The corrosion behaviours of plasma-sprayed Fe-based amorphous coatings

Introduction

Coal plays a vital role in modern electricity generation worldwide due to its geographical ubiquity and abundance, and affordability [1,2]. Coal is presently reported as an important energy source given its 26.5% share of world primary energy. The typical utility combustion boiler for burning coal provides an aggressive environment in terms of erosive wear and high-temperature corrosion for the materials [3]. In a coal-fired boiler, oxidation and sulphidation can be problematic depending on the oxygen and sulphur levels, and burning fuels with significant amounts of sodium, potassium, vanadium, etc. can result in the formation of ash and salt deposits [4]. For example, T91 ferritic steel has been widely used in the boilers of power plants worldwide but suffers from an extensive oxidation attack in the service environment given that the oxide scale penetrates deep into the substrate. The inferior resistance of bare T91 steel is attributed to the formation of porous and loosely bound Fe₂O₃-rich oxide scales following long-term service [5]. In the combustion system, Na and S react with each other to form Na₂SO₄, whereas vanadium reacts with oxygen to form V₂O₅ and other complex vanadates. These compounds, commonly known as ash, deposit on the surface of the materials and induce corrosion [6]. The boiler steel corrosion at locations below ash deposits is typically much more severe than it is in a homogenous atmosphere [7].

Attempts to extend the service lifetime and to decrease the maintenance costs of these components have resulted in the increased interest of shielding them with protective coatings. Jayaraj et al. [8] reported that a Ni-based amorphous coating exhibits good corrosion behaviour in H₂SO₄ solution by forming a stable passive layer, and the passive layer may consist of oxides and spinel-type oxides of nickel, aluminium, and chromium. Similarly, studies on Ni–20Cr coatings on an Al alloy substrate have presented the formation of protective oxides such as Cr₂O₃, Al₂O₃, SiO₂ and NiO [9]. Aluminium penetrated from the bond coat to the top coat along the splat boundaries. Oxides of chromium, nickel and aluminium are recognised as protective oxides for boiler environment. The coatings prove to be beneficial in providing better corrosion resistance under a hot salt environment such as with Na₂SO₄, K₂SO₄, NaCl and KCl. Kaur sprayed Cr₃C₂–NiCr coatings on SAE-347H boiler steels by the detonation gun spray process [10]. The coating exhibited good adherence to the boiler steels during hot salt exposure and good corrosion resistance by the formation of nickel and chromium oxides. Among these novel surface coatings, Fe-based amorphous coatings present excellent corrosion resistance along with relatively low costs due to their chemical homogeneity [4]. In addition to their excellent corrosion resistance, Fe-based amorphous coatings that exhibit high strength and hardness can be used to prevent the penetration of hot gases, molten ashes and liquids [11]. For example, the Defense Advanced Research Projects Agency in the United States has developed a kind of Fe-based amorphous coating product with ultrahigh hardness and excellent corrosion resistance, which has been widely used in ships, naval military components and nuclear radiation protection [12].

The coating thickness was reported to significantly influence the electrochemical corrosion behaviour of the coated steel [13]. A thin coating allows the electrolyte to easily pass until it has reached the coating-substrate interface [14]. Wang et al. [15] stated that the corrosion resistance of the coatings increased following a coating thickness increase from 120 to 400 μm. The reduced corrosion resistance observed in the thinner coatings was assumed to be associated with the formation of through-porosity by the connection of the individual porosities along the thickness of the coating [16].

Though Fe-based amorphous coatings are used in many fields for corrosion resistance at room temperature, they have been minimally reported in terms of their use for boilers of power plants at high temperatures. Considering the good performance of Ni-based and Cr-based coatings in hot corrosion environments [17,18] and the effect of the coating thickness on corrosion resistance, the present study prepared Fe-based amorphous coatings with high Cr and Ni contents at three different thicknesses by the plasma spraying technique on a T91 alloy substrate. The hot corrosion resistances of the coatings in hot salt were investigated. The present study is expected to generate valuable data and references for the potential research and application of these Fe-based amorphous coatings.

