Arc Thermal Spray NiCr20 Alloy Coating: Fabrication, Sealant, Heat Treatment, Wear, and Corrosion Resistances

This study presents the effect of heat treatment on porosity, phase composition, microhardness, and wear and corrosion resistances of the thermal sprayed NiCr20 coating after sealing with aluminum phosphate. The annealing temperatures were varied in a range of 400 to 1000C.The obtained results indicated the porosity of coating decreased with increasing the annealing temperature. After treatment at temperatures in range of 800-1000C, more than 90% of initial pores in the coating were successfully filled with the sealants.TheXRDdata revealed not only the formation of newphases of other compounds, but also the interactionbetween coating and sealant. After heat treatment, wear resistance of coating was 12 times higher than that without heat treatment. The corrosion test in H2SO4 solution indicated that the presence of sealant in coatings increased their corrosion resistance. From these findings, application of these NiCr20 coatings to protect steel against wear and corrosion appears very promising.


Introduction
For the thermal spray coatings in corrosive environments, their intrinsic porosity could facilitate the ingress of corrosive agents, resulting in the initiation of corrosion leading to the coating delamination.The porous structure not only deteriorates their corrosion resistance, but also decreases their mechanical properties and consequently reduces their wear resistance [1].Thus, the post-heat treatment after spraying process is necessary.This posttreatment process could be carried out by using various techniques, such as the laser or electron-beam surface melting, alloying method, hot isostatic pressing (HIP), chemical vapor deposition (CVD), and chemical vapor infiltration (CVI).However, these techniques are costly and require complex equipment.Other effective posttreatments, like the impregnation technique using polymers, inorganic compounds, or molten metals, have been applied [1].
It was reported in literature that aluminum phosphate sealant could infiltrate deeply into plasma-sprayed coating layers, inhibit the formation of pores, and increase the abrasion resistance of the coatings [1][2][3][4][5][6][7].The influence of temperature on porosity and wear resistance of thermal coating after sealing with aluminum phosphate has been also reported [8][9][10].
Aluminum phosphate sealed NiCr alloy coatings have been widely applied for corrosion protection at high temperatures [11][12][13][14].However, the effect of heat treated temperature on the adhesive strength, phase composition, and microstructures of coatings has been reported mostly for the NiCr20 alloy coatings.There was a lack of information about the effect of heat treatment on property and structure of the aluminum phosphate sealed NiCr20 coatings.
In this study, we present our research on the effect of heat treatment on porosity, phase composition, microhardness, and wear and corrosion resistances of the thermal sprayed NiCr20 coating after sealing with aluminum phosphate.

Experimental
2.1.Sample Preparation.Aluminum phosphate was prepared using orthophosphoric acid (H 3 PO 4 , 85%) and aluminum hydroxide powder (Al(OH) 3 ), with 2.3 molar ratio of P/Al.The reaction took place at 120 ∘ C till clear solution is obtained.The obtained aluminum phosphate solution has the viscosity () of 177.36 s and the density () of 1.58 g.cm −3 .NiCr20 alloy coatings have been fabricated by spraying with electric-arc equipment OSU-HESSLER 300A (OSU Hessler GmbH & Co., Germany).The optimized spraying parameters are shown in Table 1.
To seal the NiCr20 coating samples, the aluminum phosphate prepared solution is used and stabilized during 12 hours at room temperature and then heat treated with the below continuous steps (Figure 1).

