Microstructures and Corrosion Behaviors of Non-Equiatomic Al_0.32CrFeTi_0.73(Ni_1.50−xMo_x)(x = 0, 0.23) High-Entropy Alloy Coatings Prepared by the High-Velocity Oxygen Fuel Method

Abstract

The non-equiatomic Al_0.32CrFeTi_0.73(Ni_1.50−xMo_x) (x = 0, 0.23) high-entropy alloy (HEA) coatings were prepared by the high-velocity oxygen fuel (HVOF) method. The microstructures and corrosion behaviors of the HVOF-prepared coatings were investigated. The corrosion behaviors were characterized by polarization, EIS and Mott-Schottky tests under a 3.5 wt.% sodium chloride aqueous solution open to air at room temperature. The Al_0.32CrFeTi_0.73Ni_1.50 coating is a simple BCC single-phase solid solution structure compared with the corresponding poly-phase composite bulk. The structure of the Al_0.32CrFeTi_0.73Ni_1.27Mo_0.23 coating, combined with the introduction of the Mo element, means that the (Cr,Mo)-rich sigma phase precipitates out of the BCC solid solution matrix phase, thus forming Cr-depleted regions around the sigma phases. The solid solution of large atomic-size Mo element causes the lattice expansion of the BCC solid solution matrix phase. Micro-hole and micro-crack defects are formed on the surface of both coatings. The growth of both coatings’ passivation films is spontaneous. Both passivation films are stable and Cr₂O₃-rich, P-type, single-layer structures. The Al_0.32CrFeTi_0.73Ni_1.50 coating has better corrosion resistance and much less pitting susceptibility than the corresponding bulk. The corrosion type of the Mo-free coating is mainly pitting, occurring in the coating’s surface defects. The Al_0.32CrFeTi_0.73Ni_1.27Mo_0.23 coating with the introduction of Mo element increases pitting susceptibility and deteriorates corrosion resistance compared with the Mo-free Al_0.32CrFeTi_0.73Ni_1.50 coating. The corrosion type of the Mo-bearing coating is mainly pitting, occurring in the coating’s surface defects and Cr-depleted regions.

1. Introduction

High-velocity oxygen fuel (HVOF) is a practical thermal spraying process, which is widely used for the preparation of surface coatings. Compared with plasma spraying, the coatings prepared by HVOF have higher density, lower porosity, lower oxide content and higher interface-bonding strength. Its heat source comes from the combustion heat of fuel and oxygen. The particle speed is very fast, and can reach 1180 m/s. And the extremely high impact cooling rate can reach 10⁷–10⁸ K/s [1,2,3,4,5].

A. Vallimanalan et al. [6] prepared a BCC single-phase solid solution of AlCoCrMoNi high-entropy alloy (HEA) coating by HVOF, which has a better corrosion resistance compared with a traditional NiCrSiB HVOF coating. L.W.Zhang et al. [7] used HVOF to prepare the AlCoCrFeNi HEA coating, which was a BCC single-phase structure, and which had no effect on the magnetic properties of sintered NdFeB magnet substrate but greatly improved the corrosion resistance of the substrate. A. Silvello et al. [8] used HVOF to prepare the dense and high-bond strength Cantor FeCoCrNiMn HEA coating, whose porosity was very low (less than 1%), and which contained Fe oxides and MnCr oxides. The presence of oxides increased the surface hardness and wear resistance of the coating, but was detrimental to its corrosion resistance, which was better than that of low carbon steel. N.V. Abhijith et al. [9] prepared TiNbMoMnFe powder by mechanical alloying, and the corresponding coating was prepared by HVOF. The coating prepared by grinding powder for 15 h was a BCC single-phase structure with the best corrosion resistance, which was better than that of 304 L stainless steel, and which increased hydrophobicity. A. Meghwal et al. [10] prepared a AlCoCrFeNi coating by HVOF which contained a negligible amount of oxide inclusions, and whose phase structure was composed of a FeCr-rich BCC matrix and a small amount of NiAl-rich B2 phase. Compared with 316 L stainless steel, the coating had a better general corrosion resistance under filtered seawater (pH = 6.8). However, the local corrosion resistance was poor, and the surface passivation film was unstable, mainly due to the formation of Cr-rich BCC and Cr-depleted B2 phases. The selective dissolution of the Cr-depleted B2 phase eventually resulted in pitting corrosion of the coating.

