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Article

Microstructure and Corrosion Property of Prepared CoCrW Coatings on the TC4 Surface by Laser Cladding

by
Yu-Bin Sun
1,
Hao-Jie Niu
1,
Jia-Ying Wang
1,
Gui-Fu Dong
2 and
Cheng-Xin Lin
1,*
1
Naval Architecture and Ocean Engineering College, Dalian Maritime University, Dalian 116026, China
2
Hunan Automotive Engineering Vocational College, Zhuzhou 412000, China
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(10), 1687; https://doi.org/10.3390/coatings13101687
Submission received: 29 August 2023 / Revised: 13 September 2023 / Accepted: 18 September 2023 / Published: 26 September 2023
(This article belongs to the Topic Materials for Corrosion Protection)
Browse Figures
Versions Notes

Abstract

:
Ti6Al4V (TC4) is widely used in aerospace, marine equipment, and the petrochemical industry. However, the dense oxide film on the surface of this alloy will be destroyed in reducing acid solution, resulting in surface corrosion in practical application. To enhance the corrosion resistance of TC4 in marine environments, this study employed laser cladding technology to deposit a CoCrW cladding layer on the TC4 alloy surface. Experimental results validated the successful preparation of a dense, crack-free CoCrW layer. The microstructure of the CoCrW layer was characterized by predominant bulk grains and minor equiaxed crystal constituents, demonstrating a robust metallurgical bond to the matrix. Notably, the corrosion resistance of the TC4 surface witnessed a marked improvement, evident from the CoCrW coating’s increased open circuit potential, elevated electrochemical impedance spectroscopy (EIS) radius, phase angle, and impedance modulus values. The corrosion rates of both the TC4 and CoCrW cladding layers escalated with extended immersion time and increased immersion corrosion temperature. However, the CoCrW cladding layer reported minimal mass loss and the least corrosion rate. In summary, the CoCrW coating, when prepared via laser cladding on the TC4 surface, markedly bolstered corrosion resistance.

