a field emission scanning electron microscope (SEM, SUPRA55, ZEISS, Jena, Germany). The repaired areas were also observed by SEM and analyzed by an image analysis software (Image J, V 1.6, National Institutes of Health, Bethesda, MD, USA) in order to obtain porosity. The tensile tests as shown in Figure 4 were conducted on a universal tester (WDW-300E, Jinan New Gold Test Instrument Co. LTd., Jinan, China) according to EN ISO 6892-1: 2016. The thickness of the samples was 4mm, and three samples as a group were used for obtaining the average tensile strength. According to the ASTM C633 standard, the bonding strength of the samples was estimated. According to EN ISO 9015-2: 2013, the microhardness of the samples was measured on a microhardness tester (FM-300, FUTURE TECH, Tokyo, Japan) with a test load of 4.9 N for 15 s. The repaired sheets were cut into small pieces for different tests. The samples for microstructure analysis were sanded down, polished, etched with a liquid mixture (95 mL H2O, 2.5 mL HNO3, 1.5 mL HCl and 1.0 mL HF) and then investigated by an optical microscope (OM, OLYMPUS, Tokyo, Japan) and a field emission scanning electron microscope (SEM, SUPRA55, ZEISS, Jena, Germany). The repaired areas were also observed by SEM and analyzed by an image analysis software (Image J, V 1.6, National Institutes of Health, Bethesda, MD, USA) in order to obtain porosity. The tensile tests as shown in Figure 4 were conducted on a universal tester (WDW-300E, Jinan New Gold Test Instrument Co. LTd., Jinan, China) according to EN ISO 6892-1: 2016. The thickness of the samples was 4mm, and three samples as a group were used for obtaining the average tensile strength. According to the ASTM C633 standard, the bonding strength of the samples was estimated. According to EN ISO 9015-2: 2013, the microhardness of the samples was measured on a microhardness tester (FM-300, FUTURE TECH, Tokyo, Japan) with a test load of 4.9 N for 15 s. Neutral salt spray tests were carried out in accordance with ISO 11130-2017 in a salt spray test chamber. The samples had a size of 60 mm × 40 mm × 4 mm. Before testing, the surfaces of the samples were polished with 1000# sandpaper and cleaned with ethyl alcohol. The initial mass of the samples was measured by an analytical balance with an accuracy of 0.1 mg. Then, the samples were placed in the test chamber and leant against the bracket at an angle of 20° from the vertical. The test solution was a mixture of deionized water and sodium chloride (NaCl), and the concentration of sodium chloride was 50 g/L. Neutral salt spray tests were conducted at the temperature of 35 °C for 1000 h. After testing, the samples were immersed in a liquid mixture (35 mL H3PO4 + 20 g Cr2O3 + 1000 mL deionized water) at 100 °C for 10 min in order to remove the corrosion products. The rate of mass loss was calculated by the formula as follows: R-the rate of mass loss; Wb-the mass of samples before testing;  The repaired sheets were cut into small pieces for different tests. The samples for microstructure analysis were sanded down, polished, etched with a liquid mixture (95 mL H2O, 2.5 mL HNO3, 1.5 mL HCl and 1.0 mL HF) and then investigated by an optical microscope (OM, OLYMPUS, Tokyo, Japan) and a field emission scanning electron microscope (SEM, SUPRA55, ZEISS, Jena, Germany). The repaired areas were also observed by SEM and analyzed by an image analysis software (Image J, V 1.6, National Institutes of Health, Bethesda, MD, USA) in order to obtain porosity. The tensile tests as shown in Figure 4 were conducted on a universal tester (WDW-300E, Jinan New Gold Test Instrument Co. LTd., Jinan, China) according to EN ISO 6892-1: 2016. The thickness of the samples was 4mm, and three samples as a group were used for obtaining the average tensile strength. According to the ASTM C633 standard, the bonding strength of the samples was estimated. According to EN ISO 9015-2: 2013, the microhardness of the samples was measured on a microhardness tester (FM-300, FUTURE TECH, Tokyo, Japan) with a test load of 4.9 N for 15 s. Neutral salt spray tests were carried out in accordance with ISO 11130-2017 in a salt spray test chamber. The samples had a size of 60 mm × 40 mm × 4 mm. Before testing, the surfaces of the samples were polished with 1000# sandpaper and cleaned with ethyl alcohol. The initial mass of the samples was measured by an analytical balance with an accuracy of 0.1 mg. Then, the samples were placed in the test chamber and leant against the bracket at an angle of 20° from the vertical. The test solution was a mixture of deionized water and sodium chloride (NaCl), and the concentration of sodium chloride was 50 g/L. Neutral salt spray tests were conducted at the temperature of 35 °C for 1000 h. After testing, the samples were immersed in a liquid mixture (35 mL H3PO4 + 20 g Cr2O3 + 1000 mL deionized water) at 100 °C for 10 min in order to remove the corrosion products. The rate of mass loss was calculated by the formula as follows:

