Objective To study the effect of different Mo additions on the microstructure and corrosion resistance of 316L cladding layer, and further improve the seawater corrosion resistance of 316L laser cladding layer, so as to achieve high-quality laser remanufacturing repair. Methods The cladding layer was prepared by adding different proportions of spherical Mo powder (2 wt.%, 4 wt.%, 6 wt.%) to 316L stainless steel alloy powder using laser cladding technology. The macroscopic morphology, microstructure, phase composition, element distribution, grain orientation and corrosion resistance of the cladding layer were characterized by metallographic microscope, scanning electron microscope, laser confocal microscope, X-ray diffractometer and electrochemical workstation. Results When the Mo addition was less than 4%, the phase in the cladding layer did not change and was still composed of austenite. When the Mo addition reached 6%, the supercooling increased, resulting in the transformation of austenite to martensite, and a large number of incompletely melted Mo particles appeared in the cladding layer. With the increase of Mo content, the texture strength of the grains in the <001> direction decreased, and there was a tendency to grow in the <111> direction. The average grain size decreased from 235.59 µm to 184.35 µm. The increase in the proportion of small-sized grains led to the accumulation of dislocations inside the grains, and the dislocation density in the cladding layer increased. At the same time, grain refinement helped to form a denser passivation film. In the electrochemical test, with the increase of Mo content, the number of pitting pits decreased significantly, and the corrosion resistance of the cladding layer was improved. When the Mo addition was 4%, the self-corrosion current density decreased from 8.253×10-6 A/cm2 to 4.540×10-7 A/cm2 compared with the group without addition, and the passivation resistance increased from 4927 Ω·cm2 to 8702 Ω·cm2, an increase of about 75%. Conclusion Increasing the Mo element content appropriately during laser cladding of 316L can refine the grains, make the passivation film formed on the surface of the cladding layer denser, improve the ability of the passivation film to resist erosion, and further improve the seawater corrosion resistance of the 316L cladding layer.

High salinity, humidity, temperature changes and microbial activity in the marine environment can cause serious corrosion to marine engineering equipment [1-2]. The corrosion resistance of equipment is crucial to ensure the safe operation of marine engineering. 316L stainless steel has been widely used in the marine field due to its high strength, low magnetism, excellent corrosion resistance and machinability [3-4]. Therefore, for the laser remanufacturing and repair of large mechanical parts used in marine corrosion environments [5], the cost-effective 316L alloy powder is the preferred cladding material. However, even though 316L stainless steel has relatively high corrosion resistance, when exposed to seawater for a long time, chloride ions will still erode and damage the repair surface [6], resulting in corrosion, pitting or pitting [7-8], reducing the service life of the equipment.

Since Mo is the main chloride ion corrosion resistant element in standard grade 316L stainless steel [6], its content can be appropriately increased to further improve its seawater corrosion resistance. At the same time, studies have shown that the Mo element not only greatly improves the corrosion resistance of the material, but also changes the microstructure of the material, affecting mechanical properties such as hardness, strength and toughness [9-12]. Pardo et al. [13] found that the Mo element promoted the formation of insoluble oxide film (passivation film) in steel, thereby enhancing the corrosion resistance of 304 and 316L austenitic stainless steel in sulfuric acid solution. Ostovan et al. [14] studied the effect of Mo content changes on the microstructure and corrosion properties of laser welded 316L stainless steel welds. The results showed that the increase in Mo content caused Mo-rich compounds to precipitate at the grain boundaries. The precipitated phase increased the strength and hardness of the weld zone, reduced the corrosion current in the weld zone, and improved the corrosion resistance. Yuan et al. [15] found that the Mo element directly participated in the formation of the natural oxide film in duplex stainless steel. The addition of the Mo element changed the content and film thickness of the chromium oxide compound in the original passivation film, greatly inhibited the pitting corrosion formation process, and improved the pitting corrosion resistance of the duplex stainless steel surface. Yang Jiawei et al. [16] studied the effect of Mo on the microstructure and corrosion resistance of high entropy alloys by adding different contents of Mo to high entropy alloys. The results showed that the increase of Mo content improved the hardness of the cladding layer, and the corrosion resistance of the cladding layer improved with the increase of Mo content. The above research results show that adding an appropriate amount of Mo element during laser cladding can improve the microstructure of the cladding layer and enhance the strength and corrosion resistance of the cladding layer. However, there are certain limitations. The influence of the microstructure change caused by the change of Mo content on the corrosion resistance has not been clearly defined. In order to achieve high-quality laser remanufacturing and repair, it is necessary to study the mechanism of the effect of Mo content change on the microstructure and corrosion resistance of 316L cladding layer.

