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Article

Mechanistic Insights into Corrosion and Protective Coating Performance of X80 Pipeline Steel in Xinjiang’s Cyclic Freeze–Thaw Saline Soil Environments

1
College of Geology and Mining Engineering, Xinjiang University, Urumqi 830046, China
2
Xinjiang Collaborative Innovation Center for Green Mining and Ecological Restoration of Mineral Resources, Urumqi 830046, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(8), 881; https://doi.org/10.3390/coatings15080881
Submission received: 3 June 2025 / Revised: 21 June 2025 / Accepted: 27 June 2025 / Published: 28 July 2025

Abstract

This study systematically investigated the corrosion evolution and protective mechanisms of X80 pipeline steel in Xinjiang’s saline soil environments under freeze–thaw cycling conditions. Combining regional soil characterization with laboratory-constructed corrosion systems, we employed electrochemical impedance spectroscopy, potentiodynamic polarization, and surface analytical techniques to quantify temporal–spatial corrosion behavior across 30 freeze–thaw cycles. Experimental results revealed a distinctive corrosion resistance pattern: initial improvement (cycles 1–10) attributed to protective oxide layer formation, followed by accelerated degradation (cycles 10–30) due to microcrack propagation and chloride accumulation. Synchrotron X-ray diffraction analyses identified sulfate–chloride ion synergism as the primary driver of localized corrosion disparities in heterogeneous soil matrices. A comparative evaluation of asphalt-coated specimens demonstrated a 62%–89% corrosion rate reduction, with effectiveness directly correlating with coating integrity and thickness (200–500 μm range). Molecular dynamics simulations using Materials Studio revealed atomic-scale ion transport dynamics at coating–substrate interfaces, showing preferential Cl permeation through coating defects. These multiscale findings establish quantitative relationships between environmental stressors, coating parameters, and corrosion kinetics, providing a mechanistic framework for optimizing protective coatings in cold-region pipeline applications.

1. Introduction

In today’s society, a safe and stable supply of energy is crucial to the economic development and social stability of a country. As an important strategic energy base in China, Xinjiang occupies a prominent position in terms of oil and natural gas resources. Its coal, oil, natural gas, and wind energy reserves account for 30%, 15%, 19%, and 20% of the country’s total reserves, respectively, and all four phases of the West-to-East Natural Gas Pipeline Project start in Xinjiang, highlighting the important position of Xinjiang in China’s energy security and energy structure. In the transportation of oil and gas resources, pipelines play a key role [1], undertaking about 85% of the crude oil, 30% of the refined oil products, and 95% of the natural gas transmission tasks on land in China. Among many pipeline materials, X80 pipeline steel is widely used in the construction of long-distance oil and gas pipelines [2]. However, the special natural geographic environment of Xinjiang, especially the complex soil conditions, poses severe challenges in the safe operation of buried oil pipelines [3].
Xinjiang has a temperate continental climate with a large temperature difference between day and night, long sunshine hours, low precipitation, and a dry climate, resulting in widespread soil salinization [4]. The saline soil is highly saline and alkaline, which is highly corrosive to metal pipelines. Coupled with frequent freezing and thawing cycles in spring and winter, the moisture and salts in the soil undergo phase change and migration with temperature changes, further aggravating the soil corrosion of pipelines. This corrosion can lead to the decline of pipeline performance and oil leakage, causing environmental pollution and economic loss and even threatening public safety [5,6].
Scholars in China and internationally have extensively studied the corrosion of oil and gas pipelines. In different soil environments, the corrosion behavior of metal pipelines is affected by the soil chemical composition, physical properties, and climatic conditions [7,8]. For example, under the saline soil environment in the north of China, a high water content and suitable temperature will accelerate pipeline corrosion; porosity has a tendency to increase and then decrease in the corrosion impact; increased soil pH and Na+ content will exacerbate corrosion, while SO42− may inhibit corrosion under certain conditions [9,10]. In addition, the roles of temperature, humidity, and soil chemical composition changes on corrosion rate have been discussed [11,12].
Zheng et al. reported that different types of metal pipelines, such as Q235 steel, X70 steel, and X80 steel, all showed different corrosion behaviors in different soil environments [13], and Liu et al. investigated the corrosion of X80 steel in seawater environments [14]. The salt content and temperature in the soil are also key factors affecting the corrosion of oil and gas pipelines [15]. Sun et al. showed that chloride ions (Cl) and sulfate ions (SO42−) affect the corrosion behavior of steel differently: Cl usually promotes the corrosion of steel, while SO42− may enhance the chemical inertness of the metal surface corrosion and SO42− may enhance the chemical inertness of the metal surface. Meanwhile, temperature variations significantly affect the degree of corrosion, and low temperatures usually improve the corrosion resistance of steel [16]. Sun et al. further confirmed the coupled effect of salinity and temperature on the electrochemical corrosion behavior of X80 pipeline steel in saline soils [17]. Bai et al. focused on the corrosion mechanism of X80 steel in silty soils under the dual effects of salinity and temperature [18]. Their results showed that above freezing point, Cl accelerates corrosion due to its strong adsorption, especially underneath the sediments, while below freezing point, the presence of SO42− increases the risk of corrosion in pipelines by causing soil expansion through crystalline compressive stresses.
In this study, the corrosion behavior and mechanism of X80 pipeline steel in a typical freeze–thaw saline soil environment in Xinjiang were systematically investigated through indoor freeze–thaw cycling experiments combined with electrochemical testing, micro-morphological analysis, and anticorrosion coating evaluation, using X80 pipeline steel as the material [19,20]. The aim is to provide theoretical support and technical guidance for the anticorrosion design of oil pipelines in Xinjiang, help extend pipeline service life, reduce corrosion risk, and ensure safe and stable oil and gas transportation. At the same time, there is an in-depth discussion around the protective effect of anticorrosion coating and its failure mechanism, combined with simulation calculations to reveal the interaction mechanism between the coating and the steel sheet, in order to optimize the design of the anticorrosion coating and improve its protective performance in the specific environment of Xinjiang, to provide a theoretical basis.

