Study on the Corrosion Inhibition Mechanism of HEDP and Mechanical Performance Degradation of HSGPSW Under Tensile Stress

Abstract

High-strength galvanized parallel steel wire (HSGPSW) is a primary load-bearing component in cable-supported bridge structures. However, due to both human and environmental factors, corrosion during its service life is often inevitable, and in severe cases, it may threaten the structural safety of the bridge. In this study, a novel method employing the organic corrosion inhibitor hydroxyethylidene diphosphonic acid (HEDP) is proposed to mitigate the corrosion of HSGPSW. First, electrochemical accelerated corrosion tests were conducted on 48 specimens immersed in HEDP solutions to investigate the effects of three key parameters—HEDP concentration, tensile stress, and inhibition duration—on the mass loss rate of the specimens. Subsequently, tensile tests were performed on the inhibited specimens to obtain their load–displacement curves, and the maximum tensile load under the influence of HEDP was comparatively analyzed. The results show that at an HEDP concentration of 0.12 mol·L⁻¹, the inhibition efficiency reached 40.31%, but it became saturated when the concentration exceeded 0.08 mol·L⁻¹. At a tensile stress of 7.5 kN, the inhibition efficiency decreased to 13.24%, with passive film breakdown identified as the primary cause of performance degradation. Energy-dispersive spectroscopy (EDS) analysis revealed that HEDP significantly stunts zinc layer dissolution, thereby enhancing initial corrosion protection, while mechanical tests indicated that its ability to slow the degradation of tensile performance diminishes after film rupture. The corrosion inhibition mechanism is attributed mainly to the synergistic effect of anodic suppression and interfacial coordination. This study provides a new method and novel insights for the corrosion protection of high-strength galvanized HSGPSW in cable-supported bridge structures.

1. Introduction

High-strength galvanized parallel steel wire (HSGPSW), as a primary load-bearing component in cable-supported bridge systems, has been widely used in recent years due to its high yield strength, high tensile strength, and excellent durability [1,2,3]. Compared with conventional reinforcement (e.g., HRB500 with a yield strength of approximately 500 MPa), HSGPSW can achieve yield strengths exceeding 1580 MPa, significantly enhancing structural load-carrying efficiency while reducing cross-sectional dimensions, thereby enabling structural weight reduction and material savings [4,5]. However, during service, HSGPSW is inevitably exposed to harsh and complex environmental conditions. Corrosive agents, such as chloride ions, can severely degrade its mechanical properties, compromising cable load capacity and overall structural safety, and in extreme cases, leading to catastrophic bridge failures [6,7,8,9,10]. Given these challenges, it is important to recognize that HSGPSW is essentially a type of high-strength steel. HSGPSW refers to steel with a yield strength not less than 500 MPa, widely applied in buildings, bridges, automobiles, and ships. Depending on composition and properties, high-strength steel can be classified into various categories, including low-alloy high-strength steel, bainitic steel, martensitic steel, and transformation-induced plasticity steel [11,12].

Corrosion inhibitors are an effective means of protecting metals against corrosion [13,14,15]. Hydroxyethylidene diphosphonic acid (HEDP), an organic phosphonic acid inhibitor, exhibits excellent chelating and stabilizing properties. Its phosphonic acid and hydroxyl groups can form a dense protective film on the metal surface via chemisorption or coordination, blocking corrosive agents. In addition, its strong chelating ability toward Ca²⁺ and Mg²⁺ suppresses scale formation and improves passive film integrity, thereby significantly reducing both uniform corrosion rates and pitting susceptibility. Previous studies have demonstrated that HEDP performs effectively under acidic, neutral, and mildly alkaline conditions, with low environmental toxicity, making it suitable for long-term protection of steel and other metals.

