Effect of TiC mass fraction on the microstructure, microhardness, and corrosion resistance of TiC/Fe composite coatings

ABSTRACT

TiC/Fe cladding layers with different mass fractions of TiC were deposited on the surface of 45 steel by laser cladding technology, and their microstructure, microhardness, and corrosion resistance were investigated. Results show that the dendrites of the Fe35-fused cladding are composed of α-Fe solid solution, which is rich in Fe, Cr, Si, Mn, and Ni elements; the intercrystal is a cocrystal composed of α-Fe solid solution and (Cr, Fe)₇C₃, which is rich in Cr and C elements; the fused cladding is composed of columnar crystals, columnar dendrite crystals, and equiaxed crystals in the order from the bottom layer to the upper layer. In the 10% TiC-fused cladding, the bottom layer consists of columnar crystals, while the middle and upper layers are composed of equiaxed crystals. TiC is dispersed as fine, diffused small particles and small pieces within the matrix. Conversely, the fused cladding layers containing 20% and 30% TiC comprise equiaxed crystals, with TiC distributed in the matrix as agglomerated large particles and pieces. With an increase in TiC content, the bonding of TiC to the surrounding matrix diminishes. The 10% TiC-fused cladding exhibits the highest average microhardness, whereas the 20% TiC-fused cladding demonstrates the lowest average microhardness. The fused cladding without TiC addition displays the most positive corrosion potential, the lowest corrosion current density, the most stable passive film, and the best corrosion resistance. The corrosion resistance of the fused cladding diminishes following the addition of TiC; the higher the TiC content, the poorer the corrosion resistance of the fused cladding.

Keywords:
TiC/Fe fused cladding; microstructure; microhardness; corrosion resistance

