Effect of Hot Dip Plating Process Parameters on Microstructure and Properties of Zinc–10%Aluminum–Mischmetal Alloy Coated for Bridge Cable Steel Wire

Abstract

In this work, the effect of the hot dip plating process on the microstructure and properties of zinc–10%aluminum–mischmetal (Zn-10Al-MM) alloy coated for bridge cable wire was studied by changing the hot dip plating temperature, time, and cooling mode. An optical microscope was used to observe the microstructure of the coating and steel wire at low power; a GeminiSEM300 field emission scanning electron microscope was used to observe the microstructure of the coating and steel wire at high power. The hot dip planting temperature is 450 °C, the hot dip plating time is 30 s and 45 s, whether water cooling or air cooling, the hot dip coating structure is relatively uniform, and the grain size is small, the hot dip coating thickness is thick. The coating with 450 °C, 45 s, and the water-cooling process has the highest tensile strength. The corrosion resistance of the hot dip coatings under different processes is higher in the salt spray test. Based on the results of the electrochemical test and salt spray test, the best corrosion resistance of the coatings under 450 °C, 45 s, and the water-cooling process is obtained.

1. Introduction

Steel wire is an important type of steel material. Its appearance replaces ordinary rope. It has higher strength and can bear heavier workpieces, longer service life, and greatly reduces the cost of frequent replacement [1,2]. With the continuous research on steel wire, the tensile strength of steel wire is higher and higher, and its performance is more and more excellent, which is gradually applied to bridges. Suspension bridges and cable-stayed bridges are two important bridge structures. Compared with other structures, they have many advantages, such as large span, material saving, simple construction, and so on. The bridge cable is the main bearing part of these two bridges [3,4], and the core of the cable is high-strength steel wire. The research shows that the increase of sorbite content in steel wire can greatly improve the toughness and plasticity of steel wire. Cold drawing of steel wire can greatly improve the tensile strength of steel wire [5,6].

In addition to the excellent mechanical properties of steel wires, bridge cables also have high requirements for the corrosion resistance of steel wires [7,8]. The working environment of bridge cables has been in humid air for a long time, and sometimes suffered from wind and rain, so they are vulnerable to corrosion, fatigue, and other damage. Especially for some sea crossing bridges, the air of their working environment contains a large number of small sea salt particles brought by the sea breeze. When the air temperature is too low to contain the original water vapor, the excess water vapor condenses into small water droplets. Small sea salt particles dissolve in small water droplets to form a high concentration of salt fog. The complex marine environment makes the cables of the sea crossing bridge more prone to corrosion than those in other areas. Coupled with the increasingly serious air pollution, phosphorus, sulfur, and other elements contained in the air and the high chlorine atmosphere on the sea surface, the corrosion rate of bridge cables is further exacerbated [9]. As one of the main load-bearing components, bridge cables bear the large dead load and dynamic load. At the stress peak, local plastic deformation zone, defect, and microcrack of the main cable steel wire, the existence of corrosive medium accelerates the crack propagation. In the corrosion fatigue fracture, the cyclic stress and corrosion promote each other and accelerate the crack propagation [10]. With the increase of carbon content and strength, the sensitivity of corrosion fatigue cracking also increases [11]. Facing some problems, in this harsh environment, how to improve the corrosion resistance of cables has become the top priority.

Corrosion protection is the key technology to prolong and ensure the safe life of cable structure bridges. The coating material protection method and electrochemical protection method can usually be used to improve the corrosion resistance of steel materials [12,13]. Hot dip metal hot dip coating is recognized as one of the most direct and effective methods for steel protection [14]. Hot dip galvanized coating can effectively reduce the corrosion rate of steel materials and increase the durability of the matrix. It has the advantages of low cost, convenient operation, and strong adhesion with the matrix. Therefore, it has been widely used in the corrosion protection of bridge cable steel wire. With the development of industry, ordinary galvanized steel wire can not meet the requirements of the construction and operation of some long-span important bridges. Higher tensile strength and corrosion resistance have become an important goal of research.

