High temperature chlorine corrosion resistance of gradient high entropy alloy coatings prepared by high-velocity-oxygen-fuel spraying

The effect of different gradient structures on the high temperature chlorine corrosion resistance of gradient high entropy alloy coating was studied to improve the service life of the heating surface of a waste-incinerated boiler. NiCrAl ceramic composite coating (NC), high entropy ceramic composite coating (HC), and gradient high entropy alloy coating (TC) were prepared on 12Cr1MoV substrate by high-velocity-oxygen-fuel spraying. The effects of morphology and microstructure of different coatings on thermal shock resistance and high temperature chlorine corrosion resistance were studied. The results indicate that the porosity of NC coating, HC coating, and TC coating decreased in turn. In the thermal shock resistance test from 800°C to room temperature, the average number of thermal shock of TC coating can reach 21. The mass change of TC coating in 650°C high temperature Cl corrosion resistance test is less than 30 mg/cm2 after 50 hours. In summary, the gradient high entropy alloy coating with HEA as the metal bonding phase can improve the density of thermal spraying gradient coating, and enhance the thermal shock resistance and high temperature Cl corrosion resistance of the coating. Based on the comprehensive on-site application results, it can be concluded that the use of high velocity oxygen flame gradient high entropy alloy coating can effectively improve the density, thermal shock resistance, and high temperature Cl corrosion resistance of the thermal spraying gradient coating on the heating surface of the boiler, and has broad application prospects.


Introduction
In recent years, waste-to-energy incineration has become the main way of waste-efficient and harmless treatment.However, the Incineration process will cause serious high temperature corrosion on the metal parts of the boiler heating surface, and the failure of the heating surface pipeline due to high temperature corrosion will lead to frequent accidents of tube explosion and shutdown [1][2] .At present, research shows that chlorine elements in incineration garbage, such as plastic and kitchen waste, has a greater impact on corrosion of waste-incinerated boilers with high temperature [3] .The metal heating surface undergoes various types of metal corrosion such as active corrosion, electrochemical corrosion, and molten salt corrosion induced by chlorine elements on the metal surface under a high temperature chlorine corrosion environment [4] .Measures need to be taken to slow down the high temperature corrosion of the metal surface of the waste-incinerated boiler and extend the service life of the metal parts of the heating surface of the waste-incinerated boiler, such as superheater tubes and water-cooled walls.This has become one of the problems that currently hinder the development of Incineration generator units to high temperature and high pressure parameters [5] .
At present, the high temperature protective coatings applied to the heating surface of Incineration mainly include surfacing coating, ceramic coating, and gradient coating.Due to the complete melting of the coating material and metallurgical bonding with the metal substrate, the coating has a high degree of density and excellent corrosion and wear resistance [6] .However, the thickness of the flame surfacing layer is relatively large, and larger heat input can easily lead to deformation of the pipeline substrate and a decrease in mechanical properties.Ceramic coatings are mainly used for high temperature protection of boilers by high speed flame spraying to prepare ceramic phases such as NiCr or NiCrAl alloy and Cr 3 C 2 .These coatings can effectively improve the corrosion and wear resistance of metal heating surfaces at high temperature [7] .However, its brittleness is relatively high, it is prone to cracking, and the actual service life of the coating is short.
Currently, high entropy alloy coating is rarely used in the high temperature protection of boilers.It has excellent high temperature creep resistance, corrosion resistance, and wear resistance.AlCoCrFeNi coating is a HEA coating that has been studied extensively [8] .Zhou et al. prepared the FeCoCrNiAl HEA coating using the supersonic flame spraying technology has good wear resistance, about twice that of 35CrMo steel [9] .The high temperature corrosion resistance of AlCoCrFeNi HEA coating was studied earlier in this study.The HEA coating can form a dense oxide film at high temperatures [10] .The gradient ceramic coating prepared by adding hard ceramic particles to the HEA coating can have both the high adhesion strength of the metal coating and the corrosion resistance of the ceramic coating.Zhong et al. found that the gradient structure can delay the formation of cracks and the generation of residual stress at the interface in the coating and improve the interface bonding strength by comparing the Thermal spraying ZrO 2 -8% Y 2 O 3 -NiCoCrAlY gradient coating with the double-layer coating.The thermal shock resistance life of the gradient Thermal barrier coating obtained is significantly higher than that of the double-layer Thermal barrier coating [11] .Another study shows that the gradient distribution of Al in gradient structure NiCoCrAlYSi coating leads to the improvement of corrosion resistance of gradient structure NiCoCrAlYSi coating compared with ordinary coating [12] .
At present, there are few studies on the properties and applications of gradient high entropy alloy coatings, and there is no relevant report on the effect of gradient structure on the high temperature chlorine corrosion resistance of HEA.Based on the previous research in this article, ceramic phase particles were added to the AlCoCrFeNi high entropy alloy coating to prepare a high temperature protective coating with a gradient structure.The purpose is to study the influence of different gradient structures on the thermal shock resistance and high temperature corrosion resistance of gradient high entropy alloy coatings.

