Effect of Cr on Corrosion Behavior of Laser Cladding Ni-Cr-Mo Alloy Coatings in Sulfuric Acid Dew Point Corrosion Environment

Abstract

Ni-Cr-Mo alloy coatings with different content of Cr element were produced on carbon structural steel by laser cladding. Coatings exhibited similar phase composition containing γ-Ni solid solution and similar microstructure composed of primary grains and eutectics with segregated Mo. Corrosion behavior of coatings in a sulfuric acid dew point corrosion environment were investigated through the combination of immersion test, electrochemical measurements, and surface analysis. Coatings exhibited similar passive behavior and passive film with a bilayer structure consisting of a compact inner layer and a porous outer layer. An increase in Cr resulted in a more protective passive film with thicker and more compacting inner layer. However, the corrosion resistance was firstly degraded as the content of Cr increased from 18 wt.% to 22 wt.%, and then improved as the content of Cr increased from 22 wt.% to 26 wt.%. This phenomenon was attributed to the competition effect between the increase in Cr and relative decrease in Ni.

1. Introduction

It is well known that Ni-Cr-Mo alloys, such as Hastelloy C22, C276, and Inconel 625, exhibit excellent corrosion resistance in both oxidizing and reducing environments. These alloys are considered as the optimal candidates for construction or lining materials of the flue gas desulfurization (FGD), back-end ductworks, reheaters, fans, and chimneys in power plants to overcome the severe acid dew point corrosion problems caused by the condensation of flue gas [1,2,3,4,5,6,7,8,9,10,11]. Meanwhile, researchers have conducted plenty of studies that investigate the acid dewpoint corrosion resistance and behavior of Ni-Cr-Mo alloys through laboratory and filed tests [1,2,5,8,12,13]. The excellent corrosion resistance is generally attributed to the protective oxide layer and stable passivation behavior in this aggressive environment. In-depth research has proceeded to investigate the passive behavior and passive film properties of Ni-Cr-Mo alloys [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30]. Lloyd et al. confirmed that the passive film of Ni-Cr-Mo alloys in 1.0 M NaCl + 0.1 M H₂SO₄ solution consists of an inner Cr/Ni oxide layer and an outer Mo/W oxide layer [16,17,18]. Gary et al. demonstrated the corrosion and passive film properties of C22 alloy in different acid solutions [19]. Jakupi et al. related the impedance properties of passive film on C22 alloy in NaCl solution to the propertied of Cr₂O₃ barrier layer [20]. Zhang et al. observed a similarly layered structure of passive film on a C-2000 alloy in 5M NaCl solution, and claimed that the transpassive oxidation of Cr(III) to Cr(VI) in the barrier layer of the film and its subsequent dissolution leads to the loss of the Cr₂O₃-rich inner layer and the destruction of passivity [21,22]. Luo et al. confirmed that the passive film on Alloy 59 in sulfuric acid mainly consists of Cr₂O₃, Cr(OH)₃ and Ni(OH)₂ [23]. Li et al. specified the accumulation and dissolution of Cr and Mo in the passive film of Ni-Cr-Mo alloys in sulfuric solution along with the contribution of two elements on the passivity [29,30]. The Cr element is considered as the primary factor in enforcing passivity by lowering both the passivation potential and the passive dissolution current, and the presence of Mo and W in the outer regions of the passive film is thought to suppress transpassive dissolution and localized corrosion [16,17,18,27,28,29,30].

In spite of the excellent corrosion resistance and stable passivity of Ni-Cr-Mo alloys in aggressive environments, the persistently high cost of nickel and molybdenum raw materials restricts the extensive application of Ni-Cr-Mo alloys in FGD systems and other facilities [8,31,32]. Laser cladding is widely approved as an effective and cost-effective technology to obtain comparable surface properties to the same material in bulk [32,33,34,35,36]. Numerous investigations have been made studying the corrosion resistance of laser cladding Ni-Cr-Mo alloy coating in different aggressive environments [37,38,39,40]. In our previous work, the laser cladding C22 coating exhibited comparable corrosion resistance and unique intergranular corrosion in simulated sulfuric acid dew point corrosion environment comparing with a cast C22 alloy [13]. The different corrosion resistance and behavior of laser cladding coating is generally attributed to the dense oxide film, grain refinement and non-equilibrium segregation in grain boundary. However, the effect of alloying elements on the corrosion behavior of laser cladding coating have rarely been investigated, which is conceivably different from the casting or wrought alloys as a result of the unique microstructure and phase composition introduced by laser cladding technology.