1. Introduction

Metal materials find extensive applications across various industries, making the enhancement of their surface properties crucial [1,2,3]. The TC4 alloy, colloquially termed as the “space metal” and “ocean metal”, is prevalently utilized in sectors such as aerospace and national defense, owing to its commendable comprehensive performance [4]. The suitability of the TC4 alloy for aggressive marine environments hinges on its indispensable corrosion resistance. While titanium alloys can form a stable oxide film in standard corrosion environments, offering protection to the underlying metal, the juxtaposition of TC4 alloy with other metals can precipitate electrochemical corrosion in real-world scenarios [5,6,7]. This phenomenon can culminate in the damage or detachment of the oxide film. Prolonged exposure to marine conditions subjects marine engineering to the combined impact of wear and corrosion, potentially jeopardizing its integrity and lifespan [8,9,10].
To improve the corrosion resistance of titanium and titanium alloy, surface modification technology is one of the effective ways to gain the comprehensive performance of the TC4 titanium alloy. In response to the challenges presented by corrosive environments, researchers have sought methods to enhance the corrosion resistance of TC4 [11,12,13,14,15,16,17]. Various techniques, including ion implantation, electroless plating, laser cladding, plasma spraying, vapor deposition, and micro-arc oxidation, are commonly utilized to modify the surface of the TC4 alloy. Sun et al. [18] found that an electroplate Ta-10W coating improved the substrate metal corrosion resistance by slowing down the corrosion tendency and rate, increasing the charge transfer resistance and dielectric property of the double layer. Liu et al. [19] suggest that according to the plasma immersion ion implantation (PIII) technique, the TC4 sample of Ag ion dose at 1.0 × 1017 ions/cm2 has the best corrosion resistance with the lowest corrosion current density and the least porosity in a 3.5% NaCl saturated solution. Zhou et al. [20] indicated in the experiment that TC4/ZrO powder is deposited by directed energy deposition (DED). As the content of alloying element Zr increased, the grains were refined. The corrosion product detection showed that ZrO2 oxide might reduce the compactness of the TC4 passive film containing TiO2 and Al2O3, which resulted in a slight decrease in corrosion resistance. However, each surface modification technology had its inherent limitations—the ion implantation layer, for instance, is thinner; electroless plating potentially pollutes the environment; plasma spraying exhibits poor adhesion; vapor deposition demonstrates a slower forming film and deposition rate; and the films produced by micro-arc oxidation carry numerous defects.
The CoCrW alloy, a member of the Stellite alloy family, stands out for its commendable hardness and corrosion resistance, further bolstered by its non-magnetic properties [21,22,23,24,25,26]. The CoCrW alloy is widely used in various fields because of its non-magnetic and corrosion-resistant characteristics. Previous studies by Lu et al. [27,28] have evaluated the microstructure, mechanical characteristics, and electrochemical behavior of CoCrW alloys in dental applications using selective laser melting (SLM) and demonstrated that CoCrW alloys possess outstanding corrosion resistance. You et al. [29] investigated the relationship between the microstructure and mechanical anisotropy of SLM-prepared CoCrW alloys, which revealed the alloy’s dense structure and superior mechanical properties. Miyake et al. [30] studied the microstructure and heat treatment effects on LMD-prepared CoCrW alloy, which indicated that heat treatment reduces carbon content, thereby enhancing hardness and improving corrosion resistance. On the basis of the above research, it can be found that CoCrW alloy has excellent wear and corrosion resistance, and it is promising to be applied to the TC4 surface of marine engineering to avoid the failure of TC4 due to its wear and corrosion.
Cobalt chromium tungsten alloy (CoCrW) is the hard alloy that can withstand all kinds of wear and corrosion and high temperature oxidation. Given these characteristics, the present study employed laser cladding technology to incorporate a CoCrW cladding layer onto the TC4 alloy surface, aiming to bolster its corrosion resilience in marine settings. Successful deposition of a high-density, crack-free CoCrW alloy layer onto the TC4 alloy was confirmed through this method. The microstructure of the CoCrW alloy layer was analyzed using scanning electron microscopy (SEM), while the corrosion properties of CoCrW and CrNi coatings on the TC4 alloy were evaluated through an electrochemical workstation and a comprehensive immersion test. The findings underscore that this approach can markedly enhance the corrosion resistance of the TC4 alloy, holding potential for marine engineering applications.