HEDP has been widely recognized as an effective corrosion inhibitor, demonstrating remarkable performance in the protection of metallic materials. Extensive experimental studies have systematically elucidated its inhibition mechanisms and protective efficacy. Research indicates that HEDP mitigates metal corrosion through multiple pathways: In chloride-containing, highly alkaline simulated concrete pore solutions, HEDP preferentially adsorbs onto the passive film surface of 20SiMn low-alloy steel, blocking chloride ion attachment via competitive adsorption and thereby preserving the film’s integrity [16]. For cold-rolled steel, electrochemical impedance spectroscopy confirmed the superior protective properties of the Zn-HEDP composite film, whose enhanced corrosion resistance stems from increased surface coverage and binding sites during prolonged immersion [17]. Furthermore, studies have shown that HEDP exhibits significant inhibitory effects on carbon steel, with a concentration as low as 25 mg/L providing substantial protection for 20# carbon steel [18], while the Zn-HEDP hybrid system effectively suppresses corrosion of mild steel in neutral, aerated chloride solutions [19]. These findings provide critical theoretical support for the engineering application of HEDP across diverse metallic systems.

Nevertheless, little is known about the mechanical performance evolution of HSGPSW under HEDP inhibition. In this study, a novel HEDP-based corrosion mitigation method is proposed to delay the corrosion of bridge-grade HSGPSW. The effects of different stress levels and inhibitor concentrations on its corrosion behavior were systematically investigated. An electrochemical accelerated corrosion method was employed, in which current density, electrolyte composition, and externally applied tensile stress were precisely controlled to simulate the long-term durability performance of prestressed steel wires under realistic service conditions. A total of forty-eight specimens were prepared and tested in two stages: Stage I corresponded to the period from the start of inhibition until visible cracks formed in the passive film; Stage 2 extended for an equal duration beyond Stage 1. Following corrosion tests, uniaxial tensile tests were performed to obtain load–displacement curves and analyze variations in the maximum tensile load under HEDP inhibition. Furthermore, a degradation model of HSGPSW mechanical performance after HEDP inhibition was developed. The findings provide a new approach and theoretical basis for the corrosion protection of HSGPSW in cable-supported bridge systems.

2. Experimental Program

2.1. Electrochemical Corrosion Test

This study employed an electrochemical accelerated corrosion method to conduct corrosion and tensile tests on forty-eight HSGPSW specimens. The electrochemical corrosion test was divided into two stages based on inhibitor exposure duration: Stage 1 lasted from the beginning of the test until the appearance of cracks in the corrosion-inhibiting film, and Stage 2 lasted twice as long as Stage 1. Precise control of current intensity, corrosive medium composition, and applied external stress simulated the long-term durability performance of prestressed steel wires under realistic engineering corrosion conditions.

The specimens were obtained from the same batch of Japanese-manufactured New PWS high-strength galvanized steel wires with a nominal diameter of 7 mm. The trace element concentrations of HSGPW are summarized in

Table 1. Trace element concentration table of HSGPW.

Elements	C	Si	Mn	P	S	Cr
Content	0.88%	1.03%	0.40%	0.012%	0.007%	0.23%

. Diameter deviations were tightly controlled within ±0.02 mm after precise measurements. The standard tensile strength was 1570 MPa, characteristic of typical high-strength prestressing materials with excellent mechanical properties. The specimens were hot-dip galvanized with a uniform zinc coating approximately 80 μm thick, verified via multi-point eddy current thickness measurements. Prior to testing, all specimens underwent stringent cleaning procedures, including ultrasonic degreasing in acetone, rinsing with deionized water, dehydration with anhydrous ethanol, and drying to ensure surface cleanliness and test reproducibility.

The corrosive medium consisted of 3.5% NaCl solution prepared from analytical-grade NaCl and deionized water, effectively simulating typical chloride ion concentrations in seawater and realistically replicating marine corrosion conditions [20,21,22]. The initial pH of the solutions was measured and maintained uniformly across all test groups (pH 7.6–8.1) to eliminate pH-related variability in corrosion behavior. HEDP, a highly effective organic phosphonic acid-based corrosion inhibitor, was selected. Four concentration gradients were tested: 0 (control), 0.04, 0.08, and 0.12 mol·L⁻¹. All inhibitor solutions were precisely weighed and thoroughly stirred with a magnetic stirrer to ensure complete dissolution and uniform distribution.