The aim of the present study is to clarify the effect of the Cr element on the corrosion behavior of laser cladding Ni-Cr-Mo alloy coating in sulfuric acid dew point corrosion environment. On the basis of the content of the Ni, Cr and Mo elements in Hastelloy C22, Ni-Cr-Mo alloy coatings with different content of Cr element (18 wt.%, 22 wt.% and 26 wt.%) were prepared through coaxial laser cladding technology. The corrosion behavior of prepared Ni-Cr-Mo alloy coatings in 50 wt.% H₂SO₄ solution at 60 °C was investigated through the combination of immersion test, electrochemical measurements and surface analysis. The experimental results were discussed thoroughly, and the effect of the Cr element on corrosion behavior was revealed in detail.

2. Materials and Methods

2.1. Materials and Laser Cladding Process

The chemical compositions of Ni-Cr-Mo alloy powders with different content of Cr element used for laser cladding are listed in

Table 1. Chemical compositions of three Ni-Cr-Mo alloy powders in wt.%.

Material	Ni	Cr	Mo
Cr18 powders	69	18	13
Cr22 powders	65	22	13
Cr26 powders	61	26	13

. The powders with a diameter in the range of 46–150 μm were offered by Beijing General Research Institute of Mining and Metallurgy (BGRIMM), Beijing, China. The substrate for the laser cladding process was Q235 carbon structural steel plate, of which the chemical composition is 0.08 wt.% Mn, 0.37 wt.% Si, 0.16 wt.% Si and Bal. Fe, with 10 mm thickness. The coaxial laser cladding process employed a high-power fiber laser (ZKZM-3000, zKzM Laser Technology Co., Ltd., Xi’an, China) and a self-designed coaxial powder feed nozzle. The high-power fiber laser produces a collimated laser beam with a maximum 3000 W power and a beam spot of 1.4 mm in diameter. The coaxial powder feed nozzle was designed to eject three powder streams from uniformly distributed directions around the laser beam, and the powder streams were formed by argon carrier gas with a flow rate of 5 L/min. A multi-track and multilayer cladding process with an overlap ratio of 40%, and a laser scan speed of 8 mm/s was adopted to prepare 5mm thick coating specimens. The schematic of the multi-track coaxial laser cladding process was shown in our previous work [13]. The laser cladding coating specimens prepared from Ni-Cr-Mo alloy powders containing 18 wt.%, 22 wt.%, and 26 wt.% Cr element were named of Cr18, Cr22, and Cr26, respectively. The chemical composition of three Ni-Cr-Mo alloy powders used for laser cladding is listed in Table 1.

The specimens for phase composition and microstructure analysis were cut from as-prepared coatings along with substrates with the upper-surface dimension of 10 mm × 10 mm by a wire cutting machine. The upper surface of coating was polished to 2000 grit Al₂O₃ paper and 1.5 µm diamond abrasive, then ultrasonically cleaned in distilled water and alcohol. The phase composition and microstructure of the prepared Ni-Cr-Mo alloy coating were analyzed by an X-ray diffractometer (Rigaku D/Max-2400, Tokyo, Japan) and a scanning electron microscope (FEI Quanta 200F, Eindhoven, The Netherlands) equipped with energy dispersive spectroscopy (EDS) (EDAX, Pleasanton, CA, USA), respectively. The upper surfaces of specimens for scanning electron microscope (SEM) observing are etched by the aqua regia and cleaned by distilled water in advance.

The immersion testing specimens were cut from as-prepared coatings without substrates, with a dimension of 20 mm × 10 mm × 3 mm by a wire cutting machine. A 2 mm-diameter hole was drilled on the corner of each specimen, and used to be hung by a PTFE wire in test solution. Prior to immersion testing, each specimen was polished with 1500 grit Al₂O₃ paper, then ultrasonically cleaned in distilled water and alcohol.

Electrochemical working electrodes were prepared identically to the specimens for phase composition and microstructure analysis. A connecting brass rod was attached to the back of the substrate for electric connection. After being abraded to 400 grit Al₂O₃ paper and then sequentially ultrasonically cleaned in distilled water and acetone, each working electrode was fixed in epoxy resin with an attached brass rod. The 10 mm × 10 mm-dimension exposed the surface of the working electrode and was abraded to 2000 grit Al₂O₃ paper and then polished with 1.5 µm diamond abrasive. Prior to electrochemical measurements, each prepared specimen was ultrasonically cleaned in distilled water and alcohol, then stored in glass drying vessel.