In view of this, this paper takes 316L stainless steel as the object, mixes spherical Mo powder in different proportions into 316L stainless steel powder to prepare cladding layers with different Mo contents, and characterizes the relationship between Mo element content and cladding layer structure and corrosion resistance by analyzing the phase composition, microstructure, grain orientation and corrosion resistance of the cladding layer, and obtains the influence of Mo element addition on the corrosion resistance of 316L cladding layer.

1 Experiment

1.1 Experimental materials and equipment
The base material of this experiment is a 316L stainless steel plate with a size of 60 mm×100 mm×15 mm. The laser cladding powder is a 316L stainless steel powder with a particle size of 50~150 μm and a spherical Mo powder with a particle size of 45~100 μm. The two are mixed by a planetary ball mill (QM-3SP4). The spherical Mo powder and 316L powder are mixed at a mass ratio of 1:49 (2 wt.%), 1:24 (4 wt.%), and 3:47 (6 wt.%). The ball mill speed is 250 r/min and the mixing time is 2 h. Figure 1 shows the powder morphology observed under a scanning electron microscope. The chemical composition table of 316L stainless steel powder is shown in Table 1.

The laser used in this experiment is the LDF400-2000 fiber-coupled semiconductor laser based on coaxial powder feeding produced by Laserline, Germany, with a maximum power of 2 kW, a wavelength of 940-980 nm, a continuous working mode, and a multi-mode mode; the cladding head module uses the Laserline synchronous ring powder feeding cladding head, with a flat-top circular spot and a spot diameter of 4 mm; the motion module uses an industrial robot arm produced by ABB, Switzerland; Figure 2 is a schematic diagram of the laser cladding system, which mainly consists of a laser, an air compressor, a water cooler, a powder feeder, and other parts.

1.2 Experimental methods and performance characterization

Before the experiment, the powder was dried at 120℃ for 2 h in a vacuum drying oven and then stored in a sealed bag. The surface of the substrate was polished with an angle grinder and sandpaper to remove oxides and dirt on the surface of the substrate. After polishing, it was cleaned and blown dry with anhydrous ethanol for standby use. The optimized laser cladding process parameters are: P = 1400 W, scanning speed V = 5 mm/s, powder feeding rate 10 g/min, argon flow rate 10 L/min, defocusing amount 20 mm, scanning mode is transverse scanning, and the scanning strategy is shown in Figure 3(a), where the BD direction is the sample construction direction, the SD direction is the laser scanning direction, the TD direction is the tangential direction, and the overlap rate between the cladding layers is 50%.

After the laser cladding test, the sample was sampled in the transverse and longitudinal sections of the cladding layer (SD direction and TD direction) by using an electric spark wire cutting device; the wire-cut sample was mounted by a metallographic mounting test machine, and then ground, polished, and etched (the etchant was FeCl3: HCl: CH3COOH = 1: 2: 10) to make a metallographic sample. The morphology of the cladding layer was observed by optical microscopy (OM), and the distribution of the elements in the cladding layer was analyzed by scanning electron microscope ZEISS EVO18 produced by Zeiss Company of Germany and its built-in energy spectrometer, and phase analysis was performed by X-ray diffraction (XRD).

The EBSD sample was prepared by the same grinding and polishing method as the metallographic sample. After polishing to a mirror surface, it was electropolished. The electrolyte was 5% perchloric acid alcohol solution. The external electrode of the sample was immersed in the electrolyte. The constant voltage was 20 V and the electrolysis time was 23 s. After the electrolysis was completed, it was immediately cleaned with anhydrous ethanol and dried for use.