The electrochemical corrosion setup comprised a high-precision constant current power supply (model SP-305), electrode system, corrosion tank, and environmental control system. Specimens served as the anode connected to the positive terminal of the power supply. The cathode was a polished high-purity copper rod (10 mm diameter, 30 cm length). The electrode gap was fixed at 10 cm to ensure uniform electric field distribution. The corrosion tank was made of corrosion-resistant polypropylene, measuring 50 cm × 30 cm × 40 cm, filled with 3.5% NaCl solution. The immersion length of each specimen was strictly controlled to 28 cm and fixed by clamps to ensure uniform corrosion exposure. Tests were conducted in a temperature-controlled laboratory maintained at 24–25 °C with fluctuations within ±0.5 °C and relative humidity controlled at 60% ± 5%. The corrosion current was set at 0.5 A with a fluctuation within ±0.5%, determined by preliminary tests to generate observable corrosion within a reasonable timeframe without causing uncontrolled rapid corrosion. Post-corrosion, specimens were sequentially cleaned with distilled water and acid solution, dried, and weighed. Mass loss rate, corrosion rate, and inhibition efficiency were calculated according to established formulas.

The corrosion current intensity was set to 0.5 A. This value was determined through preliminary tests to ensure that observable corrosion phenomena could be generated within a reasonable time frame, while avoiding excessively rapid corrosion that might lead to the loss of experimental control. After removal, the corroded specimens were sequentially cleaned with distilled water and acid solution, then dried and weighed. The mass loss rate m’, corrosion rate v’, and inhibition efficiency $IE$ were calculated according to Equations (1) and (2).

$$m’={\displaystyle \frac{m-m_0}{m}}$$

(1)

$$v’={\displaystyle \frac{∆m}{S·h}}\times 100$$

(2)

$$IE(\%)={\displaystyle \frac{V_0-V_1}{V_0}}\times 100$$

(3)

In the equations, m denotes the mass of the HSGPSW specimen before corrosion (g); m₀ is the mass after corrosion (g); $∆m$ represents the mass loss before and after corrosion inhibition (g); S is the corrosion surface area of the HSGPSW specimen with a length of 28 cm, taken as 61.57 cm² in this study; and h is the corrosion duration (hours). $V_0$ and $V_1$ represent the corrosion rates of specimens without and with corrosion inhibitor, respectively ($\mathrm{g}{\mathrm{c}\mathrm{m}}^{-2}{\mathrm{h}}^{-1}$).

To investigate the synergistic effect of stress and corrosion, a hydraulic jack was used to apply a constant axial tensile load to the specimens. Three typical stress levels were applied: 0.0 kN (no stress), 7.5 kN (approximately 15% of ultimate tensile strength), and 15.0 kN (approximately 30% of ultimate tensile strength). Each stress level was tested with three parallel specimens to ensure statistical reliability. Loading was displacement-controlled at a rate of 0.1 mm/min until the target load was reached, which was then maintained constant until the test concluded.

2.2. Scanning Electron Microscopy Analysis

To analyze the characteristics of corrosion products on specimens treated with the HEDP inhibitor, representative samples were selected from different corrosion stages for electron microscopy observation. Specifically, specimens N-04 and T-09 were chosen from Stage 1, and specimens N-09 and T-27 from Stage 2, totaling four samples for scanning electron microscopy (SEM) analysis. The SEM tests were conducted using a Zeiss SIGMA 500 field emission SEM from Germany, primarily to observe the surface micro-morphology of the corrosion products. Energy dispersive spectroscopy (EDS) was also employed to determine the main chemical elemental composition of the corrosion layers, thereby revealing the evolution characteristics of corrosion behavior under the influence of the inhibitor.

2.3. Tensile Test

Tensile tests were conducted on the treated specimens using a WAW-1000G universal testing machine, as shown in Figure 1. The upper end of each specimen was secured using a clamp holding an acrylic rod, while the lower end was fixed with a clamp attached to a linear displacement sensor with a measurement range of 25 mm. The gauge length between the upper and lower clamps was set to 28 cm. The universal testing machine applied tensile load by moving the crosshead at a constant rate of 1.0 mm/min. Meanwhile, the relative displacement of the specimen was recorded using a UT7110Y static strain acquisition system.

3. Analysis of Experimental Results

3.1. Experimental Observations

After the application of current, the amount of material adhered to the surface of HSGPSW gradually increased with the extension of the electrolysis time, eventually forming a continuous and dense passive film on its surface. The formation rate of the passive film was significantly affected by tensile stress; as the tensile stress on HSGPSW increased, the film formation rate accelerated. Additionally, higher concentrations of the inhibitor resulted in thicker and more compact passive films. During Stage 2, both the increase in HEDP concentration and tensile stress accelerated the rupture process of the passive film, manifested by earlier occurrence of cracks or localized delamination in the film. It is noteworthy that the locations and sizes of cracks in the passive film showed certain randomness, indicating that under conditions of high HEDP concentration and elevated stress, the structural stability of the film decreased, making it prone to localized failure.

