Corrosion resistance of the cold-sprayed nickel coatings deposited on the Ni surface (substrate) without and with abrasive grit-blasting treatment of the substrate was investigated. The corundum powder with different grain sizes was used. The corrosive environment contained an acidic chloride solution. The mechanism of the corrosion of nickel was suggested and discussed. Corrosion electrochemical parameters were determined by electrochemical methods. The corrosion effect of a nickel coating depends on the grain size used to prepare the substrate. The nickel coating after the medium grit-blasting treatment of the substrate was found to be the most corrosion resistant. However, the smallest resistance on the corrosion effect should be attributed to the nickel coating on the substrate after the coarse grit-blasting treatment.
Introduction
Cold spraying (CS) is a solid-state process where the coating is formed by powder particles impacting with high kinetic energy because of their high velocity. The coating is formed when particles impacting on the substrate surface are plastically deformed and adhere to the substrate (Ref 1). However, roughness, the treatment of a substrate, and the oxide passivation layer directly affect coating adhesion (Ref 2, 3). The cold spraying as an emerging coating technique has been developed to deposit high-quality metallic coatings. The advantages presented by low temperatures in cold spraying make it possible to deposit coatings on a wide range of materials. Among them, the authors (Ref 4) by CS method covered the surface of aluminum with copper layer. It turned out that CuAl2 phase appeared on Al surface. Titanium and its alloys show excellent corrosion resistance and can be widely used for the protection of material substrates. The potentiodynamic polarization measurement reveals that cold-sprayed Ti coating has much higher corrosion resistance than carbon steel and can provide favorable protection for the steel substrate (Ref 5). Tantalum is a sought-after metal with very good resistance against corrosion in various acid solutions, salt solutions, and organic compounds even at elevated temperatures. Corrosion resistance of the CS Ta coating is ensured by the formation of the passive oxide film of Ta2O5 on surface of coating. Moreover, the electrochemical tests in concentrated alkaline solutions revealed the stable passive behavior of the CS tantalum coatings (Ref 6, 7). Zinc and its alloy coatings have been extensively used on steel substrates and components to protect them from corrosion and surface degradation in aqueous environments. The cold-sprayed zinc coatings are thick, dense and provide an efficient barrier protection for a mild steel surface without exposing the substrate to corrosion (Ref 8). The use of the CS processes on zinc coatings is still at the developmental stage. The study (Ref 9) shows that high temperatures and pressures provide necessary kinetic energy to affect a mechanical bond between the substrate and the coating powders. On the other hand, the microstructure of the CS coatings evidently points to the plastic deformation of metallic powders. Therefore, cold spray is mostly suitable for coatings on oxygen-sensitive or thermo-sensitive substrates, including magnesium. Xiong and Zhang (Ref 10) thoroughly investigated the effect of the cold-sprayed Al/Al2O3 composite coatings on magnesium alloy substrates. Mg alloys coated with the cold-sprayed Al/Al2O3 composite were found to show enhanced yield strength compared with uncoated materials, and the yield strength of coated magnesium alloys increases with the volume fraction of Al2O3 in the coatings. Considerable research (Ref 11) on coating magnesium alloys with aluminum powder using the CS technique and its effect on the corrosion resistance and mechanical properties of the Mg alloy has been done. The CS aluminum coating deposited on the magnesium alloy provided a significant protection of the substrate from the corrosion attack. Nickel and its alloys are employed in a wide range of industrial sectors where corrosion resistance is required. Koivuluoto et al. (Ref 12) investigated the corrosion resistance of Ni and NiCu coatings, which were produced by CS method. The electrochemical test results demonstrate that both coatings show comparable corrosion resistance with their bulk counterparts in a variety of solutions. Moreover, the corrosion resistance of the Ni coating increases with the gas temperature (Ref 13). In the work done by Bala et al. (Ref 14), the CS process was used to successfully deposit NiCr, NiCrTiC, and NiCrTiCRe powders on boiler steel. It turned out that all the examined coatings successfully decreased the erosion-corrosion rate of the substrate under boiler environment. Therefore, it is to be noted that the mechanical properties of the CS coatings are significantly influenced by the coating structure and the particle bonding. The overall quality of the bonding is dependent on metal and process conditions, such as material strength, particle velocity, and process temperature (Ref 15). The CS process was used to prepare of WC-Co coatings (Ref 16). The results show that there is no degradation of the WC-Co powder during the cold spray process