The microstructure of FeCrNiCoMoBSi high entropy alloy (HEA) laser cladding coating and the effect of laser power on the coating phase and electrochemical corrosion properties were investigated. The results show that the HEA coating is composed of columnar crystal bands at the bottom, equiaxed crystal bands at the top, and mixed crystal bands in the middle (composed of a mixture of columnar crystals and equiaxed crystals). The HEA coating prepared with 3000W power exhibits the lowest self-corrosion current density (0.425μA/cm2), the highest self-corrosion potential (-0.16852V) and the largest polarization impedance (69616Ω). Its impedance modulus |Z| is 1143Ω·cm’2, which is 8.65 times, 4.91 times and 7.14 times that of the laser cladding coatings with 1800, 2500 and 4500 W power, respectively, and its maximum phase angle is 76.23°, which is higher than that of the other three coatings. Comprehensive evaluation shows that the HEA coating prepared with a power of 3000 W has excellent electrochemical corrosion resistance. This is due to its single FCC crystal structure, corrosion-resistant iron-nickel alloy phase and single chromium phase, good crystallinity, refined grain size and excellent passivation effect, which makes its electrochemical corrosion performance significantly better than that of coatings prepared with other powers.
High entropy alloys are a new type of alloys, which consist of four to five or more elements mixed in an atomic fraction of 5% to 35%. These alloys have the characteristics of high entropy effect, hysteresis diffusion effect, lattice distortion effect and cocktail effect, so they have attracted much attention in the field of materials. Although high entropy alloy blocks are a common preparation method, their high cost limits their wide application in the engineering field. In contrast, high entropy alloy coatings are considered to be highly promising protective coating materials due to their excellent corrosion resistance and relatively low preparation cost.
In recent years, the use of laser cladding technology to prepare high entropy alloy coatings has become an emerging method. Qiu et al. successfully prepared Al2CoxCrCuFeNiTi high entropy alloy coatings on the surface of Q235 steel using laser cladding technology. The microstructure of the coating is mainly composed of equiaxed grains, and also contains spherical particles, white particles and “plum blossom” particles. Columnar grains are located near the bonding area. This high entropy alloy coating exhibits good corrosion resistance in 1mol/LNaOH and 0.5mol/L HCl solutions. In addition, the research group successfully deposited an AlTiVMoNb lightweight refractory high entropy alloy (RHEA) coating on a TC4 substrate by laser cladding to improve the hardness and high-temperature oxidation resistance of the TC4 alloy surface. The coating has a good metallurgical bond with the TC4 substrate, and its structure is composed of a single BCC-HEA phase. The lattice constant is aXRD = 0.31246nm, aTEM = 0.31234nm, and the microhardness of the coating is 885.5HV0.2, which is 2.52 times that of the TC4 substrate and 1.66 times that of the arc-melted alloy. However, so far, there are relatively few studies on the effects of laser cladding parameters on the corrosion properties of high entropy alloys. Therefore, this study takes the independently developed FeCrNiCoMoBSi high entropy alloy (HEA) coating as the research object, and systematically studies the influence of laser cladding power on the microstructure and electrochemical corrosion properties of FeCrNiCoMoBSi high entropy alloy coatings, aiming to provide experimental support and theoretical reference for the application of laser cladding high entropy alloy coatings in the anti-corrosion industry.
1 Experiment
1.1 Experimental raw materials
316L stainless steel, a common material for traditional petrochemical equipment, is selected as the base material, and its size is 100 mm×100 mm×15 mm. The cladding material is FeCrNiCoMoBSi high entropy alloy powder independently developed by Yanbo Additive Manufacturing (Xuzhou) Technology Co., Ltd. The powder is prepared by gas atomization method with a particle size of 45~75μm, and its composition is shown in Table 1.
1.2 Preparation of high entropy alloy coating
Before cladding, the surface of the sample is first sanded to remove the surface oxide layer; then anhydrous ethanol and acetone are used for degreasing and cleaning. The cladding test uses an RK-6000 fiber laser. The process parameters of laser cladding are as follows: argon is used as the protective gas, the gas flow rate is 9NL/min, the scanning speed is 1000 mm/min, the spot diameter is 3.4mm, the defocus is 15 mm, the overlap rate is 50%, and the powder feeding amount is 16.2g/min. In order to study the effect of different laser cladding powers on high entropy alloy coatings, 1800, 2500, 3000 and 4500 W were selected as research variables to investigate their effects on the microstructure and electrochemical corrosion performance of the coatings.