2. Materials and Methods

TC4 titanium alloy was selected as the matrix material with a dimension of 100 mm × 100 mm × 10 mm, and its nominal chemical composition is listed in Table 1.
Before initiating the laser cladding process, the matrix was cleaned using ultrasonic ethanol to remove oil contaminants. The cladding surface of the sample was then roughened by sandblasting using brown corundum with a particle size of 550 μm, aiming to boost the substrate material’s cladding absorption rate. The sandblasting equipment is shown in Figure 1.
To obtain micron-sized powders, a CoCrW alloy rod underwent gas atomization with nitrogen gas (N2). After atomization, the powder was classified into various particle size segments using a powder sieve. The selected atomized powder for this investigation had a particle size ranging from 53 μm to 150 μm and a purity of 99.99%. The atomizing dispersion equipment is shown in Figure 2.
This powder was then dried in a vacuum oven at 200 °C for two hours. Table 2 and Figure 1 illustrate the chemical compositions and morphologies of the two materials, respectively. As shown in Figure 3, it can be clearly seen that the diameter and size of the powder were quiet different—some powders were spherical and the shape of the powder particles was incomplete, which may be caused by the preparation processes.
The CoCrW powders were coaxially introduced through a powder feeder control and conveyed to the cladding zone by an inert gas (argon) at a pressure of 0.7 atmospheres for laser cladding. A DL-LPM-V type cross-flow CO2 laser processing system was employed alongside the powder feeding device. The laser parameters were set as follows: a laser power of 3 kW, laser spot diameter of 3 mm, traverse speed of 900 mm/min, overlap of 50%, and defocusing amount of +30 mm. Resultantly, a 1.2 mm thick laser cladding coating was acquired, with a post-grinding coating thickness of 0.3–0.6 mm.
The sample’s microstructure and morphology were inspected using a scanning electron microscope (SEM, Zeiss Sigma 300, Carl Zeiss AG, Oberkochen, German). EDS spectroscopy was employed to determine the elemental composition content16, and the crystals’ morphology and orientation were examined by electron backscatter diffraction (EBSD) using an Oxford Nordly max3 EBSD outfitted with an HKL-EBSD system. The phase structure of CoCrW coatings was ascertained by an X-ray diffractometer (XRD, smartlab9K, RIGAKU, Tokyo, Japan), operating at a 40 kV, 30 mA, and 1°/min scan rate, using a Cu-Kα radiation source with the angle range extending from 0° to 100°.
Electrochemical experiments employed standard three-electrode cells. Evaluation of the electrochemical properties of the cladding layer was conducted in a 3.5 wt.% NaCl solution (pH = 7), simulating a seawater environment, under 24 °C atmospheric pressure. Electrochemical testing utilized the CS2350H bipotentiostat. The setup included a saturated calomel electrode as the reference, a platinum electrode as the auxiliary, and the sample as the working electrode; the exposed area of the sample was 1 cm2. After stabilization of the open circuit potential (OCP), potentiodynamic polarization curves were derived, spanning from 0.2 V to 1.4 V, with a fixed scan rate of 0.5 mV/s and a voltage range of −1 V to 1 V. Each test was repeated three times, and the resultant data were averaged. The Tafel extrapolation technique was employed to determine the corrosion potential, corrosion current density, and corrosion rate. Electrochemical impedance spectroscopy (EIS) was applied for coating analysis in the three-electrode setup.
The corrosion resistance of the CoCrW coatings and TC4 matrix in harsh marine and acidic environments was assessed following the immersion test protocols outlined in the ASTM G1-03 [31] and ASTM G31-2012 standards [32]. To replicate the marine environment of the hull, a 3.5 wt.% NaCl solution was utilized. The acidic corrosion environments were emulated using 0.5M H2SO4, 0.5M HCl, and 0.5M HNO3 solutions. Immersion tests for both the cladding and matrix were executed at 20 °C, 50 °C, 80 °C, and 110 °C, respectively.

