With the development of polar resource exploration and polar shipping, significant attention has been directed toward materials for polar equipment and protective technologies against damage in extreme environments. To address the corrosion protection needs of marine engineering steel and the evaluation of stainless steel performance under low-temperature conditions, laser cladding technology was employed to fabricate austenitic stainless steel 316L and duplex stainless steel 2205 coatings on the surface of FH690 steel. These coatings underwent a one-year exposure test in the atmospheric environment of the Zhongshan Station in Antarctica. The results revealed that the stainless steel coatings effectively reduced the corrosion rate of the marine steel substrate. The microstructure, microhardness, tribological behavior, electrochemical corrosion behavior, and stability of the samples under polar low-temperature conditions were analyzed. Findings indicated that the 316L coating exhibited minor pitting corrosion, while the 2205 coating showed slight selective corrosion. Both coatings maintained their pre-exposure levels of microhardness and wear resistance, with a slight reduction in corrosion resistance. The laser-clad stainless steel coatings demonstrated stability in phase structure and performance in the Antarctic atmospheric exposure environment, providing effective protection to the low-temperature steel substrate. These results offer valuable support for assessing the environmental adaptability of materials used in polar equipment and advancing corrosion-resistant coating technologies.
In recent years, with global warming, resource scarcity, and environmental changes, the exploration of polar resources, the advancement of polar shipping, and the safeguarding of polar interests have garnered increasing attention from nations worldwide. Research has established that the Arctic region contains approximately 30% of the world’s undeveloped natural gas and 13% of its undeveloped oil reserves, while Antarctica hosts the world’s largest coalfield, located beneath the ice cap of East Antarctica, with an estimated reserve of about 500 billion tons. In the processes of exploration, development, and preservation of polar regions, the operational performance of high-performance polar equipment such as icebreakers, offshore platforms, and terrestrial stations is of critical importance. However, the polar environment is complex and harsh, with average annual temperatures of approximately -22.3°C in the Arctic and between -28.9°C and -35°C across the Antarctic continent. Only 1 to 4 months of the year experience monthly average temperatures between 0°C and 10°C, with extreme weather conditions lowering service temperatures to as low as -70°C. Coupled with dry gales, intense ultraviolet radiation, freeze-thaw cycles, and stormy snowfalls, polar equipment is subjected to prolonged and severe corrosion damage from low-temperature atmospheric exposure. For moving components in icebreakers, drilling rigs, and storage systems, additional damage from stress and wear loads must also be considered. Consequently, the environmental adaptability of materials for polar equipment has long been a focal point of extensive scholarly research.
Currently, metallic materials for polar equipment primarily consist of low-temperature steels, which are high-performance steels designed to exhibit excellent toughness and weldability at low temperatures. These typically include ferrite-based low-alloy steels and Fe-Cr-Ni austenitic stainless steels. Low-alloy low-temperature steels are widely utilized due to their cost-effectiveness and are commonly produced using the thermomechanical control process (TMCP), which enhances strength, toughness, weldability, and reduces carbon content. Wang Chaoyi et al. conducted welding experiments using submerged arc welding on 54 mm thick 460 MPa grade low-temperature steel for polar ships produced via TMCP. They found that at an extreme low temperature of -70°C, samples from the heat-affected zone with a single bainitic microstructure exhibited brittle fracture, whereas the base material with a dual-phase ferrite-bainite microstructure demonstrated higher fracture strength and greater resistance to crack propagation. Sun Shibin et al. investigated the tribological behavior of TMCP FH36 marine steel plates of varying thicknesses at 20°C, -5°C, and -20°C. Their findings revealed that the surface microstructure consisted primarily of ferrite and pearlite, while the mid-thickness region featured ferrite, pearlite, and granular bainite. The microstructure directly influenced hardness and wear resistance, with abrasive wear as the dominant mechanism, accompanied by fatigue and adhesive wear. As temperature decreased, localized surface hardness increased, but material detachment due to friction exacerbated wear, resulting in wider and deeper wear tracks and increased wear volume. Li et al. studied the early corrosion behavior of EH36 low-temperature steel in a simulated polar marine atmospheric environment, noting that corrosion remained in an accelerated phase at low temperatures, with a rate of 0.47 g·m⁻²·h⁻¹. High-strength FH690 low-temperature steel offers excellent low-temperature mechanical properties; however, in environments with coupled wear-corrosion damage, loose and porous corrosion products fail to resist frictional shear forces, and galvanic corrosion between the exposed substrate and wear products further accelerates degradation. The microstructure of low-alloy low-temperature steels is susceptible to changes induced by heat and mechanical forces, leading to instability in mechanical and wear properties. Additionally, the absence of passivating elements results in rapid corrosion in marine Cl⁻ environments, significantly reducing service life under coupled wear-corrosion conditions.