The electrochemical sample was a multi-pass multi-layer overlapped cladding sample. The sample size was a disc with a diameter of 14 mm and a thickness of 5 mm. After grinding and polishing, the corrosion resistance test was performed. The sampling method of the electrochemical sample is shown in Figure 3(c). The corrosion resistance of the cladding layer was tested using an electrochemical workstation CHI760E. In order to simulate the marine environment, the test solution was a 3.5 wt.% NaCl solution. According to GB/T 4029902021 “Corrosion of Metals and Alloys – Applicable Management of Electrochemical Measurement Methods for Corrosion Tests”, the electrochemical workstation was adjusted to the open circuit potential working mode for 30 min before the test to ensure the accuracy of the open circuit potential. After the test, the EIS data was fitted and analyzed using ZSimp Win software.

2 Results and Analysis

2.1 Phase Analysis of Cladding Layer
Figure 4 is a line scan of the Mo element in the cross-section of the cladding layer at different Mo contents, and the scanning method is from the top to the bottom of the cladding layer (SD direction). The results show that as the addition of spherical Mo powder gradually increases to 6%, the Mo element content in the cladding layer increases significantly, which indicates that most of the spherical Mo powder is effectively alloyed into the cladding layer during the laser cladding process and is not severely diluted by the matrix.

Figure 5 shows the XRD spectra of the cladding layer with different Mo contents. From the results, when the addition of spherical Mo powder does not exceed 4%, the main phase of the cladding layer is still austenite (γ-Fe), and no other phases except austenite appear. The austenite (γ-Fe) peak shifts from <111> to <200> and <220>. When the Mo addition is 6%, the austenite peak in the cladding layer is greatly reduced, and the transformation from austenite (γ-Fe) to martensite (α-Fe) occurs, and the martensite peak is enhanced. The reason is that the addition of a large amount of Mo element interferes with the stability of austenite in stainless steel [17]. Mo atoms, as alloying elements, can enter the lattice of austenite iron, which will cause lattice distortion, thereby affecting the stability and phase transition point of the crystal. Excessive Mo elements increase the probability of stacking faults in austenite, providing the necessary atomic migration path for martensitic phase transformation. There is no obvious Mo peak in the cladding layer, indicating that most of the added spherical Mo powder has been dissolved in the austenite during the laser cladding process and exists only in the form of solid solution, without forming an independent Mo phase or Mo-related compounds. Mo atoms are dispersed in the matrix lattice and do not produce independent diffraction peaks [16,18], which only causes the lattice parameters of the main phase to change.

2.2 Microstructure of cladding layer
Currently, some studies have shown that the crystal morphology of the alloy during laser cladding is affected by the heat dissipation direction, the liquid phase composition in the molten pool and the shape control factor [19-21]. The shape control factor is the ratio of the temperature gradient G in the crystallization direction to the solidification rate R. Different solidification conditions at different positions in the molten pool lead to differences in the final structure. The top area of ​​the cladding layer is in contact with the external environment, with the smallest temperature gradient G and the largest solidification rate R, so it is mainly composed of fine equiaxed crystals [21], as shown in Figure 6. As the amount of Mo added gradually increases, the morphology of the equiaxed crystals in the top area changes significantly. When the amount of Mo added reaches 2%, secondary dendrites begin to derive inside the grains [16]. As the amount of Mo added further increases, the number of secondary dendrites increases significantly, the grain size decreases, and the number of grain boundaries increases significantly.

As the crystallization process advances from the top to the bottom of the cladding layer, the temperature gradient G in the middle area of ​​the cladding layer begins to decrease, and the cooling rate R increases [21], resulting in the structure of the middle area being between the top and the bottom, mostly columnar crystals, as shown in Figure 7. As the amount of Mo added gradually increases, the number of secondary dendrites increases significantly, the grain size decreases, and the smaller grain size means a larger grain boundary area and grain boundary density. Grain boundaries serve as channels for the migration and diffusion of metal elements [22]. When the number of grain boundaries increases, metal elements will more easily diffuse through the grain boundaries to the surface of the material and participate in the formation of passivation films in a corrosive environment. In addition, the increased grain boundaries can also act as barriers to hinder the spread of corrosive media [23,24]. For the 316L cladding layer, the increase in the number of grain boundaries makes it easier for the passive film constituent elements such as Fe, Cr, and Mo to diffuse to the surface of the cladding layer, which will help to form a denser passive film on the surface of the cladding layer[25] and improve the corrosion resistance.

