Effect of Ni content on microstructure, microhardness and corrosion resistance of laser melted FeCrNi fused cladding layerABSTRACT
To examine the effect of different Ni content on the microstructure, microhardness, and corrosion resistance of FeCrNi fused claddings, optical microscopy, scanning electron microscopy, energy-dispersive spectroscopy, hardness testing, electrochemical measurements, and X-ray photoelectron spectrometer were conducted. At a Ni mass fraction of 5%, the microstructure consists of martensite, austenite, and a eutectic of martensite, austenite, and carbides. Increasing the Ni content to 10% results in a microstructure composed of austenite and a eutectic of austenite and carbides. At 15% Ni, the fused cladding exhibits a single-phase austenitic structure. All three fused cladding layers show lower hardness at the top surface, which is associated with grain coarsening. The average hardness is 468.8 HV0.2 at 5% Ni, increases to 529.28 HV0.2 at 10% Ni, and decreases to 399.55 HV0.2 at 15% Ni. The fused cladding containing 10% Ni exhibits the highest polarisation resistance, the lowest corrosion current density, and the best corrosion resistance.
Keywords:
FeCrNi fusion cladding; Microstructure; Microhardness; Corrosion resistance
1. INTRODUCTION
45 steel is a high-quality carbon (C) structural steel characterized by favorable mechanical properties, effective hot and cold machining performance, and high cost-effectiveness, making it the most widely utilized C structural steel in industrial applications. However, the surface hardness, wear resistance, and corrosion resistance of 45 steel are insufficient, rendering it inadequate for harsh service conditions. Therefore, surface modification technologies are employed to enhance the surface properties of 45 steel. Laser cladding technology, recognized as an advanced surface modification technique, can fulfill the requirements for high strength, hardness, corrosion resistance, and oxidation resistance at the surface of components, while also maintaining high toughness and economic feasibility of the substrate. This technology provides advantages in prolonging the lifespan of the substrate and reducing production costs. Laser cladding is characterized by concentrated heat application, rapid cooling rates, fine cladding grain structures, a robust metallurgical bond between the cladding and substrate, flexible processing, and a diverse range of cladding materials to accommodate various surface performance requirements of components [1,2,3,4]. Laser cladding technology can be utilized for the preparation of integral composite coatings on component surfaces as well as for the modification, strengthening, and repair of localized material surfaces. Considering issues such as the bond between the cladding layer and the substrate as well as cracking, iron-based alloys are typically clad onto iron-based alloy surfaces. This is because the composition of the cladding layer is similar to that of the substrate material, resulting in a stronger bond at the interface. Additionally, their coefficients of thermal expansion are comparable, making cracking and spalling less likely to occur. The chemical composition of the cladding powder influences the microstructure of the cladding layer, which, in turn, affects its properties. The incorporation of various elements into the cladding powder significantly impacts the microstructure and properties of the cladding layer. For ferrous alloys, the addition of elements such as Cr, Mo, and W promotes the formation of the ferrite phase, while elements such as C, N, Ni, and Mn facilitate the formation of the austenite phase. The ferrite phase is characterized by low hardness, good atmospheric corrosion resistance, resistance to nitric acid and brine, and high-temperature oxidation resistance. In contrast, the austenite phase exhibits medium hardness, excellent corrosion resistance, plasticity, weldability, and low-temperature toughness, and can be readily strengthened through work hardening. The martensite phase, formed from austenite after quenching, possesses high hardness, elevated C content, and lower corrosion resistance. The differing properties of materials can be attributed to their constituent phases and phase compositions [5,6,7,8,9,10]. The addition of Cr to steel has been demonstrated to enhance the material’s corrosion resistance. This element possesses passivation properties that promote the formation of a passivation film on the surface of stainless steel, thereby preventing further corrosion [11,12]. The varying amounts of Cr added to materials yield different corrosion resistance properties. According to Tamman’s law, an incremental increase in the ratio of the atomic number of the corrosion-resistant element Cr to the total atomic number of the alloy, at a stepwise ratio of n/8 (where n = 1, 2, 3, … 7), results in a corresponding increase in the material’s electrode potential, thereby enhancing its corrosion resistance. Furthermore, Cr acts as a ferrite-forming element, which can improve the mechanical properties of the material. Additionally, Cr can form alloy carbides with elements such as C, boron (B), and iron (Fe), resulting in improved hardness and wear resistance. Ni serves as an austenite-forming element that can induce a transformation in the crystal structure of steel from body-centered cubic (bcc) to face-centered cubic (fcc), thereby enhancing the plasticity, toughness, and weldability of steel while mitigating the risk of brittle fracture [13]. In duplex stainless steel, Ni primarily regulates the equilibrium of the microstructure of the two phases while considering the material’s toughness, particularly its low-temperature impact toughness [14].