1.3 Performance test and characterization
The metallographic structure of the coating was observed using an OLYMPUS OLS4100 metallographic microscope. The microstructure and element distribution of the high entropy alloy coating were observed and analyzed using a ZEISS EVO10 tungsten filament scanning electron microscope. The sample phase analysis was performed using a SmartLab SE X-ray diffractometer, with a test angle of 10°~80° and a scanning step of 0.01°. The corrosion resistance of the coating was tested using an AU-TOLAB PGSTAT302N electrochemical workstation, and the polarization curve was fitted using Nova2.1 software. A standard three-electrode system was used, in which the silver chloride electrode was the reference electrode, the surface of the sample to be tested was the working electrode, and the platinum electrode was the auxiliary electrode. The scanning potential range is -0.6~0.6 V, the scanning rate is 1 mV/s, and the frequency range of the electrochemical impedance spectroscopy test is 100 kHz~100 mHz. The corrosive medium is 3.5% NaCl solution.
2 Results and discussion
2.1 Microstructure analysis
Figure 1 shows the SEM morphology of FeCrNiCoMoBSi high entropy alloy powders. These powders show spherical and near-spherical structures, with only a trace amount of dumbbell structure. This morphology is crucial for uniform powder supply during laser cladding and helps to achieve a homogeneous structure of the coating.
Figure 2 shows the SEM morphology of the FeCrNiCo-MoBSi high entropy alloy coating after laser cladding. In Figure 2(a), the interface between the coating and the substrate is very dense, showing typical metallurgical bonding characteristics. The cross section of the coating is divided into three distinct regions, including the substrate region, the coating region, and the interface fusion region. The coating consists of a columnar crystal band at the bottom, an equiaxed crystal band at the top, and a mixed crystal band in the middle, which contains a mixed structure of columnar crystals and equiaxed crystals. In addition, the coating also shows high density with only minor defects, as shown by arrows 1 to 4 in Figure (b).
In the early stage of laser cladding, the surface temperature of the 316L substrate is relatively low, while the temperature of the laser beam and the molten powder is very high, resulting in a large temperature gradient at the interface between the substrate and the coating. According to solidification theory, in this case, the temperature gradient and supercooling of the liquid and solid are large, making the cladding area in a state of rapid heating and rapid cooling. This process causes the grains to grow upward in a direction perpendicular to the bonding interface, so the grains at the bottom tend to form a columnar crystal zone with obvious directionality. In contrast, the top of the coating has a smaller temperature gradient and a smaller supercooling due to contact with the outside world, so a mixed crystal band in the middle and an equiaxed crystal band at the top are formed. The boundary between the mixed crystal band and the equiaxed crystal band presents a certain angle with the substrate interface, which is caused by the gradual accumulation of heat during the laser cladding scanning process.
Figure 3 shows the EDS element surface scanning energy spectrum of the FeCrNiCoMoBSi high entropy alloy laser cladding coating surface. It is observed that the main elements Fe, Co, Cr, Ni and Mo on the coating surface are evenly distributed, and there is no obvious element segregation phenomenon. This may be because the high energy density of the laser causes the high entropy alloy powder to melt rapidly, and then quickly cools and solidifies, so that the elements do not have enough time to diffuse over long distances.
2.2 Phase analysis
The XRD spectra of FeCrNiCoMoBSi high entropy alloy powder and laser cladding coating are shown in Figure 4. Despite the different laser cladding powers, the FeCrNiCoMoBSi high entropy alloy laser cladding coating and powder have the same crystal structure, that is, a single face-centered cubic structure (FCC). The main phases that may exist include: Fe0.64Ni0.36, Cr, (Fe, Ni), Fe6.6Cr1.7Ni1.2Si0.2Mo0.1 and Cr0.19Fe0.7Ni0.11. Previous studies have shown that high entropy alloys with FCC crystal structure have superior corrosion resistance and surpass traditional alloys.
