In order to alleviate the high temperature corrosion problem of the heating surface of the metal pipe in the process of biomass incineration power generation and realize the long-term pure combustion/blended combustion of biomass in the boiler, the Inconel 625 alloy protective coating was prepared on the surface of TP347 steel pipe by ultra-high speed laser cladding technology. The microstructure and element segregation characteristics of the cladding layer were analyzed by SEM, EDS and other characterization methods; the long-term corrosion behavior of TP347 heat-resistant steel and Inconel 625 cladding layer at 550 ℃ was compared by a self-made high temperature corrosion test device simulating the real environment of biomass boiler. The results show that the ultra-high-speed laser cladding Inconel 625 coating after process optimization has no obvious defects inside and forms a good metallurgical bonding interface with the TP347 steel substrate; the coating structure is composed of fine solid solution γ-Ni phase and grain boundary segregation phase, and the microhardness is HV341; the component segregation inside the coating is small, and the Ni and Cr alloy elements in the grain boundary segregation phase do not decrease significantly; the corrosion weight gain of the two samples increases linearly with time, and the corrosion weight gain of the Inconel 625 coating at 550 ℃ for 500 h is only 1/62 of that of the TP347 heat-resistant steel, which is mainly due to its low Fe content and uniform interface corrosion characteristics. The preparation of Inconel 625 alloy protective coating by ultra-high-speed laser cladding can effectively improve the corrosion resistance of the heating surface of the pipeline in the biomass incineration boiler.

Under the background of my country’s in-depth promotion of carbon peak, achieving clean and low-carbon energy transformation, strictly controlling the growth of coal consumption, and increasing the proportion of non-fossil energy consumption are important tasks for industrial development during the “14th Five-Year Plan” period. In addition, the treatment of urban domestic waste, rural crop straw and other biological waste is an important livelihood issue. The traditional on-site incineration treatment not only pollutes the environment, but also causes huge energy waste [1]. Since the 1980s, foreign countries have begun to use biomass waste such as crop straw for incineration power generation, which not only reduces the consumption of coal fossil energy, but also effectively solves the problem of domestic and agricultural waste treatment [2-3]. In 2016, biomass power generation technology was widely used in developed countries such as the United States, and forestry waste power generation alone accounted for 5.5%, close to their power generation share [4]. In recent years, China has gradually attached importance to biomass incineration power generation. As of 2019, the number of domestic waste incineration plants in operation nationwide exceeded 400 [2]. However, the flue gas and solid dust produced during biomass incineration are complex in composition, including flue gas such as HCl and Cl2 and solid dust such as alkali metal salts, which cause serious corrosion of the boiler heating surface [5], and the boiler has to be shut down to replace the pipeline, shortening the boiler operation cycle [6].

Surface strengthening technology is an important means to improve the corrosion resistance of materials or parts, mainly including spraying, surfacing, electroplating and cladding [7]. OKSA et al. [8] used HVOF technology to prepare Inconel 625 coating on the economizer tube wall of the heating surface of a 40 MW circulating fluidized bed boiler, reducing the maximum corrosion rate of the tube wall from 2.3 mm per year to the micrometer level. SCHMID et al. [9] prepared Ni-based corrosion-resistant coating on the surface of boiler heating surface pipes, which significantly reduced the corrosion rate of the heating surface of biomass incineration boilers and improved the safety service performance. SONG et al. [10] studied the corrosion resistance of Ni50Cr coatings made by HVOF, cold spraying and laser cladding processes under simulated biomass conditions and found that the laser cladding coating had the best corrosion resistance. Laser cladding technology can prepare a protective coating with a metallurgical bonding interface on the surface of the substrate, while eliminating the internal pores and oxide impurities in the coating, thereby ensuring the long-term protective performance of the coating [11-12]. Compared with traditional cladding technology, ultra-high-speed laser cladding technology can achieve coating preparation at extremely high linear speed (up to 200 m/min) by regulating the powder and laser focal plane, so that the laser energy mainly acts on the powder. It has the advantages of high cladding efficiency, low dilution rate and low coating surface roughness. At the same time, it can also obtain finer grain structure and significantly improve the long-term service performance of the coating [13-14].

Therefore, the author uses the corrosion-resistant Inconel 625 alloy as raw material and uses ultra-high-speed laser cladding technology to prepare corrosion-resistant coating on the surface of TP347 heat-resistant steel. Through the high-temperature corrosion test of simulated biomass incineration, the high-temperature corrosion performance of TP347 steel and Inconel 625 cladding layer is studied, which provides a new method and basis for improving the life of biomass incineration boiler pipelines, extending the replacement cycle and reducing production costs.