Coating Characterization.
After heat treatment, the surface of coating samples, except the one exposed to electrolyte (for corrosion evaluation), was sealed with an epoxy resin in PVC plastic molds.After epoxy curing, the unsealed surface was wet-polished using SiC papers (grade down from #100 to #2000 sizes).After polishing, the samples were rinsed and sonicated in ultrasonic cleaning equipment.The samples were then dried in vacuum at 50 ∘ C. For comparative studies on the effect of annealing temperature, Table 2 presents the denotation of the studied samples in heat treatment, for 3 different types of samples (such as aluminum sealant and unsealed and sealed NiCr20 alloy coatings).
The cross-sectional microstructure and porosity of coating samples have been analyzed using optical microscope Axiovert 40 Mat with imaging multiphase software.
To identify the possible phases that present in the coating, X-ray diffraction (SIEMENS D5005X-RAY instrument, Germany) has been used at temperature of 25 ∘ C, with 2 angle scanning from 10 ∘ to 70 ∘ (scan step at 0.02 ∘ /s).
The wear resistance of the coatings was measured using TE-91 Precision Rotary Vacuum Tribometer (Phoenix Tribology Ltd, England), in accordance with the standard ASTM G99:2000 using 9CrSi dowel-steel with a hardness > 60HRC.The testing condition includes the load of 30N force, wear track radius of 20 mm, rotation speed of 382 rpm, sliding speed of 0.4 m/s, and testing duration of 900 s.
The hardness of coating (Vickers) was determined according to ISO 6507-2 standard, by using the microhardness tester FM-100 (Japan), with load of 300 g for 15 seconds [15].
The corrosion behaviour of the coatings in H 2 SO 4 solution (pH 2) was determined using the AUTOLAB PGSTAT 302N.Two methods were used: electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) with a three-electrode system.The spraying samples were working electrodes, while the counter electrode and the reference electrode were a platinum electrode and a saturated calomel electrode (SCE), respectively.In the linear polarization resistance (LPR), the spraying coating samples were polarized around their (E ocp ) open circuit potential (−30 mV to 30 mV/SCE versus OCP) by a direct current (DC) signal at a scan rate of 2 mV/s, according to ASTM G3-14 standard.For EIS measurement, the spraying samples were polarized at ±5 mV around its open circuit potential (E ocp ) by an alternating current (AC) signal with its frequency ranging from 10 −2 Hz to 10 5 Hz (7 points per decade) [16,17].samples.The possible phases that present in aluminum phosphate and sealed NiCr20 samples (at different temperatures) were shown in Tables 4 and 5, respectively.

Aluminum Phosphate Samples.
For the aluminum phosphate samples (with 2.3 molar ratio of P/Al, Figure 2), the data indicated the presence of crystalline phase AlPO 4 (orthorhombic) in all testing samples at all range of temperature.However, their peak intensity of AlPO 4 (orthorhombic) has the highest value when annealing at 1000 ∘ C. In addition, the crystalline phase AlPO 4 (hexagonal) or syn AlPO 4 could not be detected at 1000 ∘ C for aluminum phosphate samples.
Above 400 ∘ C, as seen in Figure 2, the H 2 AlP 3 O 10 .2H 2 O crystal no longer existed in the aluminum phosphate mixture.This result could be explained by the fact that H 2 AlP 3 O 10 .2H 2 O crystalline phase has been transformed to the Al 2 O 6 P 18 and Al(PO 3 ) 3 crystalline phases through the following chemical reactions [18]: On the other hand, the Al 2 P 6 O 18 (monoclinic) crystal has been observed in aluminum phosphate mixture at 400, 500, and 600 ∘ C, but then it dispersed at higher temperatures.After treatment at 800 ∘ C, the Al 2 P 6 O 18 crystalline phase could be transformed into the Al(PO 3 ) 3 crystalline phase by the following chemical reaction [18]: Besides, the Al(PO 3 ) 3 (N) crystalline phase was also found at temperatures in range from 400 to 800 ∘ C.This phase could be transformed into Al(PO 3 ) 3 (Cubic) crystal at 1000 ∘ C. The formation of Al(PO 3 ) 3 and Al 2 P 6 O 18 metaphosphate phases might enhance the wear resistance of the these thermal coatings [3][4][5].

Sealed NiCr20 Spraying Samples.
As shown in Table 5, for the sealed NiCr20 coatings, besides the similar crystalline phases of aluminum phosphate samples, other crystalline phases have been observed (Table 5), such that Al 36 P 36 O 144 phase was seen at the temperatures of 500, 600, and 1000 ∘ C.However, this phase was undetectable at 800 ∘ C, due to the formation of its amorphous state or the domination of other competing phases In contrast to aluminum phosphate samples, the AlPO 4 (orthorhombic) crystalline phase could not be detected in the sealed NiCr20 coating samples at temperatures of 400, 500, and 600 ∘ C. In addition, at 1000 ∘ C, the Ni 3 (PO 4 ) 2 crystalline phase was found on the coating, due to the following chemical interaction between the NiCr coating and aluminum phosphate [18].6.