As can be seen from Figure 4, under different laser cladding powers, the FCC crystal plane diffraction peaks of FeCrNi-CoMoBSi high entropy alloy powders and laser cladding coatings have shifted to the right, and the peak value and width of the diffraction peaks have also changed. The specific diffraction data are shown in Table 2. The phenomenon of rightward shift of the diffraction peak indicates that there are more lattice and grain boundary defects in the laser cladding coating compared with the HEA powder. HEA powder contains B and Si elements with low melting points, which have deoxidation, degassing and wetting properties, and are conducive to self-cladding. However, during the rapid cooling and solidification process of laser cladding, these elements are easily distributed on the lattice or grain boundary, resulting in an increase in defects in the coating, which causes the diffraction peak to shift to the right. However, with the increase of laser power, the FCC diffraction peak begins to shift to the left, and the lattice and grain boundary defects in the coating gradually decrease. High laser power helps to achieve better melting effect of the powder and reduce defects in the coating during cooling and solidification.
Figure 5 shows the diffraction data of each FCC crystal plane of FeCrNiCoMoBSi high entropy alloy laser cladding coating with different powers. The difference between the diffraction angles of the laser cladding coating and the powder FCC crystal plane is greater than 0, which again shows that the diffraction angle of the laser cladding coating has shifted to the right relative to the powder. As the laser power increases, the diffraction angle shows a trend of shifting to the left, as shown in Figure 5(a). This shows that increasing the laser power is conducive to reducing the lattice defects and grain boundary defects of the laser cladding coating. As shown in Figure 5(b), compared with the FeCrNiCoMoBSi high entropy alloy powder, the diffraction peaks of the laser cladding coating FCC phase, except for (200) at 2500 W, are reduced to varying degrees. Combined with Figure 4, it can be seen that when the laser cladding power is 2500 and 4500 W, diffuse peaks are found on the FCC crystal plane of the powder, such as the crystal plane marked with “*” in Table 2, indicating that the cladding coatings obtained with these two powers have an amorphous phase. When the power is 3000 W, its phase structure is the same as that of the powder, which is a single FCC phase structure with three crystal planes (111), (200) and (220) at the same time; compared with other laser powers, the cladding coating prepared by this power has relatively good crystallinity at (111) and (220).
On the other hand, it can be seen from Figure 5(c) that under different laser power conditions, the diffraction width of each FCC crystal plane of the laser cladding coating is larger than that of the powder, indicating that the grain size of the HEA powder has been refined after laser cladding. Table 2 and Figure 5(d) verify this phenomenon. The grain size of the laser cladding coating is smaller than that of the powder. However, the grain size of the coating increases with the increase of laser power, indicating that laser power is the main factor affecting the increase of grain size. However, when the laser power is 4500 W, the grain size of the coating no longer increases significantly, and even decreases, as shown in the (200) crystal plane of Figure 5 (d). The decrease in grain size is caused by the increase in coating defects due to excessive power, which will have an adverse effect on the density of the coating.
Through the comprehensive analysis of XRD diffraction, it can be obtained that the laser power has a significant effect on the diffraction angle, diffraction peak, peak broadening and grain size of the FCC crystal plane. Comprehensive evaluation shows that the coating obtained with a laser power of 3000 W has relatively good diffraction data.
2.3 Potentiodynamic electrochemical corrosion analysis
Before using the electrochemical workstation to perform polarization curve testing, the FeCrNiCoMoBSi high entropy alloy coating and 316L stainless steel substrate were immersed in the test solution for 7200s, and the test was performed after the open circuit potential was stabilized. For data processing, the current data was divided by the electrode surface area (1cm2) to obtain the current density data, and the absolute value was taken using Origin software to plot the polarization curve with the corresponding corrosion potential data in log10 logarithmic coordinates. Using the Tafel linear extrapolation method, the self-corrosion potential and the corresponding self-corrosion current density can be obtained; the polarization resistance calculation formula is shown in formula (1): Rp=βaβb/（βa+βb)*1/Icorr See formula (1) in the figure
Where: RP is the polarization resistance of the corroded metal electrode; Icorr is the self-corrosion current density; βa is the Tafel slope of the anodic polarization curve; βb is the Tafel slope of the cathodic polarization curve.