Material damage, such as wear and corrosion, typically initiates at the surface. By employing high-energy beam cladding technologies to fabricate high-performance coatings with integrated resistance to low-temperature wear and corrosion on the surface of tough, low-temperature marine engineering steel, significant improvements in the service performance of engineering equipment in extreme polar environments can be achieved. Coatings prepared via laser cladding on EH32 marine steel substrates exhibited superior hardness and wear resistance compared to the substrate after low-temperature freeze-corrosion testing at -80°C. The selection of appropriate high-performance coating materials is critical to enhancing the service life of marine steel. Stainless steel, with its excellent corrosion resistance, addresses the lack of passivating elements in low-temperature marine steel and, as an iron-based alloy, ensures robust metallurgical bonding during the cladding process. Austenitic stainless steel lacks a ductile-brittle transition at low temperatures, offering exceptional impact toughness and corrosion resistance. Duplex stainless steel provides higher strength and improved wear resistance, with controlled precipitation of secondary phases preserving good toughness. The irregular variability of polar climates complicates the simulation of atmospheric exposure corrosion tests, making long-term field atmospheric exposure in polar regions the most reliable evaluation method.
This study addresses the material requirements for polar engineering equipment and the need for protection against damage in extreme environments. Laser cladding technology was utilized to fabricate austenitic stainless steel 316L and duplex stainless steel 2205 coatings on the surface of FH690 steel, followed by exposure testing in the atmospheric environment of the Zhongshan Station in Antarctica. The microhardness, tribological behavior, electrochemical corrosion behavior, and stability of the samples under polar low-temperature conditions were analyzed to provide insights into the environmental adaptability and corrosion protection of polar equipment materials. The protective efficacy of the 316L and 2205 laser-clad coatings on FH690 steel in the Antarctic atmospheric exposure environment was investigated.
Experimental Preparation
1.1 Coating Preparation and Antarctic Exposure Conditions
The substrate material used in this experiment was FH690 steel with dimensions of 100 mm × 25 mm × 10 mm. The surface was first polished with 1500-grit sandpaper to achieve uniform scratches, followed by ultrasonic cleaning with anhydrous ethanol to remove surface impurities and oil, and then dried for subsequent use. Stainless steel alloy powders of 316L and 2205, with particle sizes ranging from 48 to 74 μm, were selected as coating materials and dried in a vacuum environment at 50°C for 24 hours prior to cladding.
The alloy powders were uniformly applied to the substrate surface using the preset powder method, with a coating thickness of approximately 2 mm and a planar dimension of 50 mm × 25 mm. A fiber-coupled semiconductor laser (RECI Laser, DAC4000) with a maximum output power of 4 kW was used for cladding. The cladding parameters were as follows: laser power of 1.6 kW, spot diameter of 2 mm, scanning speed of 800 mm/min, overlap rate of 25%, and argon atmosphere protection. After cladding, the coatings were polished with 1500-grit sandpaper to match the substrate’s condition, holes were drilled at specific locations for sample assembly, and the initial state of the samples was photographed and weighed.
The fixation of samples for atmospheric exposure in Antarctica followed the GB/T 14165-2008 standard, with the sample surface positioned at a 45° angle to the horizontal plane, as shown in Figure 1. The samples were deployed at the Zhongshan Station in Antarctica for a test duration of 1 year (December 2022 to December 2023). Upon retrieval, the samples were photographed, and those with corrosion products were immersed in a rust removal solution containing 100 mL HCl, 100 mL deionized water, and 0.3 g hexamethylenetetramine for ultrasonic cleaning. The samples were then rinsed with alcohol, dried, photographed, and weighed. Wire electrical discharge machining was used to process the samples into smaller specimens with a surface area of 10 mm × 10 mm for subsequent testing.
1.2 Sample Characterization and Performance Testing Before and After Antarctic Exposure
The coatings before and after Antarctic atmospheric exposure were characterized for morphology, composition, and phase structure using a scanning electron microscope (SEM, ZEISS Gemini300), X-ray energy dispersive spectrometer (EDS, Oxford INCA 80), X-ray diffractometer (XRD, Bruker D8 Advance), and confocal laser scanning microscope (CLSM, Keyence VK-X250).
