ABSTRACT: As research into metal additive manufacturing continues, laser cladding is gaining increasing attention in the forming and reworking of complex parts. The heating rate and cooling rate in laser processing are extremely high, which can lead to cracks and damage to the cladding layer. The risk of cracking is reduced by increasing the Si content of the laser cladding powder. This lowers the melting point of the powder, thereby reducing the heat input during processing. However, the effect of Si is not limited to lowering the melting point of the powder. In order to investigate the effect of Si on the fabrication process and the corrosion performance of cladding layers, 316L cladding layers with different Si contents were fabricated on 316L
substrates with 0.8% Si-316L, 1.2% Si-316L and 1.6% Si-316L powders, respectively. The macroscopic morphology and microstructural composition of the cladding samples were characterized by laser confocal microscopy, scanning electron microscopy, X-ray diffraction and thermogravimetric analysis.

It was found that the thickness of the laser cladding layer increased as the Si content increased. The average thickness of a single layer increased from 700 μm at 0.8% Si to 800 μm at 1.2% Si and to 900 μm at 1.6% Si, increasing by 14% and 29%. At the same time, because the laser cladding layers were stacked layer by layer, the entire layer underwent a complex thermal cycling process that greatly affected the stability of the final cladding properties. By increasing the Si content of the powder, the number of stacked layers could be reduced. This would result in a more consistent quality of the final layer.

In the meantime, the oxidation of elemental Si produced SiO2, which protected the melt pool well and reduced the oxidation of the cladding layer. The results of the TG tests showed that 0.8% Si oxidized at high temperatures and gained an average of 32% by weight, whereas 1.6% Si oxidized at a rate of only 19% by weight. Si preferentially reacted with oxygen to protect the remaining elements in the melt pool for thermodynamic reasons. This protection was enhanced by the extremely high melt pool cooling rate, ultimately leading to improved oxidation of metallic elements in the cladding at high temperature.

In addition, the corrosion resistance of the molten cladding could be improved by the oxidation of elemental Si to SiO2. The number of pits on the sample surface decreased significantly with increasing Si content in the electrochemical tests. The electrochemical results showed that the corrosion current decreased from 2.039×10’–6 A·cm–2 to 1.889×10’–6 A·cm’–2 and 1.422×10’–6 A·cm’–2 with increasing elemental Si content, while the self-corrosion potential moved in a positive direction. From the dynamic polarization curves, it could be seen that as the Si content increased, the corrosion performance increased mainly in the form of a longer passivation interval. This was most likely due to the conversion of Si to SiO2 which was enriched on the Cr2O3 passivation layer. This compensated for the lack of Cr2O3 passivation. The pitting potentials of all three increased from 0.8% Si (0.4 V) to 1.2% Si (0.6 V) and 1.6%Si (0.9 V). The impedance spectral data also showed that the impedance of the specimens increased with Si content. In particular, the passivation impedance R2 increased from 0.65×10’5 Ω·cm’2 to 3.55× 105 Ω·cm’2 and 4.08×10’5 Ω·cm’2, indicating that the oxidation of elemental Si to form SiO2 improved the passivation layer of the 316L surface layer, effectively improving the corrosion resistance of the cladding layer.

Additive manufacturing (AM) is a manufacturing technology that can be used to form complex parts. Among them, laser cladding technology (LC) and selective laser melting technology (SLM) using laser as heat source are extremely typical representatives. In recent years, the number of research papers on laser additive manufacturing technology has increased significantly. Among them, selective laser melting technology has high requirements for the working environment, and is limited by its cabin volume, making it difficult to complete the processing and manufacturing of large-sized components. At the same time, due to the extremely high forming accuracy of selective laser melting technology (the laser spot is tens of microns), its forming efficiency is low; while laser cladding technology has high forming efficiency and has unparalleled advantages over selective laser melting technology for the remanufacturing of large and complex components. The millimeter-level spot enables laser cladding technology to quickly complete the modification and repair and strengthening of the surface of large-sized complex components. However, in an open environment, problems such as oxidation inclusions are prone to occur inside the laser cladding layer, and due to its high cooling rate of 10’5~10’7/s ℃, the laser cladding layer is very prone to cracking. There are many factors that affect the quality of laser cladding. In addition to process parameters such as laser scanning rate, powder feeding rate, and laser power, the composition of the metal powder itself will also affect the quality of the cladding layer.