Fe-Cr-Ni alloys are recognized for their high strength, excellent toughness, and remarkable corrosion resistance. The elements Fe, Cr, and Ni exhibit a high degree of similarity in their natural characteristics and demonstrate a low tendency for segregation within the metallographic layer. These attributes render Fe-Cr-Ni alloys a preferred choice for surface modification, strengthening, and repair of steel materials. The Fe-Cr-Ni series stainless steel is a primary material selection for fuel cell bipolar plates due to its superior corrosion resistance and exceptional mechanical properties [15]. The Fe-Cr-Ni alloy stainless steel electrode is employed for welding corrosion-resistant high-temperature oxidation sulfide equipment. Under high-temperature conditions, the Cr element forms Cr3+ ions that migrate to the surface; these Cr3+ ions and S2–, O2– ions subsequently form a dense, stable, corrosion-resistant film that impedes ion migration and enhances corrosion resistance [16]. The corrosion behavior of Fe-Cr-Ni alloys is contingent upon the properties of the passivating film, which are significantly influenced by the Cr content of the alloy. It is widely accepted that selective dissolution of Fe and oxidation of Cr are the primary factors contributing to the formation of Cr-rich passivation layers. It is generally acknowledged that the corrosion resistance of stainless steel improves with the addition of higher levels of chromium (Cr) [17,18,19]. However, it is important to recognize that a higher Cr content does not always correlate with superior material performance [20]. In the microstructure of ferritic stainless steel, an increase in Cr content has been shown to accelerate the formation and precipitation of intermetallic compounds, martensite, and ferrite grains, resulting in a coarser grain size. These phenomena are recognized as primary factors influencing the performance of ferritic stainless steel, with embrittlement being the predominant concern [21,22,23,24]. CHAI et al. [25] investigated the effect of Cr content on the corrosion behavior of FeCoNiCrx (x = 0,0.5,1.0) alloy in H2SO4 and NaCl solutions. The results indicated that the passivation zone and breakdown point of FeCoNiCr0.5 alloy in both corrosion solutions were significantly increased compared to FeCoNiCr0, demonstrating excellent corrosion resistance. However, as the Cr content was further increased, the FeCoNiCr alloy exhibited serious localized corrosion due to Cr element segregation, leading to a decrease in corrosion resistance. It can be proposed that there exists a critical threshold regarding the corrosion resistance of stainless steel in relation to Cr content. Specifically, when the Cr mass fraction exceeds 25%, stainless steel becomes susceptible to the formation of detrimental phases, and Cr may undergo intergranular segregation. Therefore, it is essential to identify methods to mitigate segregation and harmful phases while simultaneously increasing Cr content to enhance the corrosion resistance of stainless steel. In this study, the addition of Cr powder is controlled at a mass fraction of 20% in the material design, taking into account that Ni powder contains a certain amount of Cr. Nickel (Ni) serves as a stabilizing element of the austenite phase [26,27,28], thereby conferring enhanced mechanical and corrosion properties to the alloy. Ni-based corrosion-resistant alloys exhibit a high level of resistance to a wide range of acid corrosion and stress corrosion, finding extensive applications in the petroleum, chemical, and power industries. Furthermore, Chromium and nickel exhibit greater strength and improved wear resistance in ferrite. The addition of chromium (Cr) and nickel (Ni) strengthens the atomic bonds with iron atoms, hinders dislocation motion caused by lattice distortion, and inhibits grain boundary slip and migration. These properties enhance plastic deformation resistance and grain boundary stability [29]. However, excessive Ni levels may result in increased austenite content within the microstructure, thereby reducing the mechanical strength of the material [30]. This can lead to a rise in Cr-enriched ferrite, which is prone to the formation of harmful secondary phases, thereby diminishing the material’s corrosion resistance. An Fe-based fused cladding layer containing 15.2Cr-Ni5.1 and 15.7Cr-Ni7.1 was prepared on the surface of 1Cr13 steel by YANG et al. [31], the results demonstrated that the mass fraction of austenite increased with Ni content, although the material’s hardness decreased while its corrosion resistance in a 0.5 M H2SO4 solution improved. Qiu’s research findings on the microstructure and properties of the laser-fused cladding layer of the high-entropy alloy Al2CrFeCoCuTiNix revealed a correlation between Ni content and the corrosion resistance of the fused cladding layer, it was observed that as the Ni content increased from Al2CrFeCoCuTiNi0.5 to Al2CrFeCoCuTiNi2, the corrosion resistance of the fused cladding layer initially increased, followed by a subsequent decline, with the material exhibiting the highest Ni content demonstrating the lowest corrosion resistance [32].