and a well-bonded and phase pure WC coating can be produced. Moreover, cold-sprayed coatings were produced by spraying WC-25Co powders onto Al7075-T6 and low-carbon steel substrates (Ref 17). Mechanical properties of coatings on both substrates were found to be very similar to those obtained by the high-velocity oxy-fuel (HVOF) method. On the other hand, gas pressure and gas temperature are two critical factors for obtaining high-quality coatings (Ref 18). Furthermore, a number of investigations on the characteristic mechanical and tribological properties of the cold-sprayed titanium coatings on commercial Ti-6Al-4V substrates have been performed (Ref 19-23). Most authors agree that mechanical properties of each coating mainly depend on its microstructure and thicknesses. Moreover, the surface preparation of a substrate is the essential first-stage treatment before the application of any coating. It is well-known that the correct preparation of the substrate has a significant effect on the adhesion of the coating to the metal surface. The abrasive grit-blasting treatment (ABT) of metals is widely used for the preparation of the substrate for the cold-sprayed coating process. Metal coating should protect the substrate against corrosion. Moreover, the same materials are used to regenerate the substrate surface, for example, nickel coatings are often applied to the nickel substrate. In addition, the coating and the substrate should have almost the same mechanical and corrosion properties. Therefore, we have tried to solve this interesting problem. However, in the literature, we have not found any information about the effect of the substrate (Ni(s)) treatment on the corrosion resistance of the cold-sprayed nickel coating (Ni(c)), especially in aggressive environments.
The paper deals with the corrosion resistance of the cold-sprayed nickel coatings on the nickel surface (i.e., Ni(c)/Ni(s)), without and with an abrasive grit-blasting treatment of the substrate. To treat the substrate, the corundum powder with different grain sizes was used. The corrosive environment contained an acidic chloride solution. The electrochemical method and other supporting techniques were applied.
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Experimental
Solutions
The following reagents were used to make the solutions: FLUKA analytical grade sodium chloride (NaCl) and POCH analytical grade hydrochloric acid (HCl). Water, which was distilled three times, was used as a solvent. The corrosive environment (supporting electrolyte) was obtained by mixing the sodium chloride and hydrochloric acid, so the concentration of Cl− ion was 1.2 M. The pH was 1.5. The electrolyte was not deoxygenated.
Materials and Surface Analysis
The cold spray system was used to spray nickel coatings by means of Impact Innovations 5/8 equipped with the Fanuc M-20iA robot. The nickel powder (P836, Ni > 99.5%) was purchased from the metallization firm. The size of nickel grains was in the range of 11-45 μm (average of Ni grit size was 28 μm). However, the size distribution of powders has a crucial importance on the quality of coatings. The cold spray system of Impact Innovations 5/8 equipped with the Fanuc M-20 robot was used to produce of spray nickel coatings. The rectangular specimens were cut off from the nickel sheet of dimensions 40 × 400 × 5 mm by the wire-cut electrical discharge machine (WDM). The substrates were prepared without and with the abrasive grit-blasting treatment (ABT) by corundum powder. The substrates were covered with cold-sprayed nickel coatings. Moreover, the specimen without any treatment of the substrate was marked as (NG). The surfaces after blasting treatment with the corundum of the grain size from 600 to 710 μm (type 30) as medium grit powder were called as (MG) or with the corundum of the grain size from 1700 to 2000 μm (type 12) were described as coarse grit powder (CG). The thicknesses of cold-sprayed nickel coatings were in the range of 1850-1920 μm.
The microstructure of the specimens was observed in a scanning electron microscope (SEM). SEM study was carried out at an accelerating voltage 5 kV and 5 K× magnification on a Zeiss Evo 50 XVP instrument.
X-ray diffraction was used to characterize the phase composition of materials. The phase composition of cold-sprayed coatings was tested using a D8 Discover Bruker with Co-Kα radiation with anode settings at 40 kV and 40 mA (wavelength was 0.17889 nm).
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The residual stress measurements on specimens were determined by the XRD technique using penetration limited to 5 μm. The XRD-based residual stress measurements were executed with an x-ray stress analyzer model D8 Discover. The stress analysis was done using Cu-Kα radiation with anode settings at 40 kV and 40 mA (wavelength was 0.15405 nm) using the sin2 Ψ method. Moreover, the scanning speed of 2Θ was 1°min−1 during the experiment. The residual stress level was calculated by means of x-ray diffraction based on measurements of changes in crystal lattice spacing which manifest themselves as shifts in angular positions of respective diffraction peaks. At least six measurements were necessary for the determination of every specimen.
