Laser cladding technology was used to prepare FeAlCrNiSix high entropy alloy cladding layers with different Si addition amounts on the surface of DH36 high-strength marine engineering structure steel to further improve the corrosion resistance of marine structure steel. X-ray diffractometer (XRD), optical microscope (OM) and scanning electron microscope (SEM) were used to analyze the phase composition and microstructure of the high entropy alloy cladding layer. The microhardness of the cladding layer was measured, and the corrosion behavior of the cladding layer was analyzed using polarization curves. The results show that the addition of Si element causes the phase of the cladding layer to transform from FCC+BCC phase to a single BCC phase, the dendrite size gradually decreases, and finally completely transforms into irregular equiaxed crystals. The average hardness of the cladding layer first increases and then decreases, and the highest hardness value is 430.15HV0.1. The addition of Si element also effectively improves the corrosion resistance of the cladding layer. The corrosion resistance of the cladding layer shows a trend of first increasing and then decreasing with the increase of Si element addition. Considering the electrochemical parameters comprehensively, the FeAlCrNiSi0.25 cladding layer has the best corrosion resistance.

Marine corrosion is the most serious obstacle in the process of developing and utilizing marine resources, and it causes huge economic losses every year. The marine environment is extremely complex, and its temperature, pH value, salinity, microorganisms, etc. are all important factors that cause corrosion of marine engineering structures. The marine environment is divided into the atmospheric zone, the splash zone, the tidal zone, the seawater immersion zone and the seabed mud zone. Among them, the splash zone has the most serious corrosion due to the alternation of dry and wet, sufficient oxygen supply and constant waves [1-3].

Laser cladding technology is a rapidly developing surface modification technology [4]. It can prepare a protective coating with good corrosion resistance on the surface of the workpiece at a low cost to meet the use requirements of different metal parts and equipment, and extend the service life of metal parts and equipment under extremely harsh conditions [5-7]. It can also be used to repair faulty parts and reduce their maintenance costs. It has been widely used in marine facilities, aerospace, oil extraction and medical engineering [8-10].

The concept of high entropy alloy has received widespread attention in academia since it was proposed in 2004 [11]. High entropy alloys are usually composed of 5 or more elements, and the molar fraction of each element is 5%~35%[12]. High entropy alloys are composed of simple solid solutions in phase structure and have excellent comprehensive properties, such as high strength, high hardness, and good corrosion resistance[13−14]. The use of laser cladding to prepare high entropy alloy coatings can effectively improve the performance of the alloy and meet more severe environmental conditions. GAO et al.[15] used laser cladding technology to prepare Ni60 coatings with different Fe content on Q235 substrates. The corrosion resistance of the coatings first decreased and then increased with the increase of Fe content. The coating with an Fe content (mass fraction) of 25% has good corrosion resistance, which is equivalent to that of Ni60AA alloy, but the increase of Fe content leads to the disappearance of the strengthening phase and the decrease of coating hardness. Jiang et al. [16] used laser cladding technology to prepare AlCoCrxFeNi high entropy alloy coatings on 45 steel as the substrate. When the Cr content was 1.0% and 1.5%, the coating had a single FCC phase structure. The AlCoCrxFeNi high entropy alloy coating had a higher corrosion potential, a lower passivation current, and a larger passivation potential range, which effectively improved the corrosion resistance of 45 steel. Zhou et al. [17] prepared FeCoNiCrMox laser cladding high entropy alloy coatings on the surface of 40Cr and studied the effect of Mo content on the corrosion resistance of the coating. With the increase of Mo content, the corrosion resistance of the coating first increased and then decreased. This is because excessive Mo element will cause the γ phase of the coating to precipitate, causing galvanic corrosion and reducing the corrosion resistance. NGUYEN et al. [18] prepared AlxFeMnNiCrCu0.5 high entropy alloy coating on AISI 1045 steel surface by laser cladding technology and found that Al element can promote the phase structure of coating from FCC phase to BCC phase, and the hardness of coating increases with the increase of Al element content. LIU et al. [19] prepared AlCoCrFeNiSix high entropy alloy coating on AISI 304 stainless steel surface by laser cladding. With the increase of Si element content, the coating structure gradually becomes finer, the hardness also increases, the average friction coefficient of the coating gradually decreases, and the coating has a single BCC phase structure.