In this paper, the composition of the FeCrNi fusion cladding layer is designed with the addition of Cr controlled at a mass fraction of 20%. The effects of varying the addition of Ni are investigated to determine the changes in the material’s microstructure and the resulting alterations in properties. This study will provide insights into the alloy design of FeCrNi fusion cladding layer materials.
2. EXPERIMENTAL MATERIALS AND METHODS
The model of the laser cladding machine is designated RC-LCD-4000-D-R, with the base material consisting of 45 steel. The laser parameters were established as follows: a power of 1.8 kW, a scanning rate of 10 mm/s, a lap rate of 50%, and single-layer cladding. The metal powders utilized in the experiment included Fe35 powder, Cr powder, and Ni powder, with particle sizes ranging from 100 to 270 mesh. The chemical compositions of these powders are presented in Table 1. The experimental program is detailed in Table 2. The specimens that underwent cladding are illustrated in Figure 1. All three categories of fused cladding surfaces are characterized by the absence of cracks and a robust bond to the substrate. The surfaces with Ni contents of 5% and 10% exhibit blurred boundaries between each layer, accompanied by a pronounced granularity of undissolved metal powder on the surface. Conversely, the surfaces of specimens with a Ni mass fraction of 15% are notable for their smooth, non-granular, and bright appearance. Subsequently, the specimens were cut into 10 × 10 × 8 mm blocks along the perpendicular surface line of the fused cladding. The faces perpendicular to the fused cladding direction were then sanded, polished, and corroded with aqua regia, after which the morphology was observed using an optical microscope model 4XB and a scanning electron microscopy (SEM) model Sigma300. An X-ray diffraction (XRD) analyzer (Smartlab SE) was employed for physical phase analysis. The hardness test was conducted utilizing an MHVS-1000AT hardness tester, employing a load of 200 g and a loading time of 10 s. Hardness evaluations were performed at intervals of 0.2 mm from the lower layer of the fused cladding to the upper layer. Measure data at four different locations on the same horizontal line at an equal distance from the cladding base layer, and take the average as the hardness at that thickness-distance. The electrochemical specimens were cut into 5 × 5 × 8 mm pieces, with the working surface exposed, while the backside was connected with copper wires and encased in a plastic tube filled with epoxy resin for sealing. Tafel curves and electrochemical AC impedance spectroscopy (EIS) tests were conducted using the CHI660E electrochemical workstation in a sodium chloride solution with a mass fraction of 3.5%. The counter electrode utilized was a platinum electrode, the reference electrode was a saturated KCl glycerol mercury electrode, and the specimen served as the working electrode. After electrochemical corrosion was completed, a scanning electron microscope (Sigma 300) equipped with an on-board energy dispersive spectrometer was used to examine the surface corrosion morphology and elemental distribution. An X-ray photoelectron spectrometer (Thermo Fisher Scientific Nexsa) was used to analyze the composition of the surface layer. The vacuum level in the analysis chamber was 5 × l0–9mBar, the excitation source was Al ka radiation (hv = 1486.6 eV), with a beam spot size of 30μm, operating voltage of 12 kV, and filament current of 0.4 mA. The test area was 3 × 3 mm. During spectroscopy, the total spectrum was recorded first, followed by the acquisition of high-resolution spectra in the C 1s, O 1s, Cr 2p, and Fe 2p energy ranges.
