The Effect of Preheating Temperature on the Corrosion Resistance and Porosity Defects Development Behaviour of Ni60A Coating

Abstract

The laser cladding of nickel-based fusion alloys makes them prone to cracks and defects that affect the overall performance of the coating. In this study, Ni60A coatings were prepared at different preheating temperatures (25 °C, 200 °C, 400 °C and 600 °C). The effect of the preheating temperature of the substrate on the corrosion resistance of the coating as well as on the development of defects were investigated by electrochemical tests and immersion experiments in a 65 wt% H₂SO₄ solution at 60 °C. The results indicate that preheating the substrate to 200 °C can completely eliminate cracks in the coating and reduce porosity. Preheating leads to a decrease in the corrosion resistance of the coating. The size of the porosity defects is related to the law of longitudinal development of the defects. Porosity defects with diameters smaller than 100 μm have a more pronounced tendency to expand vertically than those with diameters larger than 100 μm.

1. Introduction

Laser cladding, as an economical surface modification technology, can prepare high-performance alloy coatings on inexpensive metal substrates, which is of great significance in reducing costs and saving precious metals [1,2]. Under the action of a high-energy laser beam, the powder alloy and the thin layer of the surface of the substrate are rapidly melted and solidified, causing the powder to form a coating on the surface of the substrate that is metallurgically bonded to the substrate, in order to achieve the purpose of improving the surface properties of the substrate. Nickel-based self-fusing alloys have received widespread attention for their excellent wear and corrosion resistance as well as high-temperature oxidation resistance [3,4,5].

During laser cladding, there is a significant temperature gradient between the coating and the substrate. An uneven distribution of the temperature field can lead to defects such as porosity and cracks in the prepared coatings [6,7,8]. In addition, the mismatch of thermo-physical properties between the substrate and the coating material is a major factor contributing to the formation of cracks [6]. Substrate preheating has received extensive attention in recent years as an effective means of eliminating cracks and reducing defects [5,6,7,8,9,10,11]. R. Jendrzejewski et al. performed calculations by establishing the temperature and stress fields generated during laser cladding. They attributed the cracking of the coating to the internal stresses generated during the coating preparation process exceeding the ultimate tensile strength of the coating material. The preheating temperature of the substrate has a significant effect on reducing the stresses generated in the coating [7]. Zhang et al. conducted multilayer laser cladding experiments on Q235 steel plates (2 mm) using Ni60A powder and investigated the effect of preheating temperature on the thin-wall part. Preheating the substrate during the laser cladding process increases the temperature of the molten pool and reduces the temperature gradient between the molten pool and the substrate. When the preheating temperature is lower than 400 °C, increasing the preheating temperature is conducive to reducing the residual stress, preventing the generation of cracks within the coating, increasing the hardness of the part [10]. Wu et al. investigated the organisation and wear behaviour of nickel-based alloy coatings prepared by plasma cladding on substrates with different preheating temperatures. The results of the study showed that the coating dilution rate gradually increased with the increase in preheating temperature [11].

To date, most studies have focused on the effect of preheating on coating hardness and abrasion resistance, while there is a lack of studies related to the effect of preheating on the corrosion resistance of coatings. Ni-Cr-B-Si nickel-based alloys are widely used as coating materials in coal-fired boilers, heat exchangers, turbines and mill rolls due to their excellent corrosion resistance [12]. This series of alloy powders also has great potential for application in marine engineering such as bearings, blades, valves and desalination units [13]. However, problems such as porosity defects and cracks on the surface of the nickel-based cladding prepared by laser cladding limit its development. Substrate preheating as a technique to address cladding cracking can lead to new problems such as increased dilution rates in the cladding. The effect of preheating on the corrosion resistance of coatings and the extension behaviour of coating defects in corrosion is unknown and needs to be studied in detail.