3. Results

3.1. Microstructure of the Cladding Layer

Figure 4 displays the SEM microstructure and EDS map, emphasizing the elemental distribution at the interface between the CoCrW cladding layers and the TC4 matrix. Figure 4a–c present the surface morphology of the CoCrW cladding layer. A distinct metallurgical bonding interface between the CoCrW coating and the substrate emerges, characterized by a compact surface structure absent of cracks and pores. The magnified map from Figure 4b uncovers a characteristic microstructure and a dendritic network morphology for the CoCrW coating. Its color dominantly presents as a consistent gray, with occasional black central spots, a consequence of electron scattering during the electron scanning imaging process. Owing to uneven temperature gradients in the laser cladding regions, parts solidify at varying rates, generating a net-like morphology and dense dendritic microstructure adjacent to the coating interface, as illustrated in Figure 4a. Comparing Figure 4b,c indicates that the CoCrW coating exhibits a notably consistent and dense microstructure post laser cladding. EDS analysis was executed on various sections of the cladding layer to qualitatively determine its composition and its metallurgical bond to the matrix, as depicted in Figure 4c. EDS findings suggest the bright gray area primarily comprises Co and Cr elements, while the darker gray region also predominantly features Co and Cr, but with a marked rise in Ti content. Among them, the distribution of Ti, Co, Cr, and W in the matrix and cladding layer is shown in Figure 4f–i. Based on the binary phase diagram, Co and Ti elements are believed to constitute a Co2Ti compound. Moreover, EDS data imply the densely clustered light gray dendritic structure corresponds to the Cr hard phase, while the dark gray area, with a heightened chromium and nickel concentration, might represent a nickel-based solid solution. The element interdiffusion between the cladding layer and matrix, illustrated in Figure 4f–i, indicates a robust metallurgical bond between the TC4 matrix and cladding layer. The elemental mass percentages at the interface are detailed in Table 3.
To substantiate the crystal structure and grain orientation of the cladding layer, EBSD and XRD diffraction analysis were performed on the CoCrW cladding layer, with results illustrated in Figure 5 and Figure 6. Using the EBSD inverse pole figure (IPF) where grain color indicates grain orientation, Figure 5 suggests that the CoCrW cladding layer displays features consistent with an equiaxed grain microstructure. The grain colors within the CoCrW cladding layers demonstrate minimal variation, suggesting a uniform grain orientation within the CoCrW cladding layer at this juncture. The lack of distinct grain orientation combined with evidence of disordered grain growth indicates a microstructure dominated by bulk grains with a minor proportion of equiaxed grains. The overall grain size within the CoCrW coating is observed to be large and continuous.
Figure 6 presents the X-ray diffraction analysis of the CoCrW cladding conducted at room temperature. The diffraction pattern from Figure 6 reveals that the CoCrW cladding predominantly consists of three phases: γ-Co (with a face-centered cubic structure), Co3Cr(Ti,W)2, and CoCr(Ti,W). The second phase in the CoCrW alloy, identified by EDS analysis, is characterized as γ-Co in a solid solution, incorporating significant amounts of Cr, Ti, and W elements. The gray dendritic phase at spot 1 corresponds to Co3Cr(Ti,W)2. Due to its diminutive grain size, the white dot phase posed difficulties for EDS analysis and is slated for subsequent studies. According to the EDS findings, the predominance of the γ-Co matrix can be ascribed to the solubility of Cr, Ti, and W elements within Co. A notable distinction between spots 1 and 2 is the difference in Ti element concentration, with point 1 having an elevated Ti content and spot 2 exhibiting a reduced concentration.

3.2. Electrochemical Corrosion Behavior of Cladding Layer

3.2.1. Open Circuit Potential

Figure 7 presents the open circuit potential (OCP) test results, assessing the evolution of the self-corrosion potential of the sample without the imposition of external current. Each OCP curve manifests an initial ascent, transitioning towards equilibrium before reaching saturation. The OCP of the TC4 matrix takes a protracted duration to saturate, in contrast to the CoCrW coating, which saturates more expeditiously. This rapid saturation of the CoCrW coating suggests its swift passivation, culminating in the establishment of a stable passivation film on its surface. A shorter time for OCP to attain its steady-state value, coupled with a heightened steady-state value, signifies augmented corrosion resistance of the sample. As delineated in Figure 7, the CoCrW cladding layer achieves its steady-state value much faster than the TC4 matrix and records a notably elevated steady-state value in comparison. Such data underscores the enhanced corrosion resistance of the TC4 alloy when incorporated with the CoCrW coating, relative to the uncoated TC4 matrix.

3.2.2. Potentiodynamic Polarization Curve

Figure 8 presents the potentiodynamic polarization curves for TC4 alloys with distinct cladding layers and matrices, tested in a 3.5%wt NaCl solution. Parameters including self-corrosion potential (Ecorr), corrosion current density (Icorr), corrosion rate (Vcorr), and polarization resistance (RP) are quantified and summarized in Table 4. Both corrosion current density and self-corrosion potential are pivotal indicators when evaluating material corrosion resistance. The corrosion rate of materials is principally dictated by the corrosion current density, suggesting that materials with reduced corrosion current density undergo corrosion at a diminished rate [27]. The self-corrosion potential, on the other hand, indicates the inherent corrosion propensity of a material; a more elevated value denotes increased resistance against corrosion [33]. Analyzing Figure 8, it becomes apparent that the CoCrW cladding layer possesses reduced erosion susceptibility, elevated corrosion resistance, and provided an overarching superior performance against corrosion. This superior performance is primarily ascribed to the development of a robust oxide film on the surface, in conjunction with the cladding layer’s compact microstructure. This results in optimal metallurgical bonding and shields against corrosion by the electrolyte, ultimately augmenting the material’s resistance to corrosion. Furthermore, in the realm of corrosion electrochemistry, polarization resistance stands as a vital kinetic metric. A heightened polarization resistance directly correlates with improved corrosion resistance. Notably, the CoCrW cladding layer manifests the highest polarization resistance at 7.53 × 106, a value substantially surpassing that of the TC4 matrix, which is at 3.12 × 103.