R-the rate of mass loss;
Wb-the mass of samples before testing; Wa--the mass of samples after testing; S-the superficial area for testing. Neutral salt spray tests were carried out in accordance with ISO 11130-2017 in a salt spray test chamber. The samples had a size of 60 mm × 40 mm × 4 mm. Before testing, the surfaces of the samples were polished with 1000# sandpaper and cleaned with ethyl alcohol. The initial mass of the samples was measured by an analytical balance with an accuracy of 0.1 mg. Then, the samples were placed in the test chamber and leant against the bracket at an angle of 20 • from the vertical. The test solution was a mixture of deionized water and sodium chloride (NaCl), and the concentration of sodium chloride was 50 g/L. Neutral salt spray tests were conducted at the temperature of 35 • C for 1000 h. After testing, the samples were immersed in a liquid mixture (35 mL H 3 PO 4 + 20 g Cr 2 O 3 + 1000 mL deionized water) at 100 • C for 10 min in order to remove the corrosion products. The rate of mass loss was calculated by the formula as follows: R-the rate of mass loss; W b -the mass of samples before testing; W a -the mass of samples after testing; S-the superficial area for testing. The electrochemical tests performed on the samples were conducted on an electrochemical workstation (CHI 700E, Chinstruments Co. LTd., Shanghai, China) according to ISO 17475-2005. Three electrodes consist of working electrode (the samples), reference electrode (saturated calomel electrode) and auxiliary electrode (platinum sheet). The open circuit potentials (E OCP ) of samples before and after neutral salt spray tests were tested in 3.5% NaCl solution. Potentiodynamic polarization tests were conducted with a scanning rate of 0.33 mV/s from −0.5 V to 1.5 V versus OCP. 6 show the cross-section of samples repaired by superior metal and cold spray. For convenience, the samples repaired by superior metal and cold spray are marked as RSM and RCS, respectively. As shown in Figure 5a, the pit was filled with superior metal. There is a clear interface between the repaired areas and the substrate, and the bonding line is relatively smooth. However, some gaps are obvious in the interface. This demonstrates that the bonding strength will not be too high. Moreover, some pores with different sizes in the repaired areas (RSM) are observed easily. The microstructure of RSM is composed of white ferro-silicon and dark epoxy resin as demonstrated in Figure 6a. As observed in Figure 5b, there are very few pores and cracks in the repaired areas (RCS). As shown in Figure 6b, a mount of irregular particles A exists in the repaired areas. According to the EDS results, the irregular particles A include Al 2 O 3 and Al in the other zones B in the repaired areas. The initial morphology of pure Al powders cannot be observed due to plastic deformation caused by the high-speed impact during the repair process. Likewise, some Al 2 O 3 particles are broken by this effect. What is more is that it is observed from Figure 5b that some Al 2 O 3 particles are embedded in the substrate, so the bonding line is unsmooth and seemingly irregular. It is clear that RCS has a fine mechanical bond with the substrate. chemical workstation (CHI 700E, Chinstruments Co. LTd., Shanghai, China) according to ISO 17475-2005. Three electrodes consist of working electrode (the samples), reference electrode (saturated calomel electrode) and auxiliary electrode (platinum sheet). The open circuit potentials (EOCP) of samples before and after neutral salt spray tests were tested in 3.5% NaCl solution. Potentiodynamic polarization tests were conducted with a scanning rate of 0.33 mV/s from −0.5 V to 1.5 V versus OCP. Figures 5 and 6 show the cross-section of samples repaired by superior metal and cold spray. For convenience, the samples repaired by superior metal and cold spray are marked as RSM and RCS, respectively. As shown in Figure 5a, the pit was filled with superior metal. There is a clear interface between the repaired areas and the substrate, and the bonding line is relatively smooth. However, some gaps are obvious in the interface. This demonstrates that the bonding strength will not be too