The roughness parameters of materials were quantitatively assessed using the Taylor-Hobson Form Talysurf-120L surface profiler. The roughness was measured in two directions perpendicular to each other.
Electrodes
For testing, we used three types of working electrodes. The first type was of the substrate (nickel electrode). The second type was of the nickel electrode whose surface was covered by cold-sprayed nickel coating without abrasive grit-blasting treatment of the substrate. The third type was of the nickel electrode whose surface was covered by CS nickel coating with abrasive grit-blasting treatment of the substrate. The dimensions of all the electrodes were 10 × 30 × 5 mm. The geometric surface area of the working electrode which was contacted with the electrolyte was 2 cm2. The remaining part of the electrode surface was isolated from the electrolyte by epoxy lacquer.
The surface of the electrode was washed with bidistilled water, ultrasonically decreased in acetone, and dried at room temperature. Subsequently, the working electrode was immediately immersed in the test solution.
The saturated calomel electrode (SCE) was used as the reference. It was connected with the solution using a Luggin capillary. The capillary tip was placed about 3 mm in relation to the surface of the working electrode.
The counter electrode (9 cm2) was made of platinum foil (99.99% Pt).
Electrochemical Measurements
The electrochemical experiments were carried out in a conventional three-electrode cell. All measurements were made using a potentiostat/galvanostat PGSTAT 128N, AutoLab, the Netherlands with NOVA 1.7 software of the same producer.
The potentiodynamic polarization curves (LSV) were recorded. All measurements were carried out under a potential range from − 800 to + 300 mV versus SCE while the potential change rate was 1 mV s−1. Immediately before the start of the measurement, the electrode was maintained at potential − 800 mV for 30 s, in order to clean the surface. Potentiodynamic polarization curves were used to designate the corrosion electrochemical parameters, i.e., corrosion potential (Ecorr), corrosion current density (jcorr), cathodic (βc), and anodic (βa) Tafel slopes. Moreover, Stern and Geary formulated the following equation for the corrosion current density (Ref 24-27):
where Rp is the polarization resistance, ba and bc are the coefficients of the electrode reactions. However, the polarization resistance was calculated according to Eq 1.
The corrosion rate of the substrates was obtained using the following equation:
where M is the molecular weight of the reacting substrate, n is the number of electrons exchanged, and ρ is the density of the material.
The chronoamperometric (ChA) curves were obtained for the potential values: − 580, − 140, and + 200 mV versus SCE, respectively. The characteristic values of the electrode potentials were selected based on polarization curves.
All measurements were carried out at 25 °C (298 K), which was maintained using an air thermostat. The electrochemical experiments were started after 3 h of the immersion of the working electrode in the testing solution. All measurements were repeated three times.
Results and Discussion
Surface Parameter Study
Some parameters were identified to characterize the surface topography before and after the grit blasting of the substrate. Figure 1 presents SEM top views of the substrate without and with medium or coarse grit-blasting treatment of surfaces illustrating the roughening effect of the surface treatment.
Fig. 1
SEM surface top views of the substrate: (a) without (NG) and with: (b) medium grit-blasting (MG, corundum size of 600-710 μm), (c) coarse grit-blasting (CG, corundum size of 1700-2000 μm) treatment of surface. Arrows indicate numerous pits and craters
×
These surface defects had crater shapes caused by corundum grit-blasting process. The treatment generated a severe variation in the substrate topography. Moreover, the resulting area of surfaces was analyzed using the profilometry method. The different roughness profiles before and after grit-blasting process are given in Fig. 2.
Fig. 2
Roughness profiles and surface topography of the substrate: (a) without (NG) and with: (b) medium grit-blasting (MG, corundum size of 600-710 μm), (c) coarse grit-blasting (CG, corundum size of 1700-2000 μm) treatment of surface
×
The roughness coefficient (Ra) was measured in two directions perpendicular to each other. Then, the average value of the Ra was calculated. It turned out that Ra takes values: 0.48, 6.11, and 9.79 μm for the substrate without (NG), with medium (MG), and coarse (CG) grit-blasted treatment, respectively. Consequently, the deep craters on the Ni surface were caused by corundum coarse grit-blasting treatment as a direct effect modification of the substrate. Moreover, a specific surface topographic analysis was conducted: the determination of crater size due to grit blasting through peaks and valleys (Fig. 2). Their detailed parameters are shown in Table 1.