To simulate the sulfuric acid dew point corrosion environment in FDG systems and back-end ductworks [3,41,42,43,44], a 50 wt.% H₂SO₄ solution at 60 °C was adopted as the immersion solution and electrolyte solution. The solution was freshly prepared with deionized water and analytical grade 98 wt.% H₂SO₄.

2.2. Immersion Testing

The immersion testing designed according to ASTM G1-03 and ASTM G31-2012 was performed to study the corrosion resistance of the laser cladding Ni-Cr-Mo alloy coatings in the 50 wt.% H₂SO₄ solution [45,46]. The temperature of the testing solution was maintained at 60 °C through a water bath. Triplicate parallel specimens for each test were applied to reduce measurement errors and occasional exceptions. The duration of immersion testing in each condition consisted of six 12-h cycles up to 72 h in total. Specimens were taken out after each testing cycle to measure the mass. The process of removing corrosion products before each measurement adopted 15 vol.% HCl solution as a corrosion product remover solution, according to the chemical cleaning method specified in ASTM G1-03 and ASTM G31-2012 [45,46]. The corrosion rate was employed to evaluate the corrosion resistance, and the calculation formula is expressed as

$$\mathrm{R}=8.76\times {10}^4\times \left(\mathrm{M}-{\mathrm{M}}_1\right)∕\left(\mathrm{S}\times \mathrm{T}\times \mathrm{D}\right)$$

(1)

where R is the corrosion rate in mm·a⁻¹ (mm·year⁻¹), M is the initial mass in g, M1 is the mass after one cycle in g, S is the initial area in cm², T is the time in hours, and D is the density in g·cm⁻³ with 8.76 g·cm⁻³ for Cr18, 8.69 g·cm⁻³ for Cr22, and 8.63 g·cm⁻³ for Cr22, The densities of three specimens were calculated according to the proportion of three elements on the basis of 8.69 g·cm⁻³ for Hastelloy C22. The calculated corrosion rates are only used to compare the corrosion resistance between three specimens.

2.3. Surface Analysis

The surface morphologies and compositions of passive film on specimens after 72-h immersion testing were analyzed using SEM and XPS, respectively. A scanning electron microscope (FEI Quanta 200F, Eindhoven, The Netherlands) equipped with EDS (EDAX, Pleasanton, CA, USA) was applied to observe the surface morphology and elemental composition. Compositions of passive films and chemical states of alloying elements were analyzed through XPS measurements on an X-ray photoelectron spectrometer (K-Alpha, ThermoFisher, Waltham, MA, USA) with a monochromatic Al Kα (1486.6 eV). The X-ray gun was operated at 150 W (15 kV, 10 mA) and the photoelectron take-off angle was 90°. During the experiments, a base pressure was maintained at approximately 5 × 10⁻⁹ mbar. High-resolution spectra with an analyzed area of 400 µm² were recorded at a pass energy of 50.0 eV with an energy step of 0.1 eV, whereas survey spectra were recorded at a pass energy of 150.0 eV with an energy step of 1 eV. The values of measured binding energies were corrected by referencing to C 1s (284.8 eV). A Gaussian–Lorentzian product function and a Shirley background subtraction were employed to fit the spectra by applying Avantage software (version 5.984).

2.4. Electrochemical Measurements

All electrochemical measurements were conducted with an electrochemical workstation (CHI660e, Shanghai, China) in a standard three–electrode cell, using a pure platinum sheet as the counter electrode and a mercurous sulfate electrode (MSE) with a Luggin capillary salt bridge as the reference electrode. The temperature of electrolyte solution during measurement was maintained at 60 °C through a water bath. Open circuit potential (OCP), potentiodynamic polarization curve and electrochemical impedance spectra (EIS) of each specimen was measured. Each measurement was repeated three times and the most representative measuring results were presented for discussion in the present study.