Abdulaziz et al. [15] study found that hot dip galvanized Zn-Al coatings on the surface of steel rebars have better corrosion resistance than pure zinc coating. Zhao et al. [16] reported the liquid metal embrittlement susceptibility of Zn-Al-Mg/Sn. They found that the addition of the Sn element in the alloy coating improves the stability of the inhibition layer, and the coating had better crack propagation resistance than that without the Sn element. LeBozec et al. [17] researched the influence of the microstructure of zinc–aluminum–magnesium alloy coated steel on corrosion behavior. The results show that the corrosion resistance of the fine microstructure in the magnesium-rich region is the strongest. The results of the literature show that the corrosion resistance of hot dip zinc–aluminum coating can be improved by doping appropriate elements.

Amadeh et al. [18] found that rare earth metals can improve the corrosion resistance of hot dip zinc coating. Hence, this experiment mainly studies the influence of hot dip process parameters on the microstructure and properties of Zn-10Al-MM alloy coating of bridge cable steel wires, provides theoretical guidance for the application of hot dip process of bridge cable steel wire, and pursues better properties of hot dip coated steel wire, so as to meet the increasingly urgent needs of transportation.

2. Materials and Methods

This experiment used a bridge cable with a diameter of 6.0 mm, and high-strength steel wire is used as the base material. The steel wire is pickled with hydrochloric acid to remove the surface oxide film so that the steel wire is in the state to be plated. The chemical composition of steel wire is shown in

Table 1. Chemical composition of glossy steel wire (wt%).

Element	C	Si	Mn	Cr	Fe
Content	0.92	0.30	0.78	0.27	Bal.

.

In this experiment, the Zn-10Al-0.08MM plating solution is formed by mixing and melting pure Zn ingots and Zn-Al-MM alloy ingots (Lbis, Baiyin Nonferrous Group Co.,Ltd, Baiyin, China). The compositions of the alloy ingots are shown in

Table 2. Chemical composition of zinc ingots (wt%).

Element	Pb	Cd	Fe	Cu	Al	Zn	Impurity
Content	0.001	<0.0005	<0.001	<0.0005	0.001	Bal.	<0.005

and

Table 3. Chemical composition of zinc and aluminum ingots (wt%).

Element	Al	Si	Pb	Fe	MM	Zn	Impurity
Content	16.24	0.006	0.0014	0.016	0.1	Bal.	0.025

Note: “MM” refers to the mixture of lanthanum and cerium.

, respectively. For the experiment, put the alloy ingot into the crucible and start the resistance furnace (Wuxi Changqiao Electric Furnace Co., Ltd., Wuxi, China). After the alloy ingot is melted, prepare for immersion plating. The plated steel wire is inserted vertically into the plating solution. The hot dip plating process parameters in this experiment are shown in

Table 4. Process parameters for hot dip plating of Zn-10Al-0.08MM.

Sample	Hot Dip Plating Temperature/°C	Hot Dip Plating Time/s	Sample
450-30-W	450	30	water cooling
450-45-W	450	45	water cooling
475-30-W	475	30	water cooling
475-45-W	475	45	water cooling
450-30-A	450	30	air cooling

.

After the hot dip plating of the steel wire is completed, various types of steel wire with a length of 1 cm are cut by wire cutting, and after inlaying, they are ground and polished until the surface is smooth and scratch-free. AxioVert.A1 optical microscope was used to observe the microstructure of the hot dip coating and steel wire under low magnification, and a field emission scanning electron microscope (ZEISS, GeminiSEM300, Carl Zeiss AG, Oberkochen, Germany) was used to observe the microstructure of the steel wire and hot dip coating under high magnification. The elemental composition of the hot dip coating was analyzed by an energy spectrometer. The corroded steel wire was observed with a scanning electron microscope to observe its microscopic corrosion morphology, and the phase analysis of the corrosion products was carried out with an X-ray diffractometer (Bruker, D8-Advance, BRKR, Karlsruhe, Germany).

Microcomputer-controlled electro-hydraulic servo universal testing machine (MTS, SHT4106, MTS systems (China) Co., Ltd., Shanghai, China) was used to conduct a tensile test on steel wire samples. Take three samples of each kind, each 50 cm, test their tensile strength, and finally, take the average value.