Table 1
Geometry of the substrate parameters (according to ISO 25178)
Parameter
Specimens surface treatment
Lack grit-blasting (NG)
Medium grit-blasting (MG)
Coarse grit-blasting (CG)
Arithmetic height surface (Sa), μm
0.41
6.20
14.67
Root square height surface (Sq), μm
0.55
8.05
18.76
Skewness surface (Ssk)
− 1.50
− 0.77
− 0.44
Kurtosis surface (Sku)
25.27
4.26
3.37
Maximum peak height surface (Sp), μm
8.80
30.57
56.41
Maximum pit height surface (Sv), μm
12.09
44.78
72.37
Maximum height surface (Sz), μm
20.90
75.35
128.78
Horizontal surface, mm2
2.76
2.76
2.76
Developed surface, mm2
2.78
3.20
3.62
Depth, μm
12.18
50.93
94.30
Volume, mm3
0.0008
0.0106
0.0259
The specimen without grit-blasting (NG) treatment represents a typical very flat surface (Sa = 0.41 μm) of the substrate (Fig. 2a). The subsequent grit-blasting treatment of the substrates caused a significant diverseness in surface geometry (Fig. 2b and c). It turned out that after the coarse grit-blasting (CG) treatment, the parameter of Sa increased about thirty-six times compared to the specimen without any treatment of the surface (Table 1). A similar relationship was observed for the parameter of mean-squared surface height (Sq). Negative value of the (Ssk) asymmetry parameter of the surface indicated that all surfaces were flattened and occurring peaks were rounded. The parameter of Kurtosis (Sku) is responsible for the steep irregularities and defects. The value of Sku over 3 (Table 1) indicated that distribution of profile ordinate corresponds to higher concentration around the mean value, which is clearly visible for the NG substrate. Other parameters such as the maximum peak (Sp) and the maximum valley (Sv) or the maximum height (Sz) significantly increase with the size of applied corundum grain. Moreover, the horizontal surfaces of all specimens were the same (Table 1), but after grit blasting, the two surfaces became rough. The area of specimens MG and CG increased about 15 and 30%, respectively. However, for the CG specimen, the volume value increased about thirty-two times compared to the NG substrate. Therefore, the CG surface geometry makes it easy to create contact with striking particle. Moreover, a particle of metal powder hits the substrate in an inert gas atmosphere.
Determination of the Residual Stresses
The material modification effects of the grit-blasting treatment on the residual stresses in the substrate were studied. The results of residual stresses of the grit blasting for the substrates are given in Fig. 3 as a function of surface preparation.
Fig. 3
Residual stress of grit blasting of the substrates as a function of surface preparation, (a) longitudinal orientation, (b) transverse direction
×
The rate of the residual stresses was slightly lower for transverse direction than for the longitudinal orientation. Additionally, grit-blasting treatment induced a relative homogenous plastic deformation onto the substrate metallic surface. Moreover, grit size had no significant influence on the residual stresses of the substrates. Therefore, similar results of the residual stress were obtained ranging from medium to coarse corundum grits.
Microstructure of the Substrate and Cold-Sprayed Nickel Coating
The SEM surface microstructures of the substrate (ED) and cold-sprayed nickel coating deposited on the surface without grit-blasting (NG) treatment of the substrate are presented in Fig. 4.
Fig. 4
SEM surface microstructure: (a) substrate (ED) and (b) cold-sprayed nickel coating without grit-blasting (NG) treatment of surface
×
Some inter-splat cracks and small pores are visible on the coating microstructure (Fig. 4b). Coating characterization mainly consisted in the metallographic cross-sectional observations. However, Fig. 5 refers to SEM cross-sectional micrographs of the nickel coatings without (Fig. 5a) and with treatment of the substrate (Fig. 5b and c).
Fig. 5
SEM cross-sectional micrographs of the nickel coating: (a, a′) without (NG) and with: (b, b′) medium grit-blasting (MG, corundum size of 600-710 μm), (c, c′) coarse grit-blasting (CG, corundum size of 1700-2000 μm) treatment of the substrate
×
The micrographs revealed homogeneous connection between the substrate and the nickel coating layer for all specimens. The resulting coating showed a very dense microstructure of very low porosity. Therefore, particles of Ni impinged with a high kinetic energy and well deformed on the substrate.