In this study, Ni60A coatings were prepared on a T91 substrate using a laser cladding technique at different preheating temperatures. Experiments were carried out in an environment simulating sulphuric acid dew point corrosion during sulphur fumigation of heat exchanger tubes in the sugar industry. The overall corrosion resistance of the coating and the development of defects during corrosion were investigated. The aim of this study is to improve the corrosion resistance of heat exchanger tubes in the sugar industry during sulphur fumigation by laser cladding, which is important for improving the service life of the heat exchanger tubes and reducing the production cost in the sugar industry.

2. Materials and Methods

2.1. Experimental Materials

T91 steel plates (50 mm × 25 mm × 5 mm) were employed as the substrates and their main composition is shown in

Table 1. Chemical composition of T91 steel substrate (wt%).

Ni	Si	Mn	Mo	Co	Cr	Fe
0.25	0.340	0.450	0.890	0.28	8.22	Bal.

. Ni60A powder (diameter 43–150 μm) was used for the coating material. The powder morphology is shown in Figure 1. Ni60A powder has uniform particles, good sphericity and less satellite powder.

Table 2. Chemical composition of Ni60A powder (wt%).

Cr	Fe	Si	B	C	O	Ni
16.58	4.63	4.11	3.02	0.76	0.03	Bal.

provides the chemical composition of Ni60A powder. Prior to laser cladding, the powder was dried in a drying oven for 1 h to ensure good powder flow.

2.2. Preparation of Laser Cladding Layers

Prior to laser cladding, the surface of the substrate was sanded with coarse sandpaper to remove surface oxides, followed by degreasing with acetone and dewatering with alcohol. After preheating the T91 substrate to a temperature slightly higher than the experimental temperature using a muffle furnace, the specimen was taken to the specimen table and the temperature of the specimen was measured by an infrared thermometer. The laser cladding work was carried out when the temperature of the specimen was reduced to the experimental temperature.

The laser cladding experimental equipment used the laser surface treatment system shown in Figure 2, including a robotic arm, laser, cladding head, protective gas, voltage regulator, cooling system and powder feeding system. The laser generator was an RFL-C6000 laser (Raycus, Wuhan, China) with a laser wavelength range of 1060–1100 nm and a maximum stabilised output power of 6000 W. The coaxial powder feeding method was used to prepare the laser cladding coatings. The laser cladding process parameters are shown in

Table 3. Laser cladding process parameters.

Laser Power (W)	Cladding Speed (mm/s)	Overlap (%)	Powder Feed Rate (g/min)	Shielding Gas Flow (L/min)
1500	15	50	7.9	9

.

2.3. Corrosion Experiments

The Ni60A laser cladding was processed to a size of 10 × 10 × 5 mm³. The samples were sealed with epoxy resin to ensure that galvanic corrosion would not occur between the coating and the substrate during the experiment. The coated surface was kept bare, with a test area of 10 × 10 mm². The specimens were smoothed using 1200 grit metallographic sandpaper. All specimens were degreased with acetone and dehydrated with alcohol prior to testing.

The experiments were carried out in a 65 wt% H₂SO₄ solution at 60 °C, a condition thought to cause severe corrosion in boilers and heat exchanger tubes [14,15,16].

The electrochemical testing was performed with a Gamry 1010E electrochemical workstation. A traditional three-electrode system was used, with a platinum electrode for the auxiliary electrode and a saturated calomel electrode for the reference electrode. The test solution was 65 wt% H₂SO₄ solution. The temperature of the solution was adjusted to 60 °C using a water bath before the corrosion experiments. The EIS was recorded at open circuit potential (OCP) over a frequency range from 10,000 Hz to 0.01 Hz. Potentiodynamic polarisation curve was measured by positively scanning the potential from −0.25 V (vs. OCP) with a sweep rate of 0.4 mV s⁻¹.

2.4. Analytical Test Methodology

The crack of the Ni60A coating was characterised by dye penetration test. The macroscopic morphology of the coating surface and cross-section was photographed by a 3D morphology instrument, and the coating porosity and dilution were measured by the in-built data processing tool.