1 Experiment

1.1 Preparation of Inconel 625 coating by ultra-high-speed laser cladding

TP347 heat-resistant steel pipe (1Cr19Ni11Nb) was selected as the substrate, with a diameter of 54 mm and a wall thickness of 10 mm; the cladding powder used was spherical Inconel 625 powder produced by Tianjin Zhujin Technology Co., Ltd., with a particle size of 50-75 μm (Figure 1 (a)). The chemical composition of the raw powder is shown in Table 1. In this experiment, the ultra-high-speed laser cladding power was 4 kW, the scanning speed was 81 m/min, and argon was used as the protective gas. The specific cladding parameters are shown in Table 2. During the coating cladding process, 0.4 MPa compressed air was continuously introduced into the sample surface to accelerate the cooling of the sample. Before cladding, the substrate surface needs to be ground and polished, and the surface is wiped with alcohol to ensure the smoothness of the cladding interface. After cladding, an Inconel 625 coating with a thickness of about 1 mm is prepared on the surface of the TP347 heat-resistant steel pipe, and the surface roughness of the coating is significantly lower than that of traditional laser cladding (Figure 1 (b)).

1.2 Microstructure analysis of Inconel 625 coating

Microstructure analysis samples were cut from the TP347H pipe after ultra-high-speed laser cladding of Inconel 625 alloy coating by electric spark wire cutting; metallographic samples without obvious scratches were prepared by grinding and polishing the cross section of the cladding sample. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) were used to observe and analyze the microstructure and element distribution characteristics of the ultra-high-speed laser cladding Inconel 625 alloy coating. The hardness of the Inconel 625 cladding layer and the TP347 heat-resistant steel substrate was tested using a micro-Vickers hardness tester. The test load was 300 g and the holding time was 30 s.

1.3 Study on high-temperature corrosion behavior of coatings

The high-temperature corrosion test was carried out in a self-made simulated biomass incineration flue gas corrosion reaction device (Figure 2 (a)). The surface of the sample to be corroded was brushed with a saturated solution of KCl, NaCl and K3Na(SO4)2 (molar ratio of 3:4:1) and quickly dried to simulate the solid alkali metal salt attached to the surface of the biomass incineration boiler pipe; the sample to be corroded was placed in a tubular heating furnace, the heating temperature was set to 550℃, and the HCl and H2SO3 aqueous solutions were dripped into the high-temperature oil bath to generate HCl, SO2 gas and water vapor, and the corrosive gas was brought to the sample surface by N2 and O2 gas flow. The simulated corrosive flue gas was HCl (0.08%), SO2 (0.01%), H2O (5%), O2 (5%) and N2 (remainder) [15]. The corrosion samples were cut from the pipe by wire cutting to ensure that the arc corrosion surface area of ​​each sample was 8.5 cm2, and the samples were placed in a corundum crucible to prevent the corrosion layer from peeling off. The weight gain data of corrosion at different corrosion times were obtained by weighing the mass change before and after corrosion using an electronic balance with an accuracy of 0.01 mg. At least 3 samples were tested under each condition and the average value was taken. The corrosion interface morphology characteristics and product composition analysis of the TP347 heat-resistant steel substrate and Inconel 625 cladding layer after corrosion were characterized by SEM, EDS and XRD.

2 Experimental results and discussion

2.1 Microstructure analysis of cladding layer

The backscattered morphology and corrosion structure image of the cross section of the ultra-high-speed laser cladding Inconel 625 coating are shown in Figure 3. As shown in Figure 3 (a), there are no defects such as pores and cracks inside the ultra-high-speed laser cladding Inconel 625 alloy coating, and the coating surface is relatively flat; a good metallurgical bonding interface is formed between the cladding coating and the substrate, and the interface is uninterrupted. This shows that by optimizing the process parameters, a defect-free ultra-high-speed laser cladding Inconel 625 coating can be prepared on the surface of TP347 heat-resistant steel. Figure 3 (b) shows the microstructure of the coating after aqua regia corrosion, which is mainly composed of γ-Ni phase and precipitated phase at the grain boundary, which is consistent with the report in the literature [16]; the microstructure of the coating is mainly columnar crystals, dendrites and equiaxed grains. Due to the small energy input per unit area in the ultra-high-speed laser cladding process, the cooling rate of the molten pool increases, so that the alloy melt obtains a greater degree of supercooling, which is conducive to obtaining fine equiaxed grains. According to the grain statistics, the average grain area of ​​the ultra-high-speed laser cladding Inconel 625 alloy coating is about 45.31 μm2. Compared with the traditional cladding, its grains are significantly refined, which is conducive to improving the strength and hardness of the coating.

