Microstructure and corrosion resistance of Ni-based alloy laser coatings with nanosize CeO₂ addition

Abstract

Micron-size Ni-base alloy (NBA) powders were mixed with both 1.5 wt.% (hereinafter %) micron-size CeO₂ (m-CeO₂) and also 1.5% and 3.0% nano-size CeO₂ (n- CeO₂) powders. These mixtures were coated on low-carbon steel (Q235) by 2.0 kW CO₂ laser cladding. The effects on the microstructures, phases and electrochemical corrosion of the coatings upon the addition of m- and n- CeO₂ powders to NBA (m- and n- CeO₂ /NBA) have been investigated. The results showed that a smooth coating was prepared under suitable processing parameters (P= 2.0 kW, V= 180 mm min^{- 1}) by adding 1.5% n- CeO₂. In addition to the primary phases of γ-Ni, Cr₂₃C₆ and Ni₃B in the Ni-base alloy coating, CeNi₃ was formed in Ni-base alloy coatings with both n- CeO₂ and m-CeO₂ particles, and CeNi₅ appeared in the coating upon decreasing the size of CeO₂ particles. Well-developed dendrites were observed in the Ni-base alloy coating; directional dendrites grew at the interface in the coating upon the addition of m-CeO₂, whereas fine and multioriented dendrites grew upon decreasing the size of CeO₂ particles to the nanoscale. Actinomorphic dendrites and compact equiaxed dendrites grew from the interface to near the surface upon increasing the content of n- CeO₂ from 1.5 to 3.0%. In strongly acidic HNO₃ solution, the severe corrosion of dendrites occurred and there were many corrosion pits in the Ni-base alloy coating; intercrystalline corrosion also has a dominant role upon the addition of m-CeO₂, whereas uniform corrosion occurs in the coating as the size of CeO₂ particles is decreased to nanoscale.

Introduction

As an advanced surface modification technique, laser cladding has attracted interest in the research and application of new synthesized materials and in surface repair [1]. Laser technology used for the formation of protective coatings has gained popularity in both industry and academic research in recent years. In particular, laser cladding is a relatively new process in which alloy powders of a desired composition and a thin surface layer of a substrate material are first melted together under laser irradiation and then rapidly solidified to form a dense coating with metallurgical bonding to the base material and minimum dilution of the cladding layer [2]. Nanostructured composite coatings usually exhibit enhanced mechanical, anticorrosion and antioxidation properties compared with pure metal coatings as well as composite coatings containing micronsize particles. The improvement of these properties mainly depends on the size and percentage of the particles co-deposited as well as on the distribution of the particles in the metallic matrix. Thus, there is now a trend of using nanosized particles as an additional ingredient of composite coatings to develop enhanced mechanical, wear and corrosion properties [3, 4]. Ni-base alloys (NBA), combined with other particles, are widely used in applications where wear resistance combined with oxidation and corrosion resistance at high temperatures is required [5]. The addition of rare-earth (RE) compounds in metals realizes multiple functions, such as purification, modification and alloying, and thus can improve a range of properties of metals to various extents. An area of current interest is the application of RE compounds to surface engineering to achieve properties such as corrosion resistance [6]. However, only limited studies have been carried out on the modification of the microstructure and corrosion resistance in laser cladding resulting from the addition of RE compounds [7]. Some studies on the effect of RE oxides on the corrosion resistance of Ni-base alloy coatings in weak acid solution were carried out by Han and Lu [8] and Wang et al [9]. It is well known that Ni-base alloy possesses excellent corrosion resistance in most corrosive media except in the strongly acidic HNO₃. Thus, it is necessary to further improve the corrosion resistance of Ni-base alloy in strong acid.

In this work, CeO₂ was used as the RE oxide because of its resistance to corrosion and oxidation [10]. CeO₂ /Ni-base alloy composite coatings were prepared on Q235 low-carbon steel by laser cladding. The surface morphologies, phases, microstructure and corrosion resistance of micronsize and nanosize (m- and n-) CeO₂ /NBA composite coatings were compared. In particular, the distribution of n- CeO₂ and the effects of 1.5 wt.% (hereinafter %) and 3.0% n- CeO₂ on the microstructure and corrosion resistance of the coatings were analyzed.