At present, there is relatively little research on the corrosion behaviors of HVOF-prepared HEA coatings, and most of the coatings prepared are equiatomic HEA coatings. In the present study, the microstructures and electrochemical corrosion behaviors under a 3.5 wt.% sodium chloride aqueous condition of HVOF-prepared non-equiatomic Al_0.32CrFeTi_0.73(Ni_1.50−xMo_x) (x = 0, 0.23, labeled as Mo0 and Mo23, respectively) HEA coatings were studied.

2. Experimental Procedures

2.1. Coating Preparation

The non-equiatomic HEA coatings were fabricated by the JP5000 HVOF spraying system (JP5000, Praxair Technology Inc., Phoenix, AZ, USA). JP5000, with automatic control, easy operation and high safety is designed for production of a relatively small size, and it can work in severe environmental conditions. It can prepare high-quality coatings using high-quality kerosene as fuel, and nitrogen or argon as carrier gas. The JP5000 HVOF spraying system is mainly composed of three units, namely a spray gun, a powder feeder and a console panel. The spray gun is fixed on the robot arm. The gun barrel length is 4 inches.

Table 1. Spray parameters employed in the HVOF process.

Kerosene Flow (L/h)	Oxygen Flow (L/min)	Carrier Flow (L/min)	Feed Rate (r/min)	Stand-Off Distance (mm)
28	944	12.3	15	380

lists the main HVOF spray process parameter values used in this study, which ensure the quality of the prepared coatings.

The coating substrate was Q345 low-alloy steel. The substrate surface was grit-blasted and preheated by the spray gun’s flame before spraying with powder. The HEA powder for the HVOF coating was prepared by a gas-atomization method. The gas-atomization powder production process is mainly composed of five parts, namely, a gas supply system, a metal/alloy melting system, an atomizing nozzle, an atomizing chamber, a powder collector and a gas-extraction system. Figure 1 shows the microstructure morphology of prepared HEA powders. The morphology of both the Mo0 and Mo23 powders is similar and mainly spherical, which is conducive to ensuring the good flow of the powder during the thermal spraying process. The particle-size distribution range of both powders is about 40–65 μm.

Figure 2 gives X-ray diffraction (XRD) patterns of the Mo0 and Mo23 (a) powders and (b) bulks. The XRD spectra of both the powders show that their crystal structures are poly-phase composite structures, both composed of a BCC solid solution matrix phase and several intermetallic compound phases, as shown in Figure 2a. Compared with that of the corresponding bulks, as shown in Figure 2b, the diffraction peak intensity of the BCC solid solution matrix phase of the powders was strengthened, while the diffraction peak intensity of the intermetallic compound phases of the powders was weakened, and even clearly dissipated, as shown in Figure 2a. This was mainly due to the fact that the rapid cooling rate during gas atomization can inhibit the growth of intermetallic compound phases, but the cooling rate during gas atomization is not rapid enough to completely eliminate the precipitation of the intermetallic compound phases from the BCC solid solution matrix phase [10]. As Mo element has a relatively larger atomic radius, the solid solution of Mo into the BCC matrix phase will lead to lattice expansion, thus the lattice constant of the BCC matrix phase increases, and the corresponding diffraction peaks shift leftwards [11,12,13]. Figure 3 shows a schematic of the coating preparation process. Figure 4 shows the HVOF-prepared, non-equiatomic HEA coating samples.