Prior to polarization and EIS measurement, each specimen went through open circuit potential measurement with 10-h duration to achieve a stabilized state and passive film. According to the measured stable OCPs for three materials, potentiodynamic polarization curves were recorded at a scan rate of 0.5 mV·s⁻¹ with the potential ranges of −700 to 700 mV_mse. EIS measurements were carried out in the frequency range of 10⁵ to 10⁻² Hz with an amplitude of 10 mV peak-to-peak and AC signals at the measured open circuit potential. For analysis of the impedance data, the ZSimpWin program (version 3.60) was used to numerically fit the measured impedance data and establish equivalent electric circuits (EEC).

3. Results and Discussion

3.1. Phase Composition and Microstructure

The X-ray diffraction (XRD) patterns of laser cladding Ni-Cr-Mo alloy coatings, named as Cr18, Cr22 and Cr26, are presented in Figure 1. It can be seen that all three specimens contain γ-Ni solid solution as the primary phase. The slight broadening and shift towards a small angle of diffraction peaks of γ-Ni solid solution are attributed to the grain refinement effect of laser cladding and the solution of Cr and Mo elements, respectively [13,37,39]. The absence of an Fe element in the XRD patterns confirms the suppression of diffusion from the substrate resulting from the rapid cooling rate of the laser cladding. It is concluded that the phase composition of the studied Ni-Cr-Mo alloy coatings is independent of the content of the Cr element.

As observed on the microstructure of cross-section not presented here, the distribution of solidification type from the bottom to the top of three coatings is similar to the laser cladding C22 coating, studied in our previous work [13], which is mainly cellular, columnar, and equiaxed in turn. This distribution exhibits the typical solidification behavior of the laser cladding process reported and illustrated by other researchers [13,33,35,37,39]. Therefore, the solidification of laser cladding coatings is not discussed further in the present study. The SEM micrographs and the elemental content analyses of the upper surfaces of the as-prepared Cr18, Cr22, and Cr26 are presented in Figure 2. It could be seen that the microstructure of each specimen is mainly composed of gray primary grains and light eutectics forming on the grain boundaries. The black micropores distributing in the eutectic networks probably due to the shrinkage defects and ablation formed during the laser cladding process. The large constitutional supercooling and rapid cooling lead to the trapping of element diffusion, thus eutectics with different element compositions form on the grain boundaries and result in the microstructure with intercrystalline segregation [13,37,39]. As shown by the elemental content analyses of three specimens in Figure 2, the Mo element accumulates in the eutectics (Point 1 in Figure 2a, Point 3 in Figure 2b, and Point 5 in Figure 2c marked by crosses), while the Cr element nearly uniformly distributes in the primary grains (Point 2 in Figure 2a, Point 3 in Figure 2b, and Point 6 in Figure 2c marked by crosses) and eutectics. The difference in the content of the Cr element in three specimens is basically consistent with the content of the alloy powders. It is notable that the amount of black micropores in Cr26 is obviously bigger than that of the other two specimens. This phenomenon is attributed to relatively lower content in the Ni element performing as the host atoms of the solid solution. There is no considerable variation in the size of the primary grain and eutectics with the increase in content of the Cr element.

3.2. Corrosion Rate

The corrosion rate versus time curves of Cr18, Cr22, and Cr26 in 50 wt.% H₂SO₄ solution at 60 °C are presented in Figure 3. It could be seen that the corrosion rates of three specimens locate at an extremely low scale in the region of 0.1–0.18 mm·a⁻¹, exhibiting the good corrosion resistance in the studied environment. The Cr22 performs a slightly higher corrosion rate than the other two specimens during the whole immersion testing, while the Cr26 performs a slightly higher and lower corrosion rate than Cr18 in the early period and the ending of immersion testing, respectively. The higher corrosion rate of Cr26 in the early period is attributed it having more micropores than Cr18. The corrosion rates of all three specimens exhibit a similar decrease with time in the early period, although with a different variation trend in the late period. The corrosion rates of both Cr18 and Cr22 stabilize after 62 h, however, the latter one firstly increases and then stabilizes after the early decrease. The corrosion rate of Cr26 continues to decrease and reach a lower value than that of Cr18 by the end of the test.

The decrease in corrosion rate in the early stage is attributed to the dissolution of the air-formed oxide film and/or exposed base metal, and sequential formation of a productive passive film [13,47,48]. The stabilizing in the late period is related to the equilibrium between the dissolution and solution of passive film retarding the corrosion to a nearly constant rate [47]. Correspondingly, the continual decrease for Cr26 indicates that the passive film keeps formatting and/or thickening during the whole duration of the test. The sudden increase with Cr22 before stabilizing is thought to be due to the localized deterioration of the passive film and/or incomplete removal of corrosion products [12,49,50]. It is concluded that Cr22 exhibits a poorer corrosion resistance compared to the other two specimens, and that Cr26 exhibits a slightly better corrosion resistance compared to Cr18. The corrosion resistance of three specimens is dependent on the properties of forming the passive film, which will be discussed in the following sections.