high. Moreover, some pores with different sizes in the repaired areas (RSM) are observed easily. The microstructure of RSM is composed of white ferro-silicon and dark epoxy resin as demonstrated in Figure  6a. As observed in Figure 5b, there are very few pores and cracks in the repaired areas (RCS). As shown in Figure 6b, a mount of irregular particles A exists in the repaired areas. According to the EDS results, the irregular particles A include Al2O3 and Al in the other zones B in the repaired areas. The initial morphology of pure Al powders cannot be observed due to plastic deformation caused by the high-speed impact during the repair process. Likewise, some Al2O3 particles are broken by this effect. What is more is that it is observed from Figure 5b that some Al2O3 particles are embedded in the substrate, so the bonding line is unsmooth and seemingly irregular. It is clear that RCS has a fine mechanical bond with the substrate.  An image processing software (Image J) was used for calculating the porosity of samples repaired by superior metal and cold spray. Table 3 shows the porosity results of the surface and the cross-section of the repaired areas. The porosity of the surface of these samples is a litter higher than that of the cross-section. Obviously, the RSM samples have a higher porosity than the RCS samples. During the course of repair by superior metals, it was difficult to guarantee that there was no incoming air in the repaired areas. Furthermore, it was hard for air to escape. This can be proved by Figure 5a in which a lot of pores are observed. As a result, the porosity of the RSM sample is relatively high. During the course of repairing by cold spray, the addition of Al 2 O 3 particles will promote the plastic deformation of pure Al powders; hence, the porosity between the powder particles was decreased [15]. An image processing software (Image J) was used for calculating the porosity of ples repaired by superior metal and cold spray. Table 3 shows the porosity results o surface and the cross-section of the repaired areas. The porosity of the surface of samples is a litter higher than that of the cross-section. Obviously, the RSM samples a higher porosity than the RCS samples. During the course of repair by superior meta was difficult to guarantee that there was no incoming air in the repaired areas. Fur more, it was hard for air to escape. This can be proved by Figure 5a in which a lot of p are observed. As a result, the porosity of the RSM sample is relatively high. Durin course of repairing by cold spray, the addition of Al2O3 particles will promote the p deformation of pure Al powders; hence, the porosity between the powder particles decreased [15].  Figure 7 is the microhardness of the cross section of samples repaired by cold s Microhardness was measured along the line as marked in Figure 7. As observed from results, the average microhardness of 6A01-T5 is 93.8 HV ± 2.7 HV. The average m hardness of the RCS is 54.5 HV ± 3.4 HV. The microhardness of the RCS samples dep on its microstructure. As mentioned above, amount of Al2O3 particles with relatively hardness was embedded in the repaired areas. Furthermore, the in situ impact of A particles during the repair process will result in work hardening in the repaired a Thus, the addition of Al2O3 particles not only decreased porosity but also enhanced crohardness compared with the samples repaired with only pure Al powders [16][17][18] cording to ISO868, the shore hardness of superior metal is 90D, which is considerably relative to the substrate. This will not necessarily be beneficial to the entire performa   Figure 7 is the microhardness of the cross section of samples repaired by cold spray. Microhardness was measured along the line as marked in Figure 7. As observed from the results, the average microhardness of 6A01-T5 is 93.8 HV ± 2.7 HV. The average microhardness of the RCS is 54.5 HV ± 3.4 HV. The microhardness of the RCS samples depends on its microstructure. As mentioned above, amount of Al 2 O 3 particles with relatively high hardness was embedded in the repaired areas. Furthermore, the in situ impact of Al 2 O 3 particles during the repair process will result in work hardening in the repaired areas. Thus, the addition of Al 2 O 3 particles not only decreased porosity but also enhanced microhardness compared with the samples repaired with only pure Al powders [16][17][18]. According to ISO868, the shore hardness of superior metal is 90D, which is considerably high relative to the substrate. This will not necessarily be beneficial to the entire performance.

