Effect of inorganic silicate sealing treatment on corrosion behaviour for HVAF sprayed Fe-based amorphous coatings

Abstract

Sealing treatment is an effective, environmentally friendly, and economical coating surface modification technology. To clarify the inorganic silicate sealing mechanism and concentration dependence on corrosion behaviour, HVAF-sprayed Fe-based amorphous metallic coatings (AMCs) were sealed by Na₂SiO₃ solution with various concentrations, and their microstructure, electrochemical performance, and surface chemistry were characterised in detail. The results showed that Na₂SiO₃, in the form of a silicon-oxygen bond (Si-O), could effectively bond with the coating surface inside the pore defects. In addition, the optimal concentration was proved to be 1 mol/L, and the corresponding passivation current density could be reduced to (2.38 ± 0.33) × 10⁻⁶A/cm², which was an order of magnitude lower than that of as-sprayed coating. This was due to the incomplete filling of the sealant at lower concentrations and the release of water vapour at higher concentrations. This work aims to provide guidance for the practical application of silicate sealing treatment.

Keywords

Corrosion prevention thermal spraying Fe-based amorphous metallic coating sealing treatment passive film pore defect polarization silicate

Introduction

Fe-based amorphous metallic coatings (AMCs) generally exhibit excellent corrosion and wear resistance and are usually used to protect mild steel and other substrates from corrosion and wear. These merits make them one of the best coating materials for use in marine, spent nuclear fuel storage, oil drilling industries, and other corrosive and abrasive environments. Fe-based AMCs have been successfully prepared by using atmospheric plasma spraying (APS), vacuum kinetic spray (VKS) [1], detonation spray [2], high velocity oxygen fuel (HVOF), high velocity air fuel (HVAF), etc. Among these technologies, coatings prepared by HVAF possess low porosity, strong bonding between coatings and substrates, and high degree of amorphicity [3]. However, pore defects are inevitably formed during the spraying process since the incomplete spread of the molten particles, volume shrinkage during cooling, and the rough surface condition of the substrate [4]. The coating may delaminate and peel eventually due to pore defects, which may act as channels facilitating the osmosis of corrosive media inside the coating [5 –7]. Therefore, the porosity greatly affects the corrosion behaviour of coatings and limits their service life in practical applications.

To reduce the porosity of coatings, recently, many methods have been proposed, such as optimising preparation processes [8 –10], laser remelting [11], annealing [12], and sealing treatment [13 –16]. Among these methods, sealing treatment can effectively block the pore defects to enhance the corrosion resistance and thus has gained much attention. Recently, research and investigations of both organic and inorganic sealants were conducted. Organic sealants commonly contain epoxy resins [17], phenolic resins [18], vinyl methyl silicone [19], and sol–gel [20, 21]. They could perform a potential improvement of corrosion resistance, but the disadvantages of poor mechanical properties and complex operation procedures are hard to avoid. As for inorganic sealants, they generally generate specific chemical reactions and thus can perform higher bonding strength and superior corrosion resistance, compared to organic sealants. However, some flaws have been gradually exposed in the process of application. For example, coatings sealed with inorganic cerium salt might cause the formation of microcracks, which directly deteriorated the physical barrier property [22]. Sol–gel sealants exhibited the behaviour of shrinkage during the sealing process, resulting in poor corrosion resistance [23].

Silicate shows potential applications in the field of surface treatment. It can spontaneously transform from a silanol group (Si-OH) to siloxane bridges (Si-O) by polymerisation reaction, which can provide the ideal bonding strength between silicate and coating surface. The treatment has been successfully utilised for surface sealing of hot dip galvanised (HDG) sheets [24, 25], aluminium alloys [26 –33], and magnesium alloys [34]. It was shown that the anti-corrosion of coatings sealed with silicate depended mainly on the concentration of silicate. Kazemi et al. [27] sealed the aluminium alloy with silicate solution and found that a higher concentration contributed to a higher pitting potential (E_pit), and a lower passivation current density (I_pass), showing a splendid sealing effect. In addition, it was also believed that there existed a tendency of increasing at first and then decrease in corrosion resistance with increasing silicate concentration. Therefore, it is necessary to clarify the concentration-dependent sealing mechanism.

