| Citation: |
Wang Xu, Zhang Haiyan, Wei Wei, Gao Zhanqi, Ni Junjie, Huang Zhiquan. Effect of chromium on hot corrosion behavior of arc-sprayed NiCr coatings[J]. CHINA WELDING, 2024, 33(1): 1-6. DOI: 10.12073/j.cw.20231107016
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In this article, NiCr coatings with chromium content of 13%, 27% and 41% were prepared by arc spraying. They were exposed in molten salts (NaCl-Na2SO4) at 800 ℃ for 200 hours. The effect of chromium content on the hot corrosion behavior of the coatings was investigated. X-ray diffraction (XRD) and scanning electron microscope with energy dispersion spectrum (SEM-EDS) were used to analyze the phase compositions, morphologies and chemical compositions of the coatings. The results show that NiCr13 coating exhibited the worst hot corrosion resistance due to the low chromium content, which resulted in NiO being the major reaction product. It should be noted that the hot corrosion resistance of NiCr27 coating was better than that of NiCr41 coating. The basic fluxing of Cr2O3 lowered its protection during the hot corrosion process and led to the formation of porous Cr2O3 on the NiCr41 coating. The molten salts accelerated the oxidation reaction resulting in thicker and porous oxide scales formed on the surfaces of coatings.
According to the Chinese government’s policy, an increasing amount of municipal solid waste (MSW) should be disposed of by waste-to-energy (WTE) plants in the future[1]. In WTE boilers, the MSW is combusted, and the heat generated from the combustion process is collected and converted into electricity. However, a common problem faced by these boilers is the failure of certain components, such as superheater tubes, due to hot corrosion caused by molten salts. These failures result in unscheduled outage and economic losses[2 − 3]. With the growing adoption of WTE applications and higher operating temperatures, such failures are supposed to occur more frequently.
To address this issue, thermally sprayed coatings have shown the potential to provide better hot corrosion resistance[4 − 8]. Among these coatings, arc-sprayed coatings are particularly attractive because they can be easily manufactured on-site at a low cost. Since 1989[9], commercially available arc-sprayed NiCr-based coatings have been successfully applied to prevent hot corrosion.
Binary NiCr alloy coatings containing a minimum of 20 wt.% chromium have attracted significant attention from researchers due to their commendable corrosion resistance[10 − 13]. Varis et al.[11] reported promising performance of both NiCr49 and NiCr21 coatings produced by high velocity oxygen-fuel (HVOF) process in a KCl-K2SO4 salt mixture at 600 ℃.
Muthu et al.[12] employed HVOF process to deposit Ni -20Cr on superalloy 825. The coated superalloy 825 exhibited better corrosion resistance than the bare specimen when exposed to a Na2SO4-60%V2O5 environment at 900 ℃.
At the super heater zone of a thermal power plant boiler operating at a temperature of 700 ℃, samples coated with Ni-20Cr deposited using HVOF process[13] demonstrated improved performance in comparison to uncoated steel under erosion-corrosion conditions.
Coatings prepared by arc spraying have good performance in boiler anti-corrosion applications, but the key element Cr content has not been studied adequately on the hot corrosion resistance of the coatings, and this topic can be used to provide a reference basis for the design of anti-hot corrosion coatings.
NiCr cored wires with a diameter of 2.0 mm were specifically designed and manufactured as the raw materials for arc spraying. Table 1 presents the nominal compositions of the wires. The coatings were deposited on steel plates of SA213-T2.
| Wires | Cr | C＋Si＋Mn | Ni |
| NiCr13 | 13.0 | <1.0 | Balance |
| NiCr27 | 27.0 | <1.0 | Balance |
| NiCr41 | 41.0 | <1.0 | Balance |
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A commercial arc spraying gun, carried by a robot, was utilized to fabricate NiCr coatings. The spraying parameters are listed in Table 2. Prior to coating, the substrates were grit-blasted, cleaned with acetone, and subsequently dried using compressed air. For characterization analysis and tests, the coated samples were divided into two sizes. Block samples with dimensions of 15 mm × 10 mm × 3 mm were used for X-ray diffraction (XRD, Bruker D8 DISCOVER) and scanning electron microscope (SEM, FEI Quanta FEG650) analysis to identify the phase compositions, as well as the surface and cross-sectional morphologies of the coatings. The coatings were ground off from the substrate to obtain free-standing coatings with a diameter of 10 mm, which were prepared for hot corrosion tests.
| Current/A | Voltage/V | Stand-off distance/mm | Gas pressure/MPa | Gas type | Coating thickness/μm |
| 190 − 210 | 29 − 31 | 200 | 0.55 − 0.60 | Air | 300 − 400 |
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The mixed salt Na2SO4-10% NaCl, with a mass percentage, was chosen for the hot corrosion test in this study, which is molten at 800 ℃. The Na2SO4-NaCl salt system is frequently used to test the hot corrosion resistance of coatings and alloys[14 − 15]. Before the hot corrosion test, the free-standing coatings were cleaned with acetone and dried with compressed air. The temperature of the coatings was maintained at 120 ℃, and they were brushed with a water salt solution to obtain uniformly thick salt films on the coatings. The salt coated coatings were heated at 120 ℃ for 2 hours to remove moisture. The amounts of the salt on the coatings were 3.0−5.0 mg/cm2. The substrate SA213-T2 was used as a reference in the hot corrosion tests.
Crucibles were subjected to a heat treatment at 800 ℃ for 12 hours to eliminate any moisture prior to their use in the hot corrosion tests. The salt-coated coatings, along with the crucibles, were weighed at room temperature, and these weights were considered as the original mass. During the tests, the coating samples were placed inside the crucibles. The crucibles with the coating inside were then heated in a muffle furnace at 800 ℃ and removed from the furnace after 1 hour, 2 hours, 5 hours, and 10 hours, respectively. Subsequently, the crucibles were removed at intervals of 10 hours until the total test duration of 200 hours was completed. Two samples of each coating were utilized for the tests. After cooling to room temperature, the masses of the crucibles were measured using an