Experimental procedure

Preparation of samples

The Q235 substrate had dimensions of 80 × 80 × 10 mm³. NBA powder (NiSP475, diameter 90–130 μ m, melting point 1100 °C), whose chemical composition is shown in table 1, and 1.5% m-CeO₂ (diameter 20–50 μ m, melting point 1950 °C) [11] were mixed with ethanol in a carnelian bowl and stirred well for 40 min. In addition, NBA powder and 1.5 and 3.0% n- CeO₂ (diameter 20–40 nm) were mixed with ethanol by planetary milling for 4 h. After milling to a uniform particle size, the mixed powder was placed onto a Q235 plate to form a 1.0–1.2-mm-thick powder bed. To remove moisture, the samples were dried in an oven at 393 K for 2 h before laser cladding. As shown in figure 1, NBA powder obtained by mechanical milling is more uniform, smaller and denser than that obtained by manual grinding. The grain sizes are 90–129 μ m after manual grinding and 55–94 μ m after mechanical milling.

Table 1.

Chemical composition of Q235 steel and Ni-base alloy powder (wt.%).

Element	C	Mn	Si	S	P	Cr	B	Ni	Fe
Q235	0.17	0.08	0.37	0.039	0.036	–	–	–	Bal.
NiSP475	0.9	–	4.3	–	–	16.3	3.3	Bal.	4.2

Figure 1.

Distributions of NBA powder (a) before and (b) after mechanical milling.

Preparation of coatings

A 5 kW TJ-HL-T 5000 continuous-wave CO₂ laser system was used for cladding. The laser beam, which had a top-hat energy distribution in the focal plane, was irradiated from 10 mm above the substrate surface at atmospheric pressure. The laser beam spot was elliptical with size 6 × 3 mm. Argon (0.1 MPa pressure and 5 litres min^{- 1} flow rate) was used for shrouding the molten region to prevent oxidation during the process. By moving the substrate, we covered it with a molten layer, which formed a solid layer. The laser power was turned off for 30 s at the end of each pass before lasing at a new starting position. The laser beam tracks overlapped each other by 40%. The cladding parameters were 2.0 kW laser power, 180 mm min^{- 1} scanning speed, 110 W mm^{- 2} power density and 2 s laser irradiation on each spot.

Investigation of microstructures and corrosion resistance of coatings

The microstructures and phases of the coatings were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and x-ray diffraction analysis (XRD). The electrochemical corrosion of the coatings was measured by a CHI604b electrochemical instrument. The corrosion mechanism of the coatings was discussed on the basis of the results of SEM with energy-dispersive x-ray spectroscopy (EDS).

Results and discussion

Surface morphologies of the coatings

A coarse NBA coating surface was obtained without m- and n- CeO₂ addition when the laser power was less than 2.0 kW and the scanning speed was 240 mm min^{- 1} since a low laser energy density resulted in incompletely melted NBA powder, as shown in figure 2(a). For the same parameters as above, a smooth and level n- CeO₂ /NBA coating that was defect-free (no cracks, gas holes, etc) was obtained, as shown in figure 2(b). This implies that the absorptivity of laser energy of NBA powder is improved by adding n- CeO₂ [12]. In contrast with the coating surface obtained by adding n- CeO₂, many cinder inclusions and a layer of green oxidation film formed on the surface of m-CeO₂ /NBA, as shown in figure 2(c).

Figure 2.

Surface morphologies of the coatings; (a) NBA, (b) 1.5% n-CeO₂ /NBA and (c) 1.5% m-CeO₂ /NBA.

Phases of the coatings

As shown in figure 3, the primary phases were γ-Ni, Cr₂₃C₆ and Ni₃B in NBA coatings. In addition to the primary phases, CeNi₃ existed in m- and n- CeO₂ /NBA coatings and small peaks of CeNi₅ appeared in only the n- CeO₂ /NBA coating. CeNi₃ and CeNi₅, as novel phases, first appear in the rapidly solidifying coating. Thus, the forming mechanism of CeNi₃ and CeNi₅ phases has not been reported yet. According to Du et al's study on the thermodynamic properties of the Ce–Ni system [13], the CeNi₅ phase is formed by the invariant reaction of liquid→CeNi₅ when the temperature is more than 1613 K.