In this paper, sodium silicate (Na₂SiO₃) solution was used as the sealant for Fe-based AMCs prepared by HVAF. The corrosion behaviour of AMCs was studied by electrochemical polarisations and electrochemical impedance spectroscopy (EIS). X-ray diffractometer (XRD), Scanning electron microscopy (SEM), energy spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and confocal laser scanning microscopy (CLSM) were utilised to analyse the microstructure, surface chemical, and sealing mechanism. This paper reveals the reaction mechanism of Na₂SiO₃ solution sealant on the sealing process of the thermally sprayed amorphous metallic coatings and the dependence of the corrosion resistance of the sealed coatings on the sealant concentration. This work provides a sealing treatment method for thermally sprayed metallic coatings and also provides a basis for the practical application of thermally sprayed amorphous alloy coatings.

Experimental

Materials

In this study, Fe₅₀Cr₁₈Mo_7.5Ni_3.5P₁₂B₃C_3.5Si_2.5 (at.%) was used as the composition of AMCs. The gas atomisation was utilised to prepare the Fe-based alloy powders, which could manufacture powders at 1350°C and a dynamic pressure of 7 MPa. The powder sizes ranging varied from 15 to 49 μm. Fe-based AMCs were prepared using the AcuKote HVAF thermal spray system, and the detailed spraying parameters of the HVAF process is shown in Table 1. Mild steel plates of 400 × 200 × 5 mm³ were used as the substrate material, which was pre-treated by mechanical grinding, acetone degreasing, air drying, and sandblasting before spraying.

Table 1

Spraying parameters employed in the HVAF process.

Parameters	Conditions
Powder delivery rate (rpm)	6
Fuel pressure (MPa)	0.519
Fuel flow rate (m3 /min)	0.127–0.132
Nitrogen flow rate (m3 /min)	0.027
Hydrogen flow rate (m3 /min)	0.035
Air pressure (MPa)	0.627–0.634
Traverse velocity (mm/s)	1000
Spray distance (mm)	220

The schematic illustration of the sealing treatment is shown in Figure 1. At first, the AMC samples were sonicated in acetone for 20 min and in 95% ethanol for 10 min to remove the oil stains and esters. Subsequently, the samples were placed in the Na₂SiO₃ solution with different concentrations for 10 min. To obtain better permeability, the sealing process was kept in a vacuum drying chamber (DZF-6050, Shanghai, China) for 5 min. Finally, the samples were transferred to a 120°C-heating chamber for 30 min to cure the Na₂SiO₃. The Na₂SiO₃ concentrations used for sealing treatment were 0.25, 0.5, 1, 2, and 3 mol/L. The as-sprayed coatings and coatings sealed with different concentrations were named As-sprayed, Sealed-0.25, Sealed-0.50, Sealed-1.00, Sealed-2.00, and Sealed-3.00. All coating samples for microstructural characterisation, electrochemical measurements, and surface chemical analysis were cut into 7 × 7 × 5 mm³ pieces. The measured surfaces should be mechanically ground with 2000 SiC sandpaper and polished with 0.5 μm diamond polishing paste.

Microstructure characterisation

The scanning electron microscopy (SEM, Zeiss Supra 55) and energy dispersive spectroscopy (EDS) were utilised to characterise the microscopy of specimens. The study of phase structures was completed by a Rigaku D/Max 2400 X-ray diffractometer (XRD, D8 Advance Cu-Kα radiation). The penetration degree of the sealant was characterised by confocal laser scanning microscopy (CLSM, Olympus LEXT-OLS4000).

Electrochemical analysis

A Gamry Reference 600 + instrument was utilised to conduct electrochemical evaluations of samples. A standard three-electrode system was prepared in a 1.5-L electrolytic cell in which a platinum plate and a saturated calomel electrode (SCE) severed as the counter electrode and reference electrode, respectively. The coating samples were used as the working electrodes (WE) with a nominal area of 0.49 cm². All the experiments used 3.5 wt.% NaCl solution as electrolyte environment and was carried out in air at 298 K. The potentiodynamic polarisation tests were performed from −0.4 V_OCP to 1.2 V_SCE at a scan rate of 20 mV/min after the open circuit potential (OCP) was stabilised.

Potentiostatic polarisation tests were carried out at 0.5 V_SCE to evaluate the stability of the passive film formed upon the coating samples. Employing a sinusoidal potential perturbation of ±10 mV in the frequency range of 10 kHz–10 mHz to execute the electrochemical impedance spectroscopy (EIS). The Zview software was used to fit the data of the EIS. All electrochemical tests were performed five times.