1. INTRODUCTION

Iron and steel are essential resources for national construction and development because of their excellent comprehensive mechanical properties, working ability, low cost, and abundant resources, and they are applied in all areas of human production and livelihood. However, in the case of marine equipment, high-speed ships, oil exploration, pipelines, and other related fields, the equipment is exposed to moisture, corrosion, and friction in harsh environments, making the surface highly susceptible to damage, leading to part and equipment failure [¹, ², ³]. Surface engineering technology is typically employed to create a high-performance coating on easily damaged parts. This process not only mends failed parts to their original dimensions but also enhances the reliability and stability of the components [⁴, ⁵, ⁶]. Known for its advantages such as a small heat-affected zone on the substrate, minimal heat deformation of the parts, rapid heating and cooling speed, fine grain size of the coating, a high-density structure, and a wide range of applicable materials, laser cladding technology has emerged as an important method for repairing and strengthening part surfaces. Fe-based alloys are not only close to the composition of parts made of steel but also have a strong bond at the interface after melting, the properties and structure can be easily regulated, and they have a huge advantage in cost; thus, their research, popularization, and application have attracted much attention from scholars at home and abroad [⁷, ⁸, ⁹]. However, the low surface hardness, susceptibility to wear and corrosion, and other drawbacks of Fe-based fusion cladding make it susceptible to failure under harsh and complex operating conditions, limiting its use in demanding equipment. As a result, ceramics, such as borides, nitrides, and carbides, are commonly incorporated into Fe-based powders to provide a reinforcing phase to improve coating performance. TiC, in particular, is widely used given its exceptional properties, such as high hardness, high melting point, remarkable wear resistance, and excellent corrosion resistance, making it an ideal reinforcing phase for high-performance coatings on metal surfaces [¹⁰, ¹¹, ¹², ¹³]. However, the addition of TiC changes the microstructure, the composition of the physical phase, and the potential difference between the microstructures in fused cladding layers, resulting in a change in properties. At present, quite a lot of research is being done on TiC and the hardness and wear resistance of iron-based alloys, and the conclusions of the study are relatively uniform: that is, the addition of TiC will improve the coating’s hardness and wear resistance [¹⁴, ¹⁵, ¹⁶]. BAI et al. [¹⁷] prepared a TiC/Fe gradient coating on cast iron by in-situ solid-phase diffusion and found that the coating consisted of equiaxed TiC particles and α-Fe phase, with a volume fraction of TiC up to 95%, the phase interface and the coating/substrate interface were well combined, and the coating has high hardness and elastic modulus. XIAO et al. [¹⁸] found that the introduction of TiC in TiC-Fe-based coatings would increase the degree of deformation of the splats and eliminate the delamination structure, thus effectively improving the microhardness and wear resistance of the composite coatings, and the microhardness of the TiC-Fe-based coatings was greatly improved compared with that of the pure metal coatings. NIU et al. [¹⁹] prepared TiC ceramic coating reinforced 304 components, obtaining well-bonded and defect-free coatings with two phases of TiC and γ-Fe inside the coatings, and four TiC hard phases appeared, and the coatings had excellent wear resistance. DAS et al. [²⁰] prepared TiC /Fe coatings on AISI 1020 steel using the tungsten inert gas surfacing process and found that the hardness value of the coatings with 10% TiC addition by mass fraction was nearly two times higher than that of the substrate. ZHANG et al. [²¹] prepared coatings with compositions of Fe25-30WC, Fe25-30TiC, and Fe25-15WC+15TiC on the surface of 45 steel, and found that the hardness of Fe25-30TiC coatings and wear resistance under dry grinding conditions were also better than the other two coatings. SZYMAŃSKI et al. [²²] prepared TiC/Fe coatings on mild steel and gray cast iron by reaction casting and found that the TiC particles bonded well to both substrates, the hardness of the coatings was almost tripled, and in slurry wear experiments the weight loss of the castings with the TiC/Fe coatings was less than half that of the substrates. YANG et al. [²³] prepared Fe06 TiCx/Mo (x = 0, 0.10, 0.15, 0.20) composite coatings with different mass fractions by laser cladding technique, and it was found that the coatings mainly consisted of α-Fe, Cr-Fe, and (Fe, Ni) solid solution phases, and that the hardness was highest for the Fe06 0. 20TiC cladding, and the best abrasion resistance was obtained for the Fe06 0. 20Mo coating. There have also been some studies focusing on the corrosion aspects of coatings. KUPTSOV et al. [²⁴] prepared xTiC-Fe-CrTiNiAl (x = 0, 25, 50, 75%) coatings with different TiC contents, and found that the structure of the TiC-free coatings was inhomogeneous and cracks existed, and that after the introduction of TiC, dense and crack-free coatings without obvious defects and uniform carbide grain distribution could be obtained, and the coatings with the addition of TiC were much more resistant to static corrosion and friction corrosion in artificial seawater. TANG et al.’s [²⁵] findings indicated that the application of TiC coatings enhanced the load-bearing capacity of the iron substrate. The coatings demonstrated efficacy in enhancing erosion resistance. However, the protective effect was compromised when the TiC coatings experienced cracking. BUYTOZ et al. [²⁶] added different proportions of TiC powder to FeCrC powder to make coatings on 42CrMo4 steel plates and found that TiC improved the corrosion resistance of FeCrC coatings, and the corrosion rate was inversely proportional to the TiC content in the coatings, with the lowest corrosion rate in the specimen with the highest TiC powder concentration. ZHANG et al. [²⁷] prepared coated TiC/Fe coatings by adding Cr3C2 and Ti powders, and found that the replacement of chromium in the in-situ reaction was beneficial to the corrosion resistance of the coatings, and could reduce the susceptibility to intergranular corrosion. In conclusion, TiC/Fe coating has an important research value in the surface technology of iron and steel materials due to the similarity of composition and matrix, strong interfacial metallurgical bonding, excellent comprehensive performance, and low price. Therefore, it is of great significance to carry out the fusion coating of TiC/Fe on the surface of iron-based alloys to extend the service life of iron and steel equipment, to reduce the cost of repairing and remanufacturing such equipment, and to promote the application of iron-based ceramic composite powders.

Although TiC/Fe composite coatings have been researched quite a lot, the differences between TiC and Fe have resulted in stress concentration of the TiC phase in the matrix and low bond strength at the TiC/matrix microinterface. TiC/Fe composite coating strength and plastic toughness, corrosion resistance, between the inversion of the problem have been the constraints of TiC/Fe composite coatings’ strong toughening synergistic effect, corrosion resistance, of the full play of the bottleneck. Consequently, researchers have undertaken continuous studies on Fe/TiC composite coating materials. In this paper, the controllable preparation of (TiC + Fe) is realized by modulating the raw material powder composition. Around the TiC/Fe composite coating microstructure evolution, alloying elements in the dendrite, intergranular distribution rules, carbide phase distribution, hardness of composite coatings, corrosion resistance to carry out a systematic study, from the local area of the elements of the enrichment of the performance of the composite coatings in the microscopic point of view of the play of the research information, to solve the TiC/Fe materials, such as the full play of the comprehensive performance of the problem to provide some reference information. It is expected to provide some reference materials for solving the issues of strong toughening synergistic effect and full play of corrosion resistance of TiC/Fe materials.