The microhardness of the prepared ultra-high-speed laser cladding Inconel 625 alloy coating is about HV341, while the microhardness of the TP347 heat-resistant steel substrate is about HV184. The hardness of the ultra-high-speed laser cladding Inconel 625 coating is significantly higher than that of TP347 heat-resistant steel, which is increased by about 89%. Under normal circumstances, the hardness of Inconel 625 pipe is about HV210, while the hardness of the traditional laser cladding Inconel 625 alloy is about HV277, which is lower than the alloy coating prepared by ultra-high-speed laser cladding. This is because the solidification rate of the molten pool is fast during the ultra-high-speed laser cladding process, which is conducive to obtaining fine-grained Inconel 625 alloy, thus showing a fine-grain strengthening effect [17].

2.2 Analysis of composition segregation of cladding layer

In addition to the γ-Ni main phase, there are also a large number of grain boundary precipitation phases in the ultra-high-speed laser cladding Inconel 625 coating, indicating that there is a certain composition segregation inside the coating. In TIG cladding and traditional laser cladding alloy coatings, the segregation of internal chemical composition is common [18]. In order to explore the segregation characteristics of elements in the ultra-high-speed laser cladding Inconel 625 coating, the energy spectrum scanning and point analysis of the coating cross section are performed respectively as shown in Figure 4. The results show that Ni and Cr elements are evenly distributed in the ultra-high-speed laser cladding Inconel 625 alloy coating without obvious segregation, while Mo and Nb elements show obvious segregation and local enrichment. It is inferred that during the ultra-high-speed laser cladding process, the cooling rate of the molten pool increases, which is not conducive to the balanced diffusion of elements and the precipitation of new phases, thereby reducing the segregation of elements inside the coating.

The elemental composition of the dark γ-Ni main phase and the white bright grain boundary precipitation phase in the coating is detected by energy spectrum point analysis, as shown in Figure 5 and Table 3. It can be seen that the elemental composition of the dark main phase and the grain boundary precipitation phase is basically the same, mainly composed of alloy elements such as Cr, Fe, Ni, Mo and Nb. Compared with the dark main phase, the mass fractions of Mo and Nb in the grain boundary precipitated phase are higher (3 to 4 times higher, respectively), while the Cr and Ni elements are not significantly reduced, so the corrosion resistance of the grain boundary segregation phase will not be significantly reduced.

2.3 High temperature corrosion resistance

The variation of high temperature corrosion weight gain (WTP and WCo) of TP347 heat-resistant steel and ultra-high speed laser cladding Inconel 625 alloy coating with time at 550 ℃ is shown in Figure 6. It can be seen that the corrosion weight gain of TP347 heat-resistant steel and ultra-high speed laser cladding Inconel 625 alloy coating increases linearly with corrosion time; the corrosion rate of ultra-high speed laser cladding Inconel 625 coating is much lower than that of TP347H heat-resistant steel. In contrast, by preparing the ultra-high speed laser cladding Inconel 625 protective coating, the high temperature corrosion rate of TP347 heat-resistant steel is 1/62 of that without coating, indicating that the cladding alloy coating exhibits excellent high temperature corrosion resistance. Generally, in the corrosion process dominated by the diffusion of corrosive media, the formation of the corrosion product layer on the surface of the material will have a certain hindering effect on the corrosive media, and the corrosion rate and corrosion time usually show a parabolic growth law [19]. However, in this experiment, under the coupled corrosion of high-temperature corrosive gas and alkali metal salt medium, the corrosion weight gain of the sample increases linearly with time, indicating that the corrosion product layer has little hindering effect on the subsequent diffusion of corrosive media and corrosion reaction process, and the corrosion failure phenomenon of the material is severe.

The surface morphology of TP347 heat-resistant steel and ultra-high-speed laser cladding Inconel 625 alloy coating samples after corrosion at 550 ℃ for 500 h is shown in Figure 7. It can be seen that after high-temperature corrosion, a layer of dark corrosion products is formed on the surface of the two samples. The corrosion products on the surface of the TP347 heat-resistant steel sample show serious cracking and flaking, while the corrosion surface of the ultra-high-speed laser cladding Inconel 625 coating sample is relatively smooth. This shows that the ultra-high-speed laser cladding Inconel 625 coating has better high-temperature corrosion resistance than TP347 steel, which is completely consistent with the corrosion rate change results in Figure 6. Under the coupling of high-temperature corrosive atmosphere and alkali metal salt medium, the corrosion product layer on the surface of the two samples has a loose structure, which cannot prevent the further diffusion corrosion of the corrosive medium in the subsequent corrosion process, resulting in a linear growth relationship between corrosion weight gain.