Sample Preparation
This project used 12Cr1MoV as the spraying substrate, with a size of 40 mm × 40 mm × 3 mm, and its main components are shown in Table 1 HEA powder is atomized AlCoCrFeNi powder, the average particle size is 300 mesh, the average particle size of Al 2 O 3 ceramic powder is 5 μm, and the average size of NiCrAl powder is 300 mesh.NiCrAl ceramic composite coating (NC), high entropy alloy composite coating (HC), and gradient high entropy ceramic coating (TC) with three different structures as shown in Figure 1 were prepared by high speed flame spraying.

Structure and Performance
The experiment used Hitachi SU8020 cold field emission scanning electron microscopy to survey the microstructure and structure of the coating.The thermal shock resistance experiment of the coating was conducted using a box resistance furnace (KSL-1000) to heat and maintain the sample to 800℃ and then transferred to the water for rapid cooling and heating until the sample coating showed cracks or fell off.High temperature chlorine corrosion experiment uses high temperature molten salt corrosion to simulate the actual corrosion environment [13] .A mixed salt solution with a molar ratio of KCl: Na 2 SO 4 =1:1 was uniformly applied to the surface of the coating, with a simulated corrosion elements application amount of 3 mg/cm 2 .After drying, experiment the coating in a resistance furnace at 650°C for 60 hours, and the change in quality of each coating was recorded.

Microstructure of the coating
Three different structural ceramic coatings were cut and sampled for high speed flame spraying.The surface and cross-sectional scanning analysis results of the coatings are shown in Figure 2 and figure 3. Figure 2 shows the microstructure of ceramic coatings with different structures.It can be seen that there are massive unmelted ceramic particles stacked into a bridge-like structure on the surface of the high speed flame-sprayed NiCrAl ceramic coat.The surface of the ceramic layer is comparatively rough.The high entropy ceramic composite coating is mainly due to the significant difference in thermal expansion coefficient between metals and ceramics, resulting in obvious cracks and defects on the surface of the ceramic layer.The gradient high entropy alloy coating with HEA as a metal bonding phase has a smooth surface and no crack defect.Figure 3 is the cross-sectional morphology of ceramic coatings with different structures.NiCrAl coatings have poor bonding with ceramics, and there are many pores inside the coatings.The fusion bonding effect of high entropy ceramic composite coating is better than that of NiCrAl ceramic coating, but there are significant defects in the thinner ceramic layer.The gradient high entropy alloy coating ceramic phase with gradient structure is closely bound to the matrix, and the surface density is high, without obvious defects and holes.

Thermal shock resistance test
The cyclic thermal shock performance of three gradient coatings with different structures was tested from 800℃ to room temperature, and the average maximum number of times the coating peeled off is shown in Figure 4. From Figure 4, the thermal shock resistance of TC coating is the best with an average of 21 times, while the thermal shock resistance of NC coating is only 1/3 of that of TC coating.The main failure mode of metal-ceramic composite coatings is the detachment of the coating from the substrate.There is high temperature oxidation at the interface between the coating and the substrate, and local stress is generated during cold and hot changes due to different thermal expansion coefficients between the coating and the substrate.As the number of experiments increases, the coating gradually produces cracks until it falls off [14] .The thermal expansion coefficient of gradient high entropy alloy coating gradually changes along the thickness direction with the composition of the coating, reducing the sudden change of the thermal expansion coefficient of the coating and the substrate, so it has good adhesion and thermal shock resistance.