The neutral salt spray test can simulate the atmospheric environment above the ocean, accelerate the corrosion of the sample, monitor the change of the corrosion state of the sample, so as to obtain the speed and law of the corrosion of the sample, and qualitatively compare the corrosion resistance of different samples. The specific conditions of the test are: 5 wt% NaCl aqueous solution, the pH value of the solution is neutral at 6.5–7.5, the test temperature is 50°C, and the salt spray sedimentation rate is between 1.5–2.5 mL/ (80 cm²·h), and the samples are taken out every time period, observe the corrosion state of the surface of the sample and take pictures to record. Furthermore, the electrochemical polarization curves of coated steel wires under different processes were tested by an electrochemical workstation (CHI, CHI604D, CH Instruments Ins., Shanghai, China) to further analyze the corrosion resistance of the samples.

3. Results and Discussion

3.1. Surface Morphology and Metallographic Structure of Steel Wires by Different Hot Dip Plating Processes

The surface morphology of the prepared hot dip coated steel wire is shown in Figure 1. It can be seen that these processes can produce steel wire with complete hot dip coating and better cut surface quality. The surface of the hot dip coated wire prepared at different temperatures and times did not vary much, but the surface of the hot dip coated wire at different cooling methods was slightly different. The surface of the hot dip coated steel wire under water cooling is shiny and smooth, and is accompanied by a certain pattern after quenching; the surface of the steel wire under air cooling is grainy and less bright, similar to a frosted surface, but the hot dip coating obtained in both ways has no obvious defects and is of better quality.

Figure 2 shows the metallographic microstructure of the Zn-10Al-0.08MM hot dip coated steel wire for several different processes. The photomicrographs taken show that the hot dip coating layer has the same general morphology and is clearly divided into two layers. The Fe in the steel matrix enters the hot dip coating layer by diffusion and participates in the Fe-Al reaction to form the hot dip coating reaction layer, which gradually becomes thicker and no longer participates in the hot dip coating solidification when the diffusion of Fe reaches a certain limit, mainly by the Zn, Al elements form the hot dip coating molten layer.

Table 5. Measurement results of hot dip coating thickness under different process parameters.

Sample	Plating Thickness/μm
450-30-W	93.73
450-45-W	37.94
475-30-W	29.05
475-45-W	53.29
450-30-A	38.13

shows the results of hot dip coating layer thickness measurement, it can be seen that the highest hot dip coating layer thickness under the condition of 450-30-W, and the lowest hot dip coating layer thickness under 475-30-W. Combined with the pictures, it can be seen that the main difference is the thickness of the hot dip coating layer melting layer is large, the thickness differences of the reaction layers of several hot dip coating layers are not large, the main differences are in the hot dip coating layer melting layer, which indicates that by the substrate This shows that the content of Fe diffused from the matrix is similar, and it can be judged that the process adjustment does not affect the content of Fe diffused from the matrix, but mainly changes the thickness and quality of the hot dip layer by affecting the subsequent Zn-Al eutectic phase.

3.2. Microstructure of Hot Dip Coated Steel Wire by Different Processes

To further investigate the microstructure of the steel wire hot dip plated layer, the steel wire hot dip plated layer was examined by scanning electron microscope(SEM) and energy dispersive spectrometer(EDS). Figure 3a shows the histomorphology of the Zn-10Al-0.08MM hot dip coated steel wire under 450-45-W conditions under SEM. It can be seen that the organization of the hot dip coated layer under this process is not much different from that under the 450-30-W process, which is also divided into two layers, and there is a diffuse distribution of black Al-rich phase in both layers, and its organization is more uniformly distributed. The energy spectrum analysis of the hot dip plated layer is shown in Figure 3b, points 1 and 2 are selected in the black area of the reaction layer of the hot dip plated layer and the white area, points 3 and 4 are selected in the white area of the fused layer of the hot dip plated layer and the black area, and areas 5 and 6 are selected in the reaction layer of the hot dip plated layer and the fused layer, respectively,

Table 6. Point scan and surface scan results of 450-45-W hot dip plating.