Figure 6 shows the polarization curves of the FeCrNiCoMoBSi high entropy alloy coating prepared by different laser powers in 3.5% NaCl solution. It can be seen that the high entropy alloy coatings prepared with laser power of 3000 and 4500 W have obvious passivation zones, and the passivation of the coating prepared with laser power of 3000 W is more obvious. The passivation phenomenon occurs at a self-corrosion potential of about -102 mV, and reaches a critical value at 378 mV, which means that the coating enters the passivation state during the anodic polarization stage, that is, a passivation film is generated on the corrosion surface of the coating. The formation of the passivation film can increase the corrosion resistance of the coating. The corrosion characteristics of the passivation state can generally be expressed by the passivation interval, passivation current density and overpassivation potential. Generally speaking, the wider the passivation interval and the smaller the passivation current density, the more stable the passivation film is in 3.5% NaCl solution, and the better the protection effect on the coating. In addition, Figure 6(b) shows that metastable pitting was found in the coating prepared with a power of 3000 W, and current oscillation and potential oscillation appeared on the polarization curve. The more positive the potential, the larger the current fluctuation peak of metastable pitting. It is generally believed that metastable pores will transform into stable pores, and then a passivation film will form on the surface of the pores.
Table 3 shows the polarization parameter data of FeCrNiCoMoBSi high entropy alloy coatings prepared with different laser powers in 3.5% NaCl solution calculated by fitting the slope of the Tafel curve using Nova2.1 software. By comparison, it was found that the high entropy alloy coating prepared with a laser power of 3000 W had the lowest self-corrosion current density, the highest self-corrosion potential and polarization impedance, which were 0.425μA/cm’2, -0.16852V and 69616Ω, respectively. Generally, the higher the self-corrosion potential (Ecorr) and polarization impedance, the less likely the material is to corrode; the smaller the self-corrosion current density (Icorr), the lower the corrosion rate of the material, the smaller the corrosion tendency of the material, and the better the corrosion resistance. The results show that the coating obtained at 3000 W power has better corrosion resistance than other powers.
Figure 7 shows the electrochemical impedance spectrum of the FeCrNiCoMoBSi high entropy alloy coating in 3.5% NaCl solution measured at open circuit potential. The Nyquist plot shows that the FeCoCrNi-MoBSi high entropy alloy coatings prepared at different laser powers all exhibit a single capacitive arc feature, and the capacitive arc is in the shape of a downward-pressed semicircular arc. By comparison, it is found that the FeCrNiCoMoBSi high entropy alloy coating prepared under the condition of laser power of 3000 W has the largest capacitive reactance arc radius, which indicates that the charge transfer of the FeCrNiCoMoBSi high entropy alloy coating is difficult, that is, the electrochemical reaction resistance is greater, and the passivation film protection effect is better. This shows that the coating obtained under this power condition has better corrosion resistance.
Figure 8 is an electrochemical equivalent circuit diagram of two different electrochemical reaction processes. The Bode diagram shows that, unlike the Bode curves of 1800 and 4500 W, when the power is 2500 and 3000 W, the phase angle extreme value of the coating in the high frequency region extends all the way to the low frequency region, which is composed of the low frequency region, the medium frequency region and the high frequency region. This phenomenon can be understood as the embodiment of the superposition of two different time constants. This phenomenon may be caused by the unevenness of the surface chemical composition or surface defects. Therefore, the electrochemical reaction process of the cladding and cladding annealing coatings has two time constants. As mentioned above, the unevenness of the coating corrosion surface is shown as a depressed semicircular arc in the Nyquist diagram. The influence caused by it can be eliminated by replacing the pure capacitive element with a constant phase angle element. CPE is represented by Q in the equivalent circuit, and its impedance value is: See formula (2) in the figure
Wherein, Y0, j, ω and α represent the proportional factor, imaginary unit, angular frequency and phase shift respectively. On this basis, Rs is used to represent the impedance of the electrolyte, Rf is used to represent the impedance caused by the passivation film, Rct is used to represent the charge transfer resistance, Qf is used to represent the electrochemical response of the passivation film, and Qdl is used to represent the electrochemical response of the double electric layer structure. Therefore, the equivalent circuit involved in the electrochemical reaction can be divided into three parts, namely the electrolyte, the passivation film generated on the surface of the high entropy alloy coating, and the double electric layer structure between the high entropy alloy coating and the electrolyte. The Bode plot shows that the impedance modulus |Z| of the FeCrNiCoMoBSi high entropy alloy coating prepared with a laser power of 3000 W is larger at low frequency, and the phase angle is higher, which are 1143Ω·cm’2 and 76.23° respectively. The decreasing rate in the low frequency region is significantly slowed down but is still in the decreasing process, indicating that a dense passivation film is formed on the surface of the sample.