Microhardness was measured using a Vickers microhardness tester (Veiyee QHV-1000SPTA) at 20 randomly selected points on the coating surface, with an applied load of 200 g and a dwell time of 15 s. The average of 20 measurements was taken as the surface hardness of the coating. The linear dry sliding tribological behavior of the coatings was evaluated using a multifunctional friction and wear testing machine (Rtec MFT-5000) with an applied normal force of 10 N, a wear duration of 1800 s, a reciprocating distance of 3 mm, and a SiN ceramic ball (6.35 mm diameter) as the counterface. Wear tracks were analyzed using a three-dimensional morphometer (Bruker Contour GT-K). Corrosion behavior at 10 ± 0.1°C was assessed using an electrochemical workstation (Gamry Reference 3000) in a 3.5 wt.% NaCl solution with a three-electrode system: a platinum wire as the counter electrode, an Ag/AgCl electrode as the reference electrode, and the coating as the working electrode, encapsulated in epoxy resin to expose a 10 mm × 10 mm working area. Open circuit potential (OCP) testing was conducted for 1800 s at a sampling frequency of 0.5 s⁻¹, followed by electrochemical impedance spectroscopy (EIS) at OCP with a frequency range of 100 kHz to 10 mHz. Potentiodynamic polarization was performed at a scan rate of 1 mV·s⁻¹, starting from an initial potential of -0.3 V relative to OCP and ending when the anodic polarization current density reached 1 mA·cm⁻², yielding the Tafel polarization curve. Each tribological and electrochemical test was repeated at least three times to ensure accuracy.
2 Results and Discussion
2.1 Morphology and Mass Loss Analysis
The microscopic morphology of the coatings after preparation is shown in Figure 2. Both coatings achieved satisfactory metallurgical bonding with the substrate, exhibiting uniform and dense structures without defects such as cracks, pores, inclusions, or lack of fusion at the interface. The compositional analysis of key elements in the coatings is presented in Table 1. Cr and Mo, critical pitting-resistant elements in stainless steel, form a dense passivation film in corrosive environments, while Ni is the primary austenite-stabilizing element. Laser cladding, while achieving metallurgical bonding between the coating and substrate, introduces some dilution, with elements from the substrate migrating into the coating, resulting in slightly lower Cr and Ni contents compared to the nominal compositions of the two stainless steels.
Figure 3 illustrates the macroscopic morphology of the two stainless steel coatings in their initial state, after 1 year of exposure at the Zhongshan Station in Antarctica, and after rust removal. In their initial state, the FH690 steel substrate, 316L coating, and 2205 coating exhibited a bright metallic luster (Figures 3a, 3d) with excellent surface characteristics. After 1 year of exposure at the Zhongshan Station, the coatings remained well-bonded to the substrate without cracking or delamination. The FH690 steel substrate underwent corrosion, reacting with oxygen to form a uniform, loose oxide layer, transitioning from a metallic luster to a brownish hue (Figures 3b, 3e). The primary corrosion products of FH690 steel in a marine atmospheric environment include α-FeOOH, β-FeOOH, and Fe₃O₄. Positioned at a 45° angle to the ground, rainfall and snowfall in Antarctica caused corrosion products from the FH690 substrate to flow onto the coatings, turning some areas grayish-brown. After rust removal, the grayish-brown corrosion products on the coating surfaces disappeared, and the surface morphology of the 316L and 2205 coatings showed minimal deviation from their initial state (Figures 3c, 3f), indicating effective protection of the FH690 substrate.
The microscopic features of low-alloy steel after corrosion in the Antarctic atmospheric environment have been reported, typically forming blocky, lamellar, or petal-like corrosion products accompanied by cracks and pitting features. The microscopic morphology of the two stainless steel coatings after 1 year of atmospheric exposure at the Zhongshan Station is shown in Figure 4. The 316L coating surface exhibited numerous pitting holes, with negligible differences in metal element content inside and outside the pits, though the oxygen content was higher on the pit walls. Stainless steel relies on easily passivating elements such as Cr and Mo to form a dense oxide film to resist Cl⁻ corrosion; higher oxygen content indicates a denser passivation film, with areas of lower passivation film content being preferentially corroded. The 2205 coating surface displayed selective corrosion characteristics, with the austenite regions (B2) of lower Cr content corroding preferentially, while the ferrite regions (B1) with higher Cr content exhibited higher oxygen levels and superior passivation film quality.
