1. Introduction
The use of waste incineration for power generation not only realizes the recycling of waste-separation resources, but also achieves the purpose of environmental protection [
1]. However, elements such as Cl and S contained in the waste will cause high-temperature corrosion of the boiler heating surface. The erosion of sodium, potassium, sulfate and chloride salts is an important cause of corrosion failure of heated surface materials [
2,
3]. High-temperature corrosion of the heating surface may eventually lead to the bursting of the tube wall, threatening the safe operation of the generating unit, which has been one of the main factors limiting the promotion and development of waste-incineration technology. Therefore, it is crucial to solve the problem of high-temperature corrosion on the heating surface of waste-incineration boilers.
Laser-cladding technology is a surface-strengthening technology in which the cladding material is placed on the substrate to be clad, and then heated using a high-energy-density laser to melt the cladding material first and then rapidly solidify it to the substrate, forming a cladding layer on the surface of the substrate material. The advantage of laser-cladding technology is that it is easy to prepare high-performance cladding layers on the surface of low-cost and low-property base materials to meet the requirements of replacing some high-performance alloys. This reduces material costs and substantially improves the performance of the base material. The corrosion-resistant cladding layer obtained on the surface of boiler tubes using laser-cladding technology can effectively prevent coating failures due to peeling. Moreover, laser-cladding technology has a lower heat input, significantly reducing the heat-affected zone of the boiler tube collective, but can also effectively prevent the dilution rate being too high to affect the corrosion resistance of the laser-cladding layer. The laser-melting layer is metallurgically bonded to the substrate material without cracks and holes, which can significantly improve the wear and the corrosion resistance of the surface of the substrate material [
4,
5,
6]. High-temperature corrosion protection using laser-melting cladding technology has received extensive attention from scholars [
7,
8,
9].
In order to solve the problem of high-temperature corrosion on the heated surface of waste-incineration boilers, it is imperative to develop laser-cladding materials with better high-temperature corrosion resistance. Ni-Cr alloy can generate a Cr
2O
3 protective layer on the surface in the high-temperature corrosive environment, which makes it have better corrosion resistance and oxidation resistance [
10]. However, if the Cr content is too low, a protective Cr-rich oxide layer cannot be formed on the alloy surface [
11]. Increasing the Cr content will increase the brittleness of the alloy and reduce the mechanical properties of the alloy [
12]. Studies have shown that the addition of appropriate amounts of Si can enhance the corrosion resistance of materials by promoting the formation of dense SiO
2 in corrosion-resistant alloys. Therefore, Si is often added to stainless steels, low-alloy steels and nickel-based alloys to improve the corrosion resistance of the alloy. Liu et al. [
13] investigated the effect of Si on the corrosion resistance of weathering steels and showed that the addition of small amounts of Si contributed to the formation of network-like SiO
2, promoted the enrichment of Cr in the internal corrosion layer and improved the corrosion resistance of the alloy. Geng et al. [
14] added Si to the Fe-Co-Ni alloy and showed that the addition of Si effectively reduced the oxygen partial pressure between the outer oxide layer and the substrate, promoting the selective oxidation of Cr elements and the formation of a continuous Cr
2O
3 inner oxide layer. However, no systematic study is available on the effect of Si addition on the high-temperature corrosion performance of Ni-Cr-alloy-cladding in a simulated waste-to-energy environment.
In this paper, Ni-Cr-Si-alloy-clad layers with Si content of 0 wt.%, 1 wt.%, 3 wt.% and 5 wt.% were prepared via laser-cladding technology, and the effect of Si content on the high-temperature corrosion resistance of Ni-Cr-Si-alloy-clad layers in NaCl-KCl-K2SO4-Na2SO4 mixed salt was investigated systematically and thoroughly. The role of Si in enhancing the corrosion resistance of Ni-Cr-alloy-clad layers in a simulated high-temperature corrosive environment of a waste-to-energy plant is investigated to provide a theoretical basis for solving the high-temperature corrosion problem of boiler tubes in waste-to-energy plants and the development of corrosion-resistant-clad-layer materials.