Since most of the iron-based alloy coating materials currently used for laser cladding are directly made of self-fluxing iron-based alloy powders for thermal spraying, they are not fully compatible with laser cladding technology. During thermal spraying, the alloy will have a large temperature range from the beginning of melting to the end, thereby ensuring that the alloy has suitable fluidity in the molten pool, so that the molten alloy can form a smooth surface on the substrate surface. During laser cladding, the cooling rate of the molten alloy is fast, the temperature range is small, and the fluidity is poor. The molten alloy will solidify before it has time to spread, and a smooth cladding coating cannot be formed, resulting in problems such as rough surface, lack of gloss, and insufficient density of the coating. He Jianqun et al. designed a special laser cladding iron-based alloy powder for 45 steel, and added Si to it to make the alloy powder self-slag, self-protective, and self-fluxing. Liang Zhigang et al. studied the effect of different silicon powder addition amounts on iron-based alloy coatings under laser cladding process. The results showed that when the Si element content was high, the flowability of the molten alloy was significantly improved, but the generated silicates would agglomerate at the bottom of the molten pool, affecting the bonding between the cladding layer and the substrate. Wang Yanshu et al. showed that SiO2 formed by Si element can effectively reduce the migration of ions during corrosion, thereby improving the corrosion resistance of the material.

As a high-quality stainless steel material, 316L has extremely high application value in severe corrosive environments such as oceans and chemical industries. The remanufacturing of related parts through laser cladding technology can extend the service life of parts. However, due to the technical characteristics of fast heating and cooling of laser cladding, the standard grade of 316L alloy material is not highly compatible with its process, the forming quality of the cladding layer is poor, and defects are prone to occur inside. Based on the above reasons, in order to achieve high-quality remanufacturing and repair of 316L parts, the powder composition was optimized by controlling the Si element content (not adding Si particles) in the metal powder smelting stage, and the process characteristics of the cladding layer at a higher Si content, that is, the macroscopic forming quality and microstructure of the cladding layer, were studied. At the same time, the effect of the Si element on the corrosion performance of the 316L cladding layer was studied, providing a new method for high-quality 316L laser cladding.

1 Experiment

1.1 Material and sample preparation
The laser cladding coaxial powder feeding test device used in this paper mainly includes laser, coaxial powder feeding system, cooling system, control system, etc. The laser used is LDF400-2000 fiber-coupled semiconductor laser produced by Laserline, Germany, with a spot diameter of 4 mm. The specific process parameters are shown in Table 1. In this paper, a 2 kW semiconductor laser was used to prepare 316L cladding layers with different Si contents on a 316L substrate. The powder composition used is shown in Table 2. The morphology of powders with different Si contents observed under a scanning electron microscope is shown in Figure 1. The powder particle size is 50~150 μm. The powder was dried at 120 °C for 1 h before the experiment. The size of the 316L substrate is 60 mm× 100 mm×15 mm. Before the test, the substrate was first grinded with an angle grinder to remove the oxide film and dirt on its surface, and then cleaned with anhydrous ethanol and dried.
The specific scanning strategy and sample preparation are shown in Figure 2. The sample is constructed along the z-axis (BD), and the y-axis (SD) is the laser scanning direction. The laser scanning method is unidirectional parallel scanning, and the overlap rate of the cladding layer is 50%. The electrochemical test sample is shown in Figure 2c. The sample is a disc with a diameter of 14 mm and a thickness of 4 mm, which is cut from the full cladding layer. Its test surface is sanded and polished to ensure that the surface is smooth and scratch-free.

1.2 Test method
After the laser cladding experiment, a cross-sectional sample of the cladding layer was taken perpendicular to the laser scanning direction using a wire cutting machine, and a longitudinal section sample was taken at the center of the cladding layer parallel to the laser scanning direction. Then, the sample was mounted, ground, polished, and etched (the etchant was 3 g FeCl3+10 mL HCl+100 mL CH3CH2OH) to make a metallographic sample. The macroscopic morphology and microstructure of the cladding layer were observed by optical microscopy (OM), and the grain orientation of the corresponding cladding layer sample was characterized by electron backscatter diffraction pattern (EBSD).
The STA 449F3 model synchronous thermal analyzer was used to test the oxidation mass increment of the powder material at high temperature. The test temperature was increased from 30 ℃ to 1 500 ℃, and the heating rate was 20 /min ℃. The change curve of the powder mass during the test was recorded (TG).
The corrosion resistance of the samples at different Si element contents was evaluated by potentiodynamic polarization test. A standard three-electrode system was used, and the corrosion solution was 3.5% NaCl solution. Before the potentiodynamic polarization test, the test run was carried out at open circuit potential for 30 min to ensure the stability of the test results. The potentiodynamic polarization test was carried out in the range of open circuit potential ±1 V, with a scanning rate of 1 mV/s. In the EIS test, the initial potential was the open circuit potential, the frequency range was 10–2~105Hz, and the amplitude was 10 mV. The EIS data was fitted by ZSimp Win software.

