Cr-Ni stainless steel has excellent environmental corrosion resistance and has been widely used in the fields of petroleum, chemical industry, aerospace, marine engineering, etc. Among them, 304 stainless steel has good corrosion resistance and heat resistance, and is widely used in modern industry. However, in heavily corrosive industrial environments and heavily polluted atmospheres such as inorganic acids, its body corrosion resistance still cannot meet the requirements, and its service life needs to be extended by surface coating protection technology. Modern surface coating technologies such as vapor deposition, chemical heat treatment, electroplating, thermal spraying and laser cladding are important methods to improve the corrosion resistance of material surfaces. Studies have found that uniform and dense coatings can be prepared by electroplating and vapor deposition techniques, and the coatings have high purity and controllable composition. Meng et al. prepared a dense superhydrophobic Zn-Fe coating on the surface of a magnesium alloy by electroplating. The coating showed excellent self-cleaning, wear resistance and corrosion resistance. Compared with the magnesium alloy substrate, the corrosion resistance of the coating was improved by 87%. Shan et al. deposited CrN and CrSiN coatings on 316L stainless steel, which increased the surface hardness, improved the seawater corrosion resistance and tribological properties of the material. The coatings were prepared by chemical heat treatment, thermal spraying and other methods, and the surface accuracy and thickness were controllable, the process was simple and easy to operate. Xun Qingting et al. strengthened the surface of GCr15 steel by chemical heat treatment, and its hardness was greatly improved, and the thickness of the hardened layer reached 0.25 mm. Liu et al. successfully prepared Ag-BN coatings by plasma spraying, which reduced the friction coefficient of the coatings and improved their wear resistance.

The coatings prepared by electroplating and vapor deposition technology have weak bonding strength with the substrate and thin thickness. The surface of the thermal spray coating is rough and has a large porosity. The chemical heat treatment has high requirements for the substrate material, and the coating is difficult to meet the requirements of long-term working operation. Compared with other surface treatment technologies, laser cladding technology has the advantages of high efficiency, low dilution, and good metallurgical bonding. It is often used to prepare high-quality coatings with high hardness, strong wear resistance and corrosion resistance, which can achieve the purpose of workpiece surface repair and modification.

Laser cladding technology generally uses metal powder, ceramic powder, and metal-ceramic composite powder as cladding materials. Metal powder has good wettability with the substrate material and is easier to form a close metallurgical bond, thereby improving the process forming performance of the coating. Ouyang Changyao et al. laser clad Stellite12 cobalt-based powder on the surface of 304 stainless steel and studied the microstructure, element distribution, phase and properties of the coating. The results showed that the coating surface quality was good and had no obvious defects. It formed a metallurgical bond with the substrate, and the corrosion resistance was greatly improved compared with the substrate. Yang Wenbin et al. [23] prepared two kinds of iron-based and cobalt-based metal coatings on the surface of ER8 wheel steel. The coating surface was uniform and dense, forming a good metallurgical bond. The repaired wheel steel samples all showed good wear resistance and corrosion resistance. Compared with metals, ceramics have higher hardness, as well as better wear resistance, corrosion resistance, heat resistance and high-temperature oxidation resistance. Since the physical and chemical properties of ceramics, such as elastic modulus and thermal expansion coefficient, are quite different from those of metals, defects such as cracks and pores are easily generated during the cladding forming process, thereby affecting the bonding strength between the coating and the substrate, resulting in reduced surface quality and performance. Wang Ran et al. solved the problems of Al2O3-ZrO2 ceramic coatings, such as high brittleness and easy cracking, to a certain extent by preheating the substrate. After preheating at 300 °C, the crack sensitivity of the coating was significantly reduced, but cracks still existed. Studies have shown that the use of metal-ceramic composite coatings can solve the bottleneck problem of ceramic coatings. Metal-ceramic composite powders have the toughness and good processability of metal powders, as well as the high hardness, wear resistance, and corrosion resistance of ceramic powder materials. By selecting different types of metal and ceramic powders and adjusting the composition ratio of the two, metal-ceramic composite coatings with few defects and high bonding strength can be prepared. The intermetallic compounds and unmelted ceramic reinforcement particles in the coating structure are conducive to the specific functions of the composite coating (such as corrosion resistance, wear resistance, high temperature oxidation resistance, etc.). Commonly used metal-ceramic composite powders include Fe, Co, and Ni-based composite powders reinforced with ceramic particles such as WC, SiC, and Al2O3, which are widely used to prepare metal-ceramic composite coatings with high hardness, wear resistance, and strong corrosion resistance. Among them, Al2O3 ceramics have high melting point, high hardness, small thermal expansion coefficient, and strong physical and chemical stability. Domestic and foreign scholars have conducted extensive research on Al2O3 ceramic coatings. The results show that pure Al2O3 ceramic coatings have problems such as large porosity and weak bonding strength. Zhou Jianzhong et al. prepared Al2O3 ceramic-reinforced Fe901 metal-ceramic composite coatings using laser cladding, which effectively improved the hardness and wear resistance of the coating. Ni has good ductility and good bonding effect. By adding Ni, the deposition efficiency and mechanical properties of the coating can be effectively improved, and the pinning strength of Al2O3 particles in the composite coating can be enhanced. Al2O3-enhanced Ni-based composite coating has high hardness and bonding strength, and exhibits good surface protection characteristics. At present, the research on Ni-Al2O3 composite coating mainly focuses on its wear resistance and related mechanisms, and there are few reports on the corrosion resistance of the coating. In this paper, the pre-set powder method is used to prepare Ni-Al2O3 metal ceramic composite coating on the surface of stainless steel by laser cladding technology, in order to combine the high chemical stability of metal Ni with the high hardness strengthening effect of Al2O3, greatly reduce the corrosion reaction rate, and improve the surface hardness of the material, thereby achieving the dual goals of improving the corrosion resistance and surface hardness of 304 stainless steel.