When the Mo element addition reaches 6%, many irregular block structures appear in the middle area of ​​the cladding layer, as shown in Figure 8(a). Figures 8(b) and (c) show the EDS surface scan and line scan results of irregular block structures. The EDS results confirm that these irregular block structures are incompletely melted Mo particles. These incompletely melted Mo particles are mainly concentrated in the middle of the cladding layer. The reason for the incompletely melted Mo particles is that the amount of spherical Mo powder added is too much. The melting point of Mo is 2617 ℃, and Mo has good thermal conductivity. Laser cladding is a fast cooling and heating process. When the surface of Mo particles contacts the laser, the heat will be quickly transferred out. A large number of spherical Mo particles cannot absorb enough heat to reach the melting point in a short time, resulting in the core area of ​​the Mo particles failing to reach a sufficient melting temperature [17]. These clustered unmelted particles will lead to uneven distribution of elements in the cross section of the cladding layer, which is very likely to aggravate the electrochemical corrosion here and affect the overall corrosion resistance of the cladding layer.

2.3 Effect of Mo on Grain Orientation and Grain Size of the Cladding Layer

Figures 9(a)~(c) are grain orientation diagrams of the cladding layer in the SD direction at different Mo contents. Combined with the inverse pole diagrams of Figures 9(d)~(f), it can be seen that in the SD direction, the temperature gradient plays a dominant role in grain growth during the solidification of the molten pool, and the grains grow along the <001> direction along the temperature gradient. As the Mo addition reaches 4%, the grains in the middle area of ​​the molten pool no longer grow vertically upward along the bottom of the cladding layer, but point from the cladding layer boundary to the center of the molten pool [27]. The texture strength in the <001> direction decreases, and the grains tend to grow in the <111> direction. Figures 9(g)~(i) are KAM diagrams of the cladding layer in the SD direction. In the blue area, the orientation difference between adjacent grains is small, and the green area indicates that the orientation difference between adjacent grains is large. The brighter and more concentrated the green area in the cladding layer is, the greater the local stress here, that is, the higher the KAM value, which is often associated with high dislocation density. Dislocation is the main way for materials to accumulate stress and energy during plastic deformation. The increase in local dislocation density will lead to an increase in strength, but it may also lead to an increase in material brittleness [28,29]. In the KAM diagram, the KAM value of the cladding layer with 4% Mo addition is the highest. The reason is that the addition of Mo element makes the cladding layer grains refined. The grain boundary is an obstacle to the movement of dislocations. Smaller grains mean a higher grain boundary area, which will exert more resistance to the movement of dislocations, causing dislocations to accumulate inside the grains, increasing the dislocation density [30].

Figure 10 is a statistical diagram of the grain size of the cladding layer in the SD direction at different Mo contents. The statistical method of grain size is based on the major axis of the grain fitting ellipse. With the increase of Mo addition, the average grain size of the cladding layer decreases from 235.59 µm to 184.35 µm, the proportion of small-sized grains increases, and the grain refinement effect is obvious. The reason for the grain refinement is that during the solidification process of the alloy, when the added elements are unevenly distributed, a solute enrichment zone will be formed near the solid-liquid interface, causing local supercooling [31,32]. When the Mo content in the cladding layer increases, the local supercooling area becomes larger, thus shortening the alloy solidification process [17], and the grain growth is more restricted, but it also leads to an increase in the number of grain nuclei. With the increase in the number of nuclei, the competition between the grains becomes more intense, often resulting in a smaller grain size in the final formation.