3. EXPERIMENTAL RESULTS AND ANALYSIS
3.1. XRD analysis
Figure 2 presents the XRD diagrams of three types of fused cladding layers. Analysis of these diagrams indicates that the microstructure of the fused cladding layer with a 5% Ni mass fraction is composed of α-Fe (martensite) and γ-Fe (austenite). As the Ni mass fraction increases to 10% and 15%, the diffraction peaks corresponding to the α-Fe phase gradually diminish, resulting in a predominant microstructure characterized by the γ-Fe phase. Ni has been demonstrated to expand the austenite phase region, with the temperature range in which the γ phase exists increasing in accordance with rising Ni content. As a robust stabilizing agent for the austenite phase, Ni effectively lowers the transformation temperature from austenite to ferrite. BIAN et al. [33] concludes that the γ↔α transformation temperature decreases to below 600°C when the Ni content reaches 20%. The austenite demonstrates such stability that, upon rapid cooling, the γ↔α transformation does not occur at very low temperatures, allowing both phases to maintain the austenitic microstructure. Therefore, the microstructure undergoes austenitization as the mass fraction of added Ni increases.
3.2. Microstructure
As illustrated in Figure 3, the optical microstructure of FeCrNi fused cladding layers with varying Ni mass fractions is presented. Visual examination under the optical microscope reveals that the grain morphology and size of the three fused cladding layers are largely comparable, with all layers exhibiting a columnar dendritic crystal structure and equiaxial crystals. Specifically, the secondary dendritic crystal arms with uniform spacing grow on the columnar crystalline stems of larger dimensions, interspersed with a small number of equiaxial crystals. A comparative analysis of the microstructures within the upper layers of the three fused cladding layers indicates that the layers containing 5% and 15% Ni exhibit slight coarsening. In contrast, the layers with 10% Ni demonstrate a higher prevalence of equiaxial crystals and larger grain sizes.
Optical microscope image of FeCrNi fusion cladding layer. (a) 5% Ni lower layer, (b) 5% Ni middle layer, (c) 5% Ni upper layer, (d) 10% Ni lower layer, (e)10% Ni middle layer, (f) 10% Ni upper layer, (g) 15% Ni lower layer, (h) 15% Ni middle layer, (i) 15% Ni upper layer.
The fused cladding layer was further examined using SEM, as illustrated in Figure 4. In the lower and middle layers of the fused cladding containing a 5% Ni mass fraction, the microstructure in the darker regions exhibited a columnar dendritic crystal morphology, while a gray-white microstructure was distributed among the dendrites. A detailed inspection of the gray-white microstructure revealed a two-phase laminar structure, characterized by alternating thin white and black layers. In the lower and middle layers of the fused cladding, the spacing between the laminae was minimal; however, in the upper layer, the spacing increased, resulting in a bouquet-like formation of the laminae. The chemical composition of the dendritic and intergranular regions within the middle layer of the fused cladding was analyzed, and the results are presented in Table 3. The dendritic region (position ①) exhibited a high concentration of Fe, Ni, and Si elements, indicating the presence of a γ-Fe phase microstructure. In contrast, the intergranular region (position ②) displayed elevated levels of C, Cr, and Fe elements, suggesting a composition of α-Fe (martensite), γ-Fe, and Cr-containing carbides comprised of co-crystals. During the rapid cooling process of laser cladding, the liquid molten pool initially solidified into the γ phase. With further cooling, variations in composition and temperature facilitated the growth of secondary dendritic arms from the primary dendrite. The solidification of the γ phase led to the expulsion of Cr, C, and other elements from the surrounding liquid, resulting in a γ phase that contained higher concentrations of Cr and C compared to the first solidified γ phase. Simultaneously, excess Cr and C diffused into the surrounding γ phase, and upon reaching the eutectic temperature, a eutectic transformation occurred involving Cr, C, B, and other elements in a liquid state (L → γ-Fe + M3C/M7C3), leading to the formation of austenite and carbides in an eutectic configuration [34]. Upon completion of the solidification process, the microstructure consisted of a co-crystal composed of austenite and austenitic carbides. Subsequently, the material underwent a solid-state phase transition, wherein a portion of the γ phase within the original γ-Fe, characterized by high concentrations of Cr and C, transformed into a martensitic phase, while another portion of the γ-Fe remained unchanged. Within the austenitic grains, slate martensite formed, with numerous parallel slats creating a martensitic slat bundle, separated by an austenitic film. In the upper layer of the cladding, the slats increased in thickness and spacing, with multiple slat bundles oriented differently within a single austenite grain. Notably, XRD analysis did not detect the peaks corresponding to the eutectic carbide phase, likely due to the low carbide content and its flaky distribution within the eutectic, which resulted in inconspicuous diffraction peaks.