3.2.3. Electrochemical Impedance Spectroscopy

Figure 9 showcases the AC resistance spectra for the CoCrW cladding layer and the TC4 matrix in a 3.5 wt.% NaCl solution, elucidating their electrochemical corrosion behavior. The Nyquist plot presented in Figure 9a delineates the real part (Z′) and the imaginary part (Z″) of the impedance. Typically, the radius of the capacitive arc is employed to characterize corrosion resistance. An enlarged capacitive arc radius in the cladding layer denoted enhanced electrochemical corrosion resistance. This observation was consistent with the results from the potentiodynamic polarization curve, with the most pronounced capacitive arc radius pertaining to the CoCrW cladding layer. Furthermore, as depicted in Figure 9b,c, the impedance modulus |Z| served as a metric to evaluate the material’s corrosion resistance. An escalation in the |Z| value indicated an augmentation in the corrosion resistance of the cladding layer. The larger the |Z| value, the superior the corrosion resistance was. Similarly, the phase angle degree was utilized to denote the material’s ability to thwart electrolyte penetration. A cladding layer boasting a heightened phase angle signified more robust resistive capabilities, translating to elevated electrochemical corrosion resistance. The adherence between the impedance modulus |Z| and phase angle of the cladding layer and matrix mirrored the trends observed in both the dynamic polarization and Nyquist plots. Notably, the CoCrW cladding layer’s Bode angle surpassed that of the matrix, peaking at approximately 96°, underscoring its minimized corrosion rate and exemplary corrosion resistance. In essence, the magnified phase angle fostered an enhanced passivation degree in the cladding layer. The relationship between Bode-impedance for the cladding layer and matrix is expounded upon in Figure 9c.
In the high-frequency domain, the electrolyte impedance of the cladding layer was analogous to that of the matrix. However, in the low-frequency domain (F < 1 Hz), the impedance spectrum modulus associated with the cladding layer significantly surpassed that of the matrix. Such findings indicate a pronounced enhancement in the corrosion resistance of the CoCrW cladding layer relative to the matrix, as depicted in Figure 9c. Moreover, the maximum impedance resistance was manifested by the CoCrW cladding layer, emphasizing the minimized corrosion rate of the CoCrW coating within this experimental setting.
The Z SimpWin software (https://www.ameteksi.com/products/software/zsimpwin) was employed to select an appropriate equivalent circuit diagram corresponding to the measured electrochemical impedance, as depicted in Figure 10. Upon exposure to NaCl solution, spontaneous passivation was evident in the CoCrW cladding coating, leading to the development of a passivating film on the coating surface, thereby mitigating further infiltration by the corrosive medium. The Rp values for both the cladding layer and matrix, derived from the fitting parameters, are presented in Table 5 and Table 6. These results emphasize that the Rp value for the cladding layer significantly surpassed that of the matrix, with the Rp for the CoCrW cladding layer in NaCl solution reaching 2.65 × 107 Ω·cm2. Given that a chromium passivation film formed on the CoCrW binder phase surface, the corrosive medium was confined to permeating only the compromised passivation areas at the metal hard phase interface before initiating additional corrosion in the coating. Consequently, the CoCrW coating exhibited superior corrosion resistance.