3.3. Surface Morphology after Immersion Testing

The surface morphologies and elemental content analyses of Cr18, Cr22, and Cr26 after 72 h immersion testing in 50 wt.% H₂SO₄ solution at 60 °C are presented in Figure 4. It could be seen that the corrosion of all the three specimens performs as the preferential dissolution of primary grains and the pitting corrosion initiates at the micropores. There is also dissolution of eutectics occurring in three specimens, and the dissolution gradually increasing from the interior to the edge of eutectics. In addition, the dissolution of eutectics increases as the content of Cr element increases, presenting as the most fragmentary eutectics networks on Cr26. Nevertheless, it is hard to compare the corrosion degree between three specimens through the morphologies.

When comparing with the elemental content analyses of as-prepared specimens (Figure 2), it could be seen that the content of the Ni element in the primary grains of the three specimens (Point 2 in Figure 4a, Point 4 in Figure 4b, and Point 6 in Figure 4c marked by crosses) slightly decreases after immersion testing. Meanwhile, the content of each element in the eutectics of three specimens remains nearly unchanged, except a similar slight decrease in the content of the Ni element of individual eutectics like Point 4 in Figure 4b. This variation indicates the preferential depletion of the Ni element in primary grains performs as the main mass loss behavior. Moreover, the O element is detected in both the eutectics and primary grains of the three specimens, indicating that there is protective oxide film forming on the surface to restrain the corrosion. The observed preferential dissolution of primary grains is quite different with the intergranular corrosion performing as the preferential dissolution of eutectics reported by other researchers on the laser cladding C22 alloy coating [13,40]. It is speculated that the lack of other elements, just like Co, W, and Fe, in studied Ni-Cr-Mo alloy coatings results in the degradation of corrosion resistance of primary grain. The preferential dissolution of primary grains could be explained through the micro-scale corrosion cell system. A potential difference exists between the prime grains and the eutectics as a result of the compositional heterogeneity, which establishes micro-batteries between the prime grains and the eutectics and accelerates the dissolution of primary grain with lower potential [5,48,49].

3.4. Composition of Passive Film

The passive films formed on the Cr18, Cr22, and Cr26 after 72h immersion testing in 50 wt.% H₂SO₄ solution at 60 °C were characterized through XPS. The analyses were performed according to the binding energy published in the NIST X-ray Photoelectron Spectroscopy Database. High-resolution spectra of Ni 2p3/2, Cr 2p3/2, Mo 3d, and O 1s were registered to determine the chemical state. The curve fits for the high-resolution spectra of three specimens are presented in Figure 5.

As Figure 5 shows, the passive films of three specimens possess similar composition consisting primarily of dense oxides and porous hydroxides of Ni, Cr, and Mo—specifically, NiO, Cr₂O₃, MoO₂, MoO₃, Ni(OH)₂ and Cr(OH)₃. Thereinto, Ni(OH)₂, Cr₂O₃, and MoO₃ were the primary oxidized species of Ni, Cr, and Mo elements, respectively. In addition, there were organics, OH⁻, and O²⁻ as the primary species of the O element. The organics were introduced for C calibration during XPS detection. These observations are in agreement with the results reported by other researchers on Ni-Cr-Mo alloys, in which the passive film exhibited a bilayer structure consisting of a Cr₂O₃-dominated compact inner layer and a porous outer layer containing oxides of Mo and hydroxides of Ni and Cr [17,23,25,51,52,53,54,55]. However, the structure of passive film on the studied Ni-Cr-Mo alloy coatings needs to be confirmed through combining the results of EIS measurements. As the content of the Cr element in the coatings increases, the relative content of the Ni and Cr element, excluding the metallic state, in the passive film determined by the survey spectra of three specimens (not shown here) exhibits a drastic decrease and increase, respectively. It is speculated that the oxidized species of Cr replace those of Ni in the passive film. Meanwhile, there is an obvious decrease in relative content of Ni(OH)₂, which implies the compacting of the passive film. The decrease in relative content of OH⁻ supports this speculation. These monotonic variations in the passive film composition are not consistent with the variation in corrosion resistance presented by the result of immersion testing. This contradiction will be discussed in the following sections, in combination with the results of electrochemical measurements.