Addition of Ni mass fraction of 5% of the fused cladding layer. (a) lower layer, (b) middle layer, (c) upper layer.
The morphology of the fused cladding layer, incorporating Ni at a mass fraction of 10%, is illustrated in Figure 5. It is evident from this figure that the interlayer spacing within the eutectic microstructure of the fused cladding layer diminishes, resulting in the indistinct boundaries between the layers. The micro-area elemental analysis of the lamellar and black tissues in the upper layer of the fusion cladding is presented in Table 4. The black dendritic region identified as position ③ is enriched in Fe, Ni, manganese, and C, indicating an austenitic structure. In contrast, the lamellar region at position ④ is rich in Fe, Cr, and C, and is characterized as a co-crystal composed of austenite and carbide. During the laser cladding process, the formation of austenite dendrites in the molten pool results in the release of Cr, C, and other elements. Subsequently, eutectic reactions occur within the gaps of the primary and secondary austenite dendrites, leading to the formation of eutectic crystals composed of carbides and austenite. The increased addition of Ni significantly lowers the martensitic start transition temperature (Ms) and the martensitic end transition temperature (Mf) of the cladding layer, thus preventing the austenite-to-martensite transition. Therefore, only a eutectic structure consisting of austenite and carbides is present within the cladding.
Addition of Ni mass fraction of 10% of the fused cladding layer. (a) lower layer, (b) middle layer, (c) upper layer.
SEM observations of the fused cladding layer containing 15% Ni by mass (Figure 6) revealed a microstructure consisting of two colored phases: black and white. By integrating these observations with XRD analysis, it can be concluded that the microstructure of the fused cladding layer is characterized by a single-phase austenitic structure. This conclusion arises from the observation that as the Ni content increases, the austenite phase region expands even below room temperature, preventing the transformation from austenite to martensite. Under these conditions, the stability of austenite is enhanced, allowing for a greater solubility of alloying elements such as Cr and C within the austenitic matrix. A comparative analysis of the fused cladding layers with varying Ni mass fractions indicates that the eutectic structure exhibits significant “bulging” in the layers with 5% and 10% Ni, whereas in the 15% Ni layer, the white phase does not exhibit significant bulging but rather merges with the black phase. The regions identified as white area ⑤, interwoven black and white area ⑥, and black area ⑦ within the middle layer of the fusion cladding were subjected to micro-area elemental analysis, with results presented in Table 5. The black area ⑦ demonstrates a high concentration of Fe, Ni, Mn, and Si, confirming its identification as an austenitic phase. The relatively low concentrations of Cr and C in this region, compared to positions ⑤ and ⑥, suggest that the austenite present here is primary austenite that formed directly from the liquid phase during the laser melting process. Position ⑤ is characterized by elevated levels of Fe, Cr, and C, leading to the inference that the austenite in this region is secondary austenite. This is attributed to the growth of primary γ-Fe crystals, during which a portion of Cr and C elements become solidified within the primary crystal austenite, while the remaining elements are enriched in the vicinity of the austenite. Therefore, when the austenite re-precipitates from the liquid, it possesses a higher concentration of C and Cr. The elemental contents of C, Si, Cr, Fe, and Ni at position ⑥ fall between those of primary and secondary austenite, reflecting the competitive precipitation of primary and secondary austenite at different stages in the liquid state.
Addition of Ni mass fraction of 15% of the fused cladding layer. (a) lower layer, (b) middle layer, (c) upper layer.
3.3. Hardness
Figure 7 illustrates the hardness of fused cladding layers with varying Ni mass fractions. The data presented indicates that the hardness of the upper layer is consistently lower across all examined fused cladding layers. This observation is corroborated by the optical microstructure images, which reveal that coarsening of the microstructure occurred in the upper layer. Specifically, the grains of the fused cladding layers with Ni mass fractions of 5% and 15% exhibited slight coarsening, whereas the coarsening in the layer with a Ni mass fraction of 10% was more pronounced. This coarsening is associated with a reduction in hardness. The average hardness measurements of the fused cladding layer were determined to be 468.8 HV0.2 at a Ni mass fraction of 5%, 529.28 HV0.2 at 10%, and decreased to 399.55 HV0.2 with a further increase to 15% Ni content.