In order to improve the hardness and corrosion resistance of marine steel to cope with the complex corrosion environment of ocean splash zone, this paper further improves the excellent comprehensive properties of high entropy alloy by using Si element to prepare high entropy alloy coating with excellent corrosion resistance and mechanical properties. The effect of Si element on the microstructure, microhardness and corrosion resistance of laser cladding high entropy alloy coating was studied, and the optimal Si element addition and action mechanism were explored to further improve the performance of marine steel and extend its service life, and to open up new ideas for marine corrosion protection.

1 Experiment

1.1 Sample preparation

DH36 high-strength marine engineering structural steel was selected as the substrate, with a size of 100 mm×120 mm×10 mm. Before cladding, the surface was polished smooth with sandpaper to remove oil and oxide layer, and cleaned with acetone for use. Its chemical composition is shown in Table 1. The experiment used Fe, Al, Cr, Ni, and Si powders with a purity of 99.99% and a powder size of 50~150 μm as laser cladding materials. The required powder mass was weighed using an electronic precision balance, and the FeAlCrNiSix (x=0, 0.25, 0.5, 0.75, 1.0, respectively recorded as samples Si0, Si0.25, Si0.5, Si0.75, Si1.0) powders prepared according to the molar ratio were put into a planetary ball mill for mixing. The ball mill speed was 200 r/min and the mixing time was 8 h. The mixed powder was placed in a drying oven for 2 h and then evenly coated on the surface of the substrate. The coating material was polyvinyl alcohol glue with a coating thickness of 1.25 mm.

The experiment used laser cladding equipment assembled by Yangjiang Hardware Knife and Scissors Industry Technology Research Institute, including ABB-IRC5Single robot, nLIGHT laser, Tongfei laser water cooler, HUIRUI laser cladding head, workbench, etc. According to the results of the previous experiment, the process parameters of this experiment were set as follows: laser power 1500 W, scanning speed 6 mm/s, powder thickness 1.25 mm, and protective gas argon with a purity of 99.99%.

Figure 1 shows the macroscopic morphology of the FeAlCrNiSix high entropy alloy cladding layer with different Si element additions prepared under the optimal process parameters. During the laser cladding process, Si has a low melting point, which makes the molten pool keep warm for a longer time under the same conditions, resulting in increased molten pool fluidity, uneven size, and coarse surface ripples. Since Si has good slag-making properties, impurities are discharged at the edge during the cooling process, resulting in circular spots and pits on both sides of the cladding layer.

1.2 Analysis and characterization methods

In this experiment, the Japanese RigakuSmartLab SEX X-ray diffractometer (XRD) was used to analyze the phase composition of the high entropy alloy cladding layer; the AXio Imager A2M optical microscope (OM) was used to observe the interface morphology of the cladding layer; the ZEISS EVO 18 scanning electron microscope was used to observe and analyze the microscopic morphology of the cladding layer, and the EDS was used to detect the element content of the cladding layer; the HV-1000 microhardness tester was used to measure the hardness of the cladding layer, the load size was 100g, the loading time was 15 s, and one point was tested every 0.2 mm from the cladding layer to the substrate, and each point was tested twice to take the average value; the polarization curve of the cladding layer was tested by the Chenhua CHI660E electrochemical workstation, using a three-electrode system, the cladding layer was the working electrode, the reference electrode was a saturated calomel electrode, the auxiliary electrode was a Pt electrode, the corrosive medium was a 3.5% NaCl solution, and the temperature was (30±2) ℃. The corrosion resistance of the cladding layer was characterized by analyzing the electrochemical parameters of the cladding layer.

2 Result analysis

2.1 Phase analysis of cladding layer

Figure 2(a) shows the XRD spectrum of FeAlCrNiSix high entropy alloy cladding layer. As can be seen from Figure 2(a), the phase structure of the high entropy alloy cladding layer consists of BCC phase and FCC phase. The reason is that the mixing entropy of the high entropy alloy is large, which leads to a decrease in Gibbs free energy, and the addition of Si element further increases the mixing entropy of the high entropy alloy, making it easier for the cladding layer to generate a simple phase structure. The large entropy value makes the phase of the high entropy alloy more stable, which can avoid phase separation to the greatest extent to produce intermetallic compounds. With the increase of Si element addition, the BCC phase gradually increases, the FCC phase gradually decreases and finally completely transforms into the BCC phase. This is mainly because Si element is a promoter of BCC phase and can promote the transformation of FCC phase into BCC phase[20].