2.2. Electrochemical Measurements

The sample surface was wet-ground mechanically and polished to a mirror finish. Then the electrochemical corrosion behaviors were characterized by a polarization test, electrochemical impedance spectrum (EIS) and Mott-Schottky test by using the electrochemical workstation (CHI 660 E, Shanghai Chenhua Co., Ltd., Shanghai, China) under a 3.5 wt.% sodium chloride aqueous solution open to air at room temperature. A three-electrode electrochemical cell was used with the coating sample as the working electrode, saturated calomel electrode (SCE) as the reference electrode and platinum sheet as the counter-electrode. The exposed surface area of the sample to sodium chloride aqueous solution was 1 cm². The working electrode was first immersed in the electrolyte long enough to obtain a stable open circuit potential (OCP), then potentiostatically polarized at the −0.8 V vs. OCP for 5 min to remove possible surface oxide impurities. Subsequently, the sample was immersed in the solution to obtain a stable OCP, then EIS, potentiodynamic polarization and Mott-Schottky tests were performed. The EIS test was carried out under the stable OCP, with a frequency range of 100 kHz to 10 MHz and a sinusoidal potential amplitude of 10 mV. The impedance analysis software ZsimpWin 3.30d was used to fit the experimental data. Electrochemical corrosion parameters were obtained by the potentiodynamic polarization test at a scanning rate of 1 mV/s. The Mott-Schottky test potential range was in the passivation zone, the fixed frequency was 1 kHz, the AC voltage signal amplitude was 5 mV, and the potential scanning rate was 25 mV/step. The passivation potential range and high scanning rate can prevent the reduction and structural changes of the passivation film during the Mott-Schottky test, thus relative stability of the passivation film is maintained. It should be noted that the polarization test and the Mott-Schottky test were performed on different samples. All tests were repeated at least three times to ensure good data reliability.

3. Results and Discussion

3.1. Al_0.32CrFeTi_0.73Ni_1.50 Coating

In this section, the corrosion behavior in the 3.5 wt.% sodium chloride solution of the Al_0.32CrFeTi_0.73Ni_1.50 coating is studied, with the corresponding bulk as a contrast.

3.1.1. Structural Analysis

XRD and a scanning electron microscope (SEM) equipped with an energy disperse spectroscopy (EDS) were used to characterize the coating structure. Figure 5 shows XRD patterns of the Mo0 bulk and Mo0 coating. It can be seen that the HVOF coating was a single-phase structure of BCC solid solution, while the corresponding bulk was a poly-phase composite structure composed of a BCC solid solution matrix phase and several intermetallic compound phases, such as Cr-depleted Eta and Heusler phases. This indicates that HVOF greatly inhibits the precipitation of intermetallic compound phases, thus greatly simplifying the coating microstructure. HVOF has a very high impact cooling rate which can reach 10⁷–10⁸ K/s. The non-equilibrium solidification process of this very high cooling rate inhibits the formation of poly-phase and thus forms a simple BCC single-phase structure coating. At the same time, the sluggish diffusion effect of HEA will amplify this influence [5,10,15], and it is obvious that this resulting simple single-phase structure can improve corrosion resistance well. As shown in Figure 6, the diffraction peak intensity of the BCC solid solution matrix phase of the HVOF coating greatly increases, and the width widens, which indicates that the grain size of the BCC solid solution decreases due to the extremely high cooling rate of HVOF inhibiting grain growth, thus forming a very uniform coating structure with finer grains. At the same time, the BCC matrix phase crystal lattice of the HVOF coating expands due to the solid solution of each element into the BCC matrix phase, thereby shifting the BCC diffraction peak to the left [10,12,16,17,18]. Figure 6 gives SEM and element-mapping images of the Mo0 coating surface. It can be seen that the distribution of all elements is uniform, which is consistent with the BCC single-phase structure, and there are micro-holes on the coating’s surface. Figure 7 gives SEM images of local details of the Mo0 coating. It can be seen that there are micro-holes and micro-cracks on the coating surface, indicated by red arrows, and the spraying powder particles are flattened by supersonic impact-forming sprayed splats.