1 Experiment

1.1 Materials
The laser cladding substrate is 304 stainless steel, and its chemical composition (by mass fraction) is: S 0.002%, P 0.042%, C 0.07%, Si 0.89%, Mn 1.92%, Ni 8.1%, Cr 18.2%, and the balance is Fe. The size is 200 mm×150 mm×15 mm, and the microstructure of the substrate is shown in Figure 1. The cladding powder is commercial high-purity Ni powder (average particle size 100 nm, purity 99.0%) and Al2O3 powder (average particle size 2 μm, purity 98.0%). The mixed powder was mixed in a QM-1 horizontal grinder at a grinding speed of 250 r/min for 6 h to make the powder mixed evenly. Before cladding, the mixed powder was placed in a vacuum drying oven at 150 °C for 3 h to remove moisture. Before cladding, the substrate surface was polished with SiC sandpaper and the substrate surface was cleaned with acetone to remove grease. The substrate was preheated to 300 °C to reduce the thermal stress caused by the huge temperature gradient between the substrate and the coating. In order to ensure the stability of the composite coating, laser cladding was performed using pre-set powder, and the thickness of the pre-set powder was 0.9 mm.

1.2 Coating preparation
The cladding equipment uses a JHL-1GX-2000 laser intelligent manufacturing system with a maximum power of 2 kW. Cladding process parameters: laser power of 1.2 kW, spot diameter of 3 mm, and scanning speed of 350 mm/min. After the cladding is completed, the sample is naturally cooled to room temperature. The sample is cut along the cross section of the composite coating by wire cutting, and the sample is cleaned in anhydrous ethanol using an ultrasonic cleaner to obtain a metallographic specimen. After grinding and polishing, the sample is etched for 25 s using a mixed solution consisting of HCl (volume fraction 75%) and HNO3 (volume fraction 25%).

1.3 Coating morphology and phase characterization
The microstructure of the substrate was observed by an Eclipse MA200 optical microscope (OM), and the morphology of the composite coating and its corrosion surface was observed by a VEGA3 scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS), and energy spectrum analysis was performed. The composition of the composite coating phase was analyzed by a multifunctional X-ray diffractometer (XRD, voltage 40 kV, current 200 mA, diffraction angle 2θ of 20°~80°).