Location	Al(wt%)	Fe(wt%)	Zn(wt%)
1	33.00	10.20	56.81
2	12.27	6.79	80.94
3	5.08	0.58	94.34
4	35.39	0.27	68.77
5	22.60	8.63	68.77
6	13.51	0.42	86.07

shows the results of the point scan and the surface scan of the hot dip plated layer. The hot dip plated reaction layer contains more Fe₂Al₅ and FeAl₃ phases, while the hot dip plated fused layer is white with Zn-Al eutectic and black with Al-rich phase. Extending the plating time did not change the distribution of the elemental content and the histomorphology, and Fe was still present only in the reaction layer of the hot dip layer, but almost not in the melting layer. The grains of the reaction layer are still fine and uniformly distributed, and the hot dip plating layer is well organized.

Figure 4a shows the morphology of the 475-30-W hot dip plated layer under SEM. Compared with the 450-30-W hot dip plated layer, the immersion temperature was increased, and it can be seen that the demarcation line of the melting layer within the hot dip plated layer became relatively less obvious, and the melting layer of the hot dip plated layer was also thinner. The energy spectrum analysis of the hot dip plated layer was carried out. As shown in Figure 4b, points 1 and 2 were selected in the black and white areas of the reaction layer of the hot dip layer, points 3 and 4 were selected in the white and black areas of the molten layer of the hot dip layer, and areas 5 and 6 were selected in the reaction and molten layers of the hot dip layer. As shown in

Table 7. Point scan and surface scan results of 475-30-W hot dip plating.

Location	Al(wt%)	Fe(wt%)	Zn(wt%)
1	36.58	9.63	53.79
2	9.78	4.57	85.65
3	7.53	0.94	91.53
4	36.39	0.68	62.93
5	23.63	8.38	67.99
6	17.36	0.81	81.83

, it also consists of a reaction layer rich in Fe₂Al₅ and a fused layer with diffuse distribution of Al-rich phase, but the fused layer is very thin and the quality of the hot dip layer is average, and the distinction between the fused layers within the hot dip layer is not obvious, indicating that the formation of the hot dip layer is poor under the condition of 475 °C.

Figure 5 shows the organization of the 475-45-W hot dip plated layer under SEM. It can be seen that the organization of the hot dip plated layer is disordered, and the hot dip plated layer reaction layer fusion layer is unevenly thin and thick, the reaction layer in Figure 3a is thicker and the fusion layer is thinner, while the opposite is true in Figure 3b, and the distribution of the Fe-rich Fe₂Al₅ phase of the hot dip plated layer reaction layer is also more disordered and not uniformly diffuse. The hot dip plated layer cladding layer contains a large number of lamellar tissue enrichment areas, and the Al-rich phase of the hot dip plated layer fusion condensation layer is not distributed in a diffuse point-like manner, but forms this lamellar tissue. In the energy spectrum analysis of the hot dip plating layer, points 1,2 were selected in the white area and black area of the hot dip plating layer fusion layer, and areas 3,4 were selected in the hot dip plating layer fusion layer and hot dip plating layer reaction layer, respectively,

Table 8. Point scan and surface scan results of 475-45-W hot dip coating layer.

Location	Al(wt%)	Fe(wt%)	Zn(wt%)
1	2.17	0.37	97.46
2	31.03	1.71	67.26
3	22.47	0.37	77.17
4	20.44	9.05	70.51

shows the results of point scan and surface scan, it can be seen that the scan result of surface 3 contains 22.47% Al, while point 1 in the white area has only 2.17% Al, which proves that the dotted Al-rich phase, which is diffusely distributed in other hot dip layers, appears in this hot dip layer in a lamellar organization, and at the same time is not uniformly distributed but in agglomerates. This may be due to the fact that the simultaneous increase of the immersion plating temperature and immersion plating time resulted in a longer holding time, and the Al-rich phase formed and then started to regroup, forming this lamellar enriched zone, which often has poor tissue mechanical properties and poor hot dip plating layer quality, indicating that the immersion plating process of 475-45-W is not suitable.

Figure 6 shows the morphology of the 450-30-A hot dip plated layer under SEM. It can be seen that the overall tissue distribution of the hot dip plated layer is uniform, the hot dip plated layer is thicker, and its morphology is very close to that of the 450-30-W hot dip plated layer, which proves that the cooling method does not have much influence on the morphology of the hot dip plated layer when it is formed. For the energy spectrum analysis of the hot dip layer, points 1 and 2 were selected in the black and white areas of the reaction layer of the hot dip layer, points 3 and 4 were selected in the white and black areas of the molten layer of the hot dip layer, and areas 5 and 6 were selected in the reaction and molten layers of the hot dip layer, respectively, and the results of the point and surface sweeps are shown in

Table 9. Point scan and surface scan results of 450-30-A hot dip plating.