2. Materials and Methods

2.1. Steel Selection and Compositional Analysis

2.1.1. Steel Selection

In this study, X80 pipeline steel was selected as the main pipe material for new long-distance oil and gas pipelines in China; it is also the largest amount of steel used in the second line of the West–East Natural Gas Pipeline Project. X80 pipeline steel is widely used in long-distance and high-pressure oil and gas transmission pipelines because of its high strength, high toughness, and good weldability [21], which can effectively reduce the cost of pipeline construction and improve the safety and reliability of pipeline systems.
Rectangular coupons of API 5L X80 pipeline steel (50 × 25 × 2 mm) were machined from certified parent material procured from China National Petroleum Corporation (CNPC) Western Pipeline Company, with metallurgical compliance validated against API 5L/GB/T 9711 standards [22,23]. To ensure experimental reproducibility, all specimens were subjected to sequential mechanical polishing with silicon carbide abrasive paper (final grit, 1200) followed by ultrasonic cleaning in anhydrous ethanol for 15 min (Branson 3800 ultrasonic welder, 40 kHz, Emerson, Norwalk, CT, USA). As depicted in Figure 1, this surface preparation protocol effectively eliminated inherent oxide layers and residual contaminants, thereby mitigating potential artifacts in the subsequent corrosion behavior analysis.

2.1.2. Composition Analysis

In this test, an Optima 8000 Inductively Coupled Plasma Emission Spectrometer (ICP-OES), manufactured by PerkinElmer, Waltham, MA, USA, was used to accurately analyze the main chemical composition of X80 pipeline steel. After the appropriate pre-treatment of X80 pipeline steel samples, we used the ICP-OES instrument to quantitatively analyze the major elements in them. The chemical composition and content of X80 pipeline steel are shown in Table 1.

2.2. Soil Sample Collection and Pretreatment

2.2.1. Sample Preparation

Soil samples were collected from salinized areas of Urumqi, Shanshan, and Yili according to ISO 18400-205:2018 [24]. Cores were extracted in triplicate (0.5–1.5 m depth) using an acid-washed stainless steel auger. Samples were sealed in argon-purged containers and stored at 4 °C during transportation.
To accurately simulate the freeze–thaw saline-alkali soil environment typical of Xinjiang’s spring and winter seasons, this study set the soil water content at 10% based on an integrated analysis of the literature, meteorological data, and regional precipitation records. Five kilograms of soil were spread in layers within trays and proportionally sprayed with deionized water, followed by thorough mixing. The prepared soil was immediately sealed with epoxy resin film to prevent evaporation and contamination and then equilibrated at room temperature for 24 h to ensure ionic equilibrium and establish a controlled experimental environment. This study focuses on the mechanism of soil physicochemical properties on pipeline steel corrosion, with an emphasis on key parameters such as the moisture content, particle size distribution, and permeability.

2.2.2. Soil pH Determination

Soil pH was determined via the potentiometric method in accordance with ASTM D4972 [25]. Aqueous soil suspensions were thermostatically equilibrated at 25 ± 1 °C for 1 h prior to measurement using a calibrated pH meter with automatic temperature compensation. Triplicate measurements per sample ensured analytical accuracy within ±0.05 pH units. The test results are shown in Table 2.
As can be seen from the test results in the table, the soil pH values in all three areas are greater than 7, indicating that the soils in these areas are alkaline. These differences in pH may be related to factors such as the soil type, geologic background, and climatic conditions in each area.

2.2.3. Soil Composition Analysis

Alkaline cations in the soil, such as potassium (K+), calcium (Ca2+), sodium (Na+), and magnesium (Mg2+) ions, have an important influence on the alkalinity of the soil. In saline soils, the content of these alkaline cations is usually higher, resulting in alkaline soils. In this test, an inductively coupled plasma emission spectrometer (ICP) was used to determine the ions in soil samples from Urumqi, Shanshan, and Yili, and the results of the ICP analytical tests are shown in Table 3.
In addition, chloride ions (Cl), sulfate ions (SO42−), and nitrate ions (NO3−) in the soil are also common ions that lead to metal corrosion, and their content and distribution in the soil have an important influence. Therefore, we used ion chromatography to analyze and test the ion chromatography of the soil in three places, and the test results are shown in Table 4.
The soils in Shanshan, Urumqi, and Yili were alkaline, with the highest soil pH in the Yili region, followed by Shanshan and Urumqi. Meanwhile, the content of alkaline cations, as well as corrosive ions, in the soils differed significantly from one region to another. The high pH and moderate calcium ion content of the soil in Urumqi makes its corrosion ability in pipeline steel relatively weak; the high corrosive ion content in Shanshan leads to its strong corrosion ability in steel.