3.1.2. Potentiodynamic Polarization Tests

The potentiodynamic polarization curves of the Mo0 bulk and Mo0 coating are shown in Figure 8. Both curves clearly reveal positive hysteresis, demonstrating pitting susceptibility, while the positive hysteresis loop area of the Mo0 coating is much smaller than that of the Mo0 bulk. The passivation zone of the coating is smooth and has no current transients compared with that of the bulk. This indicates that the coating greatly reduces pitting susceptibility and enhances the stability of the passivation film compared with the bulk, which is due to the fact that the coating is mainly a BCC solid solution single-phase structure eliminating the heterogeneous non-uniformity of the bulk. The distribution of elements in the coating is uniform, and there is no Cr-depleted zone, but the micro-holes on the coating surface make it still susceptible to pitting. In addition, both curves enter the passivation region directly from the Tafel active region without a transition region, which means that the growth of both passivation films is spontaneous [10,19,20,21,22,23,24,25].

Table 2. Electrochemical corrosion parameters extracted from the potentiodynamic polarization curves for the Mo0 bulk and Mo coating.

Samples	Ecorr (VSCE)	Icorr (A/cm2)	Epit (VSCE)	Eprot (VSCE)	Ipass (A/cm2)	Epit − Eprot (VSCE)
Mo0 bulk	−0.88	8.71 × 10−6	−0.08	−0.56	5.64 × 10−5	0.48
Mo0 coating	−0.66	2.68 × 10−6	0.13	−0.19	3.92 × 10−5	0.32

lists electrochemical corrosion parameters extracted from the potentiodynamic polarization curves for the Mo0 bulk and Mo coating. It can be seen that the coating has higher corrosion potential E_corr and pitting potential E_pit, and lower corrosion current density I_corr and passivation current density I_pass compared with that of the bulk, which indicates the coating has better corrosion resistance. At the same time, the coating has a smaller E_pit − E_prot, where protection potential E_prot refers to the intersection potential of the forward and reverse scans, indicating that the coating has a better repair ability. The results are attributed to the fact that the coating is mainly a BCC solid solution single-phase structure, eliminating the Cr-depleted zone and causing elements to distribute uniformly. Figure 9 gives the corrosion morphology of the coating’s surface. It is obvious that the coating’s pitting corrosion mainly occurs in the defects, such as surface micro-holes and micro-cracks, indicated by red arrows. The chloride ions gather in the defects, attack the passivation film, and accelerate the pitting corrosion in the defects, making sprayed splats debond. Conversely, the other areas of the coating surface are smooth, uniform and not corroded, showing good corrosion resistance.

3.1.3. EIS Tests

Figure 10 gives EIS plots of the Mo0 coating and Mo0 bulk. Figure 10a,b display the corresponding experimental and fitting Nyquist and Bode plots, respectively. The coating has a greater depressed capacitive loop and maximum phase angle compared with the bulk, indicating that the coating has better corrosion resistance [26,27]. The coating and bulk both have two peaks in the phase angle plots, as shown in Figure 10b, so they both have two time constants in the impedance spectrum, and their depressed capacitive loops are both the superposition of two capacitive loops [26,28]. At the same time, considering that the corrosion type of the coating is mainly pitting corrosion at the surface micro-holes and micro-cracks, while the corrosion type of the bulk is general corrosion, Figure 11a,b give an equivalent electric circuit (EEC) model used to fit the EIS data for the bulk and coating, respectively. Constant-phase element (CPE) is related to the non-ideal capacitance behavior caused by various factors, such as the heterogeneity, porosity and roughness of the interface [27,28]. CPEf and CPEdl are substitutes for the capacitance of passive film and electrical double-layer, respectively. The impedance value of CPE can be expressed as follows [26,28]:

$$Z_{CPE}={\left[Y_0{\left(j\omega \right)}^n\right]}^{-1}$$

(1)

where $Y_0$ is the admittance magnitude; $j=\sqrt{-1}$ is the imaginary number; $\omega$ is the angular frequency; the exponent n is the dispersion coefficient of the CPE for the deviation between the system and ideal capacitance behavior, and is related to surface inhomogeneity ranging from 0 to 1. The CPE can represent resistance (n = 0), ideal capacitance (n = 1) and Warburg impedance (n = 0.5) [29]. Based on the experimental EIS data, the relevant fitting parameters for the EEC model are summarized in

Table 3. Fitting parameters for the EIS of the Mo0 coating and Mo0 bulk.