Location	Al(wt%)	Fe(wt%)	Zn(wt%)
1	43.76	8.03	48.21
2	9.74	6.25	84.02
3	3.75	0.35	95.91
4	16.81	0.28	82.91
5	22.79	7.96	69.26
6	13.97	0.28	85.75

. The element distribution is also not much different from that of the hot dip layer under water cooling. The Fe in the reaction layer of the hot dip layer is mainly in the form of Fe₂Al₅ and FeAl₃, and the Al-rich phase in the fused layer of the hot dip layer is diffusely distributed in the Zn-Al eutectic organization. In summary, the hot dip plating layer under air cooling has a uniform organization and high quality.

3.3. Tensile Strength of Steel Wire with Hot Dip and Hot Dip Coating by Different Processes

Tensile tests were conducted on the hot dip coated outline wire to test its tensile strength, and the results are shown in Figure 7. The 450-45-W hot dip coated wire had the highest tensile strength of 2031 MPa, which was only 3 MPa lower compared to 2034 MPa for galvanized wire, followed by a higher tensile strength of wire under air cooling of 2027 MPa, indicating that the tensile strength was not much related to the cooling method. The lowest tensile strength is the wire under 475-45-W process with only 1988.3 MPa, which is a significant decrease in strength. This is consistent with the previous SEM photo results. The organization of the hot dip plating layer under the 475-45-W process under SEM is extremely non-uniform and contains more lamellar brittle tissues, and the hot dip plating layer performance is very poor, so the tensile strength is low. On the contrary, the hot dip plating layer under both 450-45-W and 450-30-W processes is uniformly organized, and the Al-rich phase is evenly distributed in the hot dip plating layer, which can act as a diffuse reinforcement, leading to a higher strength of the hot dip plated layer, thus enhancing the overall strength of the wire [19]. In conclusion, the highest tensile strength of hot dip coated wire can be obtained by using an immersion plating temperature of 450 °C and an immersion plating time of 30 s. The effect of the cooling method on the strength is not significant.

3.4. Corrosion Resistance of Hot Dip Plated Steel Wire Hot Dip Plating Layer

The hot dip coated steel wires with different processes were subjected to salt spray tests and their macroscopic morphology under different salt spray corrosion stages was photographed as shown in Figure 8. As can be seen in Figure 8a, the surface of all five wires before the salt spray test was smooth and flat, without any leakage of plating inclusions. Figure 8b is the macroscopic shape of the hot dip coated steel wire after 24 h salt spray test, it can be seen that the steel wire are a small amount of salt particles attached to the surface, and no obvious corrosion conditions. Figure 8c for the salt spray corrosion 20 d after the macroscopic shape of the steel wire, at this time the hot dip coating steel surface began to appear with a small amount of white rust, proving that the steel surface of the zinc and aluminum hot dip coating began to corrode. Figure 8d for the salt spray test 40 d after the macroscopic shape of the steel wire, it is clear that the amount of white rust on the surface of the hot dip coated steel wire gradually increased. Figure 8e for the salt spray test 60 d after the macroscopic shape of the wire, the amount of white rust on the surface of the wire further increased, but it is clear that the white rust layer of 450-30-A wire is significantly thicker than the other hot dip coated wire, while the white rust layer of 450-30-W wire is relatively thin, proving that the air-cooled hot dip coated wire is deeper than the water-cooled hot dip coated wire corrosion. Figure 8f for the salt spray test 80 d after the macroscopic shape of the steel wire, white rust has been covered with hot dip coated steel wire surface, a large number of corrosion products and salt particles wrapped around the wire. Figure 8g for the salt spray test 100 d after the macroscopic shape of the wire, the wire surface has appeared very thick and dense white rust layer, these white rust tightly wrapped in the hot dip coating wire surface, has been completely invisible wire matrix, but did not appear red rust, which means that the corrosion process has been ongoing, but the wire matrix has not received damage, the hot dip coating on the wire matrix protection is still good. Figure 8h is the macroscopic appearance of the wire after 120 d of salt spray test, it can be seen that the surface of 475-30-W wire and 450-30-A wire began to show a small amount of red rust spots, but several other wires did not appear, proving that the corrosion process of the substrate of these two hot dip coated wires began to occur, and gradually exceeded the corrosion resistance of the hot dip coating range. Figure 8g for the salt spray test 150 d after the macroscopic morphology of the wire, at this time 450-30-W wire surface appears large red rust, the steel matrix has received serious corrosion, hot dip coating has completely failed. The 475-30-W wire surface still has a small amount of red spots, but there is no obvious large red rust area, the rest of the steel wire does not have red rust. In summary, the corrosion resistance of several hot dip coating is more excellent, in the salt spray test before 100 d, which can be good protection of the substrate. However the salt spray test 100 d after the air-cooled conditions of the hot dip coating on the steel wire protection capacity gradually reached the limit, 475-30-W hot dip coating thin, while the organization of the grain is coarse, relatively poor corrosion resistance, the use of 450 °C of the two hot dip coating, the organization is uniform, hot dip coating is also thick, so the corrosion resistance is very high, air-cooled hot dip coating layer organization is uniform, the thickness is also high, but it is the earliest appearing red rust; the reason needs to be further investigated. Moreover, rare earth metals can make the hot dip coating obtain dense microstructure, which is conducive to improving the corrosion resistance of the coating [18].

