**1. Introduction**

High-strength steel wire is the key load-bearing component of cable bearing bridges, such as cable-stayed bridges and suspension bridges, and will degrade during operation [1]. The reliability of a bridge in operation is deeply influenced by the corrosion degradation of its components [2]. The corrosion problem of cable systems has aroused extensive consideration from scholars. The main types of cable structure are parallel wire rope and steel strand; both are composed of single steel wire. The earliest cable steel wires were all-steel wires without coating. The cable was wrapped with a protective sleeve outside and filled with barrier materials inside, but the anti-corrosion effect was poor, as proven in practice. To resist the corrosion of environmental factors, the cable components began to have a coating on the surface of the high-strength steel wire, to isolate the corrosion medium. Early cable bearing bridges mostly used galvanized high-strength steel wire, and

**Citation:** Zhao, Y.; Su, B.; Fan, X.; Yuan, Y.; Zhu, Y. Corrosion Fatigue Degradation Characteristics of Galvanized and Galfan High-Strength Steel Wire. *Materials* **2023**, *16*, 708. https://doi.org/ 10.3390/ma16020708

Academic Editor: Ming Liu

Received: 1 December 2022 Revised: 29 December 2022 Accepted: 9 January 2023 Published: 11 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

relevant research has mainly focused on galvanized steel wire. The corrosion development law of galvanized coating is relatively clear. In recent years, Galfan coating steel wire with better corrosion resistance has gradually started to be widely used. Its corrosion resistance is better than that of galvanized steel wire, but the corrosion characteristics of this coating lack systematic research. The deterioration of the steel wire is the result of the combined effect of corrosion and fatigue. The corrosion characteristics of a steel wire coating directly affect the corrosion state of the steel substrate and the subsequent fatigue crack growth. The difference of corrosion fatigue properties between the two types of steel wires is unclear. To further improve the service performance and operation and maintenance level of a cable load-bearing structure, this work intended to conduct an experimental study on the corrosion characteristics of galvanized steel wire and Galfan steel wire, analyze the degradation characteristics of the two types of high-strength steel wire, and discuss the influence of corrosion and fatigue on the degradation of high-strength steel wire.

The degradation of high-strength steel wire is the result of the combined effect of corrosion and fatigue. Simple corrosion and fatigue degradation of steel wire is relatively slow. The pitting corrosion pits produced along with the uniform corrosion of steel wire provide conditions for the initiation of fatigue cracks, thus greatly reducing the fatigue life of steel wire; the fatigue strength of the steel wire decreases with the increase of corrosion [3,4]. Betti et al. conducted in-depth research on the deterioration mechanism of the high-strength steel wire of a suspension bridge, studied the corrosion evolution of galvanized and nongalvanized steel wire under different environmental conditions through an accelerated cyclic corrosion test, and pointed out that the uneven change of the steel wire section along the length reduced the elongation of the steel wire [5]. Nakamura, Suzumura, and others researched the influence of reagent concentration, ambient temperature, and humidity on the corrosion rate of galvanized steel wire through experiments, given the loss rate of the galvanized layer of galvanized steel wire. They pointed out that the main reason for the deterioration of the properties of corroded steel wire is the reduction of elongation, torsional strength, and fatigue strength [6–9]. Lan et al. conducted an acid salt spray test and fatigue test on high-strength steel wire and fitted the corrosion fatigue life of steel wire based on the Weibull distribution. The change trend of the fatigue life of steel wire and cable components with the development of the corrosion process is basically consistent, and the fatigue life of a stay cable decreases significantly as the corrosion degree of the steel wire increases [10]. The above research confirmed that the fatigue life of steel wire decreases due to corrosion, from practical engineering and laboratory research. Jiang et al. and Wang et al. used solutions to create corrosive environments and studied the effects of different solutions, solution concentrations, stress amplitudes, and load frequencies on fatigue life. The corrosion fatigue performance of steel wire in acidic environments was the worst, and electrochemical reaction greatly reduced the life of the steel wire [11,12]. Sun established a corrosion fatigue degradation model of steel wire based on fracture mechanics and compared it with test results, which proved that the proposed model could better simulate the evolution of corrosion fatigue of steel wire [13]. Li et al. established improved uniform corrosion and pitting models for high-strength steel wire, verified that the maximum pitting factor obeys a Gumbel distribution based on an accelerated corrosion test, fitted relevant parameters, and studied corrosion fatigue through finite element simulation [14]. Jiang et al. measured the corroded steel wire 3D profile and proposed that the pitting depth of steel wire follows a normal distribution and that the location and scale parameters increase with the degree of corrosion. A method for predicting the residual life of corroded steel wire based on 3D measurement and AFGROW software was established [15]. The basic process and principles of corrosion fatigue degradation have been confirmed by scholars.