2.3. Experimental Methods, Results, and Discussion

2.3.1. Freeze–Thaw Cycle Experiment and Polarization Curve Test

Freeze–thaw cycles were performed in a THP-408 climatic chamber (Zhihe Instruments, Jinan, China) to ensure precise temperature control [26,27]. Throughout the cycling process, the initial soil moisture content (10%) was maintained using the epoxy resin sealing method described in Section 2.2.1, effectively preventing water loss during phase transitions. The corrosion behavior of X80 steel was characterized via electrochemical measurements using a Zahner workstation. Prior to testing, steel coupons were ultrasonically cleaned in acetone, inserted into pre-equilibrated soil samples (24 h stabilization), and manually compacted under controlled moisture conditions to simulate in situ density. Following system encapsulation, specimens underwent freeze–thaw cycling between −20 °C and 20 °C (24 h/cycle). The −20 °C to 20 °C cycle was designed based on Xinjiang Meteorological Bureau data (2020–2023), with 8 h freezing/16 h thawing phases simulating diurnal variation. Cycle repetitions of 5, 10, and 15 were implemented to replicate seasonal temperature fluctuations in Xinjiang.
The polarization curve is a commonly used tool in electrochemical testing, which is based on the principle of applying an external current to break the equilibrium of an otherwise stable electrode reaction, so as to obtain a curve for the relationship between electrode potential and current density. For electrochemical testing, we employed a three-electrode configuration with an X80 steel working electrode (WE), platinum counter electrode (CE), and saturated copper/sulfate reference electrode (RE). All measurements complied with ASTM G5-14 validation protocols. Potentiodynamic polarization scans were conducted at 10 mV/s over ±250 mV vs. open-circuit potential (OCP), with scan linearity verified through correlation coefficients (R2 > 0.998) across replicate scans at 5, 10, and 20 mV/s. Electrochemical impedance spectroscopy (EIS) applied a 10 mV amplitude sinusoidal perturbation from 105 to 10−2 Hz under potentiostatic control. The Tafel extrapolation method was used to calculate the corrosion current density (Icorr) and the self-corrosion potential (Icorr), in order to quantitatively evaluate the corrosion of the steel sheet for different numbers of freezing and thawing times and in different soil environments and the kinetic characteristics of steel sheets for different freezing and thawing times and soil environments. Spatial corrosion variation was assessed by comparing electrochemical parameters and morphological features among three regional systems.

2.3.2. Microscopic Morphology Observation

In order to investigate the influence of the number of freeze–thaw cycles and different soil environments on the corrosion behavior of buried pipeline steel, this study used scanning electron microscope (SEM) technology to observe the surface microstructure of an X80 steel sheet during the corrosion process at multiple magnifications, covering corrosion features such as corrosion pits, cracks, holes, and the change in surface roughness. The surface of the steel sheet after corrosion in different conditions was observed at different magnifications, and the images of the above micro-features were emphasized and recorded. At the same time, the composition and distribution of the corrosion products were analyzed qualitatively and quantitatively with the help of an X-ray diffractometer (XRD) to determine the crystal structure and reveal whether the corrosion products contained specific compounds such as iron oxides, sulfates, or chlorides. By comparing the diffraction spectra of the different samples, we can determine whether the formation of the corrosion products originates from the reaction between the pipeline material and the soil chemical composition [28].
To capture spatial variations in corrosion behavior, systematic SEM surface scanning was performed on multiple regions of each steel specimen at magnifications ranging from 500× to 5000×. Field-of-view corrosion features (e.g., crater distribution, crack propagation paths, and corrosion product layer heterogeneity) were documented. Spatial distributions of major corrosion ions (Cl, SO42−) and corrosion products were quantified microscopically using multi-point analysis. This approach established correlations between local corrosion severity and regional soil ion concentrations.