3.2. Experimental Results

3.2.1. Corrosion Test Results

Table 2. Corrosion rate efficiency of specimens in Stage 1.

Specimen ID	C (molL−1)	σ (kN)	V′ (gcm−2h−1)
N-01	0.00	0.0	0.93
N-02			1.01
T-01	0.04	0.0	0.65
T-02			0.70
T-03	0.08	0.0	0.58
T-04			0.61
T-05	0.12	0.0	0.57
T-06			0.59
N-03	0.00	7.5	1.61
N-04			1.59
T-07	0.04	7.5	1.22
T-08			1.25
T-09	0.08	7.5	1.05
T-10			1.09
T-11	0.12	7.5	1.00
T-12			1.01
N-05	0.00	15.0	2.33
N-06			2.31
T-13	0.04	15.0	2.01
T-14			1.93
T-15	0.08	15.0	1.80
T-16			1.81
T-17	0.12	15.0	1.76
T-18			1.79

summarizes the corrosion rate efficiency of specimens in Stage 1. The corrosion rate increased with rising tensile stress but decreased with higher inhibitor concentrations. Figure 2 illustrates the relationship between HEDP concentration and the mass loss rate/corrosion inhibition efficiency in Stage 1 of HSGPSWs conditions, with the mean mass loss rate and corrosion inhibition efficiency indicated by the horizontal line segments. The results demonstrate that the mass loss rate is jointly influenced by HEDP concentration and tensile stress: under stress-free conditions, the mass loss rate significantly decreases with increasing HEDP concentration; under tensile stresses of 7.5 kN and 15.0 kN, although the overall mass loss rate increases, it still decreases with increasing inhibitor concentration at the same stress level. This indicates that HEDP exhibits good stress adaptability and corrosion inhibition performance during the intact passive film stage. In summary, the corrosion severity in Stage 1 is determined by both HEDP concentration and tensile stress; controlling the stress level and applying an appropriate amount of HEDP (e.g., 0.08 mol·L⁻¹) can effectively delay the onset of film rupture and reduce the initial corrosion rate.

The relationship between inhibitor concentration and inhibition efficiency reveals that the corrosion inhibition effect of HEDP is most pronounced under stress-free conditions, with the inhibition efficiency reaching 40.31% at a concentration of 0.12 mol·L⁻¹. However, beyond a concentration of 0.08 mol·L⁻¹, the increase in inhibition efficiency tends to plateau, showing no significant enhancement. This suggests the existence of an effective inhibition concentration threshold, corresponding to the minimum concentration required to achieve the desired corrosion rate reduction. This phenomenon can be attributed to the saturation adsorption behavior of HEDP molecules on the metal surface; once saturation is reached, further increases in inhibitor concentration do not translate into additional protective effects. With the increase in tensile stress, the inhibition efficiency gradually decreases, indicating that stress may promote the formation of corrosion products and damage the local passive film structure, thereby limiting the adsorption stability of HEDP and its overall inhibition performance.

Table 3. Mass loss rate, corrosion rate, and inhibition efficiency of specimens in Stage 2.

Specimen ID	C (molL−1)	σ (kN)	V′ (gcm−2 h−1)
N-07	0.00	0.0	0.90
N-08			0.92
T-19	0.04	0.0	0.73
T-20			0.75
T-21	0.08	0.0	0.69
T-22			0.60
T-23	0.12	0.0	0.51
T-24			0.68
N-09	0.00	7.5	1.46
N-10			1.41
T-25	0.04	7.5	1.22
T-26			1.18
T-27	0.08	7.5	1.06
T-28			1.05
T-29	0.12	7.5	1.00
T-30			0.99
N-11	0.00	15.0	2.05
N-12			2.10
T-31	0.04	15.0	1.76
T-32			1.84
T-33	0.08	15.0	1.83
T-34			1.70
T-35	0.12	15.0	1.68
T-36			1.75