According to the Bode diagram, the variation trend of impedance modulus and maximum phase angle with laser power is obtained, as shown in Figure 9. The impedance value and maximum phase angle increase with the increase of laser power, reaching a peak at 3000W and then decreasing. At 3000W, its impedance value is 8.65 times, 4.91 times and 7.14 times that of the power of 1800, 2500 and 4500W, respectively, and the increase is significant. Generally speaking, the larger the impedance modulus value of the material and the larger the maximum phase angle value, the better the corrosion resistance of the material. In summary, compared with other powers, the coating obtained with a laser power of 3000W has better electrochemical corrosion performance.
The electrochemical corrosion characteristics of FeCrNiCoMoBSi high entropy alloy laser cladding coating are related to many factors and are the result of the comprehensive effect of the intrinsic characteristics of the coating itself, such as crystal structure, phase composition, crystallinity, grain size and passivation film. First, the study shows that the single FCC crystal structure of high entropy alloy has better corrosion resistance than traditional alloys. The coating prepared at 3000 W has a single FCC structure. Secondly, the phases of this coating Fe0.64Ni0.36, Cr, (Fe, Ni), Fe6.6Cr1.7Ni1.2-Si0.2Mo0.1, Cr0.19Fe0.7Ni0.11 belong to the iron-nickel alloy series and the single chromium phase. These phases have high strength and excellent corrosion resistance, and can still maintain their stable performance in harsh environments such as high temperature, strong acid and strong alkali. Again, as mentioned above, compared with 1800 and 2500 W, the coatings prepared with 3000 W laser power shifted to the left, indicating that the lattice defects and grain boundary defects were reduced and the crystallinity was better. The amorphous phase appeared at 2500 and 4500 W, which weakened its corrosion resistance. Although the relatively large grain size is conducive to improving its corrosion resistance, the data show that the increase in grain size at 3000 W is very small compared with 1800 and 2500 W, and has no significant effect on the coating performance. On the other hand, the electrochemical corrosion results show that the passivation effect of the coating obtained at 3000 W power is significantly better than that at other powers. The combined effect of various factors makes the laser cladding coating under this power condition have better electrochemical corrosion resistance.
3 Conclusion
(1) A strong metallurgical bond is formed between the HEA laser cladding coating and the substrate. The structure of the coating presents a complex three-layer morphology, including a columnar crystal band at the bottom, an equiaxed crystal band at the top, and a mixed crystal band composed of columnar crystals and equiaxed crystals in the middle.
(2) At various laser powers, the HEA cladding coating and the original powder exhibit the same face-centered cubic structure (FCC). However, the coatings at different powers show different degrees of lattice and grain boundary defects. The coating prepared at 3000 W power has the best crystallinity, fewer grain defects and smaller grain size, and its FCC crystal plane diffraction data shows more superior characteristics.
(3) The FeCrNiCoMoBSi high entropy alloy coating prepared at 3000 W power exhibits the best electrochemical corrosion performance. This coating has the lowest self-corrosion current density (0.425μA/cm2), the highest self-corrosion potential (-0.16852V) and the largest polarization impedance (69616Ω). Compared with the coatings prepared at other laser powers, the impedance modulus (1143Ω·cm2) of the laser cladding coating prepared at 3000 W power is 8.65 times, 4.91 times and 7.14 times that of the coatings prepared at powers of 1800, 2500 and 4500 W, respectively, and the maximum phase angle (76.23°) is higher than that of the coatings prepared at other powers.
(4) The laser cladding coating prepared at 3000 W power exhibits excellent electrochemical corrosion resistance, which can be attributed to its single FCC crystal structure, corrosion-resistant Fe-Ni alloy phase and single chromium phase, excellent crystallinity, refined grain size and excellent passivation effect.
Overall, the results of this study provide strong experimental and theoretical support for the preparation and electrochemical corrosion performance of high entropy alloy coatings, and lay a solid foundation for their potential application in the field of corrosion protection industry.