2.3.3. Simulation Calculation

This study used the Materials Studio simulation platform to investigate the binding energy of the anticorrosion coating and X80 steel sheet and the penetration ability of soil ions toward the anticorrosion layer [29,30]. First of all, Materials Studio 2020 software was used to construct the model: the X80 pipeline steel cell with iron as the main component was processed by cell expansion, the molecular structure of epoxy asphalt paint was drawn and simplified by ChemDraw 21.0.0 and imported into MS software, and the molecular model was generated by simulation optimization [31].
Prior to molecular dynamics simulations, macromolecular fragments of anticorrosive materials were constructed and energy-minimized to identify the lowest-energy conformations. Geometry optimization was performed using the COMPASS III force field [32]—specifically parameterized for organic/inorganic hybrids and validated for epoxy asphalt coatings’ atomic interactions (electrostatic, van der Waals) and ion-polymer binding energies—under ultrafine convergence criteria with atomic-level electrostatic/van der Waals settings (Forcite module). The optimized structure underwent sequential annealing (273–400 K) and energy minimization to resolve conformational strain. Subsequently, an epoxy-iron bilayer cell was assembled via the ‘Build Layers’ function (Figure 2).
For environmental simulation, system density was equilibrated in the NPT ensemble (298 K, 1 atm) to replicate soil ambient pressure. The ensemble was then switched to NVT for freeze–thaw cycling (−20 °C to 20 °C) with volume constraints matching experimental conditions. Production simulations (100 ps) spanning the target freeze–thaw range (253–293 K) successfully captured chloride ion diffusion mechanisms within the coating matrix. The physical realism of ion migration was ensured through force field parameterization and thermally consistent protocol design.
Energy calculations were performed for the three structures separately.
The binding energy is calculated using the following equation:
E i n t e r a c t i o n = E t o t a l E s u r f a c e + E p o l y m e r
where Etotal is the overall energy, Esurface is the energy of steel model, and Epolymer is the energy of anticorrosive material. Through simulation, we obtained the energy values of the three structures, respectively, and the results are as follows:
Etotal = −986,853.340104 kcal/mol,
Esurface = −1,028,162.602546 kcal/mol,
Epolymer = 45,285.233362 kcal/mol
Using the formula to get the binding energy Einteraction = −3975.97092 kcal/mol, in order to more accurately measure the effect of the combination, we need to combine the energy apportioned to the unit area after conversion to get the unit area of the binding energy of −373 mJ/m2.
The binding energy between conventional metal and anticorrosion coating is usually in the range of 100~500 mJ/m2, and a negative binding energy means that there is an attractive force between the coating and the steel sheet, which is helpful for the adhesion of the coating on the surface of the steel sheet. The result that we obtained from the calculation of binding energy shows that the value of binding energy of 373 mJ/m2 is relatively high, which indicates that there is a certain bonding force between the coating and the steel sheet, the bonding strength is relatively high, and the anticorrosive effect is good.
After optimizing the cell structure of the anticorrosive material according to the above, the penetration simulation experiments were carried out for different ions, respectively [33], and different ions were imported into the cell of the anticorrosive material by using the Sorption module of MS to obtain the hybrid cell of ions and anticorrosive material, as shown in Figure 3. Then, from a number of generated hybrid cell structures, the hybrid cell structure with the lowest energy was selected for kinetic processing, and the corresponding ions were defined after obtaining the trajectory file.
Foricite analysis was used to calculate the mean square displacement of the defined ions, and the MSD data of the corresponding ions were obtained, which were fitted, the segments in which the ion shifts were more stable were selected for analysis, and the output data were used to calculate the diffusion rate of ions in the anticorrosive material [34]. The diffusion of Cl in the anticorrosive material was obvious, and the diffusion rate D was 2.095 × 10−6 cm2/s relative to the diffusivity of SO42− and NO3−, which differed significantly, with the diffusivity D of SO42− and NO3− being 1.03 × 10−7 cm2/s and 3.345 × 10−7 cm2/s.
In addition, the diffusion coefficients D of the cations in the anticorrosive materials could be obtained from the MSD of the major cations, which were D(K+) = 4.367 × 10−8 cm2/s, D(Mg2+) = 2.458 × 10−7 cm2/s, D(Ca2+) = 2.538 × 10−7 cm2/s, and D(Na+) = 3.047 × 10−7 cm2/s, respectively.
In terms of cations, the barrier properties of anticorrosive materials to different cations in saline soil are K+, Mg2+, Ca2+, and Na+ in descending order. The proportion of components with strong barrier performance to K+, Mg2+, Ca2+, etc., in the material should be increased appropriately, or substances that can form stable complexes with these ions should be added, so as to reduce the mobility of these ions in the material and improve the overall anticorrosive performance.
In terms of anions, anticorrosive materials have the strongest barrier performance against SO42− ions, NO3− the next strongest, and Cl the weakest. Researchers should pay special attention to the blocking of Cl in designing anticorrosive coatings and optimizing preventive measures against pipeline corrosion by corrosion factors in the soil.