The laser confocal morphology of the two stainless steel coatings after 1 year of atmospheric exposure at the Zhongshan Station in Antarctica is shown in Figure 5. The 316L coating exhibited numerous small pitting corrosion sites, with some small pits aggregating and coalescing into larger pits, the deepest reaching 12.89 μm. In contrast, the 2205 coating showed no pitting corrosion features, primarily undergoing slight selective corrosion, with its microscopic morphology reflecting the characteristic dual-phase structure of duplex stainless steel.
Phase analysis of the two stainless steel coatings in their initial state and after 1 year of atmospheric exposure at the Zhongshan Station (Figure 6) revealed that the 316L and 2205 coatings maintained a stable single-phase austenitic structure and a dual-phase austenitic-ferritic structure, respectively, both before and after exposure. The coating surfaces experienced only minor corrosion without significant accumulation of corrosion products. Given that the thickness of the passivation film typically does not exceed 10 nm, no additional diffraction peaks were detected. The laser-clad 316L and 2205 coatings demonstrated phase stability in the Antarctic atmospheric exposure environment.
Based on the above results, the corrosion products observed on the samples originated from the substrate, while the coatings themselves exhibited no significant changes. The mass loss method was employed to investigate the corrosion rate of the samples and evaluate the protective efficacy of the stainless steel coatings. In atmospheric exposure corrosion studies, the corrosion mass loss and corrosion rate of metallic materials are calculated using the following equations: where ω represents the corrosion mass loss per unit area (g/m²), ν denotes the corrosion rate (mm/a), m_t is the mass of the sample after rust removal (g), m_0 is the mass of the sample before exposure (g), S is the surface area of the sample (cm²), ρ is the density of the low-alloy steel (approximately 7.86 g/cm³), and t is the exposure time (h).
The calculated mass loss and average corrosion rate of FH690 steel under the protection of the two coatings are presented in Figure 7. Under the 316L coating, the mass loss of FH690 steel was 12.5 mg·cm⁻², with an average corrosion rate of 15.9 μm·a⁻¹; under the 2205 coating, the mass loss was 12.8 mg·cm⁻², with an average corrosion rate of 16.3 μm·a⁻¹. Both coatings exhibited negligible corrosion in the Antarctic atmospheric environment, providing effective protection to the FH690 steel substrate. The average corrosion rates under the two coatings were nearly identical, with all mass loss attributed to the exposed substrate. Compared to the corrosion rate of unprotected 690 MPa-grade marine steel in the Antarctic atmosphere (18.7 μm·a⁻¹), a significant reduction was achieved.
2.2 Microhardness
Figure 8 illustrates the average microhardness of the two stainless steel coating surfaces. The initial microhardness values of the 316L and 2205 coatings were 279.19 HV₀.₂ and 392.77 HV₀.₂, respectively. Typically, the microhardness of cast 316L does not exceed 200 HV₀.₂, while that of cast 2205 is approximately 300 HV₀.₂. The higher hardness of the laser-clad coatings can be attributed to two factors: first, the rapid cooling during laser cladding results in dendritic and fine equiaxed grain structures, contributing to grain refinement strengthening; second, the metallurgical bonding between the substrate and coating allows elements from the FH690 steel to mix into the stainless steel coatings, enhancing hardness. This is supported by EDS results (Table 1), which indicate dilution of Fe, reducing the content of other elements. After 1 year of atmospheric exposure at the Zhongshan Station, the microhardness of the coatings remained virtually unchanged, demonstrating excellent environmental adaptability.
2.3 Tribological Behavior
Figure 9 presents the tribological behavior of the two stainless steel coatings before and after Antarctic atmospheric exposure. Under dry sliding friction conditions, the coefficient of friction (COF) stabilized after approximately 300 s, reaching a steady value of about 0.7. After 1 year of atmospheric exposure at the Zhongshan Station, the COF of the 316L coating slightly decreased compared to its initial state, while that of the 2205 coating remained unchanged. The wear volume loss of both coatings remained consistent before and after exposure, with the 2205 coating exhibiting lower wear volume than the 316L coating. The wear track profiles of the 2205 coating were shallower than those of the 316L coating, indicating superior wear resistance. The 316L coating showed pronounced ridges at the wear track edges, resulting from plastic deformation under the sliding ball’s pressure. The wear rate (μ) of the coatings was calculated using the Archard equation: where V is the measured wear volume loss (mm³), N is the normal load (N), and d is the total sliding distance (m).
The calculated results, shown in Figure 9d, indicate that the wear rates of the 316L and 2205 coatings were approximately 8.35 × 10⁻⁶ mm³·N⁻¹·m⁻¹ and 7.85 × 10⁻⁶ mm³·N⁻¹·m⁻¹, respectively. After Antarctic atmospheric exposure, the wear rates of both coatings remained at their pre-exposure levels, demonstrating stable wear resistance.