Surface analysis

X-ray photoelectron spectroscopy (ESCALAB250) was applied for evaluating the surface chemistry of coatings. The Carbon contamination (284.6 eV for the C 1 s) was employed to calibrate binding energies, which sputtered under 2 kV and 2 μA argon ion beam at a rate of 0.2 nm/s.

Results and discussion

Microstructure characterisation

The cross-section morphology of the coating used for the sealing treatment was shown in Figure 2(a), which could be observed a uniform structure with an overall thickness of ∼400 μm. The flat molten powders were clearly visible, which implied the heated powders processed a good melting and spreading state during the thermal spraying process. It was observed the existence of pore defects in the coating, which was due to inadequate stacking and cooling shrinkage. Figure 2(b,c) shows the surface morphology of the As-sprayed coating. It could be obviously observed the sufficient spread state of splats and the existence of pore defects. The EDS (Figure 2(d-i)) demonstrated a homogeneous distribution of the main elements upon the coating surface.

The typical cross-section micromorphology of the coating (Sealed-1.00) is shown in Figure 3(a), and the surface pore defects were observed to be sealed. Figure 3(b,c) shows the surface morphology of the coating after sealing treatment. It was obviously visible that there existed sealant in pore defects. Figure 3(h,i) shows that Si and O were obviously enriched in the pore defects, but Fe, Cr, Mo, and Ni were depleted inside the pore defects. These results indicated that pore defects were effectively blocked by the sealant.

To determine the effective ingredient of sealant, the typical XRD results of the original coating (As-sprayed) and sealed coating (Sealed-1.00) are shown in Figure 4. Both the two samples exhibited similar XRD patterns with broad diffraction peaks, indicating the amorphous structure of coatings. At 2θ = 40°, there exhibited an obvious crystalline peak of Sealed-1.00, showing the formation of Si-O bonds during the sealing treatment. Its known that Na₂SiO₃ utilised in this work behaved as a salt with strong alkali and weak acid properties. When it was dissolved in an aqueous solution, the silicate ion tended to rapidly hydrolyse to generate Si(OH)₄, as shown in Equation (1). Subsequently, the single molecule Si(OH)₄ continuously polymerised and dehydrated to form a large molecule polysilicate with the structure of Si-O-Si (SiO₂) and release water vapour during the thermal curing process at 120°C, as shown in equation (2). Ultimately, the adjacent Si-O-Si cross and link with each other to form SiO₂ with three-dimensional network structure, which was the main component to fill the pore defects. (1) (2)

Electrochemical corrosion behaviour

Potentiodynamic polarisation tests

Figure 5 shows the potentiodynamic polarisation behaviours of the As-sprayed and sealed coatings, and the Tafel fitting results are shown in Table 2. As for the As-sprayed coating, the relatively low corrosion current density (I_corr) and passivation current density (I_pass) reflected the superior anti-corrosion. However, within the passivation region, it exhibited significant current fluctuations, indicating that the As-sprayed coating was prone to initiating metastable pitting corrosion. It was obviously observed that the phenomenon of metastable pitting was inhibited after the sealing treatment. With the increased of Na₂SiO₃ concentration, the self-corrosion potential (E_corr) increased to (−0.33 ± 0.04) V_SCE at first and then decreased, showing excellent corrosion resistance. I_pass could decrease to the lowest level of (2.38 ± 0.33) × 10⁻⁶ A/cm² when the Na₂SiO₃ concentration was 1 mol/L, which was about one order of magnitude lower than that of the As-sprayed coating.

Table 2

The electrochemical characteristic parameters of five time potentiodynamic polarisation tests for six coating samples.

Samples	I_pass (10⁻⁶ A/cm²)	I_corr (10⁻⁶ A/cm²)	E_corr (V_SCE)
As-sprayed	23.4 ± 0.38	4.98 ± 0.32	−0.39 ± 0.01
Sealed-0.25	12.9 ± 0.15	1.89 ± 0.17	−0.31 ± 0.05
Sealed-0.50	3.94 ± 0.41	1.28 ± 0.61	−0.26 ± 0.02
Sealed-1.00	2.38 ± 0.33	0.74 ± 0.42	−0.33 ± 0.04
Sealed-2.00	3.79 ± 0.42	0.71 ± 033	−0.38 ± 0.07
Sealed-3.00	8.54 ± 0.85	2.92 ± 0.21	−0.35 ± 0.04

The passivation current density I_pass was obtained at E= 0.5 V_SCE, and corrosion potential (E_corr) and corrosion current density (I_corr) were calculated by Tafel fitting of corresponding electrochemical polarisation curves.