Bonding Strength and Tensile Strength
The bonding strength with the substrate of the cold sprayed samples is 35.6 Mpa and that of the superior metal is 31.7 Mpa. This also illustrates that the RSM samples have a lower bonding strength with the substrate. Table 4 shows the adhesion strength and tensile strength and the elongation of samples repaired by superior metal and cold spray. As indicated, the average tensile strength of the RSM samples and the RCS samples is 138.2 MPa and 160.4 MPa, respectively, and the average elongation of them is 1.2% and 2.8%, respectively. According to EN ISO15614-2, the tensile strength of 6A01-T5 weld joints by fusion welding should be more than 147 MPa. It is obvious that the RCS samples have met the requirements, whereas the RSM samples have not.

Bonding Strength and Tensile Strength
The bonding strength with the substrate of the cold spray that of the superior metal is 31.7 Mpa. This also illustrates th lower bonding strength with the substrate. Table 4 shows the sile strength and the elongation of samples repaired by super indicated, the average tensile strength of the RSM samples an MPa and 160.4 MPa, respectively, and the average elongatio respectively. According to EN ISO15614-2, the tensile strengt fusion welding should be more than 147 MPa. It is obvious tha the requirements, whereas the RSM samples have not.  Figure 8 is the macro morphologies of the fracture of sa depicted in the Figure 8a, the RSMs were preserved in its entir bonding line. In other words, the RSMs have a low bonding   Figure 8 is the macro morphologies of the fracture of samples after tensile tests. As depicted in the Figure 8a, the RSMs were preserved in its entirety and peeled off along the bonding line. In other words, the RSMs have a low bonding strength with the substrate, resulting in the fracture that firstly occurred in the bonding interface. Clearly, the fracture position of the RCS samples occurred in the repaired areas after tensile tests. Although there is only a mechanical bond between the RCS and the substrate, the repaired areas will not fall off from the substrate under the action of stress. SEM was used for analyzing the fracture morphologies of the RCS samples, as shown in Figure 9. It can be observed that there are many dimples in the substrate (6A01 aluminum alloy). This demonstrates that the fracture mechanism of substrate is ductile fracture [19,20]. By contrast, many particles are observed evidently in the RCS, and this can be attributed to its formation mechanism. The raw powders deformed intensely due to the severe impact such that the RCSs were formed by the deposition of the powders in the form of a mechanical bond. Therefore, the fracture positions were mostly in the boundary between the powders. Moreover, it is also proved by the fracture appearance with numerous irregular Al 2 O 3 particles. As observed in Figure 9b, there exist some tiny dimples. In conclusion, the fracture mechanism of the RCS consists mainly of brittle fracture and partly of ductile fracture and that of the substrate is ductile fracture.

Neutral Salt Spray Tests
Neutral salt spray tests were conducted to study the corrosion behavior of samples repaired by superior metal and cold spray. Figure 10 is the rate of mass loss after neutral salt spray tests. As observed, the rate of mass loss of 6A01-T5 is lower than that of the RSM samples not only after neutral salt spray tests for 500 h but also for 1000 h. By contrast, the RCS samples have the lowest rate of mass loss. That is to say that the RCS samples exhibited better corrosion resistance.

Neutral Salt Spray Tests
Neutral salt spray tests were conducted to study the corrosion behavior of samples repaired by superior metal and cold spray. Figure 10 is the rate of mass loss after neutral salt spray tests. As observed, the rate of mass loss of 6A01-T5 is lower than that of the RSM samples not only after neutral salt spray tests for 500 h but also for 1000 h. By contrast, the RCS samples have the lowest rate of mass loss. That is to say that the RCS samples exhibited better corrosion resistance.

Neutral Salt Spray Tests
Neutral salt spray tests were conducted to study the corrosion behavior of samples repaired by superior metal and cold spray. Figure 10 is the rate of mass loss after neutral salt spray tests. As observed, the rate of mass loss of 6A01-T5 is lower than that of the RSM samples not only after neutral salt spray tests for 500 h but also for 1000 h. By contrast, the RCS samples have the lowest rate of mass loss. That is to say that the RCS samples exhibited better corrosion resistance.