The x-ray diffraction contributes to the description of the microstructure of the materials. However, Fig. 6 presents the x-ray diffraction patterns obtained for the nickel powder, substrate (ED), and cold-sprayed nickel coatings deposited without treatment (NG) and with medium grit-blasted (MG) or coarse grit-blasted (CG) treatment of substrate.
Fig. 6
X-ray diffraction patterns obtained for the nickel powder, substrate (ED), and cold-sprayed nickel coatings deposited on the surface without (NG) and with medium grit-blasting (MG, corundum size of 600-710 μm) or coarse grit-blasting (CG, corundum size of 1700-2000 μm) treatment of the substrate
×
The high (700 °C) temperature of preheated gas did not cause the oxidation of the CS nickel coatings. All patterns showed the same phase composition and did not reveal any differences between the substrate and the nickel coatings.
Corrosion Tests for the Substrate and Cold-Sprayed Nickel Coating
It is known that in the acidic solution, the cathodic reaction is the discharge of hydrogen ions to produce hydrogen gas or to reduce oxygen, while the anodic reaction involves the movement of metal ions from the metal surface into the solution. The different kind of coating on the surface of metal may affect either the cathodic or the anodic reaction or both simultaneously (Ref 28). Potentiodynamic polarization measurements were carried out in order to gain some knowledge about the kinetics of cathodic and anodic reactions. The potentiodynamic polarization curves for the substrate and the CS nickel coating without any treatment of the substrate in the acidic chloride solutions are shown in Fig. 7.
When the potential of the electrode was changed in the anodic direction, the adsorption layer with the participation of chloride ion was formed (according to a simplified mechanism) on the electrode surface:
The adsorption layer of (NiClOH)ads on the substrate appears for the peaks of the potential about − 160 and + 60 mV versus SCE in the case of the nickel coating. Therefore, the peak potential shifting toward a more positive value (approximately 220 mV versus SCE) indicates that the structure and properties of the nickel coating are different from the base material. This layer partially prevents the electrode from further oxidation in the chloride environment. The adsorption layer (Reaction (4)) in the acid solution dissolves as a result of a simple chemical reaction:
A similar mechanism reaction for iron in the acidic chloride solution was proposed by Chin and Nobe (Ref 33). It is to be noted that for the substrate, the anodic oxidation peak appears for the potential of about − 160 mV versus SCE on the curve (a) (Fig. 7), which corresponds to the Reaction (6). In the case of the cold-sprayed nickel coating (NG), the curve (b) (Fig. 7) of the anodic oxidation peak (Reaction (6) is visible for the potential of about + 60 mV versus SCE. The shift of the potential value toward the positive values indicates that the oxidation of nickel in the form of the CS coating is more difficult. However, although nickel is known as a corrosion-resistant metal, it quickly corrodes in many aggressive media.
The study of current response of studied redox system as a function of time at suitably selected potential is called chronoamperometry (ChA). Generally, considering the course of ChA curves, it is possible to assess the tightness of the coating which was formed on the Ni surface. Figure 8 shows chronoamperometric curves for the substrate.
Fig. 8
Chronoamperometric curves for the substrate obtained for: (a) − 580, (b) − 140, and (c) 200 mV. Solution contained 1.2 M Cl−, pH 1.5
×
The working electrode potential values were selected based on the polarization curve (Fig. 7, curve (a)). The potentials related to the Reactions: (3), (4), and (7). Moreover, similar curves (that have been omitted) were obtained under the same conditions for the cold-sprayed nickel coating surface (NG). An increase in the cathodic current density versus time of electrolysis (Fig. 8, curve (a)) was caused by the reduction in hydrogen ions (Reaction (3). In this case, the reduction reaction of H+ ions is not inhibited. Figure 8, curve (b) should be attributed to the formation of the adsorption layer of (NiClOH)ads (Reaction (4)) on the nickel surface. Initially, a slight increase in the current density was observed. As the electrolysis time passes, the anode current density increases significantly. An increase in the anodic current density means that the adsorbed layer of (NiClOH)ads is not tight and does not protect the substrate from oxidation at a satisfactory level. Moreover, increasing the potential values in the more positive direction (Fig. 8, curve (c)) causes the anode current density to increase rapidly. Probably, the adsorbed layer of (NiClOH)ads has dissolved (Reaction (5)), and the oxidation of the substrate was not inhibited. Moreover, the porous NiOads layer (Reactions (6) and (7)) also does not inhibit the oxidation reaction of the substrate.