Figure 2(b) shows a partial enlarged view of the XRD spectrum of the cladding layer. As can be seen from Figure 2(b), with the increase of Si element addition, the diffraction peak of the BCC phase gradually shifts to the right, and the lattice constant gradually decreases. Analysis shows that the atomic radius of Si element is small, which replaces other atoms in the solid solution, causing lattice contraction and lattice distortion.

2.2 Analysis of microstructure of cladding layer

Figures 3 and 4 show the optical microscope photo and SEM image of the microstructure of the FeAlCrNiSix high entropy alloy cladding layer, respectively, and Table 2 shows the EDS analysis results of different positions of the cladding layer. It can be seen from the figure that when Si element is not added, the structure of the cladding layer is a mixture of cellular crystals and dendrites, mainly coarse dendrites, fine secondary dendrites are distributed on both sides, and cellular crystals are distributed in the gaps between dendrites. This is because the nucleation rate in the cladding layer without adding Si element is relatively low, and the hysteresis diffusion effect of high entropy alloy also has a certain influence on the phase separation process, which reduces the diffusion efficiency of atoms in the cladding layer, so that the grains have enough time and space to grow. As the amount of Si added increases, the dendrites of the cladding layer transform into coarse equiaxed crystals. This is mainly due to the poor solubility of Si and other metal elements. During the crystallization process, Si is enriched at the grain boundaries, which hinders the growth of grains. As the amount of Si added continues to increase, the number of equiaxed crystals increases further and the size gradually decreases. The excess Si element is dissolved inside the grains, which increases the Si content inside the grains. Some grains will also grow with Si as the crystallization core, making the grains smaller and irregular in shape. When x=1.0, due to the relatively low melting point of Si, under the same energy input conditions, the excess heat plays a certain role in heat preservation[21], making the grain size coarser than when x=0.75.

From the EDS analysis results, it can be seen that when x=0, the element composition on the grain boundary is not much different from that in the grain boundary, and the Fe element in the substrate transitions to the cladding layer, making the Fe element content in the cladding layer higher. When x=0.25, the Si element is more enriched on the grain boundary and lower in the grain boundary. As the amount of Si element added continues to increase, the excess Si is repelled to the solid-liquid interface during crystallization and eventually enriched on the grain boundary. Combined with XRD data analysis, when x=0, the Cr element content on the grain boundary is higher than that in the grain boundary, and the content of the remaining elements at different positions is not much different. The phase composition is FCC+BCC phase, the BCC phase is Fe-Cr phase, which mainly exists on the grain boundary, and the grain boundary is mainly FCC phase. The addition of Si element promotes the transformation of FCC phase to BCC phase, and finally only BCC phase exists in the cladding layer.

2.3 Analysis of microhardness of cladding layer

Figure 5(a) shows the microhardness distribution curve of high entropy alloy cladding layer with different Si element addition. As can be seen from Figure 5(a), the microhardness from the cladding layer area, heat-affected zone to the substrate area presents a step-like distribution, that is, the cladding layer area > heat-affected zone > substrate area, and has obvious regional characteristics. The microhardness of the cladding layer area is significantly higher than that of the substrate area. The hardness is highest in the middle of the cladding layer, and the hardness is slightly lower near the surface. This is because the Si element has good slag-forming properties. During the cladding process, impurities float and gather on the surface of the cladding layer, and the heat dissipation rate of the cladding layer is faster, so that the impurities are not discharged in time and solidify, resulting in a decrease in the surface hardness of the cladding layer. Laser action can also cause the surface alloy elements to burn, thereby reducing the surface hardness [22].