summarizes the corrosion rate efficiency of specimens in Stage 2. Figure 3 illustrates the relationship between HEDP concentration and the mass loss rate/corrosion inhibition efficiency in Stage 2 for HSGPSWs. Under Stage 2 conditions, the mass loss rate of HSGPSWs increased significantly. Following the rupture of the passive film, the corrosion inhibition efficiency of HEDP declined markedly, with the lowest inhibition efficiency reaching only 13.24% under high tensile stress—substantially lower than the peak performance observed when the passive film remained intact. This indicates that once the originally protective passive film is compromised, the metal substrate is directly exposed to the corrosive environment, significantly accelerating localized corrosion development. During this stage, surface defects and stress concentration effects further weaken the adsorption stability of the inhibitor on the metal surface, not only reducing its coverage capability but also severely impairing its barrier effect against corrosion reactions, resulting in a rapid deterioration of overall inhibition performance. Therefore, the integrity of the passive film plays a critical role in the inhibition system, and its disruption greatly diminishes the protective efficacy of the corrosion inhibitor.

3.2.2. Analysis Results of Corrosion Product Characteristics

(1) Chemical elemental composition of corrosion products

Table 4. Major elemental composition of corrosion product samples.

Specimen ID	Inhibition Stage	Major Elements
		Zn	Fe	O	C	P	Na	Cl
T-09	1	52.3	3.8	28.7	9.6	2.1	1.5	2.0
N-04		35.4	21.6	25.9	9.2	0.0	3.4	4.5
T-27	2	22.8	26.7	29.4	10.2	1.3	4.4	5.5
N-09		6.9	45.2	28.1	8.4	0.00	5.7	8.8

lists the major elemental compositions of corrosion products from selected specimens. Under the combined action of 3.5% sodium chloride solution and a tensile stress of 7.5 kN, HEDP exhibited a significant inhibitory effect on the corrosion behavior of HSGPSWs. When the passive film remained intact, the specimen treated with 0.08 mol·L⁻¹ HEDP showed a zinc content as high as 52.3% on its surface, while the iron content was only 3.8%, indicating that the galvanized layer was well preserved and the substrate remained unexposed. The detection of phosphorus further confirmed that HEDP effectively adsorbed onto the metal surface to form a protective layer. In contrast, specimens without HEDP addition exhibited evident damage to the zinc layer, with iron content rising to 21.6% and significantly increased levels of corrosive ions such as Cl⁻, indicating aggravated corrosion.

When the corrosion duration was extended to twice the time of passive film rupture, although the film had not completely detached, its protective effect had significantly declined. At this stage, the zinc content in the HEDP-treated group decreased to 22.8%, while the iron content increased to 26.7%, indicating that part of the substrate had undergone corrosion and the protective effect of HEDP was weakened. In contrast, the group without HEDP exhibited more severe corrosion, with iron content reaching as high as 45.2% and the zinc layer almost entirely deteriorated.

(2) Microstructural morphology of corrosion products

Under the combined effects of 3.5% sodium chloride solution and tensile stress, the corrosion morphology on the surface of HSGPSWs exhibited significant differences at various stages. Prior to the rupture of the passive film, SEM images revealed that the steel wire surface was uniformly covered by a continuous and dense passive film, which was intact and free of obvious defects, with a smooth surface and no apparent corrosion products (Figure 4 ①). This indicates that HEDP effectively adsorbs onto the metal surface during this stage, forming a stable protective barrier that impedes the ingress of corrosive media. As the corrosion duration extended and the passive film ruptured, SEM observations showed partial detachment of the passive film in localized areas, with the steel wire surface exhibiting corrosion pits and uneven roughness. Additionally, flaky iron corrosion products were locally generated (Figure 4 ②).

3.2.3. Surface Morphology of the Specimens

After the corrosion tests, the specimens were removed, thoroughly cleaned with distilled water and acid solution, and dried. Observation of the surface morphology of the HSGPSWs revealed distinct characteristics corresponding to different corrosion stages. In Stage 1 (prior to passive film rupture), the wire surface exhibited a silvery-gray appearance and was generally smooth, with no obvious corrosion pits or cracks. This indicates that the HEDP inhibitor formed a dense adsorption film with the zinc layer during this stage, effectively isolating corrosive agents such as Cl⁻ and O₂. Consequently, the corrosion severity was mild, and the structural integrity of the parallel steel wires was maintained. The fracture surfaces of Stage 1 specimens displayed typical fracture features (Figure 5 ①), characterized by a smooth cross-section without corrosion-induced cracks, demonstrating that the material retained good ductility and fracture toughness during this stage.