2 Results and analysis

2.1 Macromorphology of cladding layer
With the increase of Si content, the melting depth and melting height of the cladding layer increased, its thickness was improved, and it was easier to metallurgically bond. Si element can form low melting point eutectic with some metal elements (Fe, Co, Ni, etc.) at high temperature, which greatly reduces the melting point of the alloy. Then, under the action of laser energy, 316L with high Si content can form a molten pool earlier. Once the metal is transformed from solid to liquid, its melt absorption rate to laser is improved again. As a result, more metal powder can be melted in the molten pool, increasing the thickness of the single-pass cladding layer. With the increase of Si content (mass fraction from 0.8% to 1.2% and 1.6%), the cladding layer melt height increases from 698 μm to 834 and 918 μm, and the fusion depth increases from 601 μm to 654 and 898 μm. The height of the single-layer cladding layer increases by 15% and 40%, respectively, as shown in Figure 3.
The macroscopic morphology of the laser cladding layer under different Si contents is shown in Figure 4a. The results of the three-dimensional profile image (Figure 4b) show that with the increase of Si content, the surface of the cladding layer is smoother and the undulation difference is reduced. The reason for this result is that the viscosity of the 316L molten pool with high Si content is lower and the flow performance is better. The Si element can make the solid phase and liquid phase of the alloy have a wider temperature range, improve the fluidity and wettability of the alloy. Studies have shown that the viscosity of the molten metal will also decrease with the increase of temperature. The molten pool of 316L with high Si content is formed earlier. In the process of the metal melt continuing to heat up and transforming into a plasma state, the viscosity of the molten pool decreases rapidly, and the flow performance is further improved. Therefore, the surface of the cladding layer of 316L with high Si content is smoother. In order to further verify the flow performance of the molten pool, a verification experiment was carried out at a lower laser power, and the overlap fusion is shown in Figure 4c. In this experiment, the powder feeding rate and scanning rate were kept unchanged, and only the laser input power was reduced. The results show that unfused pores can be seen at the junction of the 0.8% Si cladding layer, while at the same power, the 1.2% Si and 1.6% Si cladding layers are well fused and no pores appear. This result is also consistent with the conclusion mentioned above that the Si element improves the flow properties of the 316L alloy.