Figure 11 (a) ~ (c) are metallographic images of the cladding layer in the TD direction with different Mo contents. A large number of columnar crystals growing laterally appear in the top area of ​​the cladding layer due to the influence of temperature gradient and laser movement. When the Mo addition reaches 4%, the number and size of dendrites at the top of the cladding layer are weakened. This is because the addition of Mo increases the supercooling and accelerates the solidification rate of the molten pool [17,32]. Combined with the grain orientation diagram and the inverse pole diagram in Figure (11), it can be seen that the grains in the middle area of ​​the cladding layer grow vertically upward along the temperature gradient during solidification, with a strong <001> growth tendency. As the amount of Mo added increases, the supercooling increases, resulting in an accelerated solidification rate, the grains in the middle area are refined, the number of grain boundaries increases, and the grain boundary energy between the fine grains is higher. The grains tend to grow in the direction of lower energy, that is, the temperature gradient direction, so the texture strength in the <001> direction increases.

2.4 Analysis of corrosion resistance of cladding layer

Figure 12 shows the electrochemical test results of cladding layers with different Mo contents in 3.5 wt.% NaCl solution. Figure 12(a) is the dynamic potential polarization curve, and Table 2 shows the self-corrosion potential and self-corrosion current density parameters of the cladding layer. As shown in Figure 12(a) and Table 2, with the increase of Mo content, the self-corrosion potential of the cladding layer gradually increases, and the corrosion current first decreases and then increases. The smaller the corrosion current, the slower the corrosion rate of the sample in the solution and the better the corrosion resistance[35]. When the Mo addition reaches 4%, the self-corrosion current density decreases from 8.253×10-6 A/cm2 to 4.540×10-7 A/cm2 compared with the group without addition. When the Mo addition reaches 6%, the self-corrosion current density begins to increase and the corrosion resistance deteriorates. This is related to the large number of incompletely melted Mo particles appearing in the cladding layer under the 6% Mo addition. These incompletely melted particles lead to the appearance of many Mo-enriched areas in the cladding layer. The increase in the Mo content in the local area will form a galvanic cell with other areas[36]. The Mo-enriched area acts as an anode, aggravating the corrosion of other areas, resulting in a decrease in the overall corrosion resistance of the cladding layer.

Figure 12(b) and (c) are the Bode diagram and Nyquist diagram of the cladding layer with different Mo contents. The impedance radius in the Nyquist diagram is an important parameter for evaluating the corrosion resistance of metal materials. The larger the impedance radius, the more difficult the electron transfer between the electrolyte and the material under test is, and the better the corrosion resistance of the material is[16]. As the Mo addition increases, the impedance radius of the cladding layer increases first and then decreases. When the Mo addition is 4%, the impedance radius of the sample is the largest, which is consistent with the results of the polarization curve analysis. The corrosion resistance is best when the Mo addition is 4%. The larger the phase angle and impedance modulus |Z| in the Bode diagram, the more dense the passivation film on the surface of the cladding layer[23,25], the stronger the penetration resistance of the material[37], and the better the corrosion resistance. In the low-frequency band and high-frequency band, the phase angle is the highest and the impedance modulus |Z| is higher when the Mo addition is 4%, so it has better corrosion resistance. In order to measure the capacitance of the passive film on the surface of the cladding sample, the equivalent circuit R(Q(R(QR))) is used to fit the EIS results. Figure 12(d) is the equivalent circuit model, where Rs is the resistance of the NaCl solution, R1 is the transfer charge resistance, R2 is the passive film resistance, Q1 is the capacitance of the double layer, and Q2 is the passive film capacitance. The stability of the passive film is generally determined by the passive film resistance R2. The smaller the passive film resistance, the easier it is for ions to diffuse through the passive film and cause damage[38]. Combined with the fitting data, it can be seen that with the increase of Mo addition, the passive resistance R2 of the cladding layer first increases and then decreases. When the Mo addition reaches 4%, the passivation resistance R2 is 8702 Ω·cm2, which is about 75% higher than that of the group without addition. This shows that the passive film formed on the surface of the cladding layer after the addition of Mo element is more compact[11], which effectively inhibits the contact between the corrosive medium and the material surface and the progress of the corrosion process.