1.4 Characterization of coating performance
The microhardness of the cross section of the composite coating was tested by a HV 1000A microhardness tester, with a loading mass of 400 g and a loading time of 30 s. The distance between each measurement position was 0.1 mm. For the same group of samples, 3 points were tested at the same distance from the coating surface, and the average value was taken.
The composite coating was sealed with organic glue, exposing 1 mm2 of the surface, and a corrosion sample was made. The corrosion sample was placed in 1 mol/L dilute hydrochloric acid and immersed in corrosion at room temperature for 5 h. After removing the corrosion products, it was weighed, and the weight loss corrosion rate of the composite coating was calculated using the corrosion weight loss: VL= (m1- m0)/t.
Where m1 is the mass of the sample before corrosion, m0 is the mass of the sample after corrosion, and t is the corrosion time. The Ametek Parstat 4000 electrochemical workstation was used to test the potentiodynamic polarization curve of the 1 mm2 composite coating corrosion sample surface. The corrosion medium was 1 mol/L dilute hydrochloric acid solution, the reference electrode was Ag/AgCl electrode, the auxiliary electrode was Pt electrode, and the working electrode was 1 mm2 of corrosion sample. After immersion at open circuit potential for 60 min, the test was performed after stabilization. The potentiodynamic polarization test was performed in the range of −1.5~1.5 at a scanning speed of 1 mV/s, and the corrosion potential and corrosion current density of the composite coating were fitted.

2 Results and discussion

2.1 Coating morphology and phase analysis
The microstructure of the cross section of the Ni-25%Al2O3 composite coating is shown in Figure 2. As can be seen from Figure 2a, the composite coating has a uniform structure, no obvious defects such as pores and cracks, and there is an obvious metallurgical bonding area between the composite coating and the substrate. The composite coating can be divided into three parts: cladding layer (CL), metallurgical bonding zone (MBZ) and heat affected zone (HAZ). As shown in Figure 2b, the structure at the bottom of the CL zone is fine cellular crystals. As shown in Figure 2c, the center of the CL zone is a columnar crystal with directional growth. As shown in Figure 2d, the structure at the top of the CL zone is fine equiaxed crystals. Since the laser beam scans the powder for a very short time and the temperature drops quickly, the composite coating solidifies and cools quickly, forming a relatively uniform and fine structure. According to solidification theory, the morphology of the solidified structure is determined by the stability factor (G/R) of the solid-liquid interface, where G is the temperature gradient and R is the solidification rate. The bottom of the CL zone is close to the substrate, with a fast cooling rate and a large degree of supercooling, forming fine cellular crystals. During the solidification process, the cooling rate perpendicular to the bonding interface is the fastest, and the grain crystallization rate is the fastest. Therefore, columnar crystals are generated in the center of the CL zone along the direction perpendicular to the interface, as shown in Figure 2c. As shown in Figure 2d, the top of the CL zone is in contact with the air, the cooling rate is fast, the undercooling is large, and the cooling rate in all directions is the same, generating fine equiaxed crystals. During the solidification process, different cooling rates lead to different microstructures. Based on the rapid melting and solidification characteristics of laser cladding, the structure of the composite coating is significantly refined compared with the substrate. The EDS surface scanning analysis results of the composite coating (Figure 2) are shown in Figure 3. As shown in Figures 3a~c, Fe and Cr elements are evenly distributed in the coating and the substrate, and Ni is mainly distributed in the CL zone. Al and O elements (as shown in Figures 3d and e, respectively) are mainly distributed at the top of the CL zone, proving that Al2O3 particles are mainly distributed at the top of the CL zone, and the composite coating is composed of a metal layer and a ceramic layer. The key to the formation of metal-ceramic composite coatings is the dispersion of Ni and Al2O3 in the powder, and the difference in their absorption of laser energy. When the high-energy laser scans the composite powder, the powder and the substrate surface are instantly melted by high temperature. Since the melting point of Al2O3 is higher than that of Ni, most of the laser energy is absorbed by Ni powder, and Ni powder is completely melted. Part of Al2O3 powder is slightly melted, but Al2O3 remains in granular form. After the high-energy laser scans the powder, Ni powder and the substrate are completely melted to form a molten pool. Strong convection is generated in the molten pool, and Al2O3 particles are evenly dispersed. Since the density of Al2O3 particles is lower than that of the metal phase, they are mainly distributed on the top of the composite coating (as shown in Figure 4), forming a ceramic layer. Intermetallic compounds are distributed in the composite coating to form a metal layer. Since Ni has good wettability with the metal matrix, a good metallurgical bonding area is formed, making the composite coating more firmly bonded to the substrate.