2. Materials and Methods
2.1. Laser-Cladding-Layer Preparation
Si powder was added to the Ni-20Cr-alloy powder to produce a mixture of powders with Si mass fractions of 1 wt.%, 3 wt.% and 5 wt.% in that order. It was noted that Cr powder was added accordingly to maintain the Cr content at 20 wt.%. The above three alloy powders were stirred thoroughly for 10 h to obtain a homogeneous cladding powder. Laser-cladding layers of Ni-Cr-Si alloys with different Si contents were prepared using the laser-cladding-layer technique. It has been shown that the addition of Si enhances the oxidation and corrosion resistance of the alloys even if the addition is not sufficient to form a continuous SiO
2 layer [
15,
16,
17]. The Si content of most of these studies is below 5 wt.%. In addition, in the actual preparation process, it is difficult to prepare a crack-free cladding layer when the Si element addition exceeds 5 wt.%. Therefore, we chose to add Si content of not more than 5 wt.%. In order to compare the corrosion-resistance properties of cladding layers with different Si contents in more detail, we chose fused cladding layers with Si contents of 0 wt.%, 1 wt.%, 3 wt.% and 5 wt.% for comparative studies. The four cladding layers were named S0, S1, S3 and S5 according to the Si content, and the specific Ni-20Cr-xSi-alloy-powder elemental-mass fraction ratios are shown in
Table 1.
The laser-cladding system mainly consists of a fiber laser, a cooling system, a synchronized-powder-feeding system and a control system. The fiber laser used in this study is a Wuhan Raycus RFL-3000 model (Wuhan Raycus Fiber Laser Technologies, Wuhan, China) with a maximum power of 3000 W and a laser spot diameter of 1.4 mm. To prevent oxidation of the alloy powder during the cladding process, argon gas (99.9%) was used to protect the powder and the molten pool, with a flow rate of 10 L/min. Nitrogen gas was used for powder feeding, with a gas flow rate of 5 L/min. The cladding powder material was uniformly dispersed around the laser beam and delivered to the molten pool in a circular powder-feeding pattern. The base material and the cladding layer were cooled by means of air cooling. To achieve smooth and continuous output of the laser, the cladding table was controlled to move and prepare multi-layer overlapped Ni-Cr-Si-alloy-cladding coatings.
To reduce the diffusion of base-metal elements into the cladding layer and minimize the dilution effect of Fe on the corrosion resistance of the cladding layer, multi-layer cladding coatings with a thickness of approximately 4–5 mm were prepared. The cladding layer was processed into a sample size of 20 mm × 10 mm × 2 mm using wire-cutting techniques (with the sample location selected in the middle-upper part of the cladding layer). The surface of the cladding-layer sample was subsequently polished using waterproof sandpaper of 400#, 600# and 800# sequentially on a metallographic polishing machine until the surface of the cladding-layer sample was smooth. The polished samples were cleaned using an ultrasonic cleaning machine in anhydrous ethanol solution for 10 min. Subsequently, the samples were subjected to ultrasonic cleaning in acetone solution for another 10 min to thoroughly remove any oil and impurities from the sample surface. After cleaning, the samples were thoroughly dried using a hot blow dryer.
2.2. High-Temperature Corrosion Test
To simulate the high-temperature corrosion environment of municipal-waste-incineration boilers, a mixture of NaCl, KCl, Na2SO4 and K2SO4 in a mass fraction ratio of 1:1:1:1 was used as the corrosive agent. High-temperature corrosion experiments were conducted within the range of 600 °C. First, the polished and cleaned cladding-layer samples were weighed and their surface areas were measured. Then, the cladding-layer samples were placed into pre-dried alumina crucibles with the mixed salt already pre-positioned. The corrosive agent was then added to cover the surface of the samples by 2–3 mm, ensuring that all surfaces of the cladding layer were covered by the mixed salt. The crucibles were placed at the central position of a tube furnace to ensure uniform heating and corrosion temperature. The duration of this corrosion experiment was 144 h.