The above research mainly focused on the corrosion characteristics of galvanized steel wire. Compared with galvanized steel wire, galvanized aluminum high-strength steel wire has been gradually applied to engineering construction in recent years. However, the research on its corrosion behavior characteristics is relatively scarce. Xue et al. [16] studied the corrosion fatigue behavior of Galfan coating, and Cao et al. [17] studied the

effect of Nd on the corrosion behavior of Zn-5Al (wt.%) alloy in neutral 3.5wt.%NaCl solution using electrochemical impedance spectroscopy. The addition of Nd can improve the corrosion resistance of Zn-5Al alloy. The above studies are important references for the study of Galfan coating corrosion resistance performance. Nonetheless, the development of Galfan coating corrosion resistance and pitting has not been systematically studied, and the difference in the service life between the two kinds of steel wires is unclear.

Systematic studies on the corrosion resistance of Galfan wire coating and the difference between its corrosion fatigue and the fatigue of galvanized wire are few. The effect on improving the service performance of cable structures in engineering applications is unclear. In this study, the corrosion resistance of galvanized and Galfan high-strength steel wires was studied using an accelerated corrosion test. On the basis of the test results, a uniform corrosion development model of the two kinds of steel wires was established, and a dynamic distribution model of the maximum pitting coefficient was established using Gumbel distribution. The time-varying characteristics of the scale and location parameters of the maximum pitting coefficient distribution for the two types of steel wire are given. On the basis of the traffic load monitoring data of a bridge during operation, the corrosion fatigue degradation characteristics of the steel wires of the cable components in the service period were analyzed, which can provide a reference for the design and maintenance of the components of bridge structure cables.

## **2. Accelerated Corrosion Experiment**

Corrosion tests can generally be divided into two categories: one is the traditional corrosion tests under natural conditions. The real corrosion conditions of test objects can be obtained by directly exposing the test samples to the real environment, but the time cost is high and the test cycle is too long. The other is accelerated corrosion tests under a laboratory environment. By putting the test samples into a corrosion chamber and using a salt spray environment, atmospheric pressure, temperature, and other factors to accelerate corrosion, the test time is greatly shortened. With the rapid development of bridge component materials and the increasing demand for corrosion-resistance research, the salt spray test has become the most commonly used method for cable corrosion research. Based on the specification "Corrosion Test in Artificial Atmospheres—Salt Spray Test"(GB/T 10125- 2012) [18], a neutral salt spray test was selected to study the corrosion characteristics of galvanized and Galfan high-strength steel wire. The corrosion atmosphere was formed using a salt spray test chamber, and an accelerated corrosion effect was achieved by combining temperature and air pressure. Given the few parameters of the accelerated corrosion test, the concentration and continuity of the salt spray, temperature, and pressure in the test chamber were mainly guaranteed during the test.

The accelerated corrosion time and number of test pieces are presented in Table 1. With reference to existing research results, the planned test duration for galvanized steel wire was 510 h, and the test duration for Galfan steel wire was 1445 h. Each group of galvanized steel wires had 5 test pieces, and the first 10 groups of Galfan steel wire had 5 steel wires. To ensure the accuracy of data, each group of Galfan steel wires had 10 steel wires. Before placing the steel wire test pieces, all steel wires were weighed and numbered, and then the test pieces were placed in a salt spray test chamber for artificial atmospheric corrosion. The scheduled steel wire test pieces were taken out in batches, according to the planned corrosion time period, and then the corrosion products were removed using a combined chemical method and physical method, according to the corrosion product removal specification "Corrosion of Metals and Alloys–Removal of Corrosion Products from Corrosion Test Specimens" (GB/T 16545) [19]. The pickling solution was a saturated solution of NH3CH2COOH, where 1000 mL solution was prepared by mixing 250 g NH3CH2COOH and distilled water. The specific steps were as follows: (1) Put the sample into the saturated NH3CH2COOH solution (pickling solution) at 20~25 ◦C and soak it for 10 min. (2) Rub off the residual corrosion products with abrasive paper. (3) Successively put it into water and alcohol 5 times for cleaning. (4) Wipe it with a towel

and dry it after cleaning. After each group of steel wire samples was taken out at specific times, the corrosion products were removed and weighed according to the above steps.


**Table 1.** Accelerated corrosion period of high-strength steel wires.

#### *2.1. Specimens of High-Strength Steel Wire*

The high-strength steel wire used in the test was provided by a cable manufacturer. The material parameters of the steel wire samples were tested, and the results are given in Table 2. According to the mass and density of the coating, the thickness of the zinc coating was 28.05µm, and the thickness of the Galfan coating was 29.53 µm. Galvanized steel wire coatings consisted of pure zinc, and Galfan steel wire consisted of 5% aluminum zinc alloy and a small amount of mixed rare earth elements. The chemical composition of the steel wire coating is shown in Table 3.