2.2 Micromorphology of cladding layer
The micromorphology of cladding layers with different Si element contents is shown in Figure 5. It can be seen that with the increase of Si element content, the internal structure of the cladding layer has changed. In 0.8% Si, long columnar subgrains can be observed growing from the bottom of the molten pool along the thermal gradient to the top, while in 1.2% Si, the coarse columnar subgrains gradually transform into slightly long cellular subgrains. Furthermore, in 1.6% Si, most of the subgrains exist in the form of cells. In the evolution of this organization, the pinning of Si elements at the subgrain boundaries plays a key role. At the subgrain boundaries of the cellular subgrain structure, there is an enrichment of Si elements, and this enrichment product is mainly nano-silicon oxide, as shown in Figure 5d. This nanoparticle exists inside the molten pool, effectively interrupting the growth of the columnar subgrain structure during the solidification process, thereby transforming the internal solidification structure into a cellular structure.
In the laser cladding 316L stainless steel structure, there is an obvious element segregation phenomenon. The EDS line scan of the cellular subgrain structure in the transmission electron microscope STEM mode is shown in Figure 6. The bright white lines gather at the subgrain boundaries of the cellular structure. There is a segregation phenomenon of Fe, Cr, and Mo elements at the subgrain boundaries of the cellular subgrain structure, which is mainly manifested in that Cr and Mo elements will be enriched at the subgrain boundaries, and the corresponding Fe element content will decrease. The TEM bright field image of the cellular subgrain structure of the 1.6% Si cladding layer and the corresponding nanoparticle element analysis are shown in Figure 7. The white arrow in Figure 7a indicates the subgrain boundary of the structure, and there are round particles indicated by the gold arrow in this subgrain boundary. EDS analysis of Figure 7b shows that the main components of this round nanoparticle are Si and O. At the same time, this nano-
silicon oxide particle is mostly present at the subgrain boundary, and is rarely found inside the structure. The nano-oxidized phase is the first microstructural feature formed during laser cladding. Previously, similar oxidation phases were observed in the microstructure of welded and selective laser melted 316L stainless steel.
The image of a single nano-silicon oxide particle under TEM bright field is shown in Figure 8a. A large number of dislocation structures are observed around this nanoparticle. There are many sources of oxygen required for the formation of this oxide. Although there is an inert atmosphere protection during laser processing, oxidation is inevitable. As the laser moves away from the current molten pool along its scanning path, the melt cools, the oxygen solubility decreases, and solid oxide nuclei begin to form. Since the temperature at which silicon oxide begins to form (the formation temperature of SiO2 is 2 000 K) is much higher than the solidus temperature of steel (1 700 K), oxide nucleation in molten steel is generally considered to be a homogeneous process driven by a large amount of undercooling before the solidification interface in the molten pool. Therefore, increasing the cooling rate can effectively increase the nucleation rate of oxides in the melt. According to the Ostwald ripening theory, oxides may coarsen through collision and coalescence events, and larger oxides will grow in the form of aggregates of smaller oxides. The oxides are eventually wrapped by the advancing solid interface in the coldest area of ​​the molten pool and become inclusions in the solid. Of course, due to the strong convective mixing in the molten pool, the oxides are transported between the colder and hotter areas, and the formation and growth of nano-oxides in the molten pool is more complicated than what is presented here.
Such nano-particle melts usually have extremely high viscosities and tend to form spheres to reduce surface tension. In addition, such melts usually have a low wetting tendency for steel and can therefore remain undissolved in the structure. Further, the local high cooling rate during laser melting is extremely favorable to the formation of this glass phase in terms of kinetics. The critical cooling rate required to form such glass phase particles can vary from 10’2 to 10’6 K/s, depending on their volume, while the cooling rate achieved during laser melting is higher, reaching 10’5 to 10’7 K/s. Under the action of high cooling rate, these small agglomerates will further aggregate to form spherical particles with a size of nearly 500 nm. This formation process takes about 0.1 s.
At the same time, the thermal stress enriched by the nano-silicon oxide particles during solidification causes a large number of dislocation structures to exist around them, as shown in Figure 8b. Since the thermal expansion coefficient of this glass phase (0.55×10’–6 ℃‘–1) is much lower than the thermal expansion coefficient of the metal matrix (16×10–6 ℃–1), this will produce compressive stress on the nano-particles and tensile stress on the surrounding metal matrix [26]. Therefore, a dense dislocation structure will be formed in the steel matrix around the silicate nano-inclusions. This dislocation structure interrupts the growth of the columnar solidification structure, resulting in more cellular forms. Therefore, in the previous study, the grains and subgrain structures of 1.6% Si are finer than those of 0.8% Si.
The grain orientation of the cladding layers with different Si contents is shown in Figure 9. It can be found that with the increase of Si content, the grains of the cladding layer gradually become finer, which is also related to the presence of more nano-silicon oxide particles inside the cladding layer. Since the formation of nano-oxide occurs before the solidification of the grains, it can provide nucleation points for the growth of the grains, making the grains inside the cladding layer finer.