3. Results and Discussion

3.1. Corrosion Behavior Analysis

We collected and analyzed the polarization curve data of X80 steel sheets under different numbers of freeze–thaw cycles and different soil environments in order to have a more intuitive understanding of the corrosion.
Figure 4 shows the polarization curves of an X80 steel sheet under 5, 10, and 15 freeze–thaw cycles, respectively, with the horizontal coordinate being the corrosion current I (expressed in logarithmic form) and the vertical coordinate being the corrosion potential E. With the increase in the number of freeze–thaw cycles, the shapes and positions of the polarization curves change significantly, indicating that the corrosion behavior is significantly affected by the number of freeze–thaw cycles.
The self-corrosion potential of X80 steel sheets in the Shanshan area varied from −0.916 V to −1.219 V, and the self-corrosion current densities ranged from 1563 to 5321 μA·cm−2. After 5 freeze–thaw cycles, the Icorr was 1694 μA·cm−2 and the corrosion rate was high; after 10 freeze–thaw cycles, the Icorr decreased to 1563 μA·cm−2, which slowed down the corrosion rate. After 15 freeze–thaw cycles, the Icorr increased significantly to μA·cm−2, and the corrosion rate increased greatly.
Figure 4 shows the polarization curves of X80 steel sheet after 5, 10, and 15 freeze–thaw cycles in the Ili region. The self-corrosion potential of the X80 steel sheet in the Yili region is 144 μA·cm−2 after 5 freeze–thaw cycles, and the corrosion rate is relatively high; after 10 freeze–thaw cycles, the Icorr is reduced to 142 μA·cm−2, and the corrosion rate is slowed down; and after 15 freeze–thaw cycles, the Icorr slightly increased to 143 μA·cm−2, and the corrosion rate is increased.
Observing the polarization curves of the X80 steel sheet after 5, 10, and 15 freeze–thaw cycles in the Urumqi area, we can see that the self-corrosion potential of the X80 steel sheet in the Urumqi area was 138 μA·cm−2 after 5 freeze–thaw cycles, and the corrosion rate was relatively high; after 10 freeze–thaw cycles, the Icorr decreased to 78 μA·cm−2, and the corrosion rate slowed down; and after 15 freeze–thaw cycles, the Icorr increased to 144 μA·cm−2, and the corrosion rate increased. Compared with the Yili region, the corrosion degree of X80 steel in the Urumqi region is slightly lower but still significantly lower than that in the Shanshan region.
The corrosion current density of X80 steel in the soils of all three regions showed a tendency of decreasing and then increasing, indicating that its corrosion resistance was first enhanced and then weakened. This suggests that the corrosion rate of X80 steel may be temporarily slowed down by the soil structure changes caused by freeze–thaw cycles under a moderate number of freeze–thaw cycles, but the corrosion rate will increase significantly with a further increase in the number of freeze–thaw cycles.
The polarization curves of X80 steel sheets from three regions, Shanshan, Yili, and Urumqi, were comparatively analyzed, and it can be seen from Figure 5 that the shapes of the Tafel cathodic curves of all the specimens are basically similar, which indicates that the cathodic responses of the steel sheets are approximately the same.
The comparative analysis of three different geographic regions showed significant differences in the soil-induced corrosion behavior of X80 steel. Electrochemical polarization measurements showed that Shanshan had the highest corrosion current density, exceeding that of Yili and Urumqi, and that Yili had a slightly higher corrosion current density than Urumqi. The moderate corrosion rate in Yili was attributed to calcium/magnesium carbonate precipitation forming a protective layer, while the alkaline soil and optimum calcium content in Urumqi minimized corrosion.

3.2. Micro-Morphology and Corrosion Products

Through the in-depth study of the corrosion behavior of X80 steel sheet in different soil environments under different numbers of freeze–thaw cycles, with the help of high-resolution field-emission scanning electron microscope (SEM) technology, we achieved detailed observations of the microscopic morphology characteristics of the surface of e steel sheet, shown in Figure 6. The following images show, respectively, the number of freezing and thawing cycles and the micro-morphological characteristics of an X80 steel sheet in different soil regions for analysis.
After five freeze–thaw cycles, the X80 steel sheets in the Urumqi soil and the Yili soil formed a thin and uniform oxidized layer, showing the protective properties of partial surface coverage. In contrast, the Shanshan soil showed significant localized pitting, characterized by irregularly distributed cavities, as shown in Figure 6c.
After 10 freeze–thaw cycles, the corrosion pattern of X80 steel showed regional differences. The phenomenon of thickening of the oxide layer and localized delamination appeared in Urumqi soil (Figure 6d), and Yili soil maintained structural coherence despite a similar thickening of the oxide layer (Figure 6e), indicating the persistence of barrier protection. Most seriously, the pits in the Shanshan soil gradually coalesced into interconnected corrosion channels (Figure 6f), establishing a preferential pathway for corrosive media infiltration.
After 15 freeze–thaw cycles, generalized corrosion occurred in Urumqi soils (Figure 6g), characterized by the transition from localized erosion to the large-scale destruction of the oxide layer, with significantly accelerated degradation. Yili soil corrosion was slightly better, with a thickening of corrosion products (Figure 6h), while Shanshan specimens showed the most serious corrosion, exhibiting both generalized corrosion and localized corrosion, leading to obvious surface roughening (Figure 6i).
The corrosion morphology of X80 steel sheets showed obvious differences under different numbers of freeze–thaw cycles and different soil environments. With the increase in the number of freeze–thaw cycles, the degree of corrosion on the surface of the steel sheet was gradually aggravated, and the thickness of the corrosion product layer increased, but in the Shanshan area, after a number of freeze–thaw cycles, the corrosion product layer appeared to be an obvious detachment phenomenon, which led to a significant increase in the subsequent corrosion rate.
Spatially resolved synchrotron X-ray diffraction (μ-SR-XRD) with a 1 μm micro-beam and 0.001° angular resolution (BL14B1, Shanghai Synchrotron Radiation Facility, Zhangjiang Hi-Tech Park, Shanghai, China) was employed for the phase mapping of corrosion products on X80 steel surfaces across heterogeneous soil environments. Combined with SEM microstructural characterization, Rietveld refinement (GSAS-II/Jade v10.0) identified chloride-dominated phases and interfacial SO42− enriched microzones (Figure 7), quantitatively confirming sulfate–chloride coexistence through phase fraction analysis. This microchemical partitioning demonstrates ionic synergistic effects within the corrosion matrix, where chloride ions preferentially migrate to outer layers while sulfate concentrates at metal–oxide interfaces.
The presence of iron oxides (e.g., F2O3 and Fe3O4) was mainly detected in the corrosion products in Urumqi, and a small amount of calcium-containing compounds was detected in the corrosion products in addition to iron oxides, which was related to the presence of calcium ions in the soil, and the formation of a protective precipitation layer by the combination of calcium and carbonate ions had an impact on the corrosion process to some extent.
The Ili and Urumqi area corrosion products were similar to the main iron oxides, due to the high content of calcium ions and magnesium ions in the soil; in the corrosion products, we also detected more calcium-containing and magnesium-containing compounds. These compounds and iron oxides together constitute a more complete corrosion product layer, to a certain extent, slowing down the further development of the corrosion.
The Shanshan area corrosion products are more complex; in addition to iron oxides, with different freezing and thawing cycles, we detected a higher content of chlorine and sulfur elements corresponding to compounds such as ferrous chloride, ferrous sulfate, etc. This is mainly due to the Shanshan area soil having a higher content of chlorine ions and sulfate ions; these ions are involved in the corrosion reaction, leading to corrosion products appearing in the corresponding compounds.
Through the analysis of the corrosion products of the X80 steel sheet in the soil environments of the three places, combined with its microscopic images, it can be deduced that the corrosion products in the Urumqi and Yili regions are mainly dominated by iron oxides, and compounds containing calcium and magnesium are also detected, which is presumed to be the reason why the corrosion degree is relatively low in the Urumqi and Yili regions. In addition to iron oxides, the corrosion products in the Shanshan area also contain high levels of chlorine and sulfur compounds, indicating that chlorine and sulfate ions in the soil in the Shanshan area play a dominant role in the corrosion process.