Corrosion electrochemical parameters of the substrate and the CS nickel coating (NG) were designated on the basis of potentiodynamic polarization curves, which are shown in Fig. 9 as semilogarithmic scale.
The corrosion electrochemical parameters of materials are listed in Table 2.
Table 2
Corrosion electrochemical parameters of the materials
Materials
Ecorr × 10−3, V vs. SCE
jcorr × 10−3, A cm−2
− βc, V dec−1 × 10−3
βa, V dec−1 × 10−3
Substrate
− 312
0.200
170
135
Cold-sprayed nickel coating (NG)
− 348
0.030
80
190
In the case of the CS nickel coating (NG), a shift in the corrosion potential toward negative values compared to the substrate (Fig. 9, curves (a) and (b)) was observed. In addition, there was a marked decrease in the corrosion current density (jcorr) for the nickel coating (Table 2). The cathodic Tafel slope (βc) was considerably changed while the anodic Tafel slope (βa) was only slightly changed, indicating that the presence of the nickel coating changed the mechanism of the cathodic Reaction (3), but the CS nickel coating (NG) slightly affected the anodic Reactions (4) and (5).
The polarization resistance (Rp) was determined according to Eq 1. The values of Rp are listed in Table 3.
Table 3
Polarization resistance and corrosion rate of the materials
Materials
Rp, Ω cm2
υcorr, mm y−1
Substrate
163
2.15
Cold-sprayed nickel coating (NG)
814
0.32
The calculation results collected in Table 3 clearly revealed that an increase in the values of Rp for the cold-sprayed nickel coating (NG) signifies that the exchange for mass and electric charge between the electrode and the electrolyte is difficult. Moreover, the corrosion rate (Eq 2) of the CS nickel coating (NG) is seven times lower compared to the parent metal (Table 3). It seems clear that metallographic structures of the substrate and the cold-sprayed nickel coating (NG) are completely different. Probably, the structure of cold-sprayed nickel coating is more compact. As a result, penetration of the Cl ions into the Ni coating is more difficult. That is why the CS nickel coating protects the substrate against electrochemical corrosion. This problem will be discussed later in this work.
Surface Images of the Substrate and Cold-Sprayed Nickel Coating
The substrate and the cold-sprayed nickel coating without grit-blasting treatment (NG) after exposure (24 h) in an aggressive chloride solution (1.2 M Cl−) were observed in a scanning electron microscope (SEM). The SEM cross-sectional micrograph of materials is given in Fig. 10.
Fig. 10
SEM cross-sectional micrograph of the materials: (a, a′) substrate, (b, b′) cold-sprayed nickel coating without grit-blasting treatment (NG) after exposure in 1.2 M Cl−, pH 1.5. Arrows indicate numerous pits and craters
×
Figure 10(a) and (a′) shows cross-sectional images of the substrate. It can be observed that the Ni surface was strongly damaged. Numerous pits were found on the surface of the substrate due to the corrosive effect of the environment (Reactions (4) and (5)). In contrast, Fig. 10(b) and (b′) refers to the CS nickel coating without grit-blasting treatment (NG). In this case of the nickel coating (NG), few pits formed as a result of the corrosive action of Cl− in the corrosive environment are visible. However, we concluded that the cold-sprayed nickel coating is more resistant to corrosion in the chloride environment compared to the substrate. On the other hand, the compact CS nickel coating significantly limits the penetration of the electrolyte solution into the NG coating.
Corrosion Tests for the Cold-Sprayed Nickel Coatings: NG, MG, CG
The potentiodynamic polarization curves for the cold-sprayed nickel coatings without (NG) and with medium grit-blasting (MG, corundum size of 600-710 μm) or coarse grit-blasting (CG, corundum size of 1700-2000 μm) treatment of the substrate are shown in Fig. 11 as semilogarithmic scale.
Fig. 11
Potentiodynamic polarization curves (as semilogarithmic scale) for the cold-sprayed nickel coating: (a) without (NG) and with: (b) medium grit-blasting (MG, corundum size of 600-710 μm), (c) coarse grit-blasted (CG, corundum size of 1700-2000 μm) treatment of the substrate. Solutions contained 1.2 M Cl−, pH 1.5, dE/dt 1 mV s−1
×
For the comparison, the curve (a), previously cited, referring to the CS nickel coating (NG) is included in Fig. 11. In the case of the cold-sprayed nickel coatings, the anodic oxidation peaks (curves (a-c)) are visible at the potentials of about 20-50 mV (versus SCE) in Reaction (6).