Figure 3.

XRD patterns; (a) NBA coating, (b) 1.5% m-CeO₂ /NBA coating and (c) 1.5% n- CeO₂ /NBA coating.

Microstructure of the coatings

As shown in figure 4, the interface between the coating and the substrate was clearly observed. A bright and thin planar bonding was grown at the bottom of the melting pool. The width of this planar layer in m- and n-CeO₂ /NBA (2–4 μ m) was less than that of the NBA coatings (6–8 μ m) without CeO₂ addition, which indicates that the addition of CeO₂ can have the beneficial effect of maintaining the chemical composition and properties of the cladding materials. Also, the dilution of the cladding material on the substrate is evidently decreased, which is very important for laser cladding [14]. M- and n-CeO₂ /NBA coatings had a similar micromorphology exhibiting a primary phase of γ-Ni dendrites and a eutectic dendrite microstructure containing γ-Ni and Cr₂₃C₆ phases. In addition, different micromorphologies were observed in figure 4. A well-developed dendrite morphology was observed in the NBA coating; directional dendrites grew at the interface in the 1.5% m-CeO₂ /NBA coating, whereas fine and multioriented dendrites grew upon the addition of 1.5% n-CeO₂. Actinomorphic dendrites and compact equiaxed dendrites grew from the interface to near the surface in the 3.0% n-CeO₂ /NBA coating.

Figure 4.

SEM morphologies of the coatings; (a) NBA, (b) 1.5% n- CeO₂ /NBA, (c) 3.0% n- CeO₂ /NBA and (d) 1.5% m-CeO₂ /NBA.

The reason for the micromorphology change in the NBA coating by the addition of m-CeO₂ is the function of the fine-grained RE oxide since CeO₂ decomposes to a Ce ion at a higher temperature than the decomposition temperature of 2050 °C [15]. In addition, Ce is enriched as an inner adsorption element mainly at the grain boundaries, which decreases the interfacial energy and free energy of the NBA, thus reducing the rate of dendrite growth [15]. In the n-CeO₂ /NBA coating, the growth of dendrites is restricted by the formation of more nuclei since the absorptivity of NBA powder to laser energy is improved by adding n- CeO₂, which results in more compact and multiorientation growth of the dendrites [16].

As shown in figure 5(a), compact equiaxed dendrites were observed in the 3.0% n- CeO₂ /NBA coating near the top surface. Figure 5(b) shows an SEM backscattered electron image of zone α in figure 5(a). The results of EDS microanalysis at different regions in figure 5(b), such as A (dark dot in grain), B (dark dot at grain boundary), C (white microstructure in grain) and D (black microstructure in grain), are shown in table 2. Figure 5(c) shows the SEM backscattered electron image corresponding to the microstructure in figure 5(b) between zone A and zone C. The result of the EDS microanalysis of zone E (dark dot at grain boundary) is shown in figure 5(d). It is shown that cerium compounds act as heterogeneous nucleation sites and are uniformly distributed among the dendrites and at the grain boundary of the n- CeO₂ /NBA coating. It is also concluded that n- CeO₂ has an adsorbent effect among grains and a sticking effect at grain boundaries [17].

Figure 5.

Microstructure of 3.0% n-CeO₂ /NBA coating; (a) OM, (b) and (c) SEM and (d) EDS.

Table 2.

EDS results of figure 5(b).

Analyzed	O	Si	Ce	Cr	Fe	Ni
zone
A	4.26	4.55	0.97	9.00	11.33	67.96
B	6.06	0.60	1.56	64.62	5.03	7.31
C	2.36	4.36	–	6.76	16.61	67.04
D	0.96	0.07	–	62.74	3.69	12.59

Figure 6 shows the surface distribution of Ce element in the 1.5% n- CeO₂ /NBA coating. When the laser power is 2.0 kW and the scanning velocity is 180 mm min^{- 1}, multiple tracks coated with a homogeneous distribution of Ce element were obtained by ball grinding the mixture of n- CeO₂ and NBA powder.