3.3. Performance Evaluation of Anticorrosion Layers

For the anticorrosion experiments in this paper, after comprehensive consideration and comparison, we chose high-economy epoxy asphalt anticorrosion paint [35,36], as it has a simple synthesis process, is commonly used and easy to obtain on the market, it can be employed by brushing and spraying with high efficiency and relatively low environmental requirements, and it is suitable for use on construction sites in remote areas such as Xinjiang [37]. Corrosion-inhibitive bitumen coatings comprise epoxy resin (30–50 wt%, bisphenol-A type), modified coal tar pitch (20–40 wt%, PAH-rich), curing agents, functional fillers (talc/mica), and additives, synergistically enhancing the barrier properties, hardness, and weathering resistance.
The surface-treated X80 specimens were coated with corrosion-inhibitive bitumen via orthogonal deposition, with the precise control of film thickness (120 ± 5 μm) and coverage uniformity. For bilayer systems, the second coating was orthogonally applied after the complete crosslinking of the initial layer under identical ambient conditions, with equivalent protocols implemented on the reverse side. This cross-coating methodology enhanced film densification, mitigated intrinsic microdefects through overlapping concealment, and increased the barrier resistance against corrosive media penetration by >40% relative to monolayer coatings.
We prepared three groups of steel sheets coated with one and two layers of different thicknesses of epoxy asphalt anticorrosion coating and placed them in a constant temperature and humidity test chamber for freeze–thaw cycle experiments [38,39]. Due to the presence of the anticorrosion coating, we increased the number of freeze–thaw cycles to 30; the time of each freeze–thaw cycle was still 24 h [40].
After the freeze–thaw experiment, we performed morphological observation and image acquisition on the surface of the steel sheet, as shown in Figure 8, to observe the integrity of the anticorrosion coating and evaluate its anticorrosion effect. At the same time, we also carried out polarization curve tests to further evaluate the protective effect of the anticorrosion coating after freeze–thaw cycles and to reveal the protective mechanism behind the anticorrosion coating and the cause of failure.
After 30 freeze–thaw cycle tests, the anticorrosion coatings of X80 steel sheets in the soil of the Shanshan area showed significant differences (Figure 8a,b). The single-layer asphalt coating showed interfacial cracks and localized detachment, indicating that the freeze–thaw effect weakened the adhesion of the coating, while the overall integrity of the double-layer coating was well maintained, with only localized breakage, confirming that increasing the thickness of the coating can effectively block the penetration of corrosive media.
The results of the tests in Urumqi showed better corrosion protection (Figure 8c,d). The single-layer coating only produces micro-cracks and slight peeling; the double-layer coating is nearly intact, showing that the soil environment in this region has a weaker destructive effect on the coating and the double-layer coating can form a more perfect protective barrier.
The degree of corrosion in the Ili region is between those in the above two places (Figure 8e,f). The single-layer coating causes corrosion holes and the damage expansion phenomenon, and corrosion products, through pore infiltration, exacerbate local corrosion; with the double-layer coating, although there are sporadic defects, the overall protective performance is significantly improved.
In order to further verify the anticorrosion effect of the coating, we coated the steel with one layer and two layers of anticorrosion asphalt paint in different soil environments. We performed an X80 steel sheet polarization curve test, and data collection was completed after fitting; the polarization curve is shown in Figure 9.
From the polarization curves, it can be seen that using a double-layer anticorrosive asphalt paint coating rather than a single-layer coating leads to a significant reduction in corrosive media, such as moisture, Cl, and O2, significantly reducing the penetration path. Moreover, increasing the thickness of the anticorrosive coating for the corrosive components of the soil enhances its role as a barrier.