In order to determine the phase composition of the Ni-25%Al2O3 composite coating, the composite coating was analyzed by XRD. The results are shown in Figure 5. The phase of the composite coating is mainly composed of Al2O3, Fe-Ni, and Fe-Ni-Cr solid solutions. Since the atomic radius of Fe is very close to that of Cr and Ni, Fe will melt and diffuse under high-energy laser irradiation, and combine with Cr and Ni to form Fe-Ni and Fe-Ni-Cr solid solutions, which exist as austenite at high temperatures and transform into martensite after cooling. The existence of Fe-Ni and Fe-Ni-Cr solid solutions indicates that the matrix and Ni powder have been fully melted, and the Fe in the matrix has been fully diffused into the molten pool. Combined with SEM and EDS analysis, it can be seen that the Al2O3 ceramic particles are not completely melted, and most of them still exist in the form of particles, which further proves the existence of the Al2O3 ceramic phase.

The cross-section and surface morphology of the Ni-x%Al2O3 composite coating are shown in Figure 6. As shown in Figures 6a, c, e, and g, the cross-sections of the Ni, Ni-15%Al2O3, and Ni-25%Al2O3 composite coatings are dense and have no obvious defects. The Al2O3 particles are slightly melted under high-energy laser irradiation, showing a light gray irregular granular structure. The slightly melted Al2O3 particles produce a pinning effect under the bonding action of the Fe-Ni and Fe-Ni-Cr solid solutions, and are more firmly combined, thereby improving the forming effect of the composite coating. With the increase of the Al2O3 content, the number of Al2O3 particles in the composite coating gradually increases. In the cross-section of the Ni-35%Al2O3 composite coating, more pores were found, the Al2O3 particles agglomerated, and the Al2O3 particles and the intermetallic compounds produced pores, which were not firmly combined, which easily led to the reduction of the performance of the composite coating. As shown in Figures 6b, d, f, and h, there are no obvious defects on the surface of Ni, Ni-15%Al2O3, and Ni-25%Al2O3 composite coatings, while there are obvious cracks and pores on the surface of Ni-35%Al2O3
composite coatings. The cracks are mainly caused by excessive stress due to the agglomeration of Al2O3 particles and uneven element distribution. Due to the rapid melting characteristics of the composite coating, the gas generated by the reaction of elements such as C and S with O has no time to escape, thus forming pores. As shown in Figure 6, after adding an appropriate amount of Al2O3, the surface of the composite coating is dense and has no obvious defects; after adding excessive Al2O3, the composite coating is prone to defects such as pores and cracks.

2.2 Microhardness analysis
The change curve of the microhardness of the cross section of the Ni-x%Al2O3 composite coating along the depth direction is shown in Figure 7. The microhardness of the substrate is about 164HV, and the microhardness of the composite coating can reach up to 1026.3HV. The microhardness is between 760HV and 1 026HV, which is 4 to 5 times higher than that of the substrate. As shown in Figure 7, the microhardness of the composite coating decreases sharply after gradually increasing. This is because there are some defects on the shallow surface of the composite coating, resulting in low microhardness of the surface; the microstructure inside the composite coating is uniform and fine, with few defects, and there are a large number of hard phases, and the microhardness gradually increases; the microhardness of the area close to the substrate decreases sharply until it approaches the microhardness of the substrate. With the increase of Al2O3 content, the microhardness of the composite coating increases first and then decreases. When the mass fraction of Al2O3 is 25%, the microhardness of the composite coating reaches the highest value. The hardness of the composite coating is related to its surface quality and Al2O3 content. Combined with the morphology and phase analysis of the composite coating, the main reasons are: first, the laser cladding composite coating produces a large degree of undercooling during the rapid cooling process, thereby refining the microstructure of the coating, playing a fine grain strengthening role on the composite coating, and significantly increasing the microhardness of the composite coating; second, the solid solution strengthening effect of the hard phases Fe-Ni and Fe-Ni-Cr improves the microhardness of the composite coating. Combined with the EDS results (Figure 3), it can be seen that the content of Ni and Cr in the composite coating is high, and the Fe atoms in the melted matrix undergo element diffusion in the composite coating. Ni and Cr are easily dissolved in Fe to form a hard solid solution; third, the high-hardness Al2O3 ceramic particles are dispersed in the composite coating, which further improves the microhardness of the composite coating. When the mass fraction of Al2O3 reaches 35%, defects such as pores and cracks appear on the surface of the composite coating, which reduces the microhardness of the composite coating. It can be seen that the improvement of microhardness of Ni-x%Al2O3 (x≤25) composite coating benefits from the combined effects of grain refinement, solid solution strengthening and particle strengthening.