Porosity and Microhardness Measurements
As shown in Table 6, porosity values of unsealed coatings are in range from 3% to 4%, indicating the presence  The microhardness is one of main factors influencing the wear resistance of coating.Table 7 presents the hardness values of prepared coating samples.As can be seen in this table, NA4 coating sample has the highest value; thus NA4 sample is expected to have the highest abrasion resistance.The highest value of microhardness could be attributed to the presence of H 2 AlO 3 P 10 crystalline phase in the coating (NA4 sample).

Wear Resistance Study.
The weight loss could be used widely to evaluate the wear resistance of coating.Figure 5 shows the wear resistance of prepared coating samples.As shown in this figure, wear resistance of coating decreased with increasing the annealing temperatures.This result can be explained by the increasing oxidation when the annealing temperature increased, thus leading to reduction of the linkage between particles inside the coating.In addition, as mentioned in Section 3.1, H 2 O molecules in H 2 AlP 3 O 10 .2H 2 O phase had been produced above 400 ∘ C and then came out from the coating, leading also to reduction of the linkage between metallic particles inside the coating.Therefore, the weight loss could be contributed by explained by both physical abrasion and self-separation (of metallic particles).
As can be seen in Figure 5, sealed NiCr20 coating samples had higher abrasion resistant than that of unsealed samples.At 400 ∘ C, the sealed NiCr coating has the highest value of abrasion resistant.Its weight loss (9.8 mg) was 12 times lower than that of unseal coatings (126.3 mg).This finding was also coherent with the XRD analysis and microhardness study.In the sealed coatings, both crystalline phases of Al 2 P 6 O 18 and H 2 AlP 3 O 10 have significantly contributed to their increase of wear resistance.Moreover, sealed coatings have lower porosity than the unsealed ones (Figure 6) and thus their abrasion resistance might increase.

Corrosion Test.
This section examines the effect of aluminum phosphate on corrosion behavior of prepared coatings.For the comparative study, two samples of NA4 (sealed and treated at 400 ∘ C) and NC (unsealed, without heat treatment) have been selected for evaluation.
Figures 7(a) and 7(b) present the EIS data of coatings after 2 hours of immersion in H 2 SO 4 solution (pH 2) under the Nyquist and Bode plots, respectively.It was reported in literature that impedance at low frequency can represent the corrosion process of coated steel.As seen in Figure 7(b), at the low frequency, the impedance || of NA4 sample was higher than that of NC sample.This indicated that the corrosion resistance of NA4 coating was higher than that of NC coating.By sealing of aluminum phosphate, the porosity of NA4 coating became lower, thus inhibiting the penetration  7(a) could be attributed to several processes, such as (i) oxidation process of coating, (ii) charge transfer process of hydrogen reduction reaction (in acidic solution), and (iii) electrochemical reactions in the pores.
In case of plasma-sprayed alumina coating, Vetrivendan et al. [19] recently also used the aluminum phosphate sealing to improve insulation resistance of their alumina coating.In their study, three values of concentrations of P/Al (molar ratio) were 3, 10, and 15.The authors found that sealing with P/Al molar ratio 3 showed the maximum insulation resistance (∼10 13 Ω).In this work, the polarization resistance (R p ) of coatings could be measured by using the polarization curves.From the data of polarization curves, both corrosion current (i corr ) and corrosion potential (E corr ) could be calculated.
The polarization curves of coatings after 2 hours of immersion (in H 2 SO 4 solution at pH 2) are presented in Figure 8. Table 8 presents the values of R p , i corr and E corr , which were calculated from polarization curves using AUTO-LAB software.As shown in this Table, the corrosion current density (i corr ) of NA4 sample (0.42 A.cm −2 ) was 2.6 times lower than that of NC sample (1.07 A.cm −2 ).Thus, sealed NiCr alloy coating offers more protection against corrosion than the unsealed coating.