4. Conclusions

The purpose of this paper was to investigate the corrosion mechanisms behind oil pipelines in typical freeze–thaw saline–alkaline soil environments in Xinjiang in depth using experiments, tests, and simulations and to suggest practical preventive and curative measures for freeze–thaw cycles and saline–alkaline soil corrosion in buried oil pipelines based on the results of anticorrosion experiments and software simulations. The main conclusions of this paper are as follows:
(1) The pH value, major ions, and salt content of the soils in Urumqi, Shanshan, and Yili in Xinjiang were measured. The results showed that the soils in the three places were alkaline, and Yili had the highest pH value. The contents of alkaline cations and corrosive ions (chloride ions and sulfate ions) of the soils varied significantly in different areas. The soil in the Shanshan area contains more calcium, sodium, chloride, and sulfate ions and is more corrosive; the corrosive ion content of the soil in the Urumqi area is lower, and the risk of corrosion is lower.
(2) With the increase in the number of freeze–thaw cycles, the corrosion current density (Icorr) of X80 steel in the soil environments of Urumqi, Shanshan, and Yili showed a trend of decreasing and then increasing, indicating that its corrosion resistance was first enhanced and then weakened. For the three X80 steel conditions, the corrosion current density change rule is similar, and all show the trend that, according to the number of moderate freeze–thaw cycles, soil structure changes may temporarily slow down the corrosion rate, but then the corrosion rate will rise significantly.
(3) In Xinjiang, the chemical and physical properties of the soil causing X80 steel corrosion behavior show significant regional differences. In the Shanshan area, due to there being a high content of chloride ions and sulfate ions in the soil, the corrosion current density and the corrosion rate are the highest; the Ili area corrosion rate comes second, higher than that in the Urumqi area, with the strong penetration of chloride ions and sulfate ions causing crystallization and expansion in the X80 steel, which is the main reason behind the rapid corrosion. The Urumqi area soil pH value is high, and there is a moderate calcium ion content and the lowest corrosion rate. This indicates that calcium ions have a positive effect in slowing down corrosion in an alkaline environment.
(4) The corrosion morphology of an X80 steel sheet in different freeze–thaw cycles and soil environments varies significantly. With the increase in the number of freeze–thaw cycles, the degree of corrosion of the steel sheet increases and the corrosion product layer thickens. In the Shanshan area, after a number of freeze–thaw cycles, the corrosion product layer is obviously detached and the corrosion rate rises sharply. The corrosion products in the Urumqi and Yili areas are mainly iron oxides, while the Shanshan area contains a large number of chlorine-containing and sulfur-containing compounds, indicating that chloride ions and sulfate ions play a dominant role in corrosion.
(5) The effect of anticorrosion asphalt paint on the corrosion of an X80 steel sheet is remarkable. In the Urumqi area soil, a single layer of coating can effectively slow down the corrosion rate; in the Shanshan and Yili areas, due to the soil corrosivity, the double-layer coating anticorrosion effect is significantly greater than that achieved with a single layer of coating, indicating that increasing the thickness of the coating can enhance the corrosion performance. The freeze–thaw cycle of mechanical stress caused by the coating and the substrate bonding force decreasing is the main cause of coating failure, as microscopic cracks are easily formed in the coating surface, the coating, and the steel substrate interface, leading to shedding and corrosive medium penetration, creating channels.
(6) We used simulation software to calculate the binding energy and ion penetration ability; the results show that the coating and the steel plate binding force is strong and can be closely attached to the steel surface, and the anticorrosion effect is good. Through simulations, we also found that there were differences in the barrier properties of anticorrosive materials to different ions, which can provide a reference for practical applications, help optimize the coating formulation or treatment process, improve the binding energy of the coating and steel, and enhance the barrier in terms of specific ions, coating adhesion, and corrosion resistance, thus enhancing the performance and service life of the anticorrosive system.
(7) The established mechanistic framework demonstrates that freeze–thaw cycles drive corrosion via dual-phase mechanisms: initial protective oxide formation followed by microcrack propagation with chloride accumulation, with sulfate–chloride synergy governing localized corrosion in heterogeneous soils. This framework extends to diverse pipeline materials (e.g., X65/X100 steels) and cold-region saline environments (e.g., the Qinghai–Tibet Plateau). Key implementation strategies include (1) predicting corrosion kinetics transitions by quantifying material-specific electrochemical responses to freeze–thaw stress and (2) optimizing coatings using region-specific ionic ratios (Cl/SO42−) and thermal cycling parameters. Molecular dynamics-informed design—enhancing coating densification (>200 μm) and incorporating ion-blocking components to counter preferential Cl penetration—enables 62%–89% corrosion inhibition. This approach provides a foundation for cross-material, cross-environmental corrosion protection strategies in critical energy infrastructure.