Through the energy spectrum analysis, it can be seen that the corrosion products on the surface of TP347 heat-resistant steel contain a large amount of alloy elements such as Fe, Cr, and Ni in addition to the main elements of the coating alkali metal salts such as O, K, S, and Na, indicating that TP347 heat-resistant steel is seriously corroded; while the corrosion products on the surface of the ultra-high-speed laser cladding Inconel 625 coating are mainly composed of elements such as Cr, Ni, Mo, Nb, and O, indicating that its corrosion products are mainly metal oxides. The XRD spectra of the corrosion surfaces of TP347H heat-resistant steel and ultra-high-speed laser cladding Inconel 625 coating are shown in Figure 8. It can be seen that the corrosion products on the surface of TP347 heat-resistant steel are mainly composed of KNa (SO4), Fe3O4, Fe2O3 and CrO2, while the corrosion surface of the ultra-high-speed laser cladding Inconel 625 coating is mainly composed of NiO and NiCr2O4. Due to the presence of a large amount of Fe element, a large amount of Fe oxide is produced in the corrosion products of TP347 heat-resistant steel, and the volume expansion of Fe oxide is serious during the formation process, causing the surface of the corrosion product layer to bear a large tensile stress and crack and peel off. For the corrosion of the ultra-high-speed laser cladding Inconel 625 coating, the occurrence forms of Na and K elements were not observed. This is because the protection of the coating resulted in no cracking and peeling during the corrosion process, so Na salts and K salts could not be stably attached to the corrosion products. For the two samples, no Cl element was detected in the surface corrosion products. This is mainly because the compounds formed by Cl elements and alloy elements such as Ni and Cr are metastable at high temperatures, with a melting point of only about 300 °C and a relatively high saturated vapor pressure. At the test temperature, they will evaporate to the top of the coating and react with oxygen at the top to form more stable oxides, thereby forming the final reaction product [20]. It can be inferred that the ultra-high speed laser cladding Inconel 625 coating exhibits more excellent high temperature corrosion resistance, which is due to its lower internal Fe content and higher Ni content with excellent corrosion resistance to a certain extent.

In order to explore the high-temperature corrosion mechanism of ultra-high-speed laser cladding Inconel 625 alloy coating and TP347 heat-resistant steel, the cross-sectional micromorphology of the corrosion sample was further observed, as shown in Figure 9. It can be seen that the product layer of the two samples after high-temperature corrosion showed cracking, which is closely related to the linear growth law of corrosion weight gain of the two samples; the corrosion interface of the two samples showed an uneven morphology, indicating the unevenness of their corrosion resistance. For TP347 heat-resistant steel, in addition to the diffusion corrosion of austenite grains, the corrosive medium will preferentially diffuse along the grain boundary into the interior of the sample, and the diffusion depth can reach 35 μm; however, the corrosion interface of the ultra-high-speed laser cladding Inconel 625 coating is relatively uniform, and there is no phenomenon of preferential corrosion of the grain boundary segregation phase, while the surface grains are gradually corroded. The composition segregation inside the ultra-high-speed laser cladding Inconel 625 alloy coating is small, and the concentration of Ni and Cr elements in the grain boundary segregation phase has not decreased significantly. Therefore, the corrosion resistance of the coating is relatively uniform, and the overall corrosion interface of the coating is relatively flat. For TP347 heat-resistant steel, the corrosion-resistant alloy element content is low and the Fe content is high, resulting in insufficient high-temperature corrosion resistance; at the same time, the high-temperature corrosive medium preferentially diffuses and corrodes along the grain boundary, thereby accelerating its overall corrosion failure. In short, the preparation of Inconel 625 anti-corrosion coating on the surface of TP347 heat-resistant steel by ultra-high-speed laser cladding can effectively improve its resistance to high-temperature corrosion caused by biomass incineration.

3 Conclusions

1) The Inconel 625 coating prepared by ultra-high-speed laser cladding on the surface of TP347 heat-resistant steel has no obvious defects inside, the coating surface is flat, and a good metallurgical bonding interface is formed between the substrate; the cladding layer is mainly composed of γ-Ni main phase and grain boundary segregation phase, showing fine columnar crystals, dendrites and equiaxed crystal structures.

2) The internal element segregation of the ultra-high-speed laser cladding Inconel 625 coating is small, the content of Mo and Nb elements in the segregation phase at the grain boundary is high, while the content of Ni and Cr elements is not significantly reduced; the microhardness of the alloy cladding layer is HV341, which is about 89% higher than the microhardness of TP347 heat-resistant steel (HV184).

3) The high-temperature corrosion weight gain of TP347 heat-resistant steel and ultra-high-speed laser cladding Inconel 625 alloy coating increases linearly with time, but the corrosion rate of Inconel 625 cladding layer is about 1/12 of TP347 heat-resistant steel, and the corrosion weight gain of 500 h at 550 ℃ is only 1/62 of TP347 heat-resistant steel. Inconel 625 alloy coating has a good effect of resisting high-temperature alkali metal and chlorine corrosion. TP347 heat-resistant steel preferentially corrodes along the grain boundary, and a large amount of Fe oxides are formed in the corrosion products, resulting in cracking and peeling of the product layer; while the corrosion interface of Inconel 625 alloy coating is relatively flat, and there is no preferential corrosion phenomenon of grain boundary segregation phase.