**Table 2.** Steel wire sample parameters.


**Table 3.** Mass percentages of microelements of the high-strength steel wires coating (%).


The steel wire was cut to make an experimental sample, and the sample length was about 20 cm. Before the test, all steel wire samples were cleaned, dried, numbered, measured, and weighed one by one. After the preparation, they were put into the salt spray box.

#### *2.2. Test Device and Accelerated Corrosion Medium*

The test materials required for the neutral salt spray test included: steel wire sample, salt spray box, electronic balance, C2H5OH, NaCl, and pickling agent. The quality of the steel wire was measured using a high-precision electronic balance. The reagent grade used in the test was chemically pure. The parameters of the relevant instruments and chemical reagents are provided in Table 4.

**Internal Dimensions (mm)** 

**Table 4.** Instruments and chemical reagents. reagents are provided in Table 4.


*Materials* **2023**, *16*, x FOR PEER REVIEW 5 of 23

*2.2. Test Device and Accelerated Corrosion Medium* 

A Zhongte LX120 multi-functional climate and environment salt fog test chamber was adopted for the test salt fog chamber, which can realize the simulation of a salt fog environment, high temperature, and high pressure conditions, as well as the coupled effect of different conditions. The technical parameters are listed in Table 5. The experimental equipment and materials are shown in Figure 1. A Zhongte LX120 multi-functional climate and environment salt fog test chamber was adopted for the test salt fog chamber, which can realize the simulation of a salt fog environment, high temperature, and high pressure conditions, as well as the coupled effect of different conditions. The technical parameters are listed in Table 5. The experimental equipment and materials are shown in Figure 1

The test materials required for the neutral salt spray test included: steel wire sample, salt spray box, electronic balance, C2H5OH, NaCl, and pickling agent. The quality of the steel wire was measured using a high-precision electronic balance. The reagent grade used in the test was chemically pure. The parameters of the relevant instruments and chemical

**Table 5.** Technical parameters of the salt spray box. **Table 5.** Technical parameters of the salt spray box.


**Figure 1.** Equipment and materials for the salt spray test device. **Figure 1.** Equipment and materials for the salt spray test device.

According to the requirements of the specification for the neutral salt spray test and to test the reproducibility of the test equipment results, a steel reference test verification was carried out. Four defect-free CR4 grade cold-rolled carbon steel plates with a thickness of 1 mm were selected. After cleaning, the back of the samples was protected with a film, and a sample was placed at the four corners of the salt spray box for 48 h, as shown in Figure 2. The specific test parameters were set according to the specifications. The volume of the salt spray box was 0.6 m3. No solution accumulated on the top of the box on the sample, and the spray was always uniform. The pH value of the spray solution collected by the collector was in the range 6.5–7.2. The test temperature was 35 ± 1 °C, and the concentration of NaCl was 50 g/L. The settling rate of the salt spray met the specifica-According to the requirements of the specification for the neutral salt spray test and to test the reproducibility of the test equipment results, a steel reference test verification was carried out. Four defect-free CR4 grade cold-rolled carbon steel plates with a thickness of 1 mm were selected. After cleaning, the back of the samples was protected with a film, and a sample was placed at the four corners of the salt spray box for 48 h, as shown in Figure 2. The specific test parameters were set according to the specifications. The volume of the salt spray box was 0.6 m<sup>3</sup> . No solution accumulated on the top of the box on the sample, and the spray was always uniform. The pH value of the spray solution collected by the collector was in the range 6.5–7.2. The test temperature was 35 ± 1 ◦C, and the concentration of NaCl was 50 g/L. The settling rate of the salt spray met the specifications.

tions. The reference sample was taken out immediately after the test, and ammonium acetate solution and a mechanical cleaning method were used to remove the corrosion products. The mass loss per unit area obtained after weighing is presented in Table 6. The loss of each test piece was within the range of 70 <sup>±</sup> 20 g/m<sup>2</sup> required by the specification, indicating that the equipment operated normally and met the operational requirements of the neutral salt spray test. The steel wire corrosion tests were then carried out according to the specification requirements. The test solution was NaCl solution with a concentration of

50 g/L ± 5 g/L, the pH value of spray solution was 6.5 to 7.2, and the temperature was 35 ± 2 ◦C. *Materials* **2023**, *16*, x FOR PEER REVIEW 6 of 23

**Figure 2.** Steel reference test. **Figure 2.** Steel reference test.

The reference sample was taken out immediately after the test, and ammonium ace-**Table 6.** Mass loss of the cold-rolled carbon steel sheet.