Figure 13(a) shows the macroscopic morphology of the surface corrosion of the cladding layer under different Mo contents. As the Mo addition gradually increases, the number of pitting pits on the surface of the cladding layer decreases significantly. Figure 13(b) shows the microscopic morphology of pitting holes on the surface of the cladding layer. Combined with Figure 13(a), the number and size of pitting holes on the surface of the cladding layer
significantly decrease with the increase of Mo content, which is consistent with the results of the electrochemical test in Figure 12. The addition of Mo element further improves the corrosion resistance of the cladding layer. When the Mo addition reaches 4%, there are no obvious pitting pits on the surface of the cladding layer. When the Mo addition reaches 6%, it can be clearly seen in Figure 13(a)
that the surface of the cladding sample with 6% Mo addition has a local area that is more severely corroded. When the area is enlarged, a broken pitting pit can be seen, as shown in Figure 13(c). The pitting pits were scanned by EDS, and the results are shown in Figure 13(d). There is a large amount of Mo enrichment around the pitting pits, which is caused by the galvanic effect. The area enriched with Mo as an anode aggravates the corrosion of other areas [36,37], resulting in corrosion cracking.

Since the corrosion resistance test environment is a 3.5 wt.% NaCl solution simulating the marine environment, the Mo element has the following reaction during the passivation process of the 316L cladding layer: see formulas (1)-(3) in the figure.

Generally speaking, the excellent corrosion resistance of 316L stainless steel is due to its high Cr content and a small amount of Mo element. In the marine environment, Cr element often reacts with Fe and O elements during the passivation process to form Fe-Cr-O compounds to form a passivation film and cover its surface [40]. Mo element participates in the formation of the passivation film by generating a small amount of MoO2 and MoO(OH)2 compounds during the passivation process. Since the content of Mo in standard grade 316L stainless steel is relatively low, the passive film of 316L cladding layer is mainly composed of compounds composed of Fe-Cr-O. This passive film composed of Fe-Cr-O compounds is often difficult to resist the erosion of Cl- in the marine environment for a long time [6,8], and corrosion damage is still inevitable. However, when the Mo content in the cladding layer increases, the amount of MoO2 and MoO(OH)2 products generated in the passivation reaction increases, which leads to the change of the passive film composition of 316L stainless steel under Mo addition to the combined action of Fe-Cr-O, MoO2 and MoO(OH)2 compounds. The change of the corrosion resistance mechanism of 316L cladding layer is shown in Figure 14. The composite passive film formed after the Mo content increases has a stronger ability to resist Cl- erosion [41], and can resist the penetration of Cl- for a long time in the marine environment and slow down the corrosion process. In addition, the grains of 316L cladding layer are refined as the Mo content increases. Smaller grain size means larger grain boundary area and grain boundary density. The increase in grain boundary density helps to form a denser passivation film [23,25], which is more beneficial to resist the penetration of corrosive media.

3 Conclusions

1) When the Mo element addition amount does not exceed 4%, the phase of the cladding layer does not change. When the addition amount reaches 6%, the supercooling increases and changes the solidification conditions. Austenite transforms to martensite, and a large number of incompletely melted Mo particles appear in the cladding layer.

2) With the increase of Mo element content, the texture strength of the grains in the <001> direction decreases, and there is a tendency to grow in the <111> direction. The grains are refined, and the average grain size of the cladding layer decreases from 235.59 µm to 184.35 µm. Grain refinement leads to an increase in the number of grain boundaries, which helps to form a denser passivation film and improve corrosion resistance. In addition, the increase in the proportion of small-sized grains increases the grain boundary area, causing dislocations to accumulate inside the grains and increasing the dislocation density in the cladding layer.

3) With the increase in the Mo element content, the number of pitting pits decreases, and the corrosion resistance of the cladding layer is significantly improved. When the Mo addition amount is 4%, the self-corrosion current density decreases from 8.253×10-6 A/cm2 to 4.540×10-7 A/cm2 compared with the group without Mo powder addition, and the passivation resistance increases from 4927 Ω·cm2 to 8702 Ω·cm2, an increase of about 75%. The composition of the passivation film is transformed into the combined effect of Fe-Cr-O, MoO2 and MoO(OH)2 compounds. The composite passivation film has a stronger ability to resist Cl- erosion, further improving the corrosion resistance.