Experimental procedures

A T91 boiler tube was obtained and cut into several 15 × 15 × 5 mm specimens. Spraying powders with spherical morphology and a diameter of 50 ± 10 μm were prepared by water atomisation. The chemical compositions of the Fe-based powders and the T91 substrate are listed in Table 1.

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Table 1

Chemical composition of the powder and substrate.

	C	Cr	Mo	V	Si	Ni	N	Mn	Nb	P	B	Fe
Coating	3.04	15.3	8.56	…	5.31	10.83	…	…	…	0.94	0.56	Bal.
Substrate	0.1	0.1	0.4	0.2	1.0	0.4	0.05	0.5	0.1	…	…	Bal.

Prior to plasma spraying, the specimens were sandblasted with brown corundum and were then washed with acetone to clean the surface. The surface roughness of the substrate following sandblasting was measured at 10.8 ± 1.2 μm (R_a). Thermal spraying was executed by an air plasma spraying system (GP-80, China) with an optimised power level of 35 kW (700 A/50 V). The optimised pressures of the primary gas (argon) and the auxiliary gas (hydrogen) were 0.7 and 0.68 MPa, respectively. The flow rates of argon and hydrogen were 60 and 6 L min⁻¹, respectively. The spraying distance and the feeding rate of the powders were measured at 100 mm and 10 g min⁻¹, respectively. During spraying, three deposition times were selected to obtain coatings with three different thicknesses. One side of the specimens was fully coated with the Fe-based amorphous coating. The uncoated substrate surfaces were protected against hot corrosion by plasma spraying alumina coatings.

The thickness of the coatings was evaluated by means of a cross-sectional study of the sample using a micrometer scale system coupled onto a microhardness tester (HX-1000TM/LCD, China). Five measurements were collected and the average was calculated. The percentage porosity of the coatings was evaluated by SEM micrograph analysis using Image-Pro Plus version 6.1 software (Media Cybernetics).

The hot corrosion tests of the Fe-based amorphous coatings were conducted in 70 mol-% Na₂SO₄ and 30 mol-% K₂SO₄ salt heated to 700°C for 84 h to simulate the coal-fired boiler corrosion rate of the hot corrosive environment. The surfaces of the specimens were brushed with saturated 70 mol-% Na₂SO₄ and 30 mol-% K₂SO₄ solutions prior to the hot corrosion tests and dried at 200°C in a box-type resistance furnace. The above steps were repeated until the total coating weight of each specimen was measured at about 3–4 mgcm⁻², thereby maintenance the consumed salts throughout the duration of the hot corrosion tests. The specimens were then embedded in salt particles of the same composition and heated to 700°C for 84 h in a furnace. The mass gain of each specimen was measured by an electronic balance (JA5003B, China) at 2, 4, 8, 12, 24, 36, 48, 60, 72, and 84 h times intervals, respectively.

The cross-sectional microstructure of the coatings was examined by scanning electron microscopy (SEM, S-4800, Hitachi, Japan) using an energy dispersive spectrometer (EDS). The analyses of the coating phase compositions was conducted by X-ray diffraction (XRD, D/max 2500PC, Rigaku, Japan) at a radiation of Cu Kα (λ = 1.5418 Å) within the range of 20–80° (2θ).

Results

Microstructural characterisation of the coating

Figure 1 presents the cross-sectional SEM micrographs of the Fe-based amorphous coatings. Given that the same parameters were used for the coating deposition, all the coatings exhibited similar lamellar structures with a porosity of 2.4 ± 0.3%, and a coating thickness of 100 ± 6, 270 ± 22 and 380 ± 18 μm, respectively.

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Figure 1

Cross-sectional microstructures of Fe-based amorphous coatings with different thicknesses. (a) 380 ± 18 μm, (b) 270 ± 22 μm, (c) 100 ± 6 μm.