Four images were selected from the upper, middle, and lower layers of the cladding, respectively, and the carbide content was analyzed using Image image analysis software. The results are shown in Table 6. As shown in Table 6, the carbide content decreases as the Ni content increases; the average carbide contents of the three fused cladding layers were 20.14%, 16.53%, and 16.40%, respectively. When the nickel content is 5%, the microstructure of the fused cladding layer consists of martensite, austenite, and eutectic phases formed by martensite-austenite carbides. The presence of carbides in the martensite and eutectic phases plays a positive role in increasing the hardness of the fused cladding layer. Although the 5% Ni fused cladding layer has the highest carbide content, its average hardness is not the highest. This is because the carbides in the 5% Ni fused cladding layer are aggregated and relatively large in size, causing a sharp increase in hardness at the carbide aggregation sites while hardness remains lower in other areas, resulting in a relatively low average hardness value. When the nickel mass fraction was increased to 10%, the microstructure of the fused cladding layer consisted entirely of austenite and eutectic structures, the carbide content decreased to 16.53%, and no martensitic structure was observed. Despite the absence of martensite, the hardness of the fused cladding layer still increased. This is because, although the proportion of carbides decreased, they existed as fine, dispersed particles with a wider distribution area, resulting in a larger area of the eutectic structure composed of carbides and austenite, the observations in Figure 5 support this view, as the figure shows an increase in the area occupied by the eutectic structure within the fused coating. When the nickel mass fraction rises to 15%, the microstructure consists of austenite. Since austenite itself has low hardness, this leads to a decrease in the hardness of the fused cladding layer. Furthermore, a large amount of chromium, carbon, and other elements dissolve in the austenite without forming carbides, which further contributes to the reduction in the cladding layer’s hardness. LI et al. [35] developed a new Fe-Ni-Cr coating on the surface of the tough iron alloy QT500-7 and found that the coating’s hardness and wear resistance were comparable to those of the substrate. The study suggests that the solid solution formed by Fe and Cr dissolved in Ni is the reason why the coating’s hardness did not increase. Furthermore, although the carbide content in the 15% Ni fused cladding layer differs only slightly from that in the 10% Ni fused cladding layer, carbides in the 15% Ni fused cladding layer exhibit agglomeration, resulting in an uneven distribution of hardness values—which is another reason for the decrease in the cladding layer’s average hardness.
Percentage of carbides in the upper, middle, and lower layers of different fused cladding layers (%).
3.4. Corrosion resistance
Figure 8 illustrates the Tafel curves of the three fused cladding layers in a 3.5% NaCl solution, while Table 7 presents the parameters associated with these Tafel curves. As depicted in Figure 8, the corrosion current density of the fused cladding layers with Ni mass fractions of 5% and 15% exhibits a gradual increase with rising potential. In contrast, the specimen containing a Ni mass fraction of 10% demonstrates a pronounced decrease in corrosion current density when the potential reaches –0.12 V, indicating the emergence of a passivation platform. This observation suggests that the passivation film on the surface of the fused cladding layer can be rapidly restored following its disruption. The corrosion current density data indicate that the fused cladding layer with a 10% Ni mass fraction exhibits the lowest corrosion current density and optimal corrosion resistance. The layer with a 15% Ni addition presents a higher corrosion current density and inferior corrosion resistance, whereas the layer with a 5% Ni addition shows the highest corrosion current density and the least favorable corrosion resistance.
The Nyquist curves for the three specimens are presented in Figure 9. Analysis of Figure 9 shows that the three types of specimens exhibit similar capacitive arcs. The radius of the capacitive arc reflects the polarization resistance of the specimen during the electrochemical process; specifically, a larger radius indicates higher polarization resistance and a slower corrosion rate. According to Figure 9, the radius of the capacitive arc decreases in the following order: Ni10% fused cladding, Ni15% fused cladding, and Ni5% fused cladding. The equivalent circuit is modeled using the Rs(CPE–Rp) diagram shown in Figure 10, with the fitting parameters provided in Table 8. Here, Rs represents the resistance of the corrosion solution, and CPE refers to the constant phase element, which includes the capacitance C of the double layer at the interface between the metal surface layer and the passivation film, as well as its dispersion coefficient (n). Rp represents the charge-transfer resistance at the metal interface, a quantity associated with the stability of the passivation film. An increased value of Rp implies greater resistance to charge migration between the medium and the electrode, thereby indicating enhanced stability of the passivation film. Table 8 reveals that the polarization resistance of the Ni10% fused cladding layer is the highest, indicating superior stability of the passivation film. However, when the mass fraction of Ni is elevated to 15%, a decrease in polarization resistance occurs, which suggests an acceleration of the polarization reaction of the fused cladding layer in solution and a corresponding reduction in the corrosion resistance of the fused cladding layer. These findings align with the results obtained from the Tafel curve test.