High temperature chlorine corrosion resistance of coatings
This experiment used high temperature molten salt corrosion experiments to test the high temperature chloride corrosion resistance of ceramic coatings with different structures at 650℃.As shown in Figure 5, the morphology changes of ceramic coatings with different structures after high temperature chlorine corrosion are observed.NiCrAl ceramic composite coatings undergo extensive cracking and detachment after corrosion.Based on the microstructure of the coating surface, that corrosive elements penetrate the interior of the coating through surface pores, leading to coating cracking and detachment.There are more green corrosion products on the surface of high entropy ceramic composite coating, and there are only salt stains on the surface of gradient high entropy alloy coating, and no obvious corrosion traces and defects are found.
Due to the significantly lower high temperature corrosion resistance of 12Cr1MoV compared to the coatings, the experiment used uncoated 12Cr1MoV samples as the control group to accurately reflect the quality changes of the coating.The average oxidation rate  of 12Cr1MoV under different corrosion times is calculated separately.Formula (1) is substituted to calculate the actual mass change (∆ w) of the coating.where ∆m means the increase in mass of the sample (mg), Sc means the coating area (cm 2 ), and S o is the oxidation area of the uncoated part of the sample (cm 2 ).The mass change and corrosion time are then fitted with the Power function according to the following Formula (2) [15] : where ∆ w is the mass change per unit area (mg/cm 2 ), t is the corrosion time (h), and k and n are proportional coefficients.The quality changes of different coatings under high temperature chlorine corrosion are shown in Figure 6.

∆𝑤 ∆𝑚 𝑆 𝑉 𝑆
(1)  6, the quality of the coating gradually increases under a high temperature chloride corrosion environment, but the quality change of all coatings is significantly below than that of the 12Cr1MoV without coating protection.Therefore, the protective coatings have certain high temperature chloride corrosion resistance.The fitting results of the changes in high temperature chlorine corrosion quality of ceramic coatings with different structures are shown in Table 2 From the fitting results in Table 2, the correlation coefficient R 2 values of all fitting formulas are greater than 0.95, indicating a high degree of fitting correlation.It shows that the quality change of high entropy alloy coating in high temperature chlorine corrosion conforms to the Power function development law.The n value represents the development trend of corrosion, and since the n values of the coating are all less than 1, it indicates that the coating has a protective effect.The k value can indicate the severity of corrosion occurrence, and the larger the k value is, the more severe the corrosion is.The severity of coating corrosion is as follows: TC<HC<NC<12Cr1MoV.

Applications
Based on the comprehensive test results, the gradient high entropy alloy coating was used to conduct field spraying tests on two mechanical grate incinerators with a daily treatment capacity of 600 t/d in an incineration power plant.The experimental spraying area is located at the outlet of the furnace.As shown in Figure 7(a), the water wall pipes of incineration had serious high temperature corrosion before spraying protection.As shown in Figure 7, the comparison of the water-cooled wall before and after spraying shows that there are many metal oxides and coking on the water-cooled wall before spraying, and the high temperature corrosion of the metal is more severe.After sandblasting and rust removal, the water-cooled wall is sprayed with TC coating and sealed with holes.After continuous operation of the unit for about 4000 hours, sampling and analysis were conducted on the spraying area.The scanning results of the coating cross-section are shown in Figure 8: From Figure 8, it can be seen that the remaining coating is divided into two parts: the metal bottom layer and the ceramic layer.The substrate and coating parts are well bonded, while the ceramic layer and metal layer are tightly bonded.The average thickness of the remaining coating is 500 μm.The energy spectrum scanning results suggest that the coating is mainly composed of elements such as Ni, Cr, Mo, Al, Mn, V, K, and O, and there are no obvious corrosive elements such as S and Cl, indicating that the coating still has good corrosion resistance.

Conclusions 1)
NiCrAl ceramic composite coating, high entropy ceramic composite coating, and gradient high entropy alloy coating with three different structures were prepared by high speed flame spraying.The gradient high entropy alloy coating has a compact structure and the best spraying effect; 2) The gradient high entropy alloy coating has good thermal shock resistance, and the thermal shock resistance index is 3 times that of NiCrAl ceramic composite coating; 3) The mass change of the coating in the high temperature chlorine corrosion experiment meets the Power function model, and the gradient high entropy alloy coating has the best high temperature chlorine corrosion resistance.
4) According to the field test results, the gradient high entropy alloy coating has good high temperature chlorine corrosion protection performance for the boiler heating surface.The next step is to improve the performance of the coating from the perspective of spraying equipment and methods.

Figure 4 .
Figure 4. Thermal shock resistance of ceramic coatings with different structures.

Figure 6 .
Figure 6.High temperature chlorine corrosion quality of ceramic coatings with different structures.

Figure 7 .
Figure 7.Comparison of the plant before and after spraying.(a) Water wall before spraying; (b) Water wall after spraying.

Figure 8 .
Figure 8. Energy spectrum analysis of on-site coating sampling.

Table 2 .
: High temperature chlorine corrosion fitting of ceramic coatings with different structures.