Figure 6.

Surface distribution of Ce element in 1.5% n-CeO₂ /NBA; (a) micrograph of overlapping cladding and (b) Ce element: (white dots).

Corrosion resistance of CeO₂/NBA coatings

Potentiodynamic curves of the coatings in 6 M HNO₃ solution at room temperature are displayed in figure 7. It can be seen that there are no active-to-passive transition peaks for the coatings, indicating that the NBA cladding layers were passivated spontaneously in the solution. The NBA coating without added CeO₂ (curve A in figure 7) exhibited a rapid increase in current density (Icorr) with increasing corrosion potential (Ecorr) in 6 M HNO₃ solution. Also, the corrosion potential of the NBA coating in 6 M HNO₃ solution was greater than that of CeO₂ /NBA coatings. The corrosion current density was determined by the Tafel extrapolation method from the potentiodynamic curves in figure 7.

Figure 7.

Potentiodynamic curves of the coatings in 6 M HNO₃ solution.

The linear portion of the plot of anodic or cathodic polarization potential versus log (current) was extrapolated to intersect the potential line. The value of either the anodic or the cathodic current at the intersection was Icorr in the Tafel plot. Table 3 lists Ecorr (mv) Icorr (μ A cm^{- 2}) for the coatings. From the table, the corrosion potential of 1.5% n- CeO₂ /NBA was the highest. However, its corrosion current density was the lowest. In addition, the corrosion current density of 3.0% n- CeO₂ /NBA was much higher than that of 1.5% m-CeO₂ /NBA. Thus, it is shown that the corrosion resistance of NBA coatings is greatly improved by adding a suitable amount of n- CeO₂ (1.5%), which indicates that the selection of a suitable content of added n- CeO₂ is important for improving the coating properties. Nanoparticles easily amalgamate because of the surface effect, which is not well distributed during mixing and amalgamation in the melting pool. This would be a serious problem with increasing amount of nanoparticles, and would be disadvantageous for corrosion resistance. As a result, the corrosion resistance of 1.5% n- CeO₂ /NBA was greater than that of 1.5% m-CeO₂ or 3.0% n- CeO₂ /NBA.

Table 3.

Potentiodynamic curve parameters of the coatings.

Cladding material	NBA	1.5% m-CeO2 /NBA	1.5% n- CeO2 /NBA	3.0% n- CeO2 /NBA
Ecorr (mV)	683.7	708.7	715.3	691.8
Icorr (μ A cm- 2)	58.9	31.6	19.9	33.1

To discuss the effects of m-CeO₂ and n- CeO₂ on corrosion resistance, the electrochemistry corrosion morphologies and EDS results of the NBA coating and 1.5% m- and n- CeO₂ /NBA coatings are shown in figure 8 and table 4. There are many factors affecting the corrosion behavior of coatings, such as chemical composition including the oxygen content, microstructure and defects (pores and inclusions). Severe corrosion of the dendrites occurred with many corrosion pits in the NBA coating without added CeO₂. In contrast with the NBA coating, a few dendrites were still observed in m-CeO₂ /NBA. Also, the dendrites of n- CeO₂ /NBA were compact and clear.

Figure 8.

Electrochemical corrosion morphology and EDS results of the coatings; (a) NBA, (b) 1.5% m-CeO₂ /NBA and (c) 1.5% n-CeO₂ NBA.

Table 4.

Energy spectrum results for A in figure 8 (wt.%).