2. MATERIALS AND METHODS

The surface of the 45 steel plate, measuring 100 mm × 100 mm × 20 mm, was meticulously polished and cleaned prior to the fusion of Fe35 powder (100-270 mesh particle size) and TiC particles (2–4 μm diameter). Table 1 shows their compositions, and Table 2 lists the material ratio scheme.

Table 1
Chemical composition of powder (mass fraction, %).

CHEMICAL COMPOSITION	C	Mn	Mo	Si	W	B	Cr	Ni	Fe	Ti
Fe35	0.14	0.27	2.96	3.18	0.6	1.3	19.65	11.86	Bal.	–
TiC	≥19.32	0.024	–	0.02	–	–	4	–	0.05	Bal.

Table 2
Laser cladding material ratio scheme.

NUMBER	PROCESS (MASS FRACTION,%)
1	100% Fe35
2	10%TiC + 90% Fe35
3	20%TiC + 80% Fe35
4	30%TiC + 70% Fe35

The Laserline 4000 laser cladding system was operated with a laser power of 2.4 kW, an overlap rate of 50%, a scan speed of 10 mm/s, and an argon gas-protected coaxial powder feed for single-layer cladding. The fused specimens were sliced longitudinally into 10 mm × 10 mm × 20 mm blocks, polished, and subjected to aqua regia corrosion. The microstructure was examined using a model 4XB optical microscope (OM) and a Hitachi Regulus 8100 scanning electron microscope (SEM), while elemental analysis was conducted in the microarea by utilizing an energy spectrum analyzer. Additionally, a SmartLab SE X-ray diffractometer (XRD) was employed for physical phase analysis. The microhardness test was performed using an MHVS-1000AT hardness tester, carefully avoiding unmelted TiC particles, with hardness measurements taken at 0.2 mm intervals from the bottom layer to the upper layer of the fused cladding. The test used a 200 g load with a loading time of 10 s for each interval. Tafel and electrochemical impedance spectroscopy (EIS) curves were obtained using a CHI660E electrochemical workstation in a NaCl solution containing a mass fraction of 3.5%. The test employed platinum electrodes as counter electrodes and saturated calomel electrodes as reference electrodes. The electrochemical specimens were taken in 4 parallel specimens for each group, and the electrochemical specimens were prepared as follows: the specimens were cut into small pieces of 5 mm × 5 mm × 8 mm, surrounded by copper wires, and then sealed in plastic tubes using epoxy resin, leaving only a surface of 5 mm × 5 mm, and the prepared specimens are shown in Figure 1.

Figure 1
Electrochemical experiment specimen.

3. RESULTS AND DISCUSSION

3.1. XRD analysis

Figure 2 shows the XRD patterns of TiC/Fe-fused cladding with different TiC mass fractions. The Fe35-fused cladding contains α-Fe solid solution and (Cr, Fe)₇C₃ carbide. With the addition of TiC, the TiC phase appears in the fused cladding. The primary phase composition of the fused cladding remains unchanged despite the changes in TiC content.

Figure 2
The XRD patterns of TiC/Fe-fused cladding with different TiC mass fractions.