Conclusions
The main findings of this research were as follows.
(i) After post-heat treatment, XRD studies indicated the formation of various crystalline phases of aluminum phosphate.The presence of Al 2 P 6 O 18 crystal component in sealed NiCr coating might contribute to the improvement of its wear resistance.At 1000 ∘ C, the aluminum phosphate could be chemically reacted with NiCr alloy coating to form the Ni 3 (PO 4 ) 2 (Monoclinic) phase.
(ii) The porosity of aluminum phosphate sealed NiCr alloy coating (with 2.3 molar ratio of P/Al) decreased with increasing the annealing temperature.More than 90% of inherent pores have been sealed after annealing at temperature in range from 800 to 1000 ∘ C.
(iii) The abrasion resistance of NiCr coating significantly decreased with increasing the annealing temperature.After heat treatment at 400 ∘ C, the abrasion resistance of aluminum phosphate sealed NiCr coating obtained the highest value, which was 12 times higher than that without heat treatment.
(iv) In H 2 SO 4 solution at pH 2, the corrosion resistance of aluminum phosphate sealed NiCr alloy coating is much higher as compared to the unsealed coating.

Figure 1 :
Figure 1: Temperature diagram of heat treatment procedure for sealing samples.

3. 1 .
XRD Analysis.Figures2 and 3present the XDR patterns of aluminum phosphates and sealed NiCr20 coatings, after heat treatment at different temperatures, respectively.

1 -Figure 2 :
Figure 2: XRD patterns of aluminum phosphate samples with 2.3 molar ratio of P/Al, after treatment at different temperatures.

Figure 3 :
Figure 3: XRD patterns of NiCr20 coating sealed with aluminum phosphate after treatment at different temperatures.

Figure 4 :
Figure 4: Cross-section image of NiCr alloy coating sealed by aluminum phosphate and heat-treated at 400 ∘ C.
Figure 4 presents the cross-section of sealed NiCr alloy coating after heat-treatment at 400 ∘ C. As can be seen in this figure, almost all of open pores have been sealed.Porosity values for all prepared coatings are shown in Table of remained pores at different treated temperatures.Their porosity decreased with increasing the annealing temperature, due to the increasing oxidation of metallic grains in the coatings, whereas, in case of sealed coatings, their porosity values are only in range from 1.4 to 2%, with the lowest value of 1.4% when treated at 800 to 1000 ∘ C.This result could be explained by the transformation of H 2 AlP 3 O 10 .2H 2 O phase (NA4, at 400 ∘ C) into Al 2 P 6 O 18 phase (NA5, at 500 ∘ C) following the chemical reaction (1), thus leading to increase in the porosity by producing H 2 O.The higher porosity values in NA6, NA8, and NA10 samples might be explained by interaction between sealant and neat coating through the diffusion phenomenon during heat treatment.

Figure 5 :
Figure 5: Wear resistance of coating samples at different temperatures.

Figure 6 :
Figure 6: Cross-sectional structure of NiCr alloy coating unsealed (a) and sealed (b) with aluminum phosphate.

Figure 7 :Figure 8 :
Figure 7: Nyquist (a) and Bode (b) plots of NC and NA4 coating samples after 2 hours of immersion in H 2 SO 4 solution (pH 2).

Table 2 :
Denotation of the studied samples in heat treatment.

Table 3
presents the denotation of the possible phases that present in

Table 3 :
Denotation of the possible phases present in samples.

Table 4 :
Phase composition of aluminum phosphate with 2.3 molar ratio of P/Al, under different annealing temperatures.

Table 5 :
Phase composition of NA group of coating samples at different temperature.

Table 6 :
Porosity values of sealed and unsealed coatings.

Table 7 :
Microhardness values of prepared coating samples.

Table 8 :
Values of R p , i corr , E corr for coatings after 2 hours of immersion in H 2 SO 4 solution (pH 2).SO 4 solution into the steel substrate.In addition, the arcs in Figure