Element	O	Cr	Ni	Si	Fe
NBA	10.28	8.60	50.36	26.10	4.46
1.5% m-CeO2 /NBA	12.31	13.52	46.60	20.32	7.25
1.5% n- CeO2 /NBA	20.32	26.10	25.08	23.20	5.30

By comparing and analyzing the EDS results in figure 8, a large amount of Si element was found to exist at area A of both the NBA coating and the m-CeO₂ /NBA coating, which indicates that area A formed part of a grain boundary because of the effective segregation of Si at the grain boundary. Thus, at the grain boundary, the relative content of Ni element in the NBA and m-CeO₂ /NBA coatings is much larger than that in n- CeO₂ /NBA, but the relative content of Cr element is smaller. That is to say, the relative content of Ni element in the NBA and m-CeO₂ /NBA coatings sharply decreases between the dendrites of the coatings, which shows that the mainly preferential corrosion occurs at dendrites in the NBA and m-CeO₂ /NBA coatings. Thus, it is demonstrated that intercrystalline corrosion occurs both in the NBA and the m-CeO₂ /NBA coatings. According to the above analysis and the corrosion morphology shown in figure 8(c), uniform corrosion plays a leading role during the erosion of the n- CeO₂ /NBA coating. As a result, in the corrosive strongly acidic solution, the corrosion resistance of the NBA coating was greatly improved by adding 1.5% n- CeO₂.

The NBA coating possesses many active areas where the corrosion potential value is unequal. Thus, many microcells are created, which results in electrochemical corrosion readily occurring. The transfer of oxygen is also obstructed by the generated passivation film. Thus, the transfer of electrons is blocked, which inhibits the reduction at the cathode. During erosion, the cathodic reaction rate determines the entire reaction rate. Therefore, once the cathodic reaction rate decreases, the entire rate of coating corrosion is thought to decrease. CeO₂, as an active oxide, accelerates the passivation of the anode. Meanwhile, CeO₂ improves the surface state of the coatings, reducing the active area and resulting in greatly improved corrosion resistance [10].

In this work, the corrosion resistance of the 1.5% n- CeO₂ /NBA was greatest, the reasons for which are as follows: (i) Nanoparticles, which were well distributed among the dendrites and at the grain boundary in the coatings, reduce the diffusion velocity of hydrogen. The corrosion reaction of NBA was hydrogen depolarization in an acidic environment. Ni was oxidized to Ni^{2 +} at the anode, whereas H⁺ was deoxidized to H₂ at the cathode. Ce^{4 +} can act as a barrier to the permeation of H₂, which inhibited the infiltration of hydrogen in the acidic solution and reduced the mobility of hydrogen. Thus, the activation coefficient of hydrogen was reduced and the rate of reaction of the cathode was decreased [18, 19]. Thus, the hydrogen is confined and its activity is reduced by the addition of n- CeO₂, which increases the rate of cathodic polarization and decreases the rate of the cathodic reaction. (ii) Defects of the coating at the boundary were reduced because of the uniform distribution of n- CeO₂ [20]. Thus, the corrosion ratio and the rate of corrosion along the grain boundary decrease, which contribute to harmonization of the corrosion potential.

Conclusions

• A smooth coating of NBA was prepared using suitable processing parameters (P= 2.0 kW, V= 180 mm min^{- 1}) by adding 1.5% n- CeO₂. A homogeneous distribution of Ce element was obtained by mechanically grinding a mixture of n- CeO₂ and NBA powders. Also, cerium-nickel compounds were uniformly distributed among the dendrites and at grain boundaries in n- CeO₂ /NBA.

• In addition to the primary phases of γ-Ni, Cr₂₃C₆ and Ni₃B in NBA coatings, CeNi₃ was formed in NBA coatings with both m- and n- CeO₂ particles, and CeNi₅ appeared in the coating with decreasing size of CeO₂ particles.

• Well-developed dendrites were observed in the NBA coating; directional dendrites grew at the interface in the coating upon the addition of m-CeO₂, whereas fine and multioriented dendrites grew by decreasing the size of CeO₂ particles to nanoscale. Actinomorphic dendrites and compact equiaxed dendrites grew from the interface to the near surface upon increasing the content of n- CeO₂ from 1.5 to 3.0%.

• In strongly acidic HNO₃ solution, the severe corrosion of dendrites occurred and there were many corrosion pits in the NBA coating. Also, intercrystalline corrosion still played a dominant role upon the addition of m-CeO₂, whereas uniform corrosion of the coating occurred upon decreasing the size of the CeO₂ particles to nanoscale.