3.3. Total Immersion Corrosion Behavior of the Cladding Layer

The average corrosion rate per unit time was determined by assessing the mass loss of samples that were fully immersed in different solutions. The calculation was carried out using the following formula [34]:
Corrosion rate = KW/ATD
According to the requirements of laboratory immersion test designed according to ASTM G1-03 and ASTM G31-2012, K was the formula constant.
Among them, when the full immersion corrosion object was titanium alloy substrate and CoCrW cladding layer, K were 4.54 × 104, 8.76 × 104, respectively.
  • T = Immersion time, calculated by hour, accurate to 0.01 h.
  • A = unit cm2, accurate to 0.01 cm2
  • W = Mass loss, accurate to 1 mg (including for mass loss during cleaning)
  • D = density g/cm2
Table 7 presents the progressive increase in the average corrosion rates of both the TC4 substrate and CoCrW coatings over time, in tandem with the escalation of the total immersion corrosion temperature. Notably, the CoCrW coating manifested the minimal mass loss and the most diminished corrosion rate, indicating pronounced corrosion resistance. Table 7 further delineates the superior corrosion resistance of the CoCrW coating in seawater and acidic environments, as it consistently registered a reduced corrosion rate compared to the TC4 substrate. Both the CoCrW coating and the TC4 substrate displayed escalating average corrosion rates with prolonged immersion. Within identical immersion durations, the corrosion rate for the TC4 substrate surpassed that of the CoCrW coating. Specifically, the CoCrW coating consistently exhibited the most diminished corrosion rate, suggesting its robust corrosion resistance in both seawater and acidic scenarios. Such a trend insinuates enhanced corrosion resistance of the cladding layers compared to the matrix under comprehensive immersion corrosion conditions. A plausible rationale for this phenomenon could be the formation of a passivation film upon immersion of the cladding layer, which effectively mitigated corrosion, thereby amplifying the corrosion resistance.

4. Conclusions

In this research, laser cladding technology was employed to fabricate a CoCrW cladding layer on a TC4 matrix. An exhaustive examination of the microstructure and corrosion properties of both the cladding layer and the matrix was undertaken, leading to these key insights:
  • The CoCrW coating manifested a compact structure devoid of cracks, signifying a sound metallurgical integration with the matrix. Surface microstructures of the CoCrW cladding layer displayed prevalent dendrite patterns and interdendritic networks, typified by irregular dendritic accumulations set within a dark gray matrix backdrop. EBSD analyses did not identify any pronounced grain orientation within the CoCrW cladding, implying that the microstructure predominantly consists of large grains interspersed with occasional equiaxial grains. XRD assessments further verified that both the primary and secondary pronounced diffraction peaks of the CoCrW cladding layer were indistinguishably congruent, indicative of cubic microstructures. EBSD results show that CoCrW cladding layer hardly presents preferentially oriented growth.
  • OCP assessments revealed that the duration for the CoCrW coating to stabilize was markedly briefer compared to TC4, while its stabilized value substantially exceeded that of TC4. Such observations underscore enhanced corrosion resistance of the TC4 alloy with the CoCrW coating as opposed to its uncoated counterpart.
  • The CoCrW cladding layer showcased a minimal corrosion current density (1.65 × 10−7) and a heightened self-corrosion potential (−0.1503), coupled with a corrosion rate of a mere 5.363 × 10−4. From this, it can be deduced that the CoCrW cladding layer possesses diminished corrosion propensity and elevated corrosion resistance. This is attributed to the compact microstructure of the cladding layer, its adept metallurgical fusion, and the genesis of a resilient oxide film on its exterior, collectively impeding electrolytic corrosion and bolstering corrosion resistance. Additionally, the CoCrW cladding layer displayed a superior polarization resistance (7.53 × 106), which markedly surpassed that of the TC4 matrix (3.12 × 103).
  • Data gleaned from the potentiodynamic polarization curve highlighted that the capacitive arc radius, impedance modulus |Z|, and phase angle for the CoCrW cladding layer were the most pronounced. Consequently, the CoCrW cladding bore witness to the least corrosion rate, epitomizing optimal corrosion resistance.
  • Comprehensive immersion corrosion assays showed that the average corrosion rates for both the TC4 matrix and the CoCrW cladding layer escalated in tandem with immersion duration and temperature. However, the CoCrW cladding layer bore the minimal brunt in terms of mass loss and corrosion rate, reflecting its superior corrosion resistance. A plausible deduction suggests that the inception of a passivation film upon immersion of the cladding layer augments its corrosion resistance.