2.3 High-temperature oxidation resistance
The increase of Si content can significantly improve the material’s high-temperature oxidation resistance. In steelmaking, elements such as Si, Al, and Mn are often added to remove oxide inclusions produced during high-temperature smelting. From the macroscopic morphology of Figure 10, it can be found that with the increase of Si element content, the cladding layer changes from dark to bright. As far as metal materials are concerned, since the oxide layer on the metal surface blocks the inherent luster of the metal, the surface appears dark. Therefore, under the same conditions, the darker the surface color, the more intense the oxidation of the cladding layer. This shows that the oxidation of the 0.8% Si cladding layer is more intense than that of the 1.6% Si cladding layer, and this oxidation occurs during the solidification stage of the molten pool. In order to obtain the oxidation data of the metal powder during the melting and solidification process, the two powder materials of 0.8% Si and 1.6% Si were subjected to thermogravimetric tests (TG), and the results are shown in Figure 10. The 0.8% Si and 1.6% Si powders were placed in a crucible and heated at 20/min ℃. The results after 1 h of high-temperature oxidation are shown in Figure 5. Before 1000 ℃, neither of them showed obvious signs of mass increase. The oxidation of 0.8% Si powder started at 1054 ℃ and ended at 1419 ℃, with an oxidation mass increase of 32%. The oxidation of 1.6% Si powder started at 1072 ℃ and ended at 1410 ℃, with an oxidation mass increase of 19%. The temperature range of the oxidation mass increase of the two is basically consistent. However, compared with the nearly 32% oxidation mass increase of 0.8% Si powder, the oxidation mass increase of 1.6% Si powder is reduced by 41%.
Oxidation inclusions will seriously affect the final performance of the laser cladding layer. Since metal oxides are bonded by ionic bonds, their bond energy is higher than that of the metal itself, and higher energy is required to melt. Once they appear, they are extremely difficult to separate from the molten pool. Due to the poor wettability between oxides and metals, these oxides will agglomerate and become oxidation inclusions inside the cladding layer, affecting its performance.
Combined with Figure 11, the oxidation process of 1.6% Si is discussed and analyzed. After the laser acts on the surface of the substrate to form a molten pool, the oxidation of the molten pool is still inevitable despite the protection of an inert atmosphere. Due to thermodynamic reasons, the Si element first reacts to form SiO2, which will be concentrated on the surface of the molten pool, hindering the subsequent oxidation. As the temperature increases further, the continuity of these SiO2 agglomerate layers is destroyed, and subsequent oxygen is able to bypass the oxide layer, further oxidizing the metal elements in the molten pool. This corresponds to the mass increase curve in Figure 5b, which rises rapidly after stabilizing at 1450~1500℃. During the laser cladding process, the metal powder melts to form a molten pool within 10–4~10–5 s, and the cooling rate of the molten pool is 106/s℃ [30]. In this case, the SiO2 agglomerate layer will not be destroyed due to long-term high temperature testing as in the TG test, and its protective effect on the molten pool can be more clearly reflected.