Figure 11 shows that the corrosion resistance of the CS nickel surfaces depends on the method of preparation of the substrate. The nickel coating after medium grit-blasting treatment of the substrate, Fig. 11, curve (b), is the most corrosion resistant. However, the smallest resistance on corrosion effect should be attributed to the nickel coating on the substrate after coarse grit-blasted treatment, Fig. 11, curve (c). Without any doubt, the size of the corundum grains used to prepare the substrate affects the shape of the cathodic and anodic parts of the polarization curves.
In addition, the parallel cathodic Tafel curves in Fig. 11 show that the hydrogen evolution (Reaction (3)) is activation controlled and the reduction mechanism is not affected, regardless of the type of corundum which was used for the surface treatment before the deposition of the nickel coating.
Potentiodynamic polarization curves were used to determine the kinetics of the cathodic and anodic reactions. The intersection of Tafel regions of the cathodic and anodic branches (Fig. 11) gives the corrosion electrochemical parameters listed in Table 4 for the cold-sprayed nickel coating after abrasive grit-blasting treatment.
Table 4
Corrosion electrochemical parameters for the cold-sprayed nickel coatings after pretreatment
Types of corundum powder
Ecorr × 10−3, V vs. SCE
jcorr × 10−3, A cm−2
− βc, V dec−1 × 10−3
βa, V dec−1 × 10−3
Lack (NG)
− 348
0.030
80
190
Medium grit (MG, size of 600-710 μm)
− 358
0.016
105
215
Coarse grit (CG, size of 1700-2000 μm)
− 292
1.500
200
420
The analysis of the data in Table 4 revealed that the corrosion electrochemical parameters for the CS nickel coatings (MG) and (CG) were changed compared to the specimen (NG). The use of the surface treatment of medium-grained powder makes both corrosion potential and corrosion current density values decrease. However, for the specimens with coarse-grained treatment, the corrosion potential value shows a slight shift toward the noble direction, while the corrosion current density dramatically increases. The changes in slopes showed the influence of abrasive treatment of the cold-sprayed nickel coating in the both cathodic and anodic reactions. Moreover, the influence is more pronounced in the anodic polarization plots compared to that in the cathodic polarization plots.
Table 5 gives the polarization resistance and corrosion rate values for the CS nickel coatings: (NG), (MG), and (CG).
Table 5
Polarization resistance and corrosion rate for the cold-sprayed nickel coatings after pretreatment
Types of corundum powder
Rp, Ω cm2
υcorr, mm y−1
Lack (NG)
814
0.32
Medium grit (MG, size of 600-710 μm)
1914
0.17
Coarse grit (CG, size of 1700-2000 μm)
39
16.15
It has been found that the Rp and υcorr values for the nickel coatings (MG) and (CG) significantly change compared to (NG) specimen. It is worth noting that by using the medium-grained corundum powder, the corrosion rate of the treated specimen decreases twice, whereas in the case of the coarse-grained corundum powder, the corrosion rate of the substrate increases even fifty times. In general, the use of the medium-grained corundum for the treatment of the substrate contributes to the improvement in corrosion resistance of the CS nickel coating. On the other hand, the use of the coarse-grained corundum powder for the treatment of the substrate causes the corrosion resistance of the nickel layer to lower significantly. However, the use of various grit-blasting conditions showed that there was a significant effect of the substrate surface roughness on particle adherence. This was due to changes in particle anchoring mechanisms, i.e., in contact surface area of the particle-substrate. The authors (Ref 10) have come to similar conclusions. However, the coarse grit-blasting treatment of the substrate results in an increase in surface roughness, so the cold-sprayed nickel layer does not have the same thickness. Therefore, an aggressive electrolyte easily reaches the substrate, causing the corrosion of the material. Moreover, on the CS nickel coatings on the grains boundary precipitates a solid phase in the form of NiOads, which may be responsible for increasing the corrosion rate of the metal. This problem will be discussed later in this article.
Surface Images of the Substrates and Cold-Sprayed Nickel Coatings: MG, CG
The scanning electron microscope (SEM) was used to observe the images of the medium grit-blasted and coarse grit-blasted treatment of the substrate or cold-sprayed nickel coatings (MG, corundum size of 600-710 μm) and (CG, corundum size of 1700-2000 μm) before exposure in the acidic chloride solution and corrosion effect of the CS nickel coatings (MG) and (CG) after (24 h) contact with acidic chloride solution, Fig. 12.