3.2. Microstructure of fused claddings

3.2.1. Fe35-fused cladding

The OM image of the Fe35-fused cladding is shown in Figure 3. Figure 3a depicts the bottom layer of the fused cladding, where the metallurgical bonding zone has a very small width of approximately 2 μm. The fused cladding, attached to the metallurgical bonding zone, showcases columnar crystals and columnar dendritic crystals, with their growth direction oriented perpendicular to the bonding interface. Figure 3b illustrates the middle layer of the fused cladding, comprising columnar dendrites and equiaxed crystals, with a noticeable transition zone where the columnar dendrites transform into equiaxed crystals. Figure 3c shows the upper layer of the fused cladding, which has an equiaxed crystal structure. The solidification microstructure of the fused cladding layer, progressing from the bottom to the upper, reveals a series of morphological changes, including planar crystals, coarse columnar crystals, and anisotropic equiaxial crystals, characteristics indicative of a rapid solidification microstructure. At the interface of the bottom cladding layer, heat dissipates mainly through the substrate. The rapid cooling in the positive temperature gradient leads to the creation of planar crystals in the bonding zone. During solidification, near the bond interface, a negative temperature gradient prompts the growth of coarse columnar crystals and the formation of secondary dendrites in the middle layer. This middle layer showcases columnar and epitaxial growth of dendritic crystals, while the upper layer experiences accelerated cooling due to convection and conduction with the air, as well as dissipation of heat to the substrate. Consequently, the upper layer’s alloy powder reaches the melt pool before a significant portion of the melt has fused, leading to a homogeneous state in the upper layer’s microstructure, characterized by fine and uniform equiaxial crystals.

Figure 3
The OM image of the Fe35-fused cladding. (a) The bottom layer, (b) The middle layer, (c) The upper layer.

Figure 4 shows the SEM morphology of the middle layer of the Fe35 cladding, showcasing a microstructural pattern characterized by a subeutectic structure. This structure comprises dendritic and intercrystal eutectic formations. Observations indicate that the crystallization process within the same microregion of the molten pool is not synchronized. It begins with the development of coarse columnar crystals, followed by the residual liquid metal prompting renucleation and growth of the eutectic structure in the intercrystal region.

Figure 4
The SEM morphology of the middle layer of the Fe35 cladding.

The microzone composition of the middle layer of the Fe35-fused cladding was examined, as displayed in Figure 4, where position ① represents the dendrite region, and position ② represents the intercrystal eutectic organization region. The results in Table 3 show that the dendrites and intercrystal eutectic regions are predominantly enriched with Fe elements. However, the Fe content is higher in the dendrites than in the intercrystal eutectic region, while the content of Si, Mn, and Ni elements is also higher in the dendrites. The XRD results indicate that α-Fe corresponds to the diffraction peaks with the highest relative intensities, representing the matrix phase of the fused cladding layer. The dendrites are α-Fe phase solid solutions. The intercrystal structure contains significant amounts of Cr and C elements and is likely to be a eutectic structure consisting of (Cr, Fe)₇C₃ carbide and α-Fe. The intercrystal precipitation phase at position ③ is remarkably rich in C and is presumed to be the carbide phase.

Table 3
The microzone composition analysis results at different positions marked in Figure 4 (mass fraction, %).

POSITION	C	Si	Cr	Mn	Fe	Ni	Mo	W
①	9.36	1.23	6.45	0.5	77.30	4.14	0.71	0.31
②	12.94	1.07	8.09	0.31	71.79	3.64	1.71	0.45
③	34.89	0.84	4.14	0.35	56.22	2.24	1.03	0.29

3.2.2. Fe35-fused cladding with different TiC mass fractions

Figure 5 illustrates the OM image of Fe35-fused cladding with different TiC mass fractions. The bottom layer of the 10% TiC-fused cladding exhibits columnar crystals, while the middle and upper layers show equiaxed crystals. The microstructure of the 20% and 30% TiC-fused cladding consists entirely of equiaxed crystals, and all fused cladding layers show a gray granular phase after TiC addition, with the 30% TiC-fused cladding layer demonstrating significantly more granularity and higher density.

Figure 5
The OM image of Fe35-fused cladding with different TiC mass fractions. (a, b, c) 10%TiC, (d, e, f) 20%TiC, (g, h, i) 30%TiC, (a, d, g) the bottom layer, (b, e, h) the middle layer, (c, f, i) the upper layer.

The SEM morphology in Figure 6 displays the Fe35-fused cladding with 10% TiC, in which the composition test was carried out at the marked positions in the figure. The corresponding test results are presented in Table 4.

Figure 6
SEM morphology of fused cladding with 10% TiC. (a) The bottom layer, (b) The middle layer, (c) The upper layer.

Table 4
The microzone composition analysis results at different positions marked in Figure 6 (mass fraction, %).