EBSD observations were performed on the surfaces of the specimens after electrochemical corrosion, as shown in Figure 11. The corrosion morphology reveals that the pitting corrosion pits are primarily distributed at the interface between the black regions (carbide structure) and the white regions (austenitic structure)—that is, at the phase boundary between the two phases. At a Ni mass fraction of 5% in the fusion cladding layer, the matrix microstructure exhibits a composition of austenite, martensite, and austenite-martensite carbide eutectics. The variety of phases and the potential differences between these phases predispose the material to protocell reactions. The presence of two phases at the phase boundaries prioritizes corrosion, leading to a reduction in the material’s corrosion resistance. As the Ni mass fraction increases to 10%, the microstructure transitions to an austenitic structure, characterized by a eutectic composition with austenitic carbide. The reduction in phase types correlates with a decrease in the formation of galvanic cells, resulting in improved corrosion resistance. As can be clearly seen in Figure 11, the corrosion pits on the surface of the fused cladding layer with 5% Ni content are larger, and some pits appear in the austenitic phase region. This is because a large amount of Cr combines with C to form chromium-containing carbide phases, leading to chromium depletion in the austenite and the formation of corrosion pits in the austenitic region. In contrast, the corrosion pits on the surfaces of the 10% Ni and 15% Ni specimens are all located at the phase boundaries between the two phases. Micro-area chemical analysis was performed on the corroded areas, the test results are shown in Figures 12, 13, and 14, the concentrations of the various elements in Figures 12, 13, and 14 are listed in Table 9. As shown in the figures, chromium is enriched in the carbides, while chromium depletion is observed at the pitting sites. In addition, the reason why fused cladding layers with high Ni content exhibit superior corrosion resistance is that their carbide distribution is finer and more uniform, among them, the fused cladding layer with 10% Ni content has the finest and most uniformly distributed carbides. Calculations of the proportion of surface carbides in the three fused cladding layers revealed that the 5% Ni fused cladding layer had a surface carbide content of 23.28%, the 10% Ni fused cladding layer had 21.97%, and the 15% Ni fused cladding layer had 15.32%. Changes in corrosion resistance are not solely correlated with carbide content but are also related to the structural morphology of the carbides. The structural morphology of the carbides has a certain influence on the diffusion pathways of the corrosive medium, fine, dispersed carbides can prevent the formation of chromium-depleted zones caused by the aggregation of coarse carbides, thereby enhancing the material’s overall corrosion resistance and reducing the likelihood of localized corrosion. The surface layer of the 5% Ni cladding has a relatively high Cr content and a low C content, indicating that a large amount of Cr has combined with C to form chromium-containing carbides, which adversely affects the material’s corrosion resistance. When Cr combines with O to form a continuous, dense Cr2O3 protective layer (passivation layer), it enhances the material’s corrosion resistance. As the Ni content increases, the C content in the surface layer decreases, indicating that the combination of Cr and C decreases, allowing more Cr to contribute to improving the material’s corrosion resistance. The 15% Ni coating has the lowest Cr content and the highest C content; its corrosion resistance should theoretically be superior to that of the 10% Ni coating. However, due to the agglomeration and increased size of carbides in this fused cladding layer, which adversely affects the material’s corrosion resistance, the 10% Ni fused cladding layer exhibits better corrosion resistance than the 15% Ni fused cladding layer.
Figure 15 shows the XPS full spectra of the passivation films formed on fused cladding layers with different Ni contents after electrochemical corrosion in a 3.5% NaCl solution. As shown in the figure, the main components of the passivation films formed on each sample are Fe, Cr, O, and C. The presence of C is due to surface contamination of the samples; it is used solely for peak position calibration and is not included in the analysis of the passivation film composition.
XPS spectrum of the passivation layer on the fused cladding layer after electrochemical corrosion in a 3.5% NaCl solution.