Author Contributions

Conceptualization, Y.-B.S.; methodology, H.-J.N.; software, J.-Y.W.; validation, G.-F.D.; funding acquisition, C.-X.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Science and Technology Innovation Fund Project of Dalian, China (No. 2020JJ25CY016).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 13 01687 g001
Figure 1. 1212A general pressure box sandblasting machine.
Figure 1. 1212A general pressure box sandblasting machine.
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Figure 2. VIAG atomizing dispersion equipment.
Figure 2. VIAG atomizing dispersion equipment.
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Figure 3. (a) The morphology of CoCrW powder. (b) The enlarged part of the circle box in Figure 3a.
Figure 3. (a) The morphology of CoCrW powder. (b) The enlarged part of the circle box in Figure 3a.
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Coatings 13 01687 g004aCoatings 13 01687 g004b
Figure 4. SEM morphology and EDS analysis of the interface between CoCrW cladding layers and TC4 matrix. (a) Bonding surface morphology of CoCr W coating without etching. (b) Bonding surface morphology of CoCr W coating. (c) Enlarged map of area C in Figure 2b. (d,e) Element distribution of the CoCrW coating surface at points 1 and 2. (f) Map of Ti element. (g) Map of Cr element. (h) Map of Co element. (i) Map of W element.
Figure 4. SEM morphology and EDS analysis of the interface between CoCrW cladding layers and TC4 matrix. (a) Bonding surface morphology of CoCr W coating without etching. (b) Bonding surface morphology of CoCr W coating. (c) Enlarged map of area C in Figure 2b. (d,e) Element distribution of the CoCrW coating surface at points 1 and 2. (f) Map of Ti element. (g) Map of Cr element. (h) Map of Co element. (i) Map of W element.
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Figure 5. (a) EBSD image of the CoCrW coating IPF map. (b) Orientation map.
Figure 5. (a) EBSD image of the CoCrW coating IPF map. (b) Orientation map.
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Figure 6. XRD pattern of CoCrW coating.
Figure 6. XRD pattern of CoCrW coating.
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Figure 7. Open circuit potential (OCP) of coatings in 3.5%wt NaCl solution.
Figure 7. Open circuit potential (OCP) of coatings in 3.5%wt NaCl solution.
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Figure 8. Potentiodynamic polarization curve of coating.
Figure 8. Potentiodynamic polarization curve of coating.
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Figure 9. Electrochemical impedance spectroscopy (EIS) of the sample (a) Nyquist plots, (b) Bode phase angle plot, (c) Bode-impedance plot.
Figure 9. Electrochemical impedance spectroscopy (EIS) of the sample (a) Nyquist plots, (b) Bode phase angle plot, (c) Bode-impedance plot.
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Figure 10. Equivalent circuit diagrams corresponding to substrate and cladding layer. (a) TC4 matrix (R(CR)), (b) CoCrW coating R(C(R)) (C(RW)).
Figure 10. Equivalent circuit diagrams corresponding to substrate and cladding layer. (a) TC4 matrix (R(CR)), (b) CoCrW coating R(C(R)) (C(RW)).