3.5. Open Circuit Potential (OCP)

The OCP vs. time curves of Cr18, Cr22, and Cr26 in 50 wt.% H₂SO₄ solution at 60 °C are presented in Figure 6. It can be seen that there is nearly similar variation trends of OCP with time exhibited by three specimens. The negative shift in the early period indicates the degradation of air-formed oxide film and initiation of localized corrosion at micropores [40,56,57,58]. The highest extent of shifting on OCP of Cr26 is consistent with the highest number of micropores shown by the surface morphologies before and after immersion testing. Distinguishingly, a second negative shift occurs in the OCP of Cr22 after 22,500 s, which is attributed to the degradation of the formed passive film. The stabilization of the OCP after a slight positive shift in the late period indicates the formation of passive film and subsequent equilibrium between the dissolution and growth [40,52,59]. As the stable OCP values in the late period present, the Cr22 and the Cr18 display the highest and the lowest corrosion tendency, respectively. However, the difference is not remarkable in consideration of the approximate 50mV gap between the stable OCPs of Cr22 and Cr18.

3.6. Potentiodynamic Polarization Measurement

The potentiodynamic polarization curves of Cr18, Cr22, and Cr26 in 50 wt.% H₂SO₄ solution at 60 °C after OCP measurements are presented in Figure 7. All three specimens exhibit typical passivation behavior along with the active region, active–passive transition region, passive region and transpassive region, which is similar to that reported by other researchers for Ni-Cr-Mo alloys in sulfuric acid solutions [23,40,43,50]. The existence of active–passive transition and locating of stable OCP at an active region of each specimen indicate the spontaneous active behavior of studied Ni-Cr-Mo alloy coatings [12,60,61,62]. The corrosion potential of Cr18 and Cr26 locates at a similar value, which is slightly higher than that of Cr22. The current densities of Cr18 and Cr26 in the active region are nearly similar, although clearly lower than that of Cr22. The passive current density of Cr22 remains at a slightly higher value than that of Cr18, while that of Cr26 is much lower than that of the former two. Three specimens possess similar passive region and transpassive potential. The step in the active region and fluctuation in the passive region of each specimen indicates there is transformation on the passive film formed during OCP measurement, which is attributed to the bilayer structure.

By combining the results of the OCP measurements and potentiodynamic polarization measurement, it is concluded that the increase in the content of the Cr element from 18 wt.% to 22 wt.% facilitates the corrosion tendency and corrosion reaction rate, whereas the increase from 22 wt.% to 26 wt.% suppresses the corrosion tendency and corrosion reaction rate. Meanwhile, the later variation in the Cr element clearly promotes the passive ability, presenting as the decrease in anodic current density due to the ennoblement of passive film and suppressed electrochemical reaction. It is speculated that the relative decrease in the content of the Ni element dominates the influence on corrosion behavior when the content of the Cr element increases from 18 wt.% to 22 wt.%, and the further increase in the content of Cr element dominates instead by promoting the passivity. The spontaneous active behavior of the three specimens in a studied environment leads to the competitive effect of the Ni element and Cr element on the variation in corrosion behavior. This phenomenon will be further specified by combining the results of EIS in the following section.

3.7. Electrochemical Impedance Spectroscopy (EIS)

The measured EIS spectra of Cr18, Cr22, and Cr26 in 50 wt.% H₂SO₄ solution at 60 °C after OCP measurements are presented in Figure 8. There are two different equivalent electric circuits (EEC) used for analysis and fitting the EIS spectra, which are presented in Figure 9. An impedance response model consisting of three time constants is adopted to construct EECs in this study [20,24,59], expressed as:

$${\mathrm{Z}}_{\mathrm{total}}={\mathrm{Z}}_{\mathrm{f}/\mathrm{e}}+{\mathrm{Z}}_{\mathrm{f}}+{\mathrm{Z}}_{\mathrm{f}/\mathrm{m}}$$

(2)

where Z_e/f (R_e/fC_e/f), Z_f (R_fC_f), and Z_f/m (R_f/mC_f/m), represents the impedance for electrolyte/film interface, film, and film/metal interface. Considering the phase angle modulus is lower than 90° and the depression of the capacitive loop, a constant phase element CPE (Q) is used instead of the ideal capacitor C. This non-ideal behavior is attributed to various factors as surface heterogeneity, roughness, grain boundary, corrosion products adsorption and formation of porous layer [23,56]. Rs represents the solution resistance of electrolyte. Ls connected in parallel with Rs represents the inductive behavior in the high frequency region (4.5 × 10⁴ Hz to 1 × 10⁵ Hz), which is associated with the inductance of an electrochemical testing system.