Potentiostatic polarisation tests

To reveal more details about the influence of sealing treatment on the corrosion behaviours of AMCs, potentiostatic polarisation was performed for 1 h in a 3.5 wt.% NaCl solution at 0.5 V_SCE (within the passivation region). Figure 6 shows the current density-time plots for the As-sprayed and sealed coatings. For the first 300 s, the current density of all six coatings decayed continuously with time, which demonstrated the rapid formation of passive films. As for the As-sprayed coating, it showed successive current transients after 300 s, showing that the passive film was extremely unstable due to the occlusive effect inside the pore defects [35]. After the sealing treatment, the phenomenon of metastable pitting was inhibited and the curve tended to be smoother. Because the Si-O bond, detected from the XRD results, were firmly bonded to the coating surface inside the pore defects, which acted as an isolator to defend the aggressive electrolyte and avoid the massive adsorption of Cl⁻. With the increase of the Na₂SiO₃ concentration, I_pass exhibited a trendy to decrease at first and then increase, which was in keeping with the results of potentiodynamic polarisation. In summary, the sealed coating with 1 mol/L Na₂SiO₃ concentration exhibited the lowest current density and effectively eliminated metastable pitting.

Electrochemical impedance spectroscopy (EIS) tests

The EIS measurements further demonstrated that the sealing treatment could effectively improve the anti-corrosion of AMCs. Figure 7(a,b) shows the Nyquist and Bode results of As-sprayed and sealed coatings, where |Z| is the module of impedance, θ is the phase angle, and f is the frequency.

The results in Figure 7(a) show that the Nyquist curves of all coatings contain two compressed semicircular arcs, and the radius of the arcs increases after the sealing treatment, which reached the maximum of Sealed-1.00. This indicated that the 1 mol/L concentration of Na₂SiO₃ exhibited the optimal sealing effect.

Bode curves revealed typical double-time constants. Generally, the value of |Z|_10mHz was approximated as the resistance of the coating, where a higher |Z|_10mHz value stands for higher corrosion resistance. From the Bode plots in Figure 7(b), it could be found that with the increase of the Na₂SiO₃ concentration, |Z|_10mHz value became larger and reached the maximum (3.10 × 10⁴ Ω·cm²) at 1 mol/L concentration, which was an order of magnitude higher than that of the As-sprayed coating. However, with further increasing concentration of Na₂SiO₃, the |Z|_10mHz value decreased significantly. Overall, it showed an upward trend followed by a downward trend, and these results were in keeping with the results of the potentiodynamic polarisation. In this case, a two-time constant equivalent circuit diagram was utilised, as shown in the illustration of Figure 7(a), to simulate the surface electrochemical process of coatings. In this model, R_s is the solution resistance, R_c and CPE-c represent the resistance of the AMC and the non-ideal capacitance of AMC, R_t, and CPE-dl denote the charge transfer resistance and the non-ideal capacitance of double layer [36].

XPS analysis

The surface chemical analysis of the coatings was further explored by XPS, and Figure 8 shows the high-resolution spectra of Fe 2p, Cr 2p, Mo 3d, and Si 2p energy levels. The surface of coatings was dominated by Fe and Cr, where Fe 2p spectrum consisted of metal Fe, and , and Cr 2p spectrum included metal Cr, and . Cr could possess superior protective ability since it could form a dense Cr-based passive film. The Mo spectrum consisted of metal Mo, and , which could incorporate into the surface of Cr-containing oxides to prevent the active dissolution of Cr [37]. Although there existed abundant corrosion resistance elements in the As-sprayed coating, such as Cr and Mo, the presentence of pore defects might induce the deterioration of passive films or pitting corrosion inside the pore defects, which could be confirmed by the current transients in the potentiostatic polarisation. The Si 2p spectrum exhibited three characteristic peaks including Si, SiO_x, and SiO₂. After normalisation treatment of each element to the total composition, the amounts of different species can be calculated and the results are plotted in Figure 9. It was noted that Si spectra could also be identified in the surface of the As-sprayed coating with a lower content of 3.51%, which was attributed to the minor addition of Si in the Fe-based AMC [38]. With the increase of Na₂SiO₃ concentration, the Si content showed a trend of rising at first and then falling, while the contents of Fe, Cr, and Mo were almost constant. These results indicated that the optimal Na₂SiO₃ concentration was 1 mol/L.