Fig. 12
SEM surface images: (a) medium grit-blasting (MG, corundum size of 600-710 μm), (d) coarse grit-blasting (CG, corundum size of 1700-2000 μm) treatment of the substrate and cold-sprayed nickel coatings with: (b) medium grit-blasting (MG), (e) coarse grit-blasting (CG) treatment of the substrate before exposure in the acidic chloride solution or (c) medium grit-blasting (MG), (f) coarse grit-blasting (CG) treatment of the substrate after exposure in the acidic chloride solution. Solution contained 1.2 M Cl−, pH 1.5
×
It was observed that the substrates after medium grit-blasting and coarse grit-blasting treatment are completely different. The substrate (MG) is almost smooth, Fig. 12(a), whereas the nickel substrate (CG) has numerous deep scratches and dents. However, the specimens after (MG) and (CG) treatment of the substrate are tightly covered by cold-sprayed nickel coatings (Fig. 12b and e). It is clearly seen that nickel particles are pressed into the substrate. Moreover, the surface of both specimens is rough but there are no surface defects in the form of cracks or holes. The structure of the Ni coating for the coarse grit-blasting treatment of the substrate is less homogeneous than the Ni coating shown in Fig. 12b). Figure 12(c) and (f) shows the specimens after 3 h of immersion of the working electrode in the testing solution. As a result of the exposure of specimens in the aggressive electrolyte, the CS nickel coatings were destroyed. The attack by Cl− is seen to be more at grains boundary since these regions are highly susceptible to the corrosion effect in aggressive environment. The image (Fig. 12f) shows that cold-sprayed nickel coatings (MG) protected the substrate properly before the corrosion effect in the acidic chloride solution. However, in the case of the Ni coating (CG), the surface of the specimen was seriously corroded (Fig. 12f) because numerous holes and pits can be seen on the surface. Therefore, an aggressive electrolyte easily reaches the substrate, causing the corrosion of the material. The medium grit-blasting treatment induced small surface irregularities, which were erased due to bigger particle impinging. Moreover, the use of medium corundum particles (with low diameter, i.e., 600-710 μm) was beneficial in this case. In order to avoid oxide contamination, the Ni coatings were applied immediately after the preparation of the substrate.
Figure 13 presents the x-ray diffraction patterns obtained for the nickel powder and cold-sprayed nickel coatings with medium grit-blasted or coarse grit-blasted treatment of the substrate after exposure in acidic chloride solution.
Fig. 13
X-ray diffraction patterns obtained for the nickel powder and cold-sprayed nickel coatings with: medium grit-blasting (MG, corundum size of 600-710 μm) or coarse grit-blasting (CG, corundum size of 1700-2000 μm) treatment of the substrate after exposure in the acidic chloride solution. Solution contained 1.2 M Cl−, pH 1.5
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The XRD analysis was performed to identify the various phases formed during the exposure of specimens in the aggressive solution. The corrosive environment was not deoxygenated. Therefore, the presence of nickel corrosion products in the form of various nickel and oxygen compounds was found on the cold-sprayed nickel surfaces. In addition, nickel (II) oxide has been detected as one of many nickel corrosion products in the acidic chloride environment.
Conclusions
The following conclusions can be drawn from the study:
1.
The SEM cross-sectional micrographs revealed a good homogeneous connection between the substrate and the nickel coating layer for all specimens.
2.
The cold-sprayed nickel coating undergoes a multistep corrosion process, especially in the aggressive acidic chloride environment. However, the specimen surface was covered with a porous layer of nickel oxide.
3.
The SEM cross-sectional images revealed that the cold-sprayed nickel coating (NG) after exposure in the acidic chloride solution was in the better condition compared to the nickel substrate. Therefore, the CS nickel coating protects the substrate against corrosive attack.
4.
It turned out that in the aggressive chloride solution, the corrosion rate of the CS nickel coating was much less in comparison with the base metal.
5.
For the abrasive grit-blasting treatment of the substrate, the corundum powders with different grain sizes were used. It was found that ABT of the substrate changes the corrosion resistance of the nickel coating.
6.
The most corrosion resistance of the cold-sprayed nickel coating in the acidic chloride environment was obtained after the medium grit-blasted (MG) treatment of the substrate, with grains of corundum size of 600-710 μm.
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