cating that the equipment operated normally and met the operational requirements of the

#### neutral salt spray test. The steel wire corrosion tests were then carried out according to **3. Corrosion Phenomenon**

**4. Corrosion Process**  *4.1. Uniform Corrosion* 

sion.

the degree of corrosion of steel wire.

rewritten as Equation (2):

and corrosion depth of steel wire *du*.

the specification requirements. The test solution was NaCl solution with a concentration of 50 g/L ± 5 g/L, the pH value of spray solution was 6.5 to 7.2, and the temperature was 35 ± 2 °C. **Table 6.** Mass loss of the cold-rolled carbon steel sheet. **Specimen Number Specimen 1 Specimen 2 Specimen 3 Specimen 4**  Mass loss (g/m2) 75 78 77 80 **3. Corrosion Phenomenon**  The box was opened, and the corresponding test pieces were taken out, according to the time specified in the test plan. They were cleaned and dried in strict accordance with the corrosion product removal specification. Figure 3 shows the corrosion morphology of a single Galfan steel wire at different corrosion stages. The degree of corrosion of the steel wire gradually increased with time, and the corrosion resistance of the Galfan steel wire was much better than that of the galvanized steel wire. The early corrosion of the steel wire was reflected in the loss of luster of the coating, accumulation of salt on the surface of the steel wire, and the gradual production of uniformly distributed white accumulation products on the surface of the steel wire. *Materials* **2023**, *16*, x FOR PEER REVIEW 7 of 23 can be generally divided into two parts: The first part is the corrosion of the surface coating. When the corrosion depth exceeds the coating thickness, the corrosion of the second part of the steel wire matrix begins. Owing to the protection of oxidation products formed after the coating corrosion, the corrosion rate of the steel wire matrix decreases.

> reddish brown corrosion products had connected into sheets. At 1389 h, the same situation had occurred to the Galfan steel wire. At this time, uneven pits had appeared on the steel

> > / − ଵ/ଵ /

where *ψ* represents the loss rate of steel wire mass, *m*0 represents the quality of steel wire before corrosion, *l*0 represents the length of steel wire before corrosion, *m*1 represents the quality of steel wire after corrosion, and *l*1 represents the length of steel wire after corro-

=

=

Owing to the large slenderness ratio of the steel wire specimen, the sectional area of both ends of the steel wire is small and the calculation constant is large, so the length change caused by corrosion can be ignored. That is = ଵ. Therefore, Equation (1) can be

> − ଵ

According to Equation (3), the mass loss of steel wire is converted into the coating

 <sup>=</sup> − ଵ 

where *A* is the surface area of steel wire, = , *D* is the initial diameter of steel wire,

× 100% (1)

(2)

(3)

The development of the uniform corrosion of steel wire is mainly affected by two factors: the corrosion time, and the uniform corrosion rate. As the uniform corrosion depth is not easy to obtain directly, it is generally described using the volumetric method, weight-loss method, or other methods. In this study, the weight-loss method was used to describe the uniform corrosion of steel wire, which is given as Equation (1). With reference to the specification for removal of corrosion products, the chemical substances generated after steel wire corrosion can be removed without damaging the metal matrix, and the quality loss of metal in the corrosive environment can be accurately measured, to evaluate

time, producing reddish brown corrosion products. At 510 h, many reddish brown corrosion products had appeared at the middle and end of the galvanized steel wire, and the **Figure 3.** Corrosion morphology in the different stages of corrosion. *ρ* is the material density, and is the length of steel wire. **Figure 3.** Corrosion morphology in the different stages of corrosion.

=

The galvanized steel wire and the Galfan steel wire were completely covered by white corrosion products from the 264 h and 510 h, respectively. After the corrosion products were removed, part of the coating had lost luster, indicating that the coating had been corroded at this time and part of the iron matrix had begun to be exposed. At 384 h and 1016 h, some corrosion spots had appeared on the surface of the steel wire, indicating that the iron matrix under the partial coating of the steel wire had started to corrode at this time, producing reddish brown corrosion products. At 510 h, many reddish brown corrosion products had appeared at the middle and end of the galvanized steel wire, and the reddish brown corrosion products had connected into sheets. At 1389 h, the same situation had occurred to the Galfan steel wire. At this time, uneven pits had appeared on the steel wire surface after the corrosion products had been removed. The corrosion of steel wire can be generally divided into two parts: The first part is the corrosion of the surface coating. When the corrosion depth exceeds the coating thickness, the corrosion of the second part of the steel wire matrix begins. Owing to the protection of oxidation products formed after the coating corrosion, the corrosion rate of the steel wire matrix decreases.