Samples	Rs (Ωcm2)	CPEf		Rf (104 Ωcm2)	CPEdl		Rct (104 Ωcm2)	Rsp (Ωcm2)	Rp (Ωcm2)
		${\mathit{Y}}_0$ (10−5 Ω−1 cm−2 sn)	n		${\mathit{Y}}_0$ (10−4 Ω−1 cm−2 sn)	n
Mo0 coating	14.11	2.58	0.89	5.90	0.80	0.88	7.52	14.28	33,061
Mo0 bulk	13.33	5.34	0.84	0.27	1.34	0.74	2.70	---	29,700

. The coating has a smaller $Y_0$ and greater polarization resistance Rp, which indicates that the coating is more corrosion resistant than the bulk. This conclusion is consistent with the results of the EIS and polarization test.

3.1.4. Mott-Schottky Tests

Mott-Schottky tests were employed to further investigate the semiconductor property of the passive film. According to the Mott-Schottky theory, the space charge capacitance of the depletion condition can be calculated by the following equation [23,27,30,31,32]:

$${\displaystyle \frac{1}{C_s^2}} = \pm {\displaystyle \frac{2}{e\epsilon {\epsilon }_{0}N}}(E-E_{fb}-{\displaystyle \frac{kT}{e}})$$

(2)

where the positive sign represents an n-type semiconductor, the negative sign represents a p-type semiconductor, and the corresponding charge carrier density or point defect density $N$ is the electron donor density ($N_D$) and electron acceptor density ($N_A$), respectively. $C_s$ is the capacitance of the space charge under depletion conditions equal to $-1/\omega Z^{″}$, where $\omega$ is the cyclic frequency and $Z^{″}$ is the imaginary part of the impedance $Z$. $\epsilon$ is the relative permittivity of the passive film, assumed to be 15.6. ${\epsilon }_0$ is the vacuum permittivity equal to 8.85 × 10⁻¹⁴ F cm⁻¹. $e$ is the electron charge equal to 1.60 × 10⁻¹⁹ C. $T$ is the absolute temperature. $k$ is the Boltzmann constant equal to 1.38 × 10⁻²³ J/K. $E$ is the applied potential. And $E_{fb}$ is the flat band potential. Based on the slope of the linear region of the Mott-Schottky plot, the $N$ value of the passive film can be calculated. Figure 12 gives Mott-Schottky plots of Mo0 bulk and Mo0 coating passivation films. It can be seen that both passivation films are single-layer, Cr₂O₃-rich p-type structures, whose main point defects are cation vacancies [23,28,29,31,33,34,35]. The $N_A$ of the passivation film of the bulk and coating is 7.16 × 10²¹ cm⁻³ and 1.26 × 10²¹ cm⁻³, respectively, which indicates that the coating is more corrosion resistant than the bulk. The result is consistent with the above conclusions.

3.2. Al_0.32CrFeTi_0.73Ni_1.27Mo_0.23 Coating

This section mainly considers the effect of the introduction of Mo element on the corrosion behaviors of the HVOF coating in the 3.5 wt.% sodium chloride solution.

3.2.1. Structural Analysis

Figure 13 shows XRD patterns of the Mo0 coating and Mo23 coating. It can be seen that the introduction of Mo element makes (Cr, Mo)-rich Sigma phase precipitate out of the BCC solid solution matrix for the Mo23 coating, and the solid solution of large atomic radius molybdenum causes the expansion of the BCC matrix lattice, resulting in the shift of the BCC peak slightly to the left. Figure 14 shows the Sigma phase precipitating out of the BCC solid solution matrix for the Mo23 coating. EDS results show that the Sigma phase is a (Cr, Mo)-rich phase, and the Cr-depleted region is formed in the adjacent region of the Sigma phase. Figure 15 gives SEM images of local details of the Mo23 coating, showing that micro-holes and micro-cracks are formed in the coating.

3.2.2. Potentiodynamic Polarization Tests

Figure 16 gives the potentiodynamic polarization curves of the Mo0 coating and Mo23 coating.