POSITION	C	N	Si	Ti	Cr	Mn	Fe	Ni	Mo	W
④	10.47	0	0.84	0.18	4	0.55	80.65	2.67	0.48	0.16
⑤	14.7	0.91	0.26	0.39	9.27	0.6	68.59	1.66	3.08	0.54
⑥	15.06	2.93	0.94	0.34	7.4	0.54	68.30	1.85	2.28	0.36
⑦	28.45	9.17	0.7	15.62	2.79	0.36	40.30	1.32	0	1.29
⑧	32.24	0.12	0.20	27.32	2.03	0.24	34.86	0.92	1.14	0.93

In the middle layer of the cladding at position ④, a high content of Mn, Si, and Ni is observed, indicating the presence of α-Fe solid solution. At position ⑤, a high content of Cr and Mo is noted, signifying the presence of the eutectic organization. Small particles dispersed at position ⑥ and small pieces at position ⑦ are rich in Ti and C elements and are judged to be a TiC phase. In the upper layer of the cladding at position ⑧, the formation of a large quadrilateral block of TiC is observed. As depicted in Figure 6, the eutectic organization in the bottom layer of the fused cladding exhibits a predominantly fine and narrow mesh distribution, moving toward the upper layer of the fused cladding. The eutectic organization gradually increases, and agglomeration becomes apparent. TiC is diffusely distributed with fine particles in the bottom layer of the fused cladding, while TiC aggregates and forms lumps in the upper layer.

Figure 7 displays the SEM image of the Fe35-fused cladding layer with 20% TiC. The microzone composition of the middle layer was analyzed, and the results are presented in Table 5. Positions ⑨ and ⑩ exhibit a high content of Ti and C elements, indicating TiC phases, while position ⑪ is rich in C and Cr elements and demonstrates eutectic organization. The observations outlined in Figure 7 indicate minimal variation in the quantity and morphology of the eutectic structure across the bottom to upper layers of the fused cladding. The majority of TiC in the fused cladding presents as lumps within the α-Fe crystals and at the phase boundary of the eutectic structure, with a larger size than that in the 10% TiC-fused cladding. The distribution area of the eutectic structure is relatively smaller than that in the 10% TiC-fused cladding. Furthermore, the fused cladding contains a few small pores.

Figure 7
The SEM image of the Fe35-fused cladding layer with 20% TiC. (a) The bottom layer, (b) The middle layer, (c) The upper layer.

Table 5
The microzone composition analysis results at different positions marked in Figure 7 (mass fraction, %).

POSITION	C	N	Si	Ti	Cr	Mn	Fe	Ni	Mo	W
⑨	23.11	0.92	0.76	33.21	2.35	0.56	37.22	1.4	0.47	0
⑩	32.44	0.74	0	46.55	0.93	0.07	11.95	0.18	3.58	3.56
⑪	8.08	1.43	0.53	0.37	5.13	0.68	81.02	1.54	1.21	0.01

Figure 8 shows the SEM morphology of the Fe35-fused cladding layer with a TiC mass fraction of 30%. The composition tests were performed in the microregions of the middle and upper layers, and the results are shown in Table 6. The middle layer at position ⑫ is primarily composed of Fe, Ni, Mn, and Si, indicating the presence of an α-Fe phase. At positions ⑬ and ⑭, the dispersed bulk phase exhibits a depletion of Fe elements and an enrichment of Ti and C elements, indicative of a TiC phase. Poor bonding exists between the blocky TiC particles and the surrounding matrix, resulting in small gaps between them. The ⑮ position test of the upper layer of the fused cladding showed a TiC phase, which was granular, larger in size, more numerous, and more diffuse than that of the middle layer, and more hole-like defects in the Fe35 fused cladding with 30% TiC content.

Figure 8
The SEM morphology of the Fe35-fused cladding layer with 30% TiC. (a) The bottom layer, (b) The middle layer, (c) The upper layer.

Table 6
The microzone composition analysis results at different positions marked in Figure 8 (mass fraction, %).