Neutral Salt Spray Tests
Neutral salt spray tests were conducted to study the corrosion behavior of samples repaired by superior metal and cold spray. Figure 10 is the rate of mass loss after neutral salt spray tests. As observed, the rate of mass loss of 6A01-T5 is lower than that of the RSM samples not only after neutral salt spray tests for 500 h but also for 1000 h. By contrast, the RCS samples have the lowest rate of mass loss. That is to say that the RCS samples exhibited better corrosion resistance.  The surface appearances of samples before and after neutral salt spray tests are shown in Figure 11. After neutral salt spray tests for 500 h, the 6A01-T5 surface lost its original metallic luster and was covered with grey corrosion products or oxidation film. By comparison, the 6a01-T5 surface for 1000 h is a little darker. The RCS samples exhibited similar surface appearance. After neutral salt spray tests for 500 h, the substrate part is dim, and there exist a small number of corrosive pits in the RCS. Moreover, black trace from the center to the edge of samples is quite clear. This is probably because the corrosion products of the RCS were washed along the surface by the NaCl solution. After neutral salt spray tests for 1000 h, it is worth mentioning that the black trace turned white. Likewise, the RCSs are brighter than before. As observed in Figure 11c, the RSM samples suffered more severe corrosion than 6A01-T5 and the RCS samples. Many corrosive pits and tawny corrosion products are conspicuous in the RSM. The surface appearances of samples before and after neutral salt spray tests are shown in Figure 11. After neutral salt spray tests for 500 h, the 6A01-T5 surface lost its original metallic luster and was covered with grey corrosion products or oxidation film. By comparison, the 6a01-T5 surface for 1000 h is a little darker. The RCS samples exhibited similar surface appearance. After neutral salt spray tests for 500 h, the substrate part is dim, and there exist a small number of corrosive pits in the RCS. Moreover, black trace from the center to the edge of samples is quite clear. This is probably because the corrosion products of the RCS were washed along the surface by the NaCl solution. After neutral salt spray tests for 1000 h, it is worth mentioning that the black trace turned white. Likewise, the RCSs are brighter than before. As observed in Figure 11c, the RSM samples suffered more severe corrosion than 6A01-T5 and the RCS samples. Many corrosive pits and tawny corrosion products are conspicuous in the RSM. Surface morphological characteristics of samples after neutral salt spray tests are shown in Figure 12. As shown in Figure 12a, corrosive pits are conspicuous in 6A01-T5 surface. Moreover, the corrosion products around the corrosive pits are dehiscent and tend to fall off. The surface of 6A01-T5 after neutral salt spray tests for 1000 h is much smoother and largely covered with dense corrosion products. This illustrates that corrosion layers with a protective effect were formed. As depicted in Figure 12b, after neutral salt spray tests were performed for 500 h, more pits and cracks are observed in the surface of the RCS compared with that of 6a01-T5. The corrosion layers have no positive effect to protect the fresh matrix. As corrosion time increases to 1000 h, many dense corrosion products are visible in some areas, and this means that the corrosion process is at a stable stage in which the corrosion of the fresh matrix is inhibited [21][22][23]. Surface morphological characteristics of samples after neutral salt spray tests are shown in Figure 12. As shown in Figure 12a, corrosive pits are conspicuous in 6A01-T5 surface. Moreover, the corrosion products around the corrosive pits are dehiscent and tend to fall off. The surface of 6A01-T5 after neutral salt spray tests for 1000 h is much smoother and largely covered with dense corrosion products. This illustrates that corrosion layers with a protective effect were formed. As depicted in Figure 12b, after neutral salt spray tests were performed for 500 h, more pits and cracks are observed in the surface of the RCS compared with that of 6a01-T5. The corrosion layers have no positive effect to protect the fresh matrix. As corrosion time increases to 1000 h, many dense corrosion products are visible in some areas, and this means that the corrosion process is at a stable stage in which the corrosion of the fresh matrix is inhibited [21][22][23].