The porosity equation was as follows:

$$\mathrm{P}=\frac{{\mathrm{A}}_{\mathrm{D}}}{{\mathrm{A}}_{\mathrm{M}}}$$

(1)

where A_M was the measured surface area of the coating, mm². A_D was the measured area of pores present on the coating surface, mm².

The dilution rate equation was as follows [17]:

$$\mathrm{D}=\frac{{\mathrm{A}}_{\mathrm{S}}}{{\mathrm{A}}_{\mathrm{S}}+{\mathrm{A}}_{\mathrm{C}}}$$

(2)

where A_S was the melting area of the substrate cross-section, mm². A_C was the coating cross-sectional area, mm².

Fresh aqua regia solution (HCl:HNO₃ = 3:1) was used to etch the clad specimens in order to observe the microstructure. The microstructure of the coating was observed using a metallurgical microscope. The corrosion morphology of the coating was observed by scanning electron microscopy (SEM, EVO 10, Zeiss, Oberkochen, Germany). The elemental composition of the coating was analysed by means of energy dispersive spectroscopy (EDS). The phase composition was identified by X-ray diffraction (XRD, D8 Discover, Bruker, Billerica, MA, USA) using Cu Kα diffraction (λ = 1.54056 Å) with a scanning angle which ranged from 10° to 90° and scanning speed of 6°/min. The corrosion product composition was analysed by X-ray photoelectron spectroscopy (XPS, Nexsa X, Waltham, MA, USA).

In order to study the defect depth expansion behaviour during corrosion, the morphology of more than 160 porosity defects before and after corrosion was tested using a 3D profilometer. HCL solution (15 mL of hydrochloric acid (HCL, ρ = 1.19 g/mL) and deionised water to make 100 mL solution) was used to remove the corrosion product film, according to BS ISO 8407-2009 standard [18].

3. Results and Discussion

3.1. Coating Morphology and Microstructure

The macroscopic morphology of Ni60A coatings prepared by laser cladding at different preheating temperatures was obtained through dye penetration testing, as shown in Figure 3. The number of cracks in the coating decreases as the preheating temperature increases for the same laser cladding process parameters. When the preheating temperature reached 200 °C, the crack disappeared. During the laser cladding process, the powder and the surface of the substrate absorbed the laser energy and melted to form a molten pool. Due to the temperature difference between the molten pool and the substrate, energy was conducted from the molten pool region to the substrate [5]. After the substrate had been preheated, the overall temperature of the specimen rose and the temperature gradient between the melt pool and the substrate decreased [9,19]. The reduction in the temperature gradient facilitates the reduction in internal stresses in the laser cladding layer [8,9]. However, as the preheating temperature was further increased, the coating obtained by preheating at 600 °C showed obvious defects.

The results of dilution rate obtained statistically for Ni60A coatings prepared under different preheating temperature conditions are shown in Figure 4. The dilution rate of Ni60A coatings prepared at 25 °C, 200 °C, 400 °C and 600 °C was 20.60%, 21.01%, 23.27%, and 33.74%, in that order. The coating dilution rate increased as the substrate preheating temperature increased. Especially when the preheating temperature was increased from 400 °C to 600 °C, there was a significant change in the coating dilution rate, which increased by 10.47%. Preheating increases the overall temperature of the substrate and reduces its reflectivity. This facilitates the absorption of laser energy by the substrate for conversion to a molten state, expanding the extent of the melt pool and leading to increased coating dilution.

Figure 5 shows the coating morphology and EDS line scan results at the cross-section bonding line, which focuses on the distribution pattern of Cr, Fe and Ni elements. All elements show gradient changes near the bonding line between the coating and the substrate, with the elements Fe and Ni being the most evident. As the preheating temperature increases, the intensity of the Fe elemental signal rises, and the gradient of the elemental change at the binding line gradually slows down and the range of the gradient expands. The increase in preheating temperature expanded the melt pool range, and at the same time, the decrease in the temperature difference between the melt pool and the substrate reduced the cooling rate of the melt pool and prolonged the solidification time of the melt pool. The metallurgical fusion between the alloy powder and the molten pool was more adequate, which promoted the diffusion of elements between the cladding and the substrate. There are opposite undulating fluctuations in Ni and Cr signal intensities on the scan line, which was due to the enrichment of Ni and Cr at different locations [9]. Columnar crystals at the coating bonding line became smaller with increasing preheating temperature, and the development of equiaxial crystals became more significant.