2.3 Analysis of coating corrosion resistance
The weight loss corrosion rate of the Ni-x%Al2O3 composite coating after immersion in 1 mol/L dilute hydrochloric acid for 5 h is shown in Figure 8. As can be seen from Figure 8, with the increase of Al2O3 content, the weight loss corrosion rate shows a trend of first decreasing and then increasing, and the corrosion resistance shows a trend of first increasing and then weakening. The weight loss corrosion rate of the Ni-25%Al2O3 composite coating is the smallest and the corrosion resistance is the best. The polarization curve and fitting data of the Ni-x%Al2O3 composite coating are shown in Figure 9. As can be seen from Figure 9, the polarization curves of the Ni-x%Al2O3 composite coatings are similar in shape. With the increase of Al2O3 content, the corrosion potential shows a trend of first increasing and then decreasing, and the corrosion current density shows a trend of first decreasing and then increasing. The Ni-25%Al2O3 composite coating has the highest corrosion potential and the lowest corrosion current density. The corrosion potential indicates the corrosion tendency of the material. The larger the corrosion potential of the composite coating, the less likely it is to be corroded. The corrosion current density and corrosion rate indicate the quality of the corrosion resistance of the material. The smaller the corrosion current density and corrosion rate of the composite coating, the better the corrosion resistance of the composite coating. The immersion corrosion test and electrochemical test fitting data of the composite coating show that the corrosion current density and corrosion rate of the Ni-25%Al2O3 composite coating are the smallest, and the corrosion resistance is the best. The Al2O3 corrosion-resistant ceramic phase and the Fe-Ni and Fe-Ni-Cr solid solutions increase the corrosion potential of the composite coating. The Ni-25%Al2O3 composite coating has a smaller corrosion tendency and its microstructure is more uniform and dense; the Ni-35%Al2O3 composite coating has defects such as pores and cracks, and the corrosive liquid is easier to invade the interior, which aggravates the corrosion process.

The corrosion surface morphology of the Ni-x%Al2O3 composite coating immersed in 1 mol/L dilute hydrochloric acid for 5 h is shown in Figure 10. As can be seen from Figure 10a, the surface of the Ni coating is corroded more severely, the corrosion area is larger, and a continuous large-area gully-shaped corrosion area is obviously present, and the corrosion pits are deeper and larger. As can be seen from Figure 10b, the corrosion degree of the Ni-15%Al2O3 composite coating is reduced, the corrosion area is reduced, the continuous large-area gully-shaped corrosion area is reduced, the corrosion pits are shallow, the corrosion pits are small, but the number is large. The corrosion morphology of the Ni-25%Al2O3 composite coating is shown in Figure 10c. Only a small part of the composite coating surface is corroded, the continuous gully-shaped corrosion area is smaller, the corrosion pits are smaller and the number is small, and the corrosion degree is further reduced. As can be seen from Figure 10d, the corrosion degree of the Ni-35%Al2O3 composite coating is aggravated, the corrosion area is increased, the continuous large-area gully-shaped corrosion area is increased, the corrosion pit area is larger, the number is more, and the corrosion resistance of the composite coating is worse. The corrosion morphology of the composite coating further shows that with the increase of Al2O3 content, the corrosion resistance of the composite coating shows a trend of first increasing and then weakening, among which the corrosion resistance of Ni-25%Al2O3 composite coating is the best. This is because the corrosion potential of the composite coating increases first and then decreases, the corrosion tendency weakens first and then increases, the corrosion current density and corrosion rate decrease first and then increase, resulting in the corrosion degree of the composite coating first reducing and then aggravating, and the corrosion area where the pitting pit expands to form gullies first increases and then decreases.