As mentioned above, the RSMs are composed of ferro-silicon and epoxy resin. It is well-known that epoxy resin has excellent corrosion resistance. Hence, corrosion occurred on the ferro-silicon part. Figure 13 shows the surface of the RSM samples after neutral salt spray tests for 1000 h. The holes caused by the detachment of ferro-silicon were well marked in the repaired areas. Furthermore, the substrate in the RSM samples is covered with loose corrosion products and has very bad corrosion morphology. With reference to the reports [24], ferro-silicon in the repaired areas has higher corrosion potentials than the substrate (6A01 aluminium alloy). In this case, galvanic corrosion happened between the repaired areas and the substrate. The repaired areas, such as the cathode, were protected, whereas the substrate, such as the anode, corroded dramatically. Therefore, the RSM samples exhibited poor corrosion resistance. As mentioned above, the RSMs are composed of ferro-silicon and epoxy resin. It is well-known that epoxy resin has excellent corrosion resistance. Hence, corrosion occurred on the ferro-silicon part. Figure 13 shows the surface of the RSM samples after neutral salt spray tests for 1000 h. The holes caused by the detachment of ferro-silicon were well marked in the repaired areas. Furthermore, the substrate in the RSM samples is covered with loose corrosion products and has very bad corrosion morphology. With reference to the reports [24], ferro-silicon in the repaired areas has higher corrosion potentials than the substrate (6A01 aluminium alloy). In this case, galvanic corrosion happened between the repaired areas and the substrate. The repaired areas, such as the cathode, were protected, whereas the substrate, such as the anode, corroded dramatically. Therefore, the RSM samples exhibited poor corrosion resistance.

Electrochemical Tests
The repaired areas (RSM) do not have continuous conductivity since they are formed by epoxy resin and ferro-silicon. Hence, electrochemical tests cannot be carried out on the RSM. Figure 14 is the open circuit potential (EOCP) of samples before and after neutral salt spray tests. As shown in Figure 14a, the EOCP variation of the RCS is similar to that of 6A01-T5. The initial EOCP is at a relatively low level and then increases rapidly. Afterwards the EOCP gradually stabilizes downwards. However, the EOCP of 6A01-T5 fluctuates greatly and is higher in comparison to that of the RCS. In the initial stage of tests, corrosion happened promptly in the surface of samples, and a passive film was formed resulting in the rapid increase in EOCP. Then, the variation of EOCP was ascribed to the formation and destruction of the passive film. The EOCP of the RCS has a small fluctuation such that its passive film  As mentioned above, the RSMs are composed of ferro-silicon and epoxy resin. It is well-known that epoxy resin has excellent corrosion resistance. Hence, corrosion occurred on the ferro-silicon part. Figure 13 shows the surface of the RSM samples after neutral salt spray tests for 1000 h. The holes caused by the detachment of ferro-silicon were well marked in the repaired areas. Furthermore, the substrate in the RSM samples is covered with loose corrosion products and has very bad corrosion morphology. With reference to the reports [24], ferro-silicon in the repaired areas has higher corrosion potentials than the substrate (6A01 aluminium alloy). In this case, galvanic corrosion happened between the repaired areas and the substrate. The repaired areas, such as the cathode, were protected, whereas the substrate, such as the anode, corroded dramatically. Therefore, the RSM samples exhibited poor corrosion resistance.