To further investigate the reason for the high corrosion resistance of the hot dip coated steel wire, the hot dip coated steel wire was removed after 120 d of corrosion under different processes, and the corroded surface was observed by SEM and photographed, and the results are shown in Figure 9. Figure 9a is the corrosion morphology of 450-30-W hot dip coating, it can be seen that the surface corrosion products are uniformly wrapped in the surface of the wire, which has a small number of prismatic areas. Figure 9b shows the corrosion pattern of the 450-45-W hot dip coating layer, the surface of which is covered with a large number of prismatic corrosion products, and the corrosion products do not have any cracks. Figure 9c shows the corrosion pattern of 475-30-W hot dip coating, the surface corrosion products are mainly spherical corrosion products. Figure 9d shows the corrosion pattern of 475-45-W hot dip coating, which has deep cracks on the surface of corrosion products. Figure 9e for 450-30-A hot dip coating corrosion morphology, it can be seen that the corrosion products contain a large number of cracks, forming a network of cracks. Such cracks will reduce the density of corrosion products, greatly reducing the protective properties of the hot dip coating, and reducing the corrosion resistance of the hot dip coating, which may be the first air-cooled hot dip coating red rust reason. Some areas (the red wire box in Figure 9) of the corrosion products were analyzed by energy spectroscopy to analyze their elemental composition, and the results are shown in

Table 10. Elemental composition (wt%) of the surface of the hot dip coated steel wire after 120 d corrosion with different process parameters.

Sample	C	O	Al	Cl	Fe	Zn
450-30-W	6.26	18.06	0	8.24	0	67.45
450-45-W	9.27	17.99	0	8.33	0	64.41
475-30-W	5.37	23.64	0.59	5.88	0.32	64.19
475-45-W	3.71	16.13	0.40	0.62	0	78.71
450-30-A	3.49	24.53	7.74	5.77	0.93	57.53

. The elemental composition is mainly C, O, Cl, Zn, and some of the hot dip coating also contains a small amount of Al and Fe. The corrosion products were collected using physical methods and ground into powder, and the XRD analysis of the corrosion product powder was performed, and the results are shown in Figure 10 Zn₅(OH)₈Cl₂-H₂O, zinc hydroxide (Zn(OH)₂) and zinc oxide (ZnO), and also contains a small amount of aluminum oxide Al₂O₃·8Cl₂-H₂O content is relatively low and the corrosion resistance is relatively low, which is consistent with the results of the salt spray test. Meanwhile, it can be seen from Table 10 that the Al content in 450-30-A hot dip coating is higher and much larger than other hot dip coatings, which can indicate that the Al element still plays an obvious role in the hot dip coating under air-cooled conditions and salt spray corrosion at the 120 d stage.