When the composite coating is immersed in 1 mol/L dilute hydrochloric acid, Cl− easily destroys the surface passivation film, the corrosive liquid contacts the surface of the composite coating, and a corrosion galvanic cell is formed, and an electrochemical reaction occurs. Elements such as Fe, Cr, and Ni undergo oxidation reactions at the anode, lose electrons and dissolve to form free cations, and H+ undergoes reduction reactions at the cathode to generate H2 escape, resulting in corrosion pits on the corrosion surface, causing the composite coating to be further corroded. Due to the rapid melting and solidification of laser cladding, the microstructure of the composite coating is finer than that of the substrate, and the corrosion resistance of the refined structure is stronger. Therefore, the corrosion resistance of the Ni-x%Al2O3 composite coating is improved under the effect of fine grain strengthening. The Fe-Ni and Fe-Cr-Ni solid solutions firmly pin the Al2O3 particles in the composite coating, effectively bond the Al2O3 particles, and prevent the corrosive liquid from entering the composite coating through the pores near the Al2O3 particles. The solid solution strengthening effect improves the compactness of the composite coating and strengthens the corrosion resistance of the composite coating. After adding an appropriate amount of Al2O3 to the composite coating, the micro-melted Al2O3 can block the corrosion channel and reduce the corrosion area. The addition of an appropriate amount of Al2O3 can play a role in particle strengthening of the composite coating. When 35% mass fraction of Al2O3 is added, on the one hand, the excessive addition of Al2O3 causes a large number of particles to not be melted, increasing the corrosion channel and the number of corrosion galvanic cells. Therefore, the corrosion resistance of the Ni-35%Al2O3 composite coating is reduced. On the other hand, after excessive addition of Al2O3, there are a large number of pores and cracks in the composite coating, and the corrosive liquid is more likely to enter the interior of the composite coating through the pores and cracks, thereby accelerating the corrosion rate, resulting in a decrease in the corrosion resistance of the Ni-35%Al2O3 composite coating. In summary, the improvement of the corrosion resistance of the Ni-x%Al2O3 (x≤25) composite coating is the result of the combined effect of fine grain strengthening, solid solution strengthening and particle strengthening.

3 Conclusions
The high-hardness and corrosion-resistant Ni-x%Al2O3 composite coating was prepared on the surface of 304 stainless steel by laser cladding technology. The effect of Al2O3 content on the morphology, microhardness and corrosion resistance of the composite coating was studied. The main conclusions are as follows.

1) A close metallurgical bond is formed between the composite coating and the substrate. The microstructure of the composite coating is presented as fine equiaxed crystals, directional columnar crystals and cellular crystals from the surface to the inside. The Ni-x%Al2O3 (x ≤ 25) composite coating is uniform and dense without obvious defects. The Ni-35%Al2O3 composite coating has defects such as pores and cracks. The main phases of the Ni-25%Al2O3 composite coating are composed of Al2O3, Fe-Ni, and Fe-Ni-Cr solid solutions. The Al2O3 particles are mainly distributed at the top of the CL zone to form a ceramic layer. The intermetallic compounds are evenly distributed in the CL zone to form a metal layer. The Al2O3 particles are firmly pinned in the composite coating by the intermetallic compounds.

2) The microhardness of the composite coating increases first and then decreases sharply from the coating surface to the substrate. With the increase of Al2O3 content, the microhardness of the composite coating increases first and then decreases, the weight loss corrosion rate decreases first and then increases, the corrosion potential increases first and then decreases, and the corrosion current density decreases first and then increases. The Ni-25%Al2O3 composite coating has the highest microhardness and the best corrosion resistance. The improvement of the microhardness and corrosion resistance of the Ni-x%Al2O3 (x≤25) composite coating is the result of the combined effects of fine grain strengthening, solid solution strengthening and particle strengthening.