Figure 16 shows the results of narrow-region O 1s XPS spectra of fused cladding layers with different Ni contents, analyzed using Thermo Advantage, the binding energy and corresponding relative abundances of the oxidation states are shown in Table 10. The O 1s XPS spectra can be divided into four peaks, corresponding to binding energies of 529.97 eV, 531.35 eV, 531.47 eV, and 533.62 eV, Based on the O 1s binding energy values, the 529.97 eV peak is a characteristic peak of M-O compounds, corresponding to O2–, 531.35 eV and 531.47 eV are characteristic peaks of M-OH compounds, corresponding to OH−, and 533.62 eV corresponds to the characteristic peak of H2O. The H2O in the passivation film is primarily bound water, this is because when the O content in the outer layer of the passivation film is high, any residual hydrogen after the formation of metal oxides and hydroxides remains in the form of bound water. The passivation films on the surfaces of the 10% Ni fused cladding layers exhibit only characteristic peaks for M-O (529.97 eV) and M-OH (531.35 eV and 531.47 eV) compounds, with no characteristic peaks for H2O.
Figure 17 shows the narrow-region Cr 2p XPS spectrum of the passivation film on fused cladding layers with different Ni contents. After analysis using Thermo Advantage software, the Cr 2p binding energy and the relative abundance of corresponding oxidation states are shown in Table 11. The spectrum was resolved into two peaks with binding energies of 573.12 eV and 576.58 eV, corresponding to Cr and Cr2O3, respectively [36,37,38,39]. No signals for Cr hydroxides were detected. The results of peak separation on the fused cladding layer surfaces of different samples showed that the peak positions of the various oxidation states remained consistent, however, the signal at 576.58 eV was the strongest in the passivation film of the fused cladding layer with 10% Ni content, indicating the highest Cr2O3 content.
Figure 18 shows the narrow-region XPS spectra of Fe 2p3/2 in the passivation layer on fused cladding layers with different Ni contents, as analyzed using a Thermo Advantage spectrometer. The Fe 2p3/2 binding energy and the relative abundances of corresponding oxidation states are shown in Table 12. The Fe 2p3/2 spectrum can be divided into three peaks with binding energies of 706.51 eV, 710.78 eV, and 711.15 eV, corresponding to elemental Fe, Fe2O3, and FeOOH, respectively [36, 40,41,42]. Among these, the relative concentrations of Fe2O3 and FeOOH compounds in the passivation film of the 10% Ni specimen were relatively low.
During the electrochemical corrosion process, Fe and Cr elements in the passivation film dissolve along with oxides and enter the solution in ionic form, forming FeOOH, Cr2O3, Fe2O3, and other hydroxides and oxides on the surface of the passivation film, causing the film to dissolve. The iron-rich phase in the corrosion product film is characterized by poor adhesion and a loose, porous structure. These phases accumulate unevenly on the substrate surface, providing pathways for the exchange of the medium and ions, which leads to localized corrosion intensification. The high Cr2O3 content, low Fe2O3, low FeOOH, and low hydrate content in the passivation film of the 10% Ni fused cladding layer enhance the material’s corrosion resistance.
4. CONCLUSIONS
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At a Ni mass fraction of 5%, the microstructure consists of martensite, austenite, and a eutectic of martensite, austenite, and carbides. Increasing the Ni content to 10% results in a microstructure composed of austenite and a eutectic of austenite and carbides. At 15% Ni, the fused cladding exhibits a single-phase austenitic structure.
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The average hardness is 468.8 HV0.2 at 5% Ni, increases to 529.28 HV0.2 at 10% Ni, and decreases to 399.55 HV0.2 at 15% Ni. All three fused fused cladding layers show lower hardness at the top surface, which is associated with grain coarsening.
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The fused cladding layer with Ni mass fraction of 5% exhibited the highest corrosion current density and the poorest corrosion resistance. The fused cladding layer with Ni mass fraction of 10% had the most stable passivation film, the lowest corrosion current density, and the best corrosion resistance. As the Ni mass fraction increases to 15%, the single-phase austenite can dissolve more corrosion-resistant elements such as Cr, leading to the formation of Cr-depleted zones at the grain boundaries.
5. ACKNOWLEDGMENTS
The work was supported by National College Student Innovation and Entrepreneurship Projects(202511305012, 202411305027); Anhui Province College Student Innovation and Entrepreneurship Projects (S202511305089); The University Natural Science Key Project of Anhui Province Department of Education (2022AH051922).
6. DATA AVAILABILITY
All data, figures, models, and code generated or used in the course of this study are included in the submitted manuscript.
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