CLSM surface morphologies

To further elaborate the sealing mechanism, typical surface microstructures of the coating were studied by LEXT. It showed that there were many obvious pore defects on the surface of As-sprayed coating (as shown in Figure 10(a)), which consisted of the SEM results. The height profile showed that As-sprayed coatings contained pore defects with a depth of ∼2.5 μm (Figure 10(e)). The coating sealed with a lower Na₂SiO₃ concentration (0.25 mol/L) could be observed that a small number of pore defects were filled as shown in Figure 10(b), and height profile in Figure 10(f). When the Na₂SiO₃ concentration increased to moderate concentration (1 mol/L), the pore defects were completely filled (Figure 10(c)), and exhibited a depth of ∼0.5 μm (Figure 10(g)), while the coating performed the best corrosion resistance. Figure 10(d) shows the morphology of the coating which was sealed by a higher Na₂SiO₃ concentration (3 mol/L), and it could be clearly observed that ruptures occurred inside the sealant. The height profiles of the coating were depicted in Figure 10(h), which exhibited obvious fluctuates.

Concentration-dependent sealing mechanism

According to the above results, the concentration-dependent sealing mechanism can be inferred, as shown in Figure 11. XRD results show that SiO₂ with Si-O acted as the primary ingredient to seal the pore defects. It could firmly bond with the coating surface inside the pore defects and form the structure of M-O-Si, as shown in Figure 11(b). This structure could avoid the accumulation and adsorption of Cl⁻ and inhibit metastable pitting corrosion, as seen in Figures 5 and 6.

When the Na₂SiO₃ concentration is low (<1 mol/L), a small amount of sealant cannot possess enough ability to fill the pore defects upon the surface. When the Na₂SiO₃ concentration is equal to 1 mol/L, the sealant can exhibit superior sealing performance, as indicated by the XPS analysis and CLSM surface morphology. When the Na₂SiO₃ concentration exceeds 1 mol/L, a large amount of sealant can penetrate into pore defects to participate in a violent polymerisation reaction, resulting in the formation of excess water vapour. It escaped rapidly from the pore defects and damaged the formed SiO₂, as shown in Figure 11(e). This process can deteriorate the sealing performance and corresponding corrosion behaviour.

Conclusion

In this study, Fe-based AMCs were sealed by Na₂SiO₃ solution with different concentrations were prepared. The sealing mechanism and concentration dependence on corrosion behaviour were systematically investigated. The following conclusions were drawn.

Na₂SiO₃ sealing treatment could significantly reduce the porosity of the AMCs. The Si-O bond formed after the condensation of Si–OH bond was tightly bonded with the coating surface inside the pore defects. The corrosion current density and passivation current density of the sealed coatings can be effectively reduced, and metastable pitting could be eliminated.

The sealed effect is very sensitive to the concentration of sealant. The potentiodynamic, potentiostatic, EIS, XPS, and CLSM results showed that the sealed coating with the Na₂SiO₃ concentration of 1 mol/L exhibited optimal corrosion resistance. The incomplete filling of the sealant at lower concentrations and the release of water vapour at higher concentrations accounted for the inferior anti-corrosion of the other sealed coatings. This work provides guidance for the selection of sealant types and concentrations for practical applications of coatings.

Footnotes

Acknowledgements

The authors wish to express the gratitude to Mr. D.B. Wang from IMR, CAS, China, for the assistance in improving the efficiency of data processing

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability

The raw/processed data required to reproduce these findings are available from the corresponding author upon request.

CRediT authorship contribution statement

Jinghui Ren: Methodology, investigation, writing – original draft, writing – review and editing, and resources. Tianrun Li: Methodology, data curation, and resources. Suode Zhang: Methodology, data curation, resources, and writing – review and editing. Min Xu: Methodology, data curation, resources, and writing – review and editing. Jianqiang Wang: Conceptualisation, methodology, writing – original draft, writing – review and editing, resources, and project administration.

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