2.4 Corrosion performance of cladding layer
The corrosion resistance of samples with different Si content was evaluated by potentiodynamic polarization test, and the electrochemical results are shown in Figure 12. Here, the experimental data are fitted by EEC with 2 time constants. Among them, R1 mainly represents the charge transfer impedance, while R2 represents the passivation film resistance. Considering that the corrosion of the material involves both electron transfer and charge transfer, the sum of the two R is the resistance of the entire sample. Since the values ​​of the three n1 are close to 1, it is proved that a constant phase element should be used to characterize the inhomogeneity of the charge distribution of the double-layer structure caused by the inhomogeneity of the surface. The test results of the polarization curve and impedance spectrum show that the results of forged 316L and 0.8% Si are similar. In the cladding layer samples, with the increase of Si element content, the current density decreased from 2.039×10’–6 A/cm’2 to 1.889×10’–6 A/cm’2 and 1.422× 10’–6 A/cm’2
, and the self-corrosion potential shifted positively. This shows that with the increase of Si content, the corrosion resistance of the cladding layer has improved. It can be seen from Figure 12a that with the increase of Si content, the improvement of corrosion resistance is mainly reflected in the widening of the passivation range, and the pitting potential increases from 0.4 V of 0.8% Si to 0.6 V of 1.2% Si and 0.9 V of 1.6% Si. At the same time, it can be seen from Figure 12c that with the increase of Si content, the impedance of the sample is also increasing.
Table 3 shows that R1 has little change, increasing from 13.09 Ω·cm2 to 14.82 and 15.46 Ω·cm’2, while the passivation impedance R2 has increased from 0.65×10‘5 Ω·cm’2 to 3.55×10‘5 and 4.08×10’5 Ω·cm’2. This also shows that the improvement of the corrosion resistance of the cladding layer is mainly caused by the increase of R2 representing the passivation impedance.
Figure 13a is a typical pitting hole, from which it can be seen that the corrosion in the box area is relatively light. The EDS results show that these areas have obvious Si element enrichment, and Na and Cl elements also remain. This indicates that the oxide layer formed by the Si element hinders the corrosion. Figure 13b shows the macroscopic morphology of the sample after electrochemical testing. It can be seen that with the continuous increase of Si content, the number of pitting holes on the sample surface decreases significantly. Combined with the polarization curve analysis in Figure 12, due to the extension of the passivation interval of 1.6% Si, the surface of the sample was destroyed and penetrated in the final stage of the entire electrochemical test, so there were fewer pitting holes remaining on the surface. In contrast, the passivation interval of 0.8% Si was short and the pitting damage was serious. Figure 13d shows the evolution of the cellular structure in electrochemical corrosion. From left to right, the pitting damage first occurred inside the cellular structure. The evolution of this structure is related to the segregation of elements in the structure. Figure 6 shows that there is corrosion-resistant Cr element segregation at the subgrain boundary. At the same time, Si element will form nano-SiO2 particles under the action of laser and precipitate at the subgrain boundary. The segregation of these elements leads to differences in the corrosion performance of various parts of the cellular structure. Therefore, the cellular structure is first destroyed because of its poor internal quality. As the corrosion damage continues (as shown in Figure 13d3), only the more corrosion-resistant three-dimensional network subgrain boundary structure remains inside the pitting hole. Finally, as the pitting holes expand, the internal pH value further decreases, the corrosion damage intensifies, and the network subgrain boundary structure is also eroded and destroyed.
Since SiO2 is extremely resistant to aqua regia, in order to verify the existence of the SiO2 layer on the surface of the sample, aqua regia and hydrofluoric acid nitric acid aqueous solution were used to etch 0.8% Si and 1.6% Si samples, respectively. The results are shown in Figure 13e. The results show that the 0.8% Si cladding layer has a clear cellular structure under the etching of aqua regia, while the 1.6% Si cladding layer has no obvious effect under the etching of aqua regia. After switching to etching with hydrofluoric acid nitric acid aqueous solution, a clear subgrain structure framework appeared. This is because HF can react with SiO2 to generate gas SiF4, which in turn destroys the SiO2 passivation layer on the surface, so that the subgrain structure at the bottom can be corroded. This also confirms the existence of the SiO2 passivation layer, and it is deposited more evenly on the surface of the metal substrate.
In summary, combined with the existing pitting theory of 316L stainless steel, a schematic diagram of corrosion evolution is drawn as shown in Figure 14. As shown in Figure 14a, stainless steel with high Cr content has extremely strong corrosion resistance due to the Fe-Cr-O passivation layer on the surface. However, due to the discontinuity of this passivation layer (usually affected by inclusion defects such as MnS), the weak parts will be corroded first. After introducing a higher Si content, the generated SiO2 covers the original Fe-Cr-O passivation layer, effectively improving the corrosion resistance of 316L stainless steel. Figure 14b describes the most mainstream pitting damage evolution of 316L stainless steel. Usually, 316L stainless steel only goes through four stages: 3, 4, 5, and 6. Due to the presence of MnS, defect points appear in the passivation layer Fe-Cr-O, and the MnCr2O4 particles contained in MnS form micro-batteries with MnS, resulting in the dissolution of MnS. As the dissolution proceeds, MnS peels off as a whole, resulting in defects in the passivation layer, exposing the metal matrix at the bottom, which is eventually destroyed by corrosion. These defects will cause corrosion to proceed first. In contrast, high Si content 316L stainless steel has additional stages 1 and 2, and only by destroying this layer of SiO2 can corrosion damage proceed further. Therefore, increasing the Si content in 316L can effectively improve its corrosion resistance.

3 Conclusions
1) The increase in Si content can reduce the melting point of the powder, thereby increasing the thickness of the single-layer laser cladding layer. The melting height of the single-layer cladding layer increased from 700 μm for 0.8% Si to 830 μm for 1.2% Si and 920 μm for 1.6% Si.
2) The increase in Si content can improve the high-temperature oxidation resistance of the metal powder, thereby reducing the oxidation degree of the cladding layer prepared by it. Among the elements in 316L, Si has the strongest thermodynamic activity and preferentially reacts to form SiO2, which hinders subsequent oxidation. In the TG test, 0.8% Si increased its oxidation mass by 32% at 1500℃ for 1 h, while 1.6% Si increased its oxidation mass by only 19%.
3) The increase in Si content can improve the corrosion resistance of the cladding layer, and the newly generated SiO2 passivation layer effectively improves the continuity of the Cr2O3 passivation layer. In the electrochemical test, with the increase in Si content, the corrosion damage on the surface of the cladding layer was alleviated, the number of pitting holes decreased, and the pitting potential increased from 0.4 V of 0.8% Si to 0.6 V of 1.2% Si and 0.9 V of 1.6% Si, and the corrosion resistance of the cladding layer was improved.
4) With the increase in Si content, the grains of the cladding layer were refined, and columnar crystals were transformed into equiaxed crystals. The nano-oxide particles generated in situ in the cladding layer are enriched with a large number of dislocations, which interrupt the growth of long columnar grains and refine the grains inside the cladding layer.