In Stage 2, the specimen surfaces exhibited obvious roughness and appeared predominantly dark gray, with localized spotted or streaked corrosion marks. Fine corrosion pits were observed on the surface of the parallel steel wires. These observations indicate that the passive film was partially compromised under prolonged corrosion and tensile stress, leading to a weakened inhibition effect of HEDP. Corrosive media penetrated the interface between the zinc layer and the substrate, causing significant localized corrosion. Overall, corrosion in Stage 1 was mild, whereas it intensified markedly in Stage 2, confirming the critical role of passive film integrity in the corrosion inhibition performance of HEDP. The fracture surfaces of Stage 2 specimens were rough and exhibited step-like features locally (Figure 5 ①), indicative of a brittle fracture tendency. This suggests that corrosive media initiated crack sources in stress concentration regions, thereby weakening the ductile fracture mechanisms of the metal substrate.

3.2.4. Tensile Test Results

The relationship between the mass loss rate and maximum tensile load is illustrated in Figure 6. During Stage 1 (intact passive film stage), the maximum tensile load of the specimens showed a slight increase with increasing mass loss rate. In this stage, a moderate increase in inhibitor concentration exerted a minor positive effect on the maximum tensile load, with an improvement of less than 1%, indicating that the inhibitor can slightly enhance mechanical performance by stabilizing the surface structure when the passive film remains intact. Upon entering Stage 2 (passive film rupture stage), the maximum tensile load decreased significantly as the mass loss rate increased. When the inhibitor concentration exceeded 0.08 mol·L⁻¹, the variation in maximum tensile load was less than 2%, suggesting a saturation effect in the improvement of ultimate load-bearing capacity with increasing concentration. Notably, further increases in inhibitor concentration led to a slight decline in maximum tensile load, which is attributed to uneven swelling and rupture of the passive film at high concentrations, accelerating the propagation of localized corrosion, resulting in earlier film rupture and enlarged damaged areas. Corrosion pits formed during this stage had a pronounced adverse effect on tensile strength, and the protective effect of the inhibitor was insufficient to counteract the damage accumulation caused by stress concentration.

4. Corrosion Inhibition Mechanism and Engineering Recommendations

4.1. Corrosion Inhibition Mechanism

Figure 7 presents the schematic diagram of the corrosion inhibition mechanism. The corrosion inhibition effect of HEDP on HSGPSWs primarily arises from the strong coordination capability conferred by multiple phosphonic acid groups (-PO₃H₂) and hydroxyl groups (-OH) within its molecular structure. On the surface of the galvanized steel wire, HEDP chemically adsorbs onto Zn²⁺ ions through its phosphonic acid groups, forming stable five- or six-membered chelate rings via P–O–Zn coordination bonds. This promotes a dense and orderly molecular arrangement on the zinc coating, constructing a compact mono- or multilayer adsorption film. This layer physically isolates corrosive agents such as water, oxygen, and chloride ions from the zinc substrate, while covering active anodic dissolution sites on the surface. Consequently, it significantly inhibits the anodic dissolution of zinc (Zn → Zn²⁺ + 2e⁻), which serves as one of the primary inhibition mechanisms. Simultaneously, the dense adsorption layer impedes the diffusion of dissolved oxygen (O₂) to cathodic regions and suppresses its reduction reaction (O₂ + 2H₂O + 4e⁻ → 4OH⁻), thereby mitigating cathodic depolarization and reducing the overall electrochemical corrosion rate. Additionally, HEDP can form low-solubility, stable complex precipitates with corrosion-produced Zn²⁺ ions at the interface or in solution. These HEDP-Zn chelate structures exhibit superior density, stability, and adhesion compared to conventional zinc oxide or hydroxide corrosion product layers, effectively blocking the penetration of aggressive ions such as Cl⁻ and enhancing protective performance. The hydroxyl groups (-OH) further improve the water solubility and molecular polarity of HEDP, optimizing its diffusion and adsorption at the interface, and potentially cooperating in metal coordination to further strengthen the stability and coverage of the adsorption film.