Author Contributions

Methodology, G.C. and Y.W.; Software, Y.D.; Validation, S.Z.; Formal analysis, B.W.; Data curation, C.X.; Writing—original draft, Y.W.; Writing—review and editing, G.C.; Visualization, X.Z.; Supervision, G.C.; Project administration, G.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by Introduction Plan for “Tianchi Talent” in Xinjiang Uygur Autonomous Region, Project Number: 51052401536.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All relevant data are contained within the paper, and the data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. X80 steel sample.
Figure 1. X80 steel sample.
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Figure 2. Binding of crystalline cells.
Figure 2. Binding of crystalline cells.
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Figure 3. Mixed cell structure of multiple ions.
Figure 3. Mixed cell structure of multiple ions.
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Figure 4. Polarization curves of X80 steel sheets with different numbers of freeze–thaw cycles.
Figure 4. Polarization curves of X80 steel sheets with different numbers of freeze–thaw cycles.
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Figure 5. Comparison of polarization curves of X80 steel under different conditions.
Figure 5. Comparison of polarization curves of X80 steel under different conditions.
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Figure 6. Surface microscopic morphology of X80 steel sheet with different numbers of freeze–thaw cycles. (a,d,g) Urumqi; (b,e,h) Yili; (c,f,i) Shanshan.
Figure 6. Surface microscopic morphology of X80 steel sheet with different numbers of freeze–thaw cycles. (a,d,g) Urumqi; (b,e,h) Yili; (c,f,i) Shanshan.
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Figure 7. XRD patterns of corrosion products of steel sheets in different regions.
Figure 7. XRD patterns of corrosion products of steel sheets in different regions.
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Figure 8. Morphological characteristics of X80 steel surface under different conditions. (a,b) Shanshan; (c,d) Urumqi; (e,f) Yili.
Figure 8. Morphological characteristics of X80 steel surface under different conditions. (a,b) Shanshan; (c,d) Urumqi; (e,f) Yili.
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Figure 9. Morphological characteristics of X80 steel surface in different conditions.
Figure 9. Morphological characteristics of X80 steel surface in different conditions.
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Table 1. X80 steel main chemical composition and content.
Table 1. X80 steel main chemical composition and content.
ElementFeMnSiMoCOther
Content97.39%1.83%0.28%0.22%0.063%<0.22%
Table 2. Soil pH test results.
Table 2. Soil pH test results.
Serial NumberSoil SamplespH Value
1Urumqi7.58
2Shanshan7.66
3Yili7.80
Table 3. ICP test results.
Table 3. ICP test results.
Serial NumberSoil SamplesK+Ca2+Na+Mg2+
g/kgg/kgg/kgg/kg
1Urumqi0.0170.0570.0740.892
2Shanshan0.0450.4560.0330.756
3Yili0.0740.1060.0800.274
Table 4. Ion Chromatograph analysis results.
Table 4. Ion Chromatograph analysis results.
Serial NumberSoil SamplesClSO42−NO3−
g/kgg/kgg/kg
1Urumqi0.011.130.03
2Shanshan0.571.280.17
3Yili0.060.470.17
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MDPI and ACS Style

Cheng, G.; Wang, Y.; Dai, Y.; Zhang, S.; Wei, B.; Xiao, C.; Zhang, X. Mechanistic Insights into Corrosion and Protective Coating Performance of X80 Pipeline Steel in Xinjiang’s Cyclic Freeze–Thaw Saline Soil Environments. Coatings 2025, 15, 881. https://doi.org/10.3390/coatings15080881

AMA Style

Cheng G, Wang Y, Dai Y, Zhang S, Wei B, Xiao C, Zhang X. Mechanistic Insights into Corrosion and Protective Coating Performance of X80 Pipeline Steel in Xinjiang’s Cyclic Freeze–Thaw Saline Soil Environments. Coatings. 2025; 15(8):881. https://doi.org/10.3390/coatings15080881

Chicago/Turabian Style

Cheng, Gang, Yuqi Wang, Yiming Dai, Shiyi Zhang, Bin Wei, Chang Xiao, and Xian Zhang. 2025. "Mechanistic Insights into Corrosion and Protective Coating Performance of X80 Pipeline Steel in Xinjiang’s Cyclic Freeze–Thaw Saline Soil Environments" Coatings 15, no. 8: 881. https://doi.org/10.3390/coatings15080881

APA Style

Cheng, G., Wang, Y., Dai, Y., Zhang, S., Wei, B., Xiao, C., & Zhang, X. (2025). Mechanistic Insights into Corrosion and Protective Coating Performance of X80 Pipeline Steel in Xinjiang’s Cyclic Freeze–Thaw Saline Soil Environments. Coatings, 15(8), 881. https://doi.org/10.3390/coatings15080881

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