Hot corrosion behaviors

Figure 2 illustrates the mass gain of the specimens embedded in the corrosion reagents at every corrosion cycle. The mass gain of the substrate linearly increased to 21.6 mg within the initial 12 h time interval, and generated a corresponding average gain rate of 1.8 mg h⁻¹. The increase of the hot corrosion time from 12 to 24 h slowed the mean mass gain rate of the substrate to 0.69 mg h⁻¹. A further increase in the corrosion time again reduced the mean mass gain rate of the substrate to 0.094 mg h⁻¹, where it was maintained.

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Figure 2

Corrosion kinetic curves of Fe-based amorphous coatings and T91 substrate tested in Na₂SO₄ + K₂SO₄ salts heated to 700°C for 84 h.

Similarly, the corrosion behaviour of the coatings presented three stages (0–4 h, 4–12 h and 12–84 h) with lower mass gain rates as compared to that of the substrate. For all three coating thicknesses, the mean mass gain rate of each stage was lower than that of the substrate. Moreover, an increase in the coating thickness resulted in a decrease in the average mass gain rate. The mean gain rate of the first stage was calculated to be 1.64, 1.32 and 1.21 mg h⁻¹ for the 100, 270 and 380-μm-thick coatings, respectively. In terms of the second stage, the mean gain rates of the three thicknesses decreased to 0.25, 0.197 and 0.169 mg h⁻¹, respectively. A further reduction in the mean gain rate to 0.051, 0.020 and 0.018 mg h⁻¹, respectively, was observed in the third stage of the hot corrosion process.

No significant mean gain rate difference was observed in the coatings with thicknesses of 270 and 380 μm following a hot corrosion time of more than 12 h, thereby suggesting that the Fe-based amorphous coatings with a thickness within the range of 270 and 380 μm can provide effective protection to the substrate. The surface morphology of the 380-μm-thick coating following the cyclic hot corrosion in the salt environment is presented in Figure 3(a). No apparent cracks were formed on the coating during the hot corrosion test, which may be attributed to the relative compatibility of the thermal expansion coefficients of the coating and the substrate. The EDS spectrum of the selected region is presented in Figure 3(b), wherein a high concentration of Fe and O elements were observed, thereby indicating the possible formation of Fe-rich oxides. In addition to Fe and O, Na, K, S, Cr and Ni were also observed on the surface of the coating following the hot corrosion test, thereby indicating the formation of Fe, Cr and Ni oxides and sulphides.

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Figure 3

(a) Surface micrograph and (b) EDS spectra of the coating with thickness 380 μm after hot corrosion in Na₂SO₄ + K₂SO₄ salts.

Figure 4 presents the XRD patterns of the 380-μm-thick coating before and after hot corrosion at 700°C for 84 h. Before hot corrosion, a broad halo peak was observed. In addition, the sprayed coatings exhibited an obvious crystalline peak, which was later verified as the supersaturated α-Fe solid solution phase. The surface of the coating following hot corrosion exhibited the presence of Fe₂O₃, NiO, NiCr₂O₄ and Cr₂O₃.

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Figure 4

XRD patterns of the 380-μm-thick coating before and after hot corrosion.

The cross-sectional images of the coatings after hot corrosion are presented in Figure 5. A dark strip region was observed in Figure 5(a) at the interface of the 100-μm-thick coating and the substrate, thereby indicating the occurrence of oxidation and corrosion on the substrate surface and suggesting damage on the substrate surface, which agrees with the kinetics curve presented in Figure 2. The coatings did not exhibit dark strips at the interfaces following a thickness increase to 270 and 380 μm, respectively, as presented in Figure 5(b,c). An apparent oxidation was observed at the boundary between the splats in all the coatings, especially around the voids of the coatings.

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Figure 5

SEM cross-sectional images of the coatings after hot corrosion. (a) 100 ± 6 μm, (b) 270 ± 22 μm, (c) 380 ± 18 μm.

A high amount of oxygen and sulphur was observed at the interfaces between the coating and substrate, and along splat–splat interfaces, as presented in Figure 6. That is to say, these regions enhance the formation of oxides and sulphides. The EDS spectra of the surface of the oxidised coating exhibited a small amount of sulphur given that the sulphates easily permeated into the coating during the hot corrosion testing.