Electrochemical Tests
The repaired areas (RSM) do not have continuous conductivity since they are formed by epoxy resin and ferro-silicon. Hence, electrochemical tests cannot be carried out on the RSM. Figure 14 is the open circuit potential (EOCP) of samples before and after neutral salt spray tests. As shown in Figure 14a, the EOCP variation of the RCS is similar to that of 6A01-T5. The initial EOCP is at a relatively low level and then increases rapidly. Afterwards the EOCP gradually stabilizes downwards. However, the EOCP of 6A01-T5 fluctuates greatly and is higher in comparison to that of the RCS. In the initial stage of tests, corrosion happened promptly in the surface of samples, and a passive film was formed resulting in the rapid increase in EOCP. Then, the variation of EOCP was ascribed to the formation and destruction of the passive film. The EOCP of the RCS has a small fluctuation such that its passive film

Electrochemical Tests
The repaired areas (RSM) do not have continuous conductivity since they are formed by epoxy resin and ferro-silicon. Hence, electrochemical tests cannot be carried out on the RSM. Figure 14 is the open circuit potential (E OCP ) of samples before and after neutral salt spray tests. As shown in Figure 14a, the E OCP variation of the RCS is similar to that of 6A01-T5. The initial E OCP is at a relatively low level and then increases rapidly. Afterwards the E OCP gradually stabilizes downwards. However, the E OCP of 6A01-T5 fluctuates greatly and is higher in comparison to that of the RCS. In the initial stage of tests, corrosion happened promptly in the surface of samples, and a passive film was formed resulting in the rapid increase in E OCP . Then, the variation of E OCP was ascribed to the formation and destruction of the passive film. The E OCP of the RCS has a small fluctuation such that its passive film is more stable. After neutral salt spray tests, the E OCP of 6A01-T5 is still a little higher than that of the RCS. Accordingly, galvanic corrosion happened between RCS and the substrate. The repaired areas, such as the anode, corroded acutely, whereas the substrate areas, such as the cathode, were protected. What is more is that a small increase in E OCP is distinct in the RCS after neutral salt tests for 500 h. That is to say that galvanic corrosion was weakened due to the decrease in E OCP difference between RCS and the substrate. is more stable. After neutral salt spray tests, the EOCP of 6A01-T5 is still a little higher than that of the RCS. Accordingly, galvanic corrosion happened between RCS and the substrate. The repaired areas, such as the anode, corroded acutely, whereas the substrate areas, such as the cathode, were protected. What is more is that a small increase in EOCP is distinct in the RCS after neutral salt tests for 500 h. That is to say that galvanic corrosion was weakened due to the decrease in EOCP difference between RCS and the substrate. The potentiodynamic polarization curves of samples after neutral salt spray tests for 500 h and 1000 h are shown in Figure 15, and the related electrochemical parameters are provided in Table 5. As depicted, the RCSs have lower Ecorr and higher Icorr compared with 6A01-T5 during neutral salt spray tests. Thus, RCS shows higher corrosion tendency at all times. When corrosion time increased to 1000 h, it is observed that the Ecorr of RCS increases to −0.926 V/SCE and Icorr decreases to 1.004 × 10 −6 A/cm 2 . Thus, there is a downtrend in corrosion tendency indicating that the corrosion products formed on the surface of the RCS have protective effects. This is according to the analysis above. The potentiodynamic polarization curves of samples after neutral salt spray tests for 500 h and 1000 h are shown in Figure 15, and the related electrochemical parameters are provided in Table 5. As depicted, the RCSs have lower E corr and higher I corr compared with 6A01-T5 during neutral salt spray tests. Thus, RCS shows higher corrosion tendency at all times. When corrosion time increased to 1000 h, it is observed that the E corr of RCS increases to −0.926 V/SCE and I corr decreases to 1.004 × 10 −6 A/cm 2 . Thus, there is a downtrend in corrosion tendency indicating that the corrosion products formed on the surface of the RCS have protective effects. This is according to the analysis above.    The electrochemical results have indicated the occurrence of galvanic corrosion between the RCS and the nearby substrate. In order to prove the fact that RCS protects its nearby substrate by corroding as the sacrificial anode, the surface of the substrate around the RCS after neutral salt tests for 1000 h is shown in Figure 16. As indicated, the substrate around it is almost covered with thin corrosion layers that are smooth. As shown in the magnified image, there are numerous small and shallow pits. By contrast, 6A01-T5 after neutral salt tests for 1000 h shows a more terrible appearance than the substrate around RCS. This also demonstrates that the substrate around it is protected by RCS during the corrosion process. The RCS samples have the lowest rate of mass loss, although the RCSs are sacrificed. This is because the RCSs are small, and the substrate around it corroded slightly in comparison to the 6A01-T5 samples. In addition, Al and Al 2 O 3 in RCS also have excellent corrosion resistance [25][26][27].  The electrochemical results have indicated the occurrence of galvanic corrosion between the RCS and the nearby substrate. In order to prove the fact that RCS protects its nearby substrate by corroding as the sacrificial anode, the surface of the substrate around the RCS after neutral salt tests for 1000 h is shown in Figure 16. As indicated, the substrate around it is almost covered with thin corrosion layers that are smooth. As shown in the magnified image, there are numerous small and shallow pits. By contrast, 6A01-T5 after neutral salt tests for 1000 h shows a more terrible appearance than the substrate around RCS. This also demonstrates that the substrate around it is protected by RCS during the corrosion process. The RCS samples have the lowest rate of mass loss, although the RCSs are sacrificed. This is because the RCSs are small, and the substrate around it corroded slightly in comparison to the 6A01-T5 samples. In addition, Al and Al2O3 in RCS also have excellent corrosion resistance [25][26][27]. In conclusion, the corrosion mechanism of the RCS samples can be described in Figure 17. The RCS samples are immersed in NaCl solution during neutral salt spray tests. As a result, Al is easy to dissolve due to the existence of Cl − . Al 3+ is formed by the following reaction, which is the anodic reaction.