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Table 1. Chemical composition of the TC4 substrate.
Table 1. Chemical composition of the TC4 substrate.
ElementAlVFeOSiCNHTi
Content (wt.%)5.50–6.803.50–4.500.300.200.150.100.050.01Bal.
Table 2. Chemical composition of the CoCrW alloy powders.
Table 2. Chemical composition of the CoCrW alloy powders.
ElementCrWNiFeCo
Content (wt.%)29.559.730.620.20Bal
Table 3. CoCrW coating element mass percent.
Table 3. CoCrW coating element mass percent.
Point 1 Point 2
Elementwt.%Elementwt.%
Ti36.41Ti28.37
Cr11.94Cr26.72
Co49.30Co39.86
W2.35W5.06
Table 4. Polarization curve corrosion properties of CoCrW cladding layer in NaCl solution.
Table 4. Polarization curve corrosion properties of CoCrW cladding layer in NaCl solution.
Ecorr (V)Icorr (A/cm−2)Vcorr (mm × a−1)Rp (Ω × cm2)
CoCrW coating−0.15031.65 × 10−70.00053637.53 × 106
TC4 Matrix−0.47465.10 × 10−50.00411173.12 × 103
Table 5. Electrochemical impedance parameters of TC4 matrix.
Table 5. Electrochemical impedance parameters of TC4 matrix.
TC4 matrixRs (Ω·cm2)Cdl (Ω·cm−2·sn)ndlRct (Ω·cm2)
29.559.730.620.20
Table 6. Electrochemical impedance parameters pf CoCrW coating.
Table 6. Electrochemical impedance parameters pf CoCrW coating.
CoCrW CoatingRs (Ω·cm2)Cdl (Ω·cm−2·sn)ndlRct (Ω·cm2)Cb (Ω·cm−2·sn)NbRp (Ω·cm2)Zw (sec5·cm−2)
93.782.46 × 10−70.923.12 × 1034.72 × 10−70.932.65 × 1073.68 × 10−7
Table 7. The average corrosion rates of TC4 matrix; CoCrW coatings increased with time and total immersion corrosion temperature.
Table 7. The average corrosion rates of TC4 matrix; CoCrW coatings increased with time and total immersion corrosion temperature.
MaterialImmersion Time/hTemperature/°CSolutionMass Loss/gCorrosion Rate/(mm/h)
TC4 Matrix9620NaCl0.5254.17
9650NaCl0.7072.92
9680NaCl1.20125.01
96110NaCl2.80291.67
9620HCl0.4850.03
9650HCl0.7477.08
9680HCl1.40145.83
96110HCl3.50364.58
9620H2SO40.4850.05
9650H2SO40.8285.42
9680H2SO41.39144.79
96110H2SO44.79498.96
9620HNO30.8487.50
9650HNO30.8891.67
9680HNO32.60270.83
96110HNO35.40562.50
TC4 + CoCrW cladding layer9620NaCl0.4243.75
9650NaCl0.5052.08
9680NaCl1.01105.21
96110NaCl1.80187.50
9620HCl0.3738.54
9650HCl0.5254.17
9680HCl0.98102.08
96110HCl1.50156.25
9620H2SO40.3435.42
9650H2SO40.5052.08
9680H2SO40.99103.13
96110H2SO41.79186.46
9620HNO30.4647.92
9650HNO30.5759.38
9680HNO30.98102.08
96110HNO31.80187.50
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MDPI and ACS Style

Sun, Y.-B.; Niu, H.-J.; Wang, J.-Y.; Dong, G.-F.; Lin, C.-X. Microstructure and Corrosion Property of Prepared CoCrW Coatings on the TC4 Surface by Laser Cladding. Coatings 2023, 13, 1687. https://doi.org/10.3390/coatings13101687

AMA Style

Sun Y-B, Niu H-J, Wang J-Y, Dong G-F, Lin C-X. Microstructure and Corrosion Property of Prepared CoCrW Coatings on the TC4 Surface by Laser Cladding. Coatings. 2023; 13(10):1687. https://doi.org/10.3390/coatings13101687

Chicago/Turabian Style

Sun, Yu-Bin, Hao-Jie Niu, Jia-Ying Wang, Gui-Fu Dong, and Cheng-Xin Lin. 2023. "Microstructure and Corrosion Property of Prepared CoCrW Coatings on the TC4 Surface by Laser Cladding" Coatings 13, no. 10: 1687. https://doi.org/10.3390/coatings13101687

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