Figure 10 depicts the wear track morphology of the two stainless steel coatings after 1 year of exposure at the Zhongshan Station, with EDS point scan results provided in Table 2. The wear track width of the 316L coating was 565.72 μm, while that of the 2205 coating was 495.71 μm, consistent with the greater mass loss observed for the 316L coating. Morphologically, both coatings exhibited ploughing grooves and transfer layers in the wear tracks, indicating the occurrence of abrasive and adhesive wear. The 316L coating showed a higher prevalence of transfer layers, with adhesive wear being more prominent, whereas the 2205 coating displayed more pronounced ploughing grooves, suggesting abrasive wear as the dominant mechanism. The transfer layers exhibited extremely high oxygen content, attributed to frictional heat during reciprocating wear promoting the oxidation of passivating elements such as Cr and Mo.
2.4 Electrochemical Corrosion Behavior
Figure 11 shows the potentiodynamic polarization curves of the two stainless steel coatings, with electrochemical corrosion parameters listed in Table 3. After 1 year of atmospheric exposure at the Zhongshan Station, the potentiodynamic polarization curve of the 316L coating showed minimal change in trend, though the pitting breakdown potential (E_b, initial 536.8 mV, post-exposure 503.7 mV) shifted slightly earlier, and the passive current density (i_p) doubled. The passivation interval (ΔE) of the 2205 coating remained approximately 1300 mV, but the i_p increased from 2.455 μA·cm⁻² to 4.177 μA·cm⁻² post-exposure. After exposure, the corrosion resistance of both the 316L and 2205 coatings declined to varying degrees, attributed to surface defects induced by the corrosive Antarctic atmosphere.
Figure 12 presents the electrochemical impedance spectroscopy (EIS) results for the two stainless steel coatings. After 1 year of atmospheric exposure, the Nyquist plots (Figure 12a) of the 316L and 2205 coatings showed reduced capacitive arc radii, indicating a decrease in charge transfer resistance and passivation film stability. In the Bode plots (Figure 12b), the impedance modulus (|Z|) at 0.1 Hz, which typically reflects the polarization resistance of the material in the solution, decreased post-exposure for both coatings, signifying reduced corrosion resistance. Additionally, a larger phase angle and broader range in the mid-frequency region indicate greater passivation film stability. After exposure, the mid-frequency phase angle of the 316L coating narrowed and diminished, while that of the 2205 coating also decreased, reflecting a decline in passivation film quality. Given the presence of two time constants in the corrosion process, a double-layer model (inset in Figure 12a) was used to fit the data, as shown in Table 4. The impedance of the porous outer layer (R_p) was significantly lower than that of the inner layer (R_c), indicating that the electrode reaction resistance of the coatings was primarily governed by the charge transfer step. Post-exposure, the R_c of both coatings decreased. Despite a slight reduction in corrosion resistance after Antarctic atmospheric exposure, the laser-clad coatings maintained a stable passivation state and low corrosion rate, continuing to provide effective protection to the low-temperature marine steel.
3 Conclusion
In this paper, 316L austenitic stainless steel and 2205 duplex stainless steel coatings were prepared on the low-temperature marine steel FH690 substrate by laser cladding technology. The coatings were exposed to the atmosphere for 1 year at Zhongshan Station in Antarctica. The protective effect, microstructure, hardness, friction and wear, and electrochemical corrosion behavior of the two coatings were analyzed. The results are as follows:
(1) Slight pitting occurred on the surface of the 316L coating, and slight selective corrosion occurred on the surface of the 2205 coating. Both stainless steel coatings can maintain a stable phase structure, which plays a good protective role on the FH690 steel substrate and reduces the atmospheric corrosion rate of the substrate.
(2) The microhardness of the two coatings hardly changed; the friction coefficient was stable at about 0.7, and the wear rates of 316L and 2205 coatings were maintained at about 8.35 and 7.85×10-6 mm3·N-1·m-1, respectively; the 316L coating was mainly subjected to adhesive wear, while the 2205 coating was mainly subjected to abrasive wear. The two coatings were able to maintain stable mechanical and wear resistance before and after Antarctic exposure.
(3) A small amount of corrosion defects were generated on the surface of the two coatings, resulting in an increase in the passive current density, an early breakdown potential of the 316L coating, and a decrease in the passivation film impedance of the two coatings, but they were still able to maintain a good passivation effect and a low corrosion rate.