Table 4. Electrochemical corrosion parameters extracted from the potentiodynamic polarization curves for Mo0 coating and Mo23 coating.

Samples	Ecorr (VSCE)	Icorr (A/cm2)	Epit (VSCE)	Eprot (VSCE)	Ipass (A/cm2)	Epit − Ecorr (VSCE)	Epit − Eprot (VSCE)
Mo23 coating	−0.64	5.19 × 10−6	0.21	−0.15	3.31 × 10−5	0.85	0.36
Mo0 coating	−0.66	2.68 × 10−6	0.13	−0.19	3.02 × 10−5	0.79	0.32

lists the electrochemical corrosion parameters extracted from the potentiodynamic polarization curves for the Mo0 coating and Mo23 coating. It can be seen that both coatings have positive hysteresis loops, indicating that they are both susceptible to pitting, and both their passivation regions are smooth and have no current transients, indicating that the passivation film stability of both coatings is good. Moreover, the hysteresis loop area of the Mo23 coating is clearly larger, indicating that the Mo23 coating has stronger pitting susceptibility compared with the Mo0 coating. Both coating surfaces have micro-holes and micro-cracks, making them susceptible to pitting, while the Sigma phase precipitates out of the BCC matrix for the Mo23 coating to form Cr-depleted regions, making pitting susceptibility greater compared with the Mo-free BCC single-phase Mo0 coating. This is contrary to the conclusion of the corresponding poly-phase composite bulk, namely, that the introduction of Mo element reduces the pitting susceptibility of the corresponding Mo-bearing poly-phase composite Mo23 bulk, as shown in Figure 17, causing the potentiodynamic polarization curve of the Mo23 bulk to show a smaller hysteresis loop area, I_corr, I_pass and a more stable passivation region, compared with the Mo-free poly-phase composite Mo0 bulk. Figure 17 proves that the introduction of Mo improves the corrosion resistance of the corresponding bulk. At the same time, the growth of the passivation films is spontaneous for both coatings and bulks, per the above analysis.

As shown in Table 4, although the Mo23 coating has larger corrosion potential E_corr, pitting potential E_pit and a wider passivation interval E_pit − E_corr, its corrosion current density I_corr and passivation current density I_pass are larger compared with the Mo0 coating, indicating that the introduction of Mo does not improve the corrosion resistance of the coating, but deteriorates the corrosion resistance of the coating. This result is the opposite of the corresponding bulk. At the same time, the Mo0 coating has a smaller E_pit − E_prot, indicating that the Mo-free Mo0 coating has a better repair ability. Figure 18 gives the corrosion morphology of the Mo23 coating surface, showing that the corrosion type is mainly pitting occurring in the coating’s surface defects and Cr-depleted regions.

The pitting resistance equivalent number (PREN) approach can be used to assess corrosion resistance. The PREN can be defined as wt.%Cr + 3.3 wt.%Mo, taking into account the positive effect of Mo and Cr on corrosion resistance [31,36,37,38,39]. For the corresponding poly-phase composite bulk, according to PREN, the introduction of Mo can improve its corrosion resistance, which has been proven by the potentiodynamic polarization curves of the corresponding poly-phase composite bulks, as shown in Figure 17. However, PREN is mainly based on the content of Cr and Mo, and lacks the consideration of the effect of microstructures on corrosion behaviors [38,39]. Some studies have shown that the introduction of Mo precipitates the (Cr, Mo)-rich Sigma phase from the single-phase matrix, thus deteriorating the corrosion resistance of HEA [39,40], which is consistent with the results of the corresponding coating in this study.

3.2.3. EIS Tests

Figure 19 gives EIS plots of the Mo0 coating and Mo23 coating. Figure 19a,b display the corresponding experimental and fitting Nyquist and Bode plots, respectively. The Mo23 coating has a smaller depressed capacitive loop and maximum phase angle compared with the Mo-free Mo0 coating, indicating that the Mo-bearing Mo23 coating has worse corrosion resistance. Both coatings have two peaks in the phase angle plots, as shown in Figure 19b, so they both have two time constants in the impedance spectrum, and their depressed capacitive loops are both the superposition of two capacitive loops [26,28]. At the same time, considering that the corrosion type of both coatings is mainly pitting corrosion, the EEC model given in Figure 11b is used to fit the EIS data of both coatings. The relevant fitting parameters for the EEC model are summarized in

Table 5. Fitting parameters for the EIS of the Mo0 coating and Mo23 coating.

