Tribocorrosion Properties of NiCrAlY Coating in Different Corrosive Environments.

Atmospheric plasma spraying (APS) was taken to fabricate the NiCrAlY coating. The corrosion-wear properties of NiCrAlY coating was measured respectively under deionized water, artificial seawater, NaOH solution and HCl solution. Experimental results presented that the as-sprayed NiCrAlY coating consisted of Ni3Al, nickel-based solid solution, NiAl and Y2O3. In deionized water, the coating with the lowest corrosion current density (icorr) of 7.865 × 10−8 A/cm2 was hard to erode. Meanwhile, it presented a lower friction coefficient and the lowest wear rate. In HCl solution, NiCrAlY coating gave the highest corrosion current density (icorr) of 3.356 × 10−6 A/cm2 and a higher wear rate of 6.36 × 10−6 mm3/Nm. Meanwhile, the emergence of Al(OH)3 on the coating surface could reduce the direct contact between the counter ball and sample effectively, which was conducive to the lowest friction coefficient of 0.24.


Introduction
In engineering fields, wear often occurs under different corrosive circumstances leading to the degradation rate of engineering parts [1]. For instance, some mechanical parts utilized in the marine atmosphere, pulping and mining, suffer the collaborative destruction of corrosion and wear [2][3][4][5][6]. Meanwhile, the synergism of corrosion and wear decreases the service life of the material. In the process of friction, the passive film on the worn surface could be destroyed by friction force and the new passive film is hard to form, which would make the material suffering more serious damage. Normally, the corrosion-wear material loss is greater than the sum of corrosion and wear. Therefore, it is very imperative to improve the corrosion-wear resistance property of mechanical parts in different corrosive environments. To meet this requirement, the protective coatings are applied to protect the mechanical parts without changing the external structure. MCrAlY (M = Cobalt and/or Nickel) alloys with excellent oxidation resistance, corrosion resistance and wear resistance performance have been widely used in nuclear power, automotive and marine industries acting as the protective coatings [1,[7][8][9][10][11][12][13][14][15]. J. Chen et al. investigated the tribocorrosion behavior of NiCoCrAlYTa coating in corrosion. The results showed that this kind of coating presented an extremely dense structural characteristic and excellent tribological performance in NaOH and HCl solutions [1]. M. Marcu et al. studied the microstructure and oxidation resistance of as-sprayed NiCrAlY/Al 2 O 3 coating. The results presented that the as-sprayed NiCrAlY/Al 2 O 3 coating has the best cyclic oxidation resistance with an oxidation rate of 2.62 × 10 −12 g 2 ·cm −4 ·s −1 at high temperature and good adhesion during the cyclic oxidization treatment [8]. Current researches mainly focus on the oxidation resistance, corrosion, mechanical and tribological performance of the coatings [16][17][18][19][20][21][22]. These materials are also used for reciprocating parts in corrosive environments [23], so the research of the wear-corrosion resistance is still important in the process of sliding. However, few researches pay attention to the synergy of corrosion and wear [24], and its mechanism is still unclear.
In this work, the tribocorrosion properties of NiCrAlY coating were studied and the synergistic mechanisms between wear and corrosion in different corrosive environments were discussed in detail. The objective of this paper is to research how corrosive environments affect the tribological behavior of NiCrAlY coating and the interaction degree between corrosion and wear. This research would provide usable direction to the NiCrAlY coating application in corrosive environments.

Coating Preparation
Gas atomized spherical Ni 22 Cr 10 Al 1.0 Y (wt.%) powder (53-106 µm) was bought from Sulzer Metco (Winterthur, Switzerland). The NiCrAlY coating was prepared by atmospheric plasma spraying (APS). The Inconel 718 alloy was sand-blasted, then ultrasonically cleaned with ethanol before spraying. The coating thickness was about 300 µm. The specific spraying parameters presented were: flow rate of Ar was 40 L/min; flow rate of H 2 was 5 L/min; spraying angle was 90 • ; feed rate of the powder was 42 g/min; voltage was 60 V; the current was 500 A and spray distance was 110 mm.