Figure 6 shows the XRD patterns of Ni60A coatings prepared at different preheating temperatures. The composition of the phases in the coatings mainly include γ-Ni, Ni₃Fe, NiFe, M₂₃C₆ (M = Cr, Fe), CrB and Ni₃Si. The standard diffraction peaks of γ-Ni (2θ = 44.369°, PDF#01-1258), Ni₃Fe (2θ = 44.216°, PDF#88-1715) and NiFe (2θ = 44.141°, PDF#03-1109) were compared with the maximum diffraction peaks in coatings with different preheating temperatures. Although the preheating of the substrate does not change the composition of the phases, the increase in preheating temperature causes the diffusion of Fe elements into the coating, resulting in a subsequent increase in the fraction of Fe-containing phases in the coating. The XRD diffraction peaks of the coatings tended to move closer to the diffraction peaks of the Fe-containing phase as the preheating temperature increased.

3.2. Effect of Preheating Temperature on the Corrosion Resistance of Coatings

The surface morphology of the coatings prepared with different preheating temperatures after immersion in 65 wt% H₂SO₄ solution at 60 °C for 120 h is shown in Figure 7. The coatings all exhibit intergranular corrosion and pitting. The eutectic at the grain boundaries corrodes preferentially and progresses to the interior of the crystal, with only a few eutectics remaining. The pitting of crystals may be caused by the dissolution of Fe compounds in the sulphuric acid solution. The increased preheating temperature of the substrate coarsens the crystals and increases intergranular corrosion.

The SEM morphology and EDS surface scanning results before and after corrosion of the coatings prepared by preheating the substrate at 200 °C and 400 °C are shown in Figure 8.

Table 4. Elemental differences within crystals and at grain boundaries (wt%).

Site	Ni	Cr
P1	76.74	23.26
P2	92.01	7.99
P3	98.13	1.87
P4	99.66	0.34

shows the EDS results for the labelled points. In the Ni60A coating, there was a significant enrichment of Cr elements at the grain boundary locations, while the content of Ni elements at the grain boundary locations was significantly lower than that inside the crystals. A significant decrease in Cr content occurred at the grain boundaries and at the intergranular level after the corrosion of the coatings. Combined with the morphology seen in Figure 8, it can be assumed that the coating will corrode preferentially in 65 wt% H₂SO₄ solution at 60 °C for the Cr element.

Figure 9 shows the electrochemical experimental results of Ni60A coatings prepared at different preheating temperatures in 65 wt% H₂SO₄ solution at 60 °C. The higher the preheating temperature, the smaller the diameter of the coated EIS capacitor semicircle. The higher the preheating temperature, the radius of the capacitive arc and the coating’s self-corrosion potential shifted negatively with the increase in preheating temperature. This indicates that the corrosion resistance of the coating decreases as the preheating temperature increases. Preheating the substrate increased the dilution rate of the coating, which may lead to a decrease in the corrosion resistance of the coating. Figure 9b shows that no passivation of the Ni60A coating occurred. Although there is a “plateau phase” in the anode polarisation curve, the current density exceeds 10 mV. The higher anodic current density in the “plateau phase” of the coating with the substrate not preheated compared to the coating prepared after substrate preheating may be attributed to the presence of cracks in the coating that increase the reaction area.