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Figure 6

Cross-sectional EDS spectra of the coating with thickness 100 μm after hot corrosion in Na₂SO₄ + K₂SO₄ salts.

Discussion

The mass gain during hot corrosion is closely related to the oxidation and corrosion behaviours of the coating and substrate. The rapid increase in the mass gain of all the samples at the initial hot corrosion stage may be as result of the rapid oxygen pick-up during the diffusion of oxygen through the molten salt layer [9]. Given the high affinity of chromium to oxygen, the formation of Cr₂O₃ also contributed to the mass gain in the earlier stages of hot corrosion. The high content of Cr in the Fe-based amorphous composite coatings characterises them as one of the best alloy coatings to resist hot corrosion in molten sulphates given that Cr preferentially reacts with O₂ in molten sulphates to form Cr₂O₃, which acts as a diffusion barrier to the inward diffusion of the corrosive medium [19].

During hot corrosion, a series of specific reactions occurred prior to the formation of Cr₂O₃ [20].

Firstly, certain specific reactions occurred in the molten fluids. (1) (2) The Cr ions then reacted with at the coating interface, thereby forming the corresponding oxides. (3) Following the formation of uniform and compact oxide film on the surface of the coating, the growth of corrosion products decreased because the stable oxide film hindered the permeation of the corrosive medium in the salt towards the interior of the coatings. In the stable corrosion stage, the average mass gain rate was maintained at a low level, as presented in Figure 2. The element Ni reacted with Cr and O and formed NiO and NiCr₂O₄, which were confirmed by the EDS and XRD analyses. The protective behaviour of the coated specimens may be a result of the formation of nickel chromium oxide.

The through-porosity of the coating is sensitive to the coating thickness. An increase in the thickness resulted in an increase in the coating corrosion resistance for the Na₂SO₄ + K₂SO₄ hot salts. The observed substrate corrosion beneath the coating, which was a result of the through-porosity, was previously confirmed by Zhang et al. [16] through the 3D tomography reconstruction of the coated sample. The through-coating porosity of the presented hot corrosion test was too small to allow the corrosion medium to penetrate through the coating. Therefore, the substrate was not damaged at coating thicknesses of 270 and 380 μm, as presented in Figure 5(b,c), thereby demonstrating that an increase in the coating thickness generates a significant decrease in the through-coating porosity and effectively protecting the substrate at high-temperature corrosive environments.

The voids, pores and boundaries of the splats are typical features present in plasma sprayed coatings. The amorphous coating was reported to exhibit preferential corrosion in the defects [21]. During the hot corrosion process, the entrapped air results in the oxidation of the splat boundaries and contributes to the initial weight gain [22]. The oxides were also reported to form at the splat boundaries, thereby creating a denser coating and decelerating the diffusion of the oxidising medium to the internal portion of the coating, which in turn stabilises the mass gain and oxidation rate as the exposure time progresses [23]. During the hot corrosion test, the Cr₂O₃, NiO and NiCr₂O₄ films easily formed on the surfaces of the splats, and provided effective protection by hindering the diffusion of the cations [24].

However, the thin coating had many through-coating porosities such that the corrosive liquid reached the metal substrate. The mechanism for the mass gains under this condition is different than that of the coatings with a thickness of 270 and 380 μm, respectively. Many of the iron corrosion products simultaneously formed during the hot corrosion test, with the exception of the Cr oxides and Ni oxides. The following reactions were observed during the process [25]: (4) (5) (6) (7)

As the corrosive liquid reached the metal substrate, the corrosion reaction became more complex and generated more corrosion products. Although the T91 substrate and the Fe-based coating have homogeneous properties, the combining mode of the plasma thermal spray coating was primarily exhibited mechanical bonding. Therefore, spallation may be considered the final service failure under the specified hot corrosion conditions at an insufficient protective coating thickness.

Conclusions

(i) An increase in the coating thickness generated a decrease in the average mass gain rate in the hot corrosion tests. In the stable stage, an average mass gain rate of 0.094, 0.051, 0.020 and 0.018 mg h⁻¹ was measured for an uncoated substrate with a coating thickness of 100, 270 and 380 μm, respectively.