4.2. Engineering Application Recommendations

Long-term Corrosion Protection: Inject thixotropic HEDP gel containing 0.08 mol·L⁻¹ HEDP under high pressure to fill the gaps between steel wires, forming a self-healing protective film. Structural Repair: Employ carbon fiber reinforced polymer (CFRP) straps as crack-protection wraps, concurrently installing temperature and humidity sensors (with an alarm threshold at RH > 60%) and corrosion monitoring probes (with a warning set at Zn²⁺ concentration > 30 ppm). Intelligent Operation and Maintenance: Conduct quarterly unmanned aerial vehicle (UAV) inspections equipped with magnetic memory detection devices to scan stress concentration zones in the suspension cables. Combine these inspections with AI-based rust spot recognition via micro-cameras inside the protective wraps (detection accuracy of 0.1 mm²), enabling dynamic adjustment of the HEDP gel replenishment cycle.

Maintenance Recommendations: After protective wrap repair, remotely monitor sensor data monthly and automatically activate the dehumidification system when temperature or humidity exceed set thresholds. Every six months, remove the protective wrap to inspect the condition of the HEDP film layer—normally characterized by a dense blue-gray appearance—and replenish HEDP gel in areas with localized peeling. Replace the volatile corrosion inhibitor membrane bag annually and analyze corrosion rates from probes, aiming to maintain rates below 0.02 mm/year.

5. Conclusions

This study investigated the corrosion characteristics and mechanical properties of steel wires through electrochemical corrosion testing and tensile mechanical testing. The main research processes and findings are summarized as follows:

(1) The results indicate that under stress-free conditions, HEDP exhibits optimal corrosion inhibition at a concentration of 0.12 mol·L⁻¹, achieving an inhibition efficiency of up to 40.31%. However, beyond a concentration of 0.08 mol·L⁻¹, the increase in inhibition efficiency tends to plateau, suggesting the presence of an adsorption saturation effect. Under high tensile stress, the inhibition efficiency decreases significantly, dropping to a minimum of 13.24%. This highlights that the integrity of the passive film is a critical factor in maintaining the corrosion inhibition performance of HEDP; its rupture leads to reduced stability of the adsorption layer and a substantial decline in protective capability.

(2) Under the combined conditions of 3.5% sodium chloride solution and 7.5 kN tensile stress, HEDP at a concentration of 0.08 mol·L⁻¹ demonstrated significant corrosion inhibition for HSGPSWs. During the intact passive film stage, the zinc content reached 52.3%, while the iron content was only 3.8%. In contrast, the iron content increased to 21.6% in specimens without HEDP. When the corrosion duration extended to twice the time of passive film rupture, the zinc content in the HEDP-treated group decreased to 22.8%, and the iron content increased to 26.7%. Meanwhile, the iron content in the untreated group rose sharply to 45.2%, with the zinc layer nearly completely deteriorated. These results indicate that HEDP effectively mitigates early-stage corrosion, but its protective capability significantly diminishes following passive film damage.

(3) During the corrosion tests, the evolution of corrosion and mechanical properties of HSGPSWs exhibited distinct stage-specific characteristics. In Stage 1 (intact passive film), corrosion was mild, with a dense adsorption film formed by HEDP effectively inhibiting corrosion. The specimen surfaces remained smooth, and the maximum tensile load slightly increased with mass loss rate, with an improvement of less than 1%. In Stage 2 (passive film rupture), corrosion intensified markedly, characterized by rough surfaces and pronounced corrosion pits. Correspondingly, the maximum tensile load decreased significantly with increasing mass loss rate. Moreover, when the HEDP concentration exceeded 0.08 mol·L⁻¹, its beneficial effect on mechanical performance tended to saturate or even slightly decline.

(4) HEDP forms stable P–O–Zn coordination chelate structures with the zinc surface through its phosphonic acid groups, establishing a dense adsorption film that effectively inhibits zinc anodic dissolution and cathodic depolarization reactions. Additionally, HEDP can complex with Zn²⁺ ions to produce low-solubility precipitated films, significantly enhancing the film’s density and impermeability. This multi-faceted corrosion inhibition mechanism confers robust protection to HSGPSWs, with the dominant effects arising from the synergistic action of anodic inhibition and interfacial chelation.