Coating	Rs (Ωcm2)	CPEf		Rf (104 Ωcm2)	CPEdl		Rct (104 Ωcm2)	Rsp (Ωcm2)	Rp (Ωcm2)
		${\mathit{Y}}_0$ (10−5 Ω−1 cm−2 sn)	n		${\mathit{Y}}_0$ (10−4 Ω−1 cm−2 sn)	n
Mo0 coating	14.11	2.58	0.89	5.90	0.80	0.88	7.52	14.28	33,061
Mo23 coating	13.59	3.62	0.77	4.33	6.10	0.86	7.11	13.29	26,911

. The Mo23 coating has larger $Y_0$ and smaller Rp, which indicates that the Mo23 coating is less corrosion resistant than the Mo0 coating. The introduction of Mo deteriorates the corrosion resistance of the HVOF coating. This conclusion is consistent with the results of the EIS and polarization test.

3.2.4. Mott-Schottky Tests

Figure 20 gives Mott-Schottky plots of the Mo0 coating and Mo23 coating passivation films. It can be seen that both passivation films are single-layer Cr₂O₃-rich p-type structures, whose main point defects are cation vacancies. The $N_A$ of the passivation film of the Mo0 coating and Mo23 coating is 1.26 × 10²¹ cm⁻³ and 3.10 × 10²¹ cm⁻³, respectively, which indicates that the Mo-free Mo0 coating is more corrosion resistant than the Mo-bearing Mo23 coating, and the introduction of Mo deteriorates the corrosion resistance of the HVOF coating. This result is consistent with the above conclusions.

4. Conclusions

At present, there is relatively little research on the corrosion behaviors of HVOF-prepared HEA coatings, and most of the coatings prepared are equiatomic. In the present study, the microstructures and corrosion behaviors of HVOF-prepared non-equiatomic Al_0.32CrFeTi_0.73(Ni_1.50−xMo_x) (x = 0, 0.23) HEA coatings were studied. The corrosion behaviors were characterized by polarization, EIS and Mott-Schottky tests under a 3.5 wt.% sodium chloride aqueous solution open to air at room temperature. The key findings of this study are summarized as follows:

(1)

The Al_0.32CrFeTi_0.73Ni_1.50 coating is a simple BCC single-phase solid solution structure compared with the corresponding poly-phase composite bulk. The structure of the Al_0.32CrFeTi_0.73Ni_1.27Mo_0.23 coating, with the introduction of Mo element, is such that the (Cr, Mo)-rich Sigma phase precipitated out of the BCC solid solution matrix phase, thus forming Cr-depleted regions around the sigma phases. The solid solution of large atomic-size Mo element caused the lattice expansion of the BCC solid solution matrix phase. Micro-hole and micro-crack defects were formed on the surface of both coatings.

(2)

The growth of both passivation films of the Al_0.32CrFeTi_0.73Ni_1.50 coating and the Al_0.32CrFeTi_0.73Ni_1.27Mo_0.23 coating was spontaneous. Both passivation films were stable and Cr₂O₃-rich P-type single-layer structures.

(3)

The Al_0.32CrFeTi_0.73Ni_1.50 coating had better corrosion resistance than the corresponding bulk. The pitting susceptibility of the coating was greatly reduced. The corrosion type of the coating was mainly pitting, occurring in the coating surface defects.

(4)

The Al_0.32CrFeTi_0.73Ni_1.27Mo_0.23 coating, with the introduction of Mo element, increased pitting susceptibility and deteriorated corrosion resistance compared with the Mo-free Al_0.32CrFeTi_0.73Ni_1.50 coating. The corrosion type of the Mo-bearing coating was mainly pitting, occurring in the coating surface defects and Cr-depleted regions.