POSITION	C	N	Si	Ti	Cr	Mn	Fe	Ni	Mo	W
⑫	11.9	11.8	3.35	0.56	2.01	0.45	68.59	1.2	0.14	0
⑬	33.71	0	0.05	43.71	1.8	0.05	14.36	0.22	4.05	2.05
⑭	26.47	0	0.2	29.41	2.43	0.37	35.95	0.54	2.95	1.68
⑮	34.22	0.01	0.06	35.21	2.14	0.09	21.57	0.47	4.28	1.95

3.3. Microhardness

Figure 9 shows the microhardness of Fe35 cladding with different TiC mass fractions. When no TiC is added, the Fe35-fused cladding layer exhibits an average microhardness of 608.24 HV0.2. Upon the inclusion of 10% TiC, the average microhardness increases to 725.61 HV0.2. However, as the TiC content rises to 20%, the microhardness decreases to 585.34 HV0.2. When the TiC content reaches 30%, the average microhardness of the fused cladding layer is 621.18 HV0.2. After a mass fraction of 10% TiC is added, the microhardness of the fused cladding layer increases, which is due to the addition of TiC part in the decomposition. The decomposition of Ti, C, and other atoms will be solidly dissolved in the α-Fe solid solution, resulting in a certain strengthening effect. The TiC regenerated after dissolution and the unmelted small particles of TiC are diffusely distributed in the matrix as hard phases, which play the roles of pinning grain boundaries and hindering grain boundary slip and are favorable for increasing the microhardness. The SEM morphology indicates that the highest content of eutectic organization is found in the fused cladding layer with the addition of 10% mass fraction of TiC, which is beneficial for the hardness of the fused cladding layer. The decrease in microhardness of the 20% TiC-fused cladding layer is attributed to the aggregation of TiC in the cladding layer. The distribution of bulk TiC reduces the distribution area of fine diffuse TiC, so the small particles of TiC have limited effects in pinning the grain boundaries and hindering grain boundary slip. The microhardness test was performed by avoiding bulk TiC sites. Hence, the 20% TiC-fused cladding does not improve the matrix hardness, even though the total TiC mass fraction is higher. In particular, the presence of hole-like defects in the 20% TiC-fused cladding reduces the hardness of the material. When the TiC content is further increased to 30%, although the pore defects remain, their microhardness improvement is enhanced by the corresponding increase in TiC small particles due to the increased TiC content, resulting in the average microhardness of the 30% TiC fusion-coated layer being higher than that of the 20% TiC fusion-coated layer. QI et al. [²⁸] found that the hardness was lower than that of the Fe55-fused cladding after the addition of 5%, 10%, and 15% TiC to Fe55. The hardness continued to decrease with the increase in TiC content, which was considered to be caused by grain coarsening. The effect of the coarsened fused cladding microstructure on the microhardness was greater than that of the improvement of the second phase TiC. Grain coarsening also occurs after TiC addition in the current experiment.

Figure 9
The microhardness of Fe35 cladding with different TiC mass fractions.

3.4. Corrosion resistance

The Tafel curves of Fe35-fused cladding with different TiC mass fractions are shown in Figure 10. The curves were fitted, and the fitting results are shown in Table 7. The corrosion potential and corrosion current density are −0.595 V and 1.137 × 10⁻⁵A·cm⁻², respectively, when TiC is not added. However, with TiC addition, the corrosion potential is negatively shifted, the corrosion current density is increased, and the corrosion resistance is decreased. A higher mass fraction of TiC results in even worse corrosion resistance of the fused cladding layer. Although TiC itself has a high potential and excellent corrosion resistance, its addition to the Fe35-fused cladding results in the formation of Cr-rich carbides, the high dislocation density around the TiC particles, and the existence of a high potential difference between the matrix and the TiC phase; these phenomena lead to preferential corrosion at the phase boundary between the two phases and reduce the corrosion resistance of the fused cladding [²⁹]. The SEM image presents that when TiC is added, the fused cladding layer shows hole defects, the TiC particles are poorly bonded to the surrounding substrate, and small gaps exist between them, which reduce the integrity of the fused cladding layer. As the electrochemical corrosion proceeds, Cl^- in the solution can reach the deeper part of the fused cladding layer through the gaps and holes, which weakens the corrosion resistance of the fused cladding layer.

Figure 10
The Tafel curves of Fe35-fused cladding with different TiC in 3.5 mass·%NaCl solution.

Table 7
The fitting results of the tafel curves.