Due to the tendency of corrosion products to easily peel off, the weight-loss method was used to measure the high-temperature corrosion resistance of the cladding layer, in order to obtain more-accurate experimental results. After corrosion, the samples were acid-washed. The acid-washing process involved removing the samples, cooling them to room temperature and cleaning off the surface corrosive agent and some corrosion products using deionized water through ultrasonic cleaning. Subsequently, 25 wt.% hydrochloric acid (concentration: 8.082 mol/L) was used for ultrasonic cleaning in a water bath at 80 °C to thoroughly remove the corrosion products from the sample surface. After the samples were thoroughly cleaned and dried, their post-corrosion masses were measured using an electronic balance (precision: ±0.01 mg) to determine the mass change after corrosion. The corrosion weight loss per unit area of the cladding layer can be calculated using the following formula:
where m
0 is the initial mass of the cladding-layer sample (mg), m
1 is the mass of the sample after completion of the corrosion test and removal of the surface corrosion products through acid washing (mg), and A is the original surface area of the cladding-layer sample (cm
2).
2.3. Representation
Physical analysis of the original phase composition of the cladding and corrosion products was carried out via X-ray diffraction (XRD, Raku D/Max-2400, RAMPF Tooling Solutions, Grafenberg, Germany), observation of the surface and cross-sectional morphology of the corrosion products was carried out via scanning electron microscopy (SEM, Quattro-S, FEI, Thermo Fisher Scientific, Waltham, MA, USA) and characterization of the elemental composition and distribution in typical areas was carried out via X-ray energy spectrometry (EDS, EDAX, Mahwah, NJ, USA).
4. Discussion
In the case of S1, a continuous corrosion layer formed on the surface, with a relatively uniform outer corrosion layer and an average thickness of approximately 5 μm. The inner corrosion layer exhibited denser corrosion products, but a clear transverse crack could be observed between the inner and outer corrosion layers, without spalling of the outer corrosion layer. Three regions (4, 5 and 6) were selected for elemental analysis along the cross-section. The EDS results showed that the outer corrosion layer was primarily composed of O (26.9 wt.%) and Cr (46.5 wt.%) elements, with a higher Na element content of 14.8 wt.%, indicating the enrichment of corrosive media in the outer corrosion layer in the mixed-salt environment. The Ni-element content increased below the crack in the inner corrosion layer, with a lower concentration of corrosive-media elements, suggesting that although there was a deep crack, the continuous outer corrosion layer effectively hindered the penetration of corrosive media. In the region near the substrate, represented by region 6, the Ni- and Cr-element contents were similar to those of the matrix, but the S element content was high at 11.8 wt.%, indicating that a small amount of S had diffused into the cladding matrix through the cracks in the surface corrosion layer, resulting in internal corrosion. The elemental-distribution results revealed that the outer corrosion layer of S1 mainly consisted of O and Cr elements, while Ni elements primarily accumulated around the inner corrosion layer.
Compared to S1, the surface morphology of the outermost corrosion-product layer changed significantly in S3 and S5, with a more porous corrosion layer, spalling of the outer corrosion layer, and the formation of numerous pores in the inner corrosion layer. The average thickness of the spalled-off region was approximately 10 μm. According to EDS element analysis, the dark-colored corrosion-product regions 7 and 11 in the outer corrosion layer were mainly composed of O and Cr elements, confirmed via XRD analysis as predominantly Cr2O3. The light-colored regions 8 and 12 in the corrosion layer were primarily composed of Ni elements, with lower concentrations of O, S and Cl elements, indicating partial spalling of the outer corrosion layer and the presence of products similar to the matrix. In the molten-salt environment, the Cr2O3-corrosion layer formed on the surface of the cladding gradually dissolved, leading to the observation of large-scale spalling and the highest corrosion loss in S0. When SiO2-corrosion products formed during high-temperature corrosion with the addition of Si, they partially inhibited the dissolution of the Cr2O3-corrosion layer, thereby impeding the penetration of corrosive media. However, when the Si content was too high, the surface corrosion layer became porous and cracked, allowing corrosive S elements to penetrate into the cladding matrix through the defects and react with the alloying elements. Region 10 in the inner corrosion layer of S3 exhibited a high S-element content of 19.4 wt.%, and XRD analysis indicated that the corrosion-product layer was mainly composed of Ni3S2. Furthermore, a comparison of the elemental-distribution results along the cross-section revealed that Si elements primarily accumulated in the outer corrosion layer. The addition of Si in the Ni-20Cr-alloy cladding facilitated the formation of Cr2O3 during high-temperature corrosion, thereby improving the corrosion resistance of the cladding layer, consistent with the observed results in this experiment.