Characterization
The micromorphologies of cross-section and worn surface of this coating were measured by field emission scanning electron microscopy (FE-SEM, Tescan Mira 3, Bron, Kohoutovice, Czech Republic). A Philips X'Pert-MRD X-ray diffractometer (XRD; Cu-K a radiation, current 150 mA, potential 40 kV, Philips, Eindhoven, The Netherlands) was utilized to analyzed phase composition. The phase compositions on the worn surface were analyzed by Czemy-Tumer Labram HR800 Raman spectrometer (Horiba, Paris, France).

Tribocorrosion Tests
The tribocorrosion experiments were tested in deionized water (pH = 7), artificial seawater (pH = 8.2), 0.1 M NaOH solution (pH = 13) and 0.1 M HCl solution (pH = 1), with reciprocating ball-on-disk tribometer (UMT, Karlsruhe, Germany). The schematic diagram is shown in Figure 1. The polytetrafluoroethylene (PTFE) does not corrode as it is chemically inert to corrosion. So it acted as the solution cell material. The Al 2 O 3 ceramic ball acted as the counter ball, whose diameter was 5 mm. Before the friction experiment, the surface of the coating was burnished till the roughness close to 0.5 µm. The tests were performed at the conditions below: room temperature, 5 N normal load, 0.8 mm/s sliding speed, 3.5 mm amplitude and 60 min duration. Repeated experiments were tested in every corrosive environments. The color 3D laser scanning microscope (VK-9710, Keyence, Osaka, Japan) and SEM were utilized to analyze the worn surface. The wear rate was got by W = V/LF, where W represented the wear rate (mm 3 /Nm), V represented the wear volume loss (mm 3 ), L represented the sliding distance (m) and F represented the load (N).               Table 1. The polarization resistance value (Rp) is calculated by Stern-Geary equation:

Electrochemical Performance of NiCrAlY Coating
Results indicate that the corrosion potential (Ecorr) of NiCrAlY coating under deionized water is the highest of −0.428 V (vs. SCE). However, the Ecorr of the coating in artificial seawater, HCl and NaOH shift to −0.516 V (vs. SCE), −0.559 V (vs. SCE) and −0.535 V (vs. SCE) respectively. Simultaneously, the corrosion current density (icorr) of this coating in deionized water shows the lowest of 7.865 × 10 −8 A/cm 2 . Generally speaking, corrosion current density, whose rate is often used as corrosion rate, is a crucial reference to evaluate corrosion resistance [13,27]. Therefore, the coating under deionized water with the lowest corrosion rate is hard to corrode. The coating in HCl presenting the highest corrosion current density is very easy to be corroded. At the same time, the coating in deionized water has the highest βa, βc and Rp of 0.072 V/dec, 0.049 V/dec and 1.610 × 10 5 Ω respectively, which further illustrates that the coating in deionized water holds a good corrosion resistance.    Table 1. The polarization resistance value (R p ) is calculated by Stern-Geary equation:  Figure 4 gives the potentiodynamic polarization curves of NiCrAlY coating sliding conditions in different corrosive solutions. Key test parameters such as the corrosion potential (Ecorr), corrosion current density (icorr), anodic and cathodic Tafel slopes (βa and βc) are obtained from Figure 4 and shown in Table 1. The polarization resistance value (Rp) is calculated by Stern-Geary equation:

Electrochemical Performance of NiCrAlY Coating
Results indicate that the corrosion potential (Ecorr) of NiCrAlY coating under deionized water is the highest of −0.428 V (vs. SCE). However, the Ecorr of the coating in artificial seawater, HCl and NaOH shift to −0.516 V (vs. SCE), −0.559 V (vs. SCE) and −0.535 V (vs. SCE) respectively. Simultaneously, the corrosion current density (icorr) of this coating in deionized water shows the lowest of 7.865 × 10 −8 A/cm 2 . Generally speaking, corrosion current density, whose rate is often used as corrosion rate, is a crucial reference to evaluate corrosion resistance [13,27]. Therefore, the coating under deionized water with the lowest corrosion rate is hard to corrode. The coating in HCl presenting the highest corrosion current density is very easy to be corroded. At the same time, the coating in deionized water has the highest βa, βc and Rp of 0.072 V/dec, 0.049 V/dec and 1.610 × 10 5 Ω respectively, which further illustrates that the coating in deionized water holds a good corrosion resistance.  Simultaneously, the corrosion current density (i corr ) of this coating in deionized water shows the lowest of 7.865 × 10 −8 A/cm 2 . Generally speaking, corrosion current density, whose rate is often used as corrosion rate, is a crucial reference to evaluate corrosion resistance [13,27]. Therefore, the coating under deionized water with the lowest corrosion rate is hard to corrode. The coating in HCl presenting the highest corrosion current density is very easy to be corroded. At the same time, the coating in deionized water has the highest β a , β c and R p of 0.072 V/dec, 0.049 V/dec and 1.610 × 10 5 Ω respectively, which further illustrates that the coating in deionized water holds a good corrosion resistance.  Figure 5 shows the friction curves and wear rate of NiCrAlY coating in different corrosive solutions. The friction coefficient (COF) of the coating under the NaOH solution was the highest, with a value of 0.46. In artificial seawater and deionized water, it was 0.37 and 0.26, respectively. Surprisingly, the COF reduced to 0.24 and remains steady in HCl solution. Nevertheless, the NiCrAlY coating has a high wear rate (WR) of 6.36 × 10 −6 mm 3 /Nm in the HCl solution. This phenomenon is likely to show the high corrosion rate of coating in HCl solution ( Figure 4). The synergistic effect of corrosion and wear in a corrosive environment leads to the loss of large material, which usually larger than the synergistic effect of the sum of corrosion and wear [28,29]. So, the coating under the HCl solution presents a more obvious wear rate. The coating in the NaOH solution has the highest wear rate of 6.89 × 10 −6 mm 3 /Nm. At the same time, the coating in deionized water gives the lowest WR of 2.36 × 10 −6 mm 3 /Nm, which is caused by the lowest corrosion rate of coating in deionized water (Figure 4).   Figure 5 shows the friction curves and wear rate of NiCrAlY coating in different corrosive solutions. The friction coefficient (COF) of the coating under the NaOH solution was the highest, with a value of 0.46. In artificial seawater and deionized water, it was 0.37 and 0.26, respectively. Surprisingly, the COF reduced to 0.24 and remains steady in HCl solution. Nevertheless, the NiCrAlY coating has a high wear rate (WR) of 6.36 × 10 −6 mm 3 /Nm in the HCl solution. This phenomenon is likely to show the high corrosion rate of coating in HCl solution ( Figure 4). The synergistic effect of corrosion and wear in a corrosive environment leads to the loss of large material, which usually larger than the synergistic effect of the sum of corrosion and wear [28,29]. So, the coating under the HCl solution presents a more obvious wear rate. The coating in the NaOH solution has the highest wear rate of 6.89 × 10 −6 mm 3 /Nm. At the same time, the coating in deionized water gives the lowest WR of 2.36 × 10 −6 mm 3 /Nm, which is caused by the lowest corrosion rate of coating in deionized water ( Figure 4).   (Figure 6a,e). Therefore, the COF and WR are lower ( Figure 5). It further illustrates that the coating in deionized water shows excellent corrosion and wear resistance. The worn surface of NiCrAlY coating in HCl corrosive solution is very rough and has serious corrosion ( Figure 6c). So, the coating obtains high WR under HCl corrosive solution ( Figure 5). The worn track of NiCrAlY coating in NaOH corrosive solution is the deepest and widest (Figure 6d,f). Therefore, this coating has the worst tribological performance ( Figure 5).   (Figure 6a,e). Therefore, the COF and WR are lower ( Figure 5). It further illustrates that the coating in deionized water shows excellent corrosion and wear resistance. The worn surface of NiCrAlY coating in HCl corrosive solution is very rough and has serious corrosion ( Figure 6c). So, the coating obtains high WR under HCl corrosive solution ( Figure 5). The worn track of NiCrAlY coating in NaOH corrosive solution is the deepest and widest (Figure 6d,f). Therefore, this coating has the worst tribological performance ( Figure 5). To further research the influence of corrosive solution upon the corrosion-wear property of NiCrAlY coating, Raman analysis is tested. Figure 7 shows the Raman spectra of the worn surface of NiCrAlY coating in different corrosive solutions. The Al2O3, Cr2O3 and NiO are the main phases on the worn surface of NiCrAlY coating after sliding in deionized water, artificial seawater and NaOH solution. Nevertheless, the worn surface of NiCrAlY coating observes the new phase of Al(OH)3 after sliding in HCl corrosive solution [30]. The results indicate that the NiCrAlY coating has suffered serious corrosion in the HCl corrosive solution because of the existence of stronger and more numerous peaks [1]. The corrosion products are easily worn out during the friction process. So the wear rate of the coating under HCl solution is very high ( Figure 5).  Figure 8 shows the corrosion-wear mechanisms of NiCrAlY coating in the HCl solution. The surface becomes very smooth because the corrosion-wear effect with the mix of oxides and To further research the influence of corrosive solution upon the corrosion-wear property of NiCrAlY coating, Raman analysis is tested. Figure 7 shows the Raman spectra of the worn surface of NiCrAlY coating in different corrosive solutions. The Al 2 O 3 , Cr 2 O 3 and NiO are the main phases on the worn surface of NiCrAlY coating after sliding in deionized water, artificial seawater and NaOH solution. Nevertheless, the worn surface of NiCrAlY coating observes the new phase of Al(OH) 3 after sliding in HCl corrosive solution [30]. The results indicate that the NiCrAlY coating has suffered serious corrosion in the HCl corrosive solution because of the existence of stronger and more numerous peaks [1]. The corrosion products are easily worn out during the friction process. So the wear rate of the coating under HCl solution is very high ( Figure 5). To further research the influence of corrosive solution upon the corrosion-wear property of NiCrAlY coating, Raman analysis is tested. Figure 7 shows the Raman spectra of the worn surface of NiCrAlY coating in different corrosive solutions. The Al2O3, Cr2O3 and NiO are the main phases on the worn surface of NiCrAlY coating after sliding in deionized water, artificial seawater and NaOH solution. Nevertheless, the worn surface of NiCrAlY coating observes the new phase of Al(OH)3 after sliding in HCl corrosive solution [30]. The results indicate that the NiCrAlY coating has suffered serious corrosion in the HCl corrosive solution because of the existence of stronger and more numerous peaks [1]. The corrosion products are easily worn out during the friction process. So the wear rate of the coating under HCl solution is very high ( Figure 5).    Figure 8 shows the corrosion-wear mechanisms of NiCrAlY coating in the HCl solution. The surface becomes very smooth because the corrosion-wear effect with the mix of oxides and hydroxides formed by electrochemical reactions (Figures 6 and 7). In terms of the potential values, aluminum is the least noble element and the order of potentials follows Ni > Cr > Al [1]. So the aluminum element is more likely to be corroded at first. The following electrochemical reactions could explain the process of Al(OH) 3 formation:

Lubrication Behavior of Al(OH) 3 on NiCrAlY Coating in HCl Solution
Materials 2020, 13, x FOR PEER REVIEW 7 of 9 hydroxides formed by electrochemical reactions (Figures 6 and 7). In terms of the potential values, aluminum is the least noble element and the order of potentials follows Ni > Cr > Al [1]. So the aluminum element is more likely to be corroded at first. The following electrochemical reactions could explain the process of Al(OH)3 formation: H2O + 2e − → H2 +2OH − (4) Terryn et al. [31] illustrated that the generation of Al(OH)3 is related to local pH changes in the hydrogen reduction region. Hence, the Al(OH)3 could be formed where the hydrogen evolution occurs. Furthermore, when the local pH rises to above 9, Al 3+ ions will react with excessive OH − ions and forms aluminate anions [1]. Aluminate anions cannot maintain stable in HCl corrosive solution and will precipitate as Al(OH)3 ( Figure 8). This reaction can be described as follows: AlO2 − + H + + H2O → Al(OH)3 Thus, it inexistences the Al(OH)3 on the worn surface of NiCrAlY coating in NaOH solution in the process of sliding but the following reaction [32]: Of course, in addition to the Al dissolution, according to the standard of electrode potentials, Cr element is also dissolved at the anodic cycle and is electrochemically oxidized to Cr2O3, which is well consistent with the micro-Raman results (Figure 7) [1]. The oxidation reaction process can be illustrated as follows [30]: The above oxidation reactions and metal dissolution explain the smooth surface. Al(OH)3 can be evenly distributed on the smooth worn surface and effectively reduce the direct contact of counter ball and sample. At the same time, the frictional shear stress can form the lubricating layer on the worn surface, which can obviously reduce the friction coefficient of coating in HCl corrosive solution Terryn et al. [31] illustrated that the generation of Al(OH) 3 is related to local pH changes in the hydrogen reduction region. Hence, the Al(OH) 3 could be formed where the hydrogen evolution occurs. Furthermore, when the local pH rises to above 9, Al 3+ ions will react with excessive OH − ions and forms aluminate anions [1]. Aluminate anions cannot maintain stable in HCl corrosive solution and will precipitate as Al(OH) 3 ( Figure 8). This reaction can be described as follows: Thus, it inexistences the Al(OH) 3 on the worn surface of NiCrAlY coating in NaOH solution in the process of sliding but the following reaction [32]: Of course, in addition to the Al dissolution, according to the standard of electrode potentials, Cr element is also dissolved at the anodic cycle and is electrochemically oxidized to Cr 2 O 3 , which is well consistent with the micro-Raman results (Figure 7) [1]. The oxidation reaction process can be illustrated as follows [30]: 2Cr + 3H 2 O → Cr 2 O 3 + 6H + + 6e − The above oxidation reactions and metal dissolution explain the smooth surface. Al(OH) 3 can be evenly distributed on the smooth worn surface and effectively reduce the direct contact of counter ball and sample. At the same time, the frictional shear stress can form the lubricating layer on the worn surface, which can obviously reduce the friction coefficient of coating in HCl corrosive solution [1]. Therefore, the COF of NiCrAlY coating in the HCl corrosive solution is the lowest of 0.24 ( Figure 5).

Conclusions
In this work, the corrosion-wear properties of NiCrAlY coating were studied under deionized water, artificial seawater, 0.1 M HCl solution and 0.1 M NaOH. The main conclusions are given as follows: (1) The NiCrAlY coating is composed of Ni 3 Al, nickel-based solid solution, NiAl and Y 2 O 3 .
(2) In deionized water, the NiCrAlY coating with the lowest corrosion current density of 7.865 × 10 −8 A/cm 2 is hard to erode. Meanwhile, it presents a lower friction coefficient and the lowest wear rate. (3) In HCl corrosive solution, the coating gives the highest corrosion current density (i corr ) of 3.356 × 10 −6 A/cm 2 and a higher wear rate of 6.36 × 10 −6 mm 3 /Nm. (4) In HCl corrosive solution, the emergence of Al(OH) 3 on the coating surface could reduce the direct contact between the counter ball and sample effectively, which is conducive to the lowest friction coefficient of 0.24.