Figure 5(b) shows the average hardness of the high entropy alloy cladding layer with different Si element addition amounts. With the increase of Si element addition amount, the average hardness of the cladding layer first increases and then decreases. The highest hardness value of the Si0.75 cladding layer is 430.15HV0.1. The reason for the analysis is that the grain size decreases with the increase of Si element addition amount, which makes the hardness of the cladding layer increase accordingly. The solid solution strengthening effect of lattice distortion caused by the addition of Si element and the grain boundary strengthening effect caused by Si element enrichment at the grain boundary are also the reasons for the increase in the hardness of the cladding layer [23]. However, with the further increase of Si element addition amount, the grain size becomes larger and the hardness decreases.

2.4 Corrosion resistance analysis of cladding layer

Figure 6 shows the polarization curves of cladding layers with different Si element addition amounts in 3.5% (mass fraction) solution measured by electrochemical technology. Table 3 shows the fitting parameters of the polarization curves of cladding layers with different Si element addition amounts measured in 3.5% solution.

As shown in Figure 6 and Table 3, the self-corrosion potential φcorr (φcorr, Si0.75＜φcorr, Si1.0＜ φcorr, Si0＜φcorr, Si0.5＜φcorr, Si0.25) of the high entropy alloy cladding layer with different Si contents showed different degrees of positive and negative movement, the corrosion tendency first decreased and then increased, the self-corrosion potential of Si0.75 coating was the lowest and the corrosion tendency was the smallest; the self-corrosion current density Jcorr
(Jcorr, Si1.0＜Jcorr, Si0＜Jcorr, Si0.25＜Jcorr, Si0.75Jcorr, Si0.5) first decreased and then increased, and the corrosion rate first decreased and then increased.

In the polarization curve, there was an obvious activation-passivation transition. The current density in the passivation range did not increase with the increase of potential, and the performance was relatively stable. At this time, the generation rate of the passivation film was greater than the dissolution rate, forming a dense passivation film, which inhibited the dissolution process of the anode on the surface of the cladding layer. As the applied voltage continues to increase, the passivation film is broken down by Cl−, and the current density continues to increase. The self-corrosion potential of Si0.25 and Si0.75 cladding layers is higher than that of the cladding layer without Si element added, and the corrosion resistance is improved. Compared with Si0.75, Si0.25 has a lower self-corrosion potential and a similar passivation current density, but a smaller self-corrosion current density and a larger passivation area, and Si0.25 has better corrosion resistance. As shown in Table 3, under the same corrosion conditions, the self-corrosion current density increases first and then decreases. The difference between the self-corrosion current density of Si0 and Si0.25 is the smallest, and the corrosion rate is not much different. However, Si0.25 has a higher corrosion potential and a larger passivation range. A comprehensive analysis of the polarization curve of the cladding layer and the fitting parameters of the polarization curve shows that the addition of Si element can effectively improve the corrosion resistance of the cladding layer, and the Si0.25 cladding layer has the best corrosion resistance.

3 Conclusions

1) With the increase of Si addition, the phase of the cladding layer changes from BCC+FCC phase to single BCC phase. This is because the high entropy effect of high entropy alloy promotes the formation of simple phase structure, and the addition of Si element can improve the mixing entropy of the cladding layer, and Si element can also promote the formation of BCC phase.

2) The addition of Si element can effectively refine the grains of the cladding layer, but excessive Si will make the grains coarse. Since Si element is mainly enriched at the grain boundary during crystallization, it hinders the growth of grains, and the cladding layer structure changes from dendrite to equiaxed crystal with the increase of Si element addition.

3) With the addition of Si element, the hardness of the cladding layer increases first and then decreases. The hardness of Si0.75 cladding layer reaches the highest value of 430.15HV0.1, which is mainly because Si element causes grain boundary strengthening and solid solution strengthening, while excessive Si element makes the grain size larger, and the hardness of the cladding layer decreases accordingly.

4) With the addition of Si element, the self-corrosion potential of the cladding layer first shifts positively and then negatively, the corrosion tendency first decreases and then increases, the self-corrosion current density first decreases and then increases, the corrosion rate first decreases and then increases, the width of the passivation zone of the cladding layer increases, and the activation-passivation transition is obvious. When the Si element addition amount is 0.25 mol, the corrosion resistance of the cladding layer is the best.