Figure 11 shows the morphology of corrosion products under high magnification, mainly still divided into prismatic and spherical two, Figure 11a,b for prismatic corrosion products, Figure 11c,d for spherical corrosion products. In Figure 9b, it is obvious to see that the surface of the hot dip coating has a large number of prismatic corrosion products, these corrosion products are well isolated from the corrosive medium and the substrate, effective protection of the hot dip coating. In Figure 9e, it is obvious to see that the corrosion products have a large number of cracks on the surface, and their densities are low, which cannot play an effective barrier role, so the corrosion resistance will be reduced, which is also consistent with the results of the previous analysis. Therefore, it can be shown that the prismatic Zn₅(OH)₈Cl₂-H₂O has an important protective effect on the corrosion resistance of the hot dip coating during the 120 d stage of corrosion. The 450-45-W immersion plating process was used to obtain more prismatic corrosion products and enhance the corrosion resistance of the hot dip plating layer.

The electrochemistry polarization curves of coated steel wires under different processes were obtained. The results are shown in Figure 12, and the parameters of the polarization curves are shown in

Table 11. Electrochemical polarization curve parameters of coated steel wire under different process parameters.

Sample	Ecorr/V	Jcorr/mA·cm−2
450-30-W	−1.071	2.06 × 10−3
450-45-W	−1.079	8.10 × 10−4
475-30-W	−1.188	3.74 × 10−3
475-45-W	−1.038	1.51 × 10−3
450-30-A	−1.064	9.63 × 10−4

. The corrosion current density from low to high is 450-30-A steel wire <450-45-W steel wire <475-45-W steel wire <450-30-A steel wire <475-30-W steel wire. The corrosion current density can represent the corrosion resistance of coated steel wire to a certain extent. Therefore, according to the results of electrochemical experiments, 450-45-W steel wire and 450-30-A steel wire have the lowest corrosion current density and the best corrosion resistance, while 475-30-W steel wire has a higher corrosion current density and the corrosion resistance is relatively poor. According to the results of the salt spray test, 450-30-A steel wire began to show red rust at first after 100 d of salt spray time, which proved that the matrix was corroded first and the coating was the first to fail, followed by red rust spots on 475-30-W steel wire, which means that the corrosion resistance of 475-30-W steel wire is relatively poor, which is consistent with the results of electrochemistry.

4. Conclusions

In this experiment, the Zn-10Al-0.08MM hot dip plating layer was prepared by adjusting the process parameters during hot dip plating to control the organization and properties of the hot dip plating layer, and the conclusions of the study are as follows.

(1) The hot dip plating layer with immersion plating temperature at 450 °C and immersion plating time of 30 s and 45 s, regardless of water cooling or air cooling, the hot dip plating layer organization is more uniform and the grain size is fine, and the thickness of the hot dip plating layer is higher. The thickness of the outer layer of the hot dip layer was greatly reduced and the organization was not uniform enough when the immersion plating time was 30 s at 475 °C. When the immersion plating temperature was 45 s, the reaction layer of the hot dip layer was extremely thin, the distribution of the Fe-rich phase was disordered, and the Al-rich phase in the outer layer of the hot dip layer was precipitated in the form of agglomerated lamellae, and the organization uniformity was extremely poor.

(2) The tensile strength of the hot dip plated layer with 450 °C, 45 s, and the water-cooling process is the highest. At 475 °C, 45 s and the water-cooling process has the lowest tensile strength, which is related to the poor organization of the hot dip plated layer and also to the change of the properties of the wire matrix after heat. The change of process parameters does not have a significant effect on the torsional strength, and the number of torsions of hot dip plated layers under several processes is 22–24, which meets the requirements of GB/T 17101-2019.

(3) The hot dip plating layer under different processes showed high corrosion resistance in the salt spray test. The results of the integrated electrochemical experiment and salt spray test showed that the best corrosion resistance was 450 °C, 45 s, under the water-cooling process. The corrosion products of the hot dip coating layer are still dominated by Zn₅(OH)₈Cl₂-H₂O, and the role of Al₂O₃ in the air-cooled hot dip coating layer is relatively higher, but the corrosion products of the air-cooled hot dip coating layer are less dense and contain a large number of cracks.