Figure 10 shows the XPS patterns of the coating prepared by preheating at 200 °C after immersion in 65 wt% H₂SO₄ solution at 60 °C for 120 h. The measured values of intensity are denominated by black lines, with the simulated patterns marked by red lines. The XPS peaks of Ni 2p3/2, Cr 2p3/2, S 2p and O 1s were fitted and a total of six chemicals including Ni, NiO, NiSO₄, Cr, Cr₂O₃ and Cr₂(SO₄)₃ were detected. The binding energies in the Ni 2p3/2 spectra correspond to metal Ni (852.6 eV) [20], NiSO₄ (856.8 eV) [21], NiO (864.1 eV) [22] and a Ni satellite peak(858.0 eV) [23]. The binding energies in the Cr 2p3/2 spectra correspond to the metals Cr (574.2 eV) [24], Cr₂(SO₄)₃ (578.6 eV) [25] and Cr₂O₃ (580.1 eV) [26]. The binding energies in the O 1s spectra correspond to Cr₂O₃ (531.1 eV) [27], NiO (531.0 eV) [28], NiSO₄ (532.2 eV) [29] and Cr₂(SO₄)₃ (532.1 eV) [30].

Numerous studies have shown that the corrosion of nickel–chromium alloys in dilute sulphuric acid produces a dense inner layer of NiO and Cr₂O₃ as well as a loose outer-layer of Ni(OH)₂ and Cr(OH)₃ on the surface of the material, where the dense inner layer acts as a protective layer [31,32,33]. However, in more concentrated sulphuric acid solutions, NiO and Cr₂O₃ further react with sulphuric acid to form NiSO₄ and Cr₂(SO₄)₃ [34,35]. Compared to Cr₂(SO₄)₃, NiSO₄ has a low solubility in H₂SO₄ solution and is more easily deposited on the coating surface [36]. The reaction equations are as follows:

$$\mathrm{N}\mathrm{i}+{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\to \mathrm{N}\mathrm{i}\mathrm{O}+{\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_3$$

(3)

$$2\mathrm{C}\mathrm{r}+{3\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\to {\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_3+{3\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_3$$

(4)

$$\mathrm{N}\mathrm{i}\mathrm{O}+{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\to \mathrm{N}\mathrm{i}{\mathrm{S}\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}$$

(5)

$${\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_3+{3\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4\to {\mathrm{C}\mathrm{r}}_2{{(\mathrm{S}\mathrm{O}}_4)}_3+{3\mathrm{H}}_2\mathrm{O}$$

(6)

$${\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_3\leftrightarrow {\mathrm{H}}_2\mathrm{O}+{\mathrm{S}\mathrm{O}}_2\uparrow$$

(7)

NiO and Cr₂O₃ were first generated on the surface of the Ni60A coating at 60 °C in 65 wt% H₂SO₄ solution. NiO and Cr₂O₃ are further converted to NiSO₄ and Cr₂(SO₄)₃ in concentrated H₂SO₄ solution. NiSO₄ has a lower solubility than Cr₂(SO₄)₃ in concentrated H₂SO₄ solution, which can form a corrosion product film more easily on the surface of the coating, thus providing better protection for the surface of the coating.

3.3. Corrosion Behaviour of Porosity Defects

Figure 11 shows the porosity of the coatings prepared at different preheating temperatures. The porosity showed a decreasing and then increasing trend with the increase in preheating temperature. The lowest porosity was 2.12% for the coating prepared by preheating the substrate at 200 °C. Laser melting coatings with a porosity of less than 2%–3% are generally considered to be low-porosity coatings, so a coating with a porosity of 2.12% obtained by preheating at 200 °C is acceptable [37,38,39]. During molten pool solidification, tiny bubbles inside the molten pool are affected by the Marangoni driven flow and overcome the effect of gravity to move towards higher temperatures in the direction of the temperature gradient [40]. Preheating of the substrate extends the solidification time of the melt pool, allowing bubbles to escape and porosity to decrease. However, when the preheating temperature is further increased to 600 °C, the viscosity of the melt pool decreases, which is more favourable for the entry of protective gases into the melt pool [39,41]. The number of tiny bubbles entrapped inside the melt pool increases, and the bubbles that have not yet escaped from the molten pool after solidification form porosity defects inside the coating, leading to an increase in the porosity of the coating.