In this experiment, O
2 can freely cross the mixed salt and react with the cladding layer, and the chemical reaction equation is as follows:
where M is Ni, Cr and other elements in the Ni-Cr-Si alloy cladding layer, the generated NiO, and Cr
2O
3 attached to the surface of the cladding layer, preventing the mixed salt and further O
2 penetration into the cladding layer, thus slowing down the process of high-temperature corrosion reaction.
In the mixed salt (NaCl:KCl:K
2SO
4:Na
2SO
4 = 1:1:1:1) at 600 °C, the cladding layer might undergo the following reactions during the high-temperature corrosion process:
where R stands for alkali metal elements such as Na and K. The Cl
2 generated at high temperature passed through the oxide layer to the surface of the cladding layer and reacted with the metal elements to generate chlorides. This corrosion mechanism of accelerated oxidation of the alloy caused by the synergistic action of chloride and Cl
2 with O
2 is called the “active oxidation” mechanism. Therefore, the corrosion mechanism of the chloride salt in the mixed salt on the cladding layer is as follows:
Therefore, NiO, Cr
2O
3, etc., were detected in the S0 corrosion layer, but the product layer was loose and it was difficult to achieve the effect of hindering the corrosion medium. After adding Si to the cladding layer, SiO
2 was generated [
9] (Reaction (8)) during the high-temperature corrosion process. Although SiO
2 was not detected after corrosion, the EDS-analysis results show that SiO
2 existed in the corrosion layer, which could protect and greatly reduce the corrosion loss of the cladding layer.
Electrochemical corrosion played a major role as the mixed salt had undergone melting [
18]. A large number of studies have shown that the corrosion of the alloy will be aggravated by the addition of alkali metal chloride salts to the sulfate. This is due to the fact that NaCl and KCl can react with the Cr
2O
3 generated on the surface of the cladding layer, which reacts as follows [
19]:
In addition, SO
4− released from the sulfates (K
2SO
4 and Na
2SO
4) in the salt mixture causes the presence of O
2− in the molten-salt environment, with the following reaction.
Likewise, this induces the dissolution of the Cr
2O
3 generated on the surface of the cladding layer, with the following reaction:
The dissolved-oxide layer was loose and porous and thus not protective, resulting in a large number of cavities at the corrosion-product layer–cladding layer interface, and the corrosion layer appeared to spall off. The loose corrosion products could not block the mixed salt contact with the cladding layer, so that chloride ions could constantly consume the metal elements of the cladding layer through the above reaction. Sulfate ions could also react with the metal elements of the cladding layer to generate metal sulfides [
4]. Once the corrosion layer is dissolved and does not have protective properties, Ni-sulfide particles appear between the broken oxide layer, thus causing more severe corrosion. However, the addition of a certain amount of Si to the Ni-20Cr-alloy-cladding layer significantly improves the high-temperature corrosion resistance of the cladding layer in the mixed salt. In addition, Cr
2O
3 was detected on the surface of all samples, and the cross-sectional morphology shows that a continuous Cr-rich oxide layer was attached to the surface of the cladding layer, which could provide a good protection effect. The cross-sectional morphology of the cladding layer shows that the surface of the cladding layer of the four Ni-Cr-Si alloys at all temperatures is dominated by the Cr
2O
3 layer, and the addition of Si improves the stability of Cr
2O
3 in the molten salt. Katharina et al. [
20] and Wang et al. [
21] pointed out that “Si can act as a nucleation site for Cr and accelerates the formation of dense Cr
2O
3 layer. Moreover, the addition of Si enhances the activity of Cr in the matrix”, which is consistent with the conclusions obtained from this work. Additionally, the experimental results of Zhang et al. [
22] confirmed that the presence of Si promotes the formation of a continuous dense Cr
2O
3 layer on the surface. Although a continuous SiO
2 protective layer is not generated, its presence in the corrosion layer can effectively impede the penetration of the corrosive medium, thus improving the corrosion resistance of the cladding layer in NaCl-KCl-Na
2SO
4-K
2SO
4 salt.