As Figure 8 shows, there is a similar depressed capacitive arc covering the high frequency region in the Nyquist plots of each specimen, which is associated with the capacitive response of the passive film. Meanwhile, there is a broad phase angle peak covering the corresponding high frequency region in the Bode plots of each specimen, especially the two shoulders presented by the phase angle peak of Cr18. This indicates the existence of two time constants associated with the bilayer structure of the passive film [56,61]. Thus, it can be confirmed that the passive film of the studied Ni-Cr-Mo alloy coatings obtains a bilayer structure consisting of a Cr₂O₃-dominated compact inner layer and a porous outer layer containing oxides of Mo and hydroxides of Ni and Cr. The Nyquist and Bode plots of three specimens present two slightly different shapes in the low frequency region. Whereas, there are two slightly different shapes presented by EIS spectra of three specimens in the low frequency regions. An incomplete capacitive arc with a larger diameter covers the low frequency region in the Nyquist plots of Cr18, while a depressed capacitive arc with a smaller diameter for Cr22 and Cr26. Meanwhile, there is a higher phase angle peak located at the lower frequency along with higher impedance modulus covering the low frequency region in the Bode plots of Cr18. This difference implies the transition of surface state and electrochemical behavior as the content of the Cr element in the coating increases from 18 wt.% to 22 wt.%.

Thus, two types of EEC are necessary to fit plots and reveal the changes in electrochemical behavior, as EEC A relating to the response of Cr18 and EEC B to Cr22 and Cr26 show in Figure 9. All the measured EIS spectra can be well fitted using the related EEC as shown in Figure 8. The similar series connection of R_e/fC_e/f and R_fC_f is related to the impedance response in high frequency region, indicating the effect of bilayer structure of the passive film on the electrochemical behavior. The R_e/fC_e/f represents the charge transfer process through the electrolyte/film interface in the defects of the porous outer layer, and the R_fC_f represents the electrochemical reaction within the inner layer [20,23,47,63,64,65]. A Warburg impedance (W) connected in parallel with R_e/fC_e/f is included to represent the reaction product diffusion through the electrolyte or porous outer layer, which was proposed by other researchers in studying the passive behavior of the Ni-Cr-Mo alloys [20,23,61]. No reasonable EEC fitting to measure the EIS spectra could be achieved without the inclusion of the Warburg element. The R_f/mC_f/m represents the behavior at the metal/film interface related to the impedance response in the low frequency region, and the connection types between the R_f/mC_f/m and other impedance elements in EEC A and EEC B are different. The series connection of R_f/mC_f/m and R_f presented by EEC A indicates that the metal surface of Cr18 is exposed due to the incomplete passive film on Cr18, while the series connection of R_f/mC_f/m and R_fC_f presented by EEC B indicates that the metal surface of Cr22 and Cr26 are overcovered by the passive film leading to the existence of metal/film interface [40,51,66]. This phenomenon suggests that the increase in the content of the Cr element improves the compactness of the passive film and localized corrosion resistance.

In considering the electrochemical behavior and the passive film structure of the three specimens, the fitted results of R_e/fC_e/f, R_fC_f and R_f/mC_f/m are listed in

Table 2. EIS fitted results for Cr18, Cr22, and Cr26 in 50 wt.% H₂SO₄ solution at 60 °C.