Due to the potential difference between the RCS and the substrate around it, the anodic reaction in the RCS is accelerated. Hence, more Al dissolves and more Al 3+ formed in the RCS. There are still tiny pores, although the tamping of Al2O3 particles decreases the In conclusion, the corrosion mechanism of the RCS samples can be described in Figure 17. The RCS samples are immersed in NaCl solution during neutral salt spray tests. As a result, Al is easy to dissolve due to the existence of Cl − . Al 3+ is formed by the following reaction, which is the anodic reaction.
Due to the potential difference between the RCS and the substrate around it, the anodic reaction in the RCS is accelerated. Hence, more Al dissolves and more Al 3+ formed in the RCS. There are still tiny pores, although the tamping of Al 2 O 3 particles decreases the porosity of RCS. Moreover, the cathodic reaction that follows occurs in the gaps between Al and Al 2 O 3 or around the micro-pores [28,29].
OH − + Al 3+ → Al(OH) 3 , Accordingly, the Al region is covered with Al(OH) 3 , which is close to Al 2 O 3 particles. Subsequently, some Al(OH) 3 react with H 2 O and transformed into Al 2 O 3 ·nH 2 O. As corrosion time increases, more and more corrosion products are formed. It is not easy for Cl − to permeate into the fresh matrix, and this results in its enrichment on the surface. Then, the reaction between Al(OH) 3 and Cl − happens as follows [30,31].
AlCl 3 has a negative effect on the formation of stable corrosion layers and causes the production of holes. The reactions of Equations (2)-(5) also happen in the substrate around the RCS, although these areas are protected. As dense corrosion layers formed and the potential difference between RCS and the substrate around it gradually disappeared, the corrosion reactions were weakened. Accordingly, the Al region is covered with Al(OH)3, which is close to Al2O3 particles. Subsequently, some Al(OH)3 react with H2O and transformed into Al2O3•nH2O. As corrosion time increases, more and more corrosion products are formed. It is not easy for Cl − to permeate into the fresh matrix, and this results in its enrichment on the surface. Then, the reaction between Al(OH)3 and Cl − happens as follows [30,31].
AlCl3 has a negative effect on the formation of stable corrosion layers and causes the production of holes. The reactions of Equation (2) to (5) also happen in the substrate around the RCS, although these areas are protected. As dense corrosion layers formed and the potential difference between RCS and the substrate around it gradually disappeared, the corrosion reactions were weakened.

Conclusions
(1) The samples repaired by cold spray have lower porosity and higher tensile strength than the samples repaired by superior metal. The samples repaired by cold spray have met the requirements for mechanical properties. (2) The samples repaired by cold spray have the lowest rate of mass loss and show better corrosion resistance than 6A01-T5. Its repaired areas will protect the nearby substrate by corroding as the sacrificial anode.

Conclusions
(1) The samples repaired by cold spray have lower porosity and higher tensile strength than the samples repaired by superior metal. The samples repaired by cold spray have met the requirements for mechanical properties.
(2) The samples repaired by cold spray have the lowest rate of mass loss and show better corrosion resistance than 6A01-T5. Its repaired areas will protect the nearby substrate by corroding as the sacrificial anode.