Figure 12 shows the initial dimensions of porosity defects and their statistical results for random statistics on the surface of coatings prepared with different preheating temperatures. The thickness of the coatings prepared in this study was about 1.1 mm, so defects on the surface of the coating would not penetrate the coating. The porosity defects’ diameters are mainly distributed between 50 and 150 μm. The proportion of porosity defects with diameters larger than 150 μm was significantly increased on claddings prepared at substrate preheating temperatures up to 400 °C and 600 °C, reaching 32% and 44%, respectively. The increase in the preheating temperature of the substrate leads to a higher average temperature of the melt pool during the melting process and a longer solidification time. It is more favourable for the tiny bubbles in the molten pool to fuse with each other and form large-diameter porosity defects in the coating after the molten pool solidifies. Therefore, the increase in the preheating temperature of the substrate increases the percentage of large-diameter defects in the coating.

Figure 13 shows the difference in morphology before and after the corrosion of porosity defects in coatings prepared at different preheating temperatures. The change in the diameter of porosity defects in coatings prepared by preheating the substrate at 400 °C and 600 °C were more pronounced when the initial diameter of the defects was less than 100 μm. The change in the depth of the porosity defects in coatings prepared by preheating the substrate at 400 °C and 600 °C were more pronounced when the initial diameter of the porosity defects was greater than 100 μm. Overall, porosity defects with initial diameters less than 100 μm have a more pronounced tendency to expand both laterally and vertically than porosity defects with initial diameters greater than 100 μm. Figure 14 shows the development of porosity defects in the coatings prepared at different preheating temperatures after 120 h of immersion in 65 wt% H₂SO₄ solution at 60 °C. More than 45 porosity defects were counted for each preheating temperature condition and standard deviations (SD) and coefficients of variation (COV) were calculated. Porosity defects with initial diameters less than 100 μm and more than 100 μm have different transverse and longitudinal extension behaviours. The variation in the width of the porosity defects in the coatings prepared by the substrate without preheating and with preheating at 200 °C is close to each other. The variation in the width of the porosity defects in the coatings prepared by preheating the substrate at 400 °C and 600 °C was more pronounced. The variation in the width of the porosity defects in the coatings prepared by preheating the substrate at 400 °C and 600 °C was more pronounced. Figure 14b shows that the variation in the depth of the pore defects within an initial diameter of less than 100 μm is more significant than that of the pore defects greater than 100 μm. The variation in the depth of porosity defects in the coating with a diameter of more than 100 μm is strongly influenced by the preheating temperature of the substrate. The longitudinal extension behaviour of the porosity defects in the prepared coatings was more pronounced when the preheating temperature of the substrate exceeded 200 °C.

4. Conclusions

This paper studies the effect of substrate preheating temperature on the surface morphology, microstructure, and corrosion resistance of Ni60A coatings. The conclusions are as follows:

(1)

Preheating the substrate can effectively eliminate cracks in the coating. When the preheating temperature of the substrate reached 200 °C, the prepared coatings were free of cracks.

(2)

Preheating of the substrate leads to an increase in the dilution rate of the prepared coating, which promotes the mutual diffusion of the elements of the coating and the substrate, which in turn leads to a decrease in the corrosion resistance of the coating.

(3)

NiO and Cr₂O₃ generated on the surface of the Ni60A coating in a 65 wt% H₂SO₄ solution at 60 °C will further react with sulphuric acid to form NiSO₄ and Cr₂(SO₄)₃ in this environment.

(4)

Preheating the substrate affects the size of the coating porosity defects. The size of a porosity defect is related to its corrosion law. Porosity defects with diameters less than 100 μm have more pronounced transverse and longitudinal extension behaviour than those with diameters more than 100 μm. The percentage of porosity defects in coatings with diameters exceeding 150 μm increases with increasing preheating temperatures.

(5)

The coatings prepared by preheating the substrate at 200 °C were free of cracks and had the lowest porosity. Similar dilution rates and corrosion resistance compared to coatings prepared without preheating of the substrate. Substrate preheating of 200 °C is the optimum preheating temperature for the preparation of Ni60A coatings.