Specimen	Cr18	Cr22	Cr26
Re/f/Ω·cm2	315.8	176.1	153.4
Qe/f/F·cm2	2.225 × 10−2	8.280 × 10−5	3.684 × 10−5
ne/f	0.9628	0.9308	1.000
Rf/Ω·cm2	0.5830	43.98	127.2
Qf/F·cm2	7.542 × 10−4	1.243 × 10−1	2.716 × 10−2
nf	0.5941	0.8429	0.6577
Rf/m/Ω·cm2	184.9	0.3492	0.3582
Qf/m·cm2	2.806 × 10−5	7.049 × 10−12	2.725 × 10−14
nf/m	0.9852	0.4173	0.8730
Errors	4.769 × 10−4	2.695 × 10−4	3.229 × 10−4

. It could be seen that the resistance and capacitance of each RC element exhibit different variations with the increase in the content of the Cr element in the coating. The decrease in both R_e/f and Q_e/f indicates the change in the composition of the porous outer layer [63], which is consistent with the decrease in Ni(OH)₂ presented by the XPS analysis. Meanwhile, the decrease in R_e/f indicates the facilitation of the charge transfer progress through the electrolyte/film interface. The increase in R_f accompanying the decrease in Q_f suggests that the inner layer becomes thicker and more protective, resulting from the more oxidized species of Cr in the passive film [20,23,65]. The drastic decrease in R_f/m and increase in Q_f/m as the content of the Cr element increases from 18 wt.% to 22 wt.% resulted from the transformation of the state of metal/film interface. The metal surface of Cr18 is exposed to electrolytes due to the incomplete passive film, while the metal surface of Cr22 is overcovered by passive film. The decrease in n_f/m from 0.9853 to 0.4173 synchronously confirms this transformation. According to the resistance of each RC element, it is speculated that the controlling behavior of the corrosion of Cr18 is the charge transfer process through the electrolyte/film interface in the defects of the porous outer layer and the electric double layer on the exposed metal surface, however, with Cr22 and Cr26, it is the combination of the charge transfer process and the electrochemical reaction of passive film.

By comprehensively studying the results of immersion testing, surface analysis and electrochemical measurements, it is confirmed that the Cr element performs a different effect on the corrosion behavior of laser cladding Ni-Cr-Mo coatings under different content of the Cr element in the coating. As the content of the Cr element increases from 18 wt.% to 22 wt.%, comparing between Cr18 and Cr22, there is a degradation in the corrosion resistance presented by the higher corrosion rate, more negative OCP and corrosion potential, and slightly higher passive current density of Cr22. It is attributed to the effect of relative decrease in the content of the Ni element, which results in the decrease in Ni(OH)₂ in the porous outer layer of passive film leading to the facilitation of charge transfer progress. In spite of the increase in content of the Cr element leading to the thicker and more protective inner layer of the passive film, the consequent improvement in corrosion resistance is not a match for the degradation resulted from the decrease in the content of the Ni element. As the content of the Cr element increases from 22 wt.% to 26 wt.%, comparing between Cr22 and Cr26, there is an improvement in the corrosion resistance presented by the lower corrosion rate, more positive OCP and corrosion potential, and much lower passive current density of Cr22. This opposite variation in corrosion resistance is attributed to the predominant effect of increase in the content of the Cr element on the passive film properties. It is concluded that the increase in the content of the Cr element results in the more protective passive film of laser cladding Ni-Cr-Mo alloy coatings by thickening and compacting the inner layer. However, the presented corrosion resistance depends on the competition with the effects of the relative decrease in the content of the Ni element. In addition, it is speculated that there is critical content of the Cr element, beyond which the corrosion resistance of the laser cladding Ni-Cr-Mo alloy coatings exhibits considerable improvement.

4. Conclusions

Ni-Cr-Mo alloy coatings with different content of Cr element (18 wt.%, 22 wt.%, and 26 wt.%) were produced on Q235 steel by laser cladding. The phase composition, microstructure, and corrosion behavior in sulfuric acid dew point corrosion environment (50 wt.% H₂SO₄ at 60 °C) were investigated. The main conclusion obtained are presented below:

(1)

All the Ni-Cr-Mo alloy coatings with different content of Cr element exhibit similar phase composition and microstructure. The increase in Cr results in more black micropores in the eutectic networks;

(2)

The passive film forming on each Ni-Cr-Mo alloy coating exhibits a bilayer structure consisting of a Cr₂O₃-dominated compact inner layer and a porous outer layer containing oxides of Mo and hydroxides of Ni and Cr. The increase in Cr leads to the decrease in Ni(OH)₂ and the replacing of oxidized species of Ni with those Cr;

(3)

The inner layer becomes thicker and more compact with the increase in Cr, leading to the more protective passive film;

(4)

The corrosion resistance of Ni-Cr-Mo alloy coating is degraded and then improved with an increase in Cr. This phenomenon is attributed to the competition effect between the increase in Cr and relative decrease in Ni.