SPECIMEN	ICORR/A·cm−2	ECORR/V
1	1.137 × 10−5	−0.595
2	1.526 × 10−5	−0.669
3	3.375 × 10−5	−0.843
4	4.367 × 10−5	−0.847

The EIS curves of Fe35-fused cladding layers with different TiC mass fractions are shown in Figure 11. The capacitive arcs of the four specimens are similar. The size of the capacitive arc radius can correspond to the polarization resistance of the specimens in the electrochemical process; the larger the capacitive arc radius, the larger the polarization resistance, and the slower the corrosion rate. The Fe35 cladding exhibits the largest radius of the capacitive arc, indicating the highest stability of the passive film. However, with the addition of TiC, the stability of the passive film decreases. The greater the amount of TiC added, the worse the stability of the passive film becomes. R_s{Q₁[R₁(Q_filmR_film)]} was utilized to fit the equivalent circuit, resulting in the parameter values presented in Table 8. The circuit incorporates various elements, including R_s (solution resistance), Q₁ (constant phase angle element consisting of a bilayer capacitance Y₀₁ and a dispersion coefficient n₁ of the metal surface), R₁ (bilayer transfer resistance), Q_film (constant phase angle element corresponding to the corrosion product film, which includes the capacitance Y₀₂ of the rust film double layer and the dispersion coefficient n₂), and R_film (corrosion product film resistance). The sum of R₁ and R_film is considered the resistance of the polarization reaction, with higher polarization resistance indicating a more stable passive film. Table 8 reveals that the Fe35-fused cladding layer exhibits the highest polarization resistance. After the addition of TiC, the polarization resistance appears to decrease. Specifically, the polarization resistance decreases rapidly with the increase in TiC content, indicating that the polarization reaction of the fused cladding layer in the solution is accelerated and that the corrosion resistance of the fused cladding layer is decreased. This finding is consistent with the results of the Tafel curve test.

Figure 11
The EIS curves of Fe35-fused cladding layers with different TiC mass fractions in 3.5 mass·%NaCl solution.

Table 8
The fitting results of EIS curves in R_s{Q₁[R₁(Q_filmR_film)]} equivalent circuits.

SPECIMEN	RS/(Ω.cm2)	Q1		R1/(Ω.cm2)	QFILM		RFILM/(Ω.cm2)
		Y01/(S.sn.cm−2)	N1		Y02/(S.sn.cm-2)	N2
1	12.78	2.6134 × 10−5	0.7786	27455	4.7212 × 10−3	0.8602	2.385 × 1014
2	12.56	4.4988 × 10−5	0.7593	2908	8.1721 × 10−5	0.5245	8817
3	9.672	5.4884 × 10−5	0.7272	5186	9.6856 × 10−5	0.6562	3163
4	8.209	9.0830 × 10−5	0.7389	788.5	2.3824 × 10−6	1.4491	1379

4. CONCLUSIONS

• (1)

  The Fe35-fused cladding dendrite consists of α-Fe solid solution, while the intercrystal is a cocrystal composed of α-Fe solid solution and (Cr, Fe)₇C₃. The fused cladding layer consists of columnar crystals, columnar dendritic crystals, and isometric crystals from the bottom to the upper.

• (2)

  The bottom layer of the 10% TiC-fused cladding is a columnar crystal, while the middle and upper layers are isometric crystals. TiC is distributed in the matrix as finely dispersed small particles and small lumps. The 20% and 30% TiC-fused cladding layers are equiaxial crystals, and TiC is distributed in the matrix as agglomerated large particles and clusters. As the TiC content increases, the bonding of TiC to the surrounding matrix decreases.

• (3)

  The cladding layer containing 10% TiC exhibits the highest average microhardness, whereas the cladding layer with 20% TiC shows the lowest average microhardness. TiC is dispersed within the matrix in the form of fine particles, contributing positively to the improvement of the matrix’s microhardness. However, increasing the TiC content becomes unfavorable for enhancing microhardness because of the presence of agglomerated lumps, the appearance of holes in the cladding, and a deterioration in the bonding properties between the TiC phase and the matrix.

• (4)

  The fused cladding layer without TiC addition demonstrates the most positive corrosion potential, the lowest corrosion current density, and the most stable passive film, resulting in the highest corrosion resistance. However, the corrosion resistance of the fused cladding layer decreases with the introduction of TiC, deteriorating further with an increase in TiC content.

5. ACKNOWLEDGMENTS

The work was supported by The University Natural Science Key Project of Anhui Province Department of Education (2022AH051922); The High-quality Scientific Research Cultivation Project of Bengbu University (2021pyxm03).

6. BIBLIOGRAPHY

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Publication Dates

• Publication in this collection
  25 Aug 2025
• Date of issue
  2025

History

• Received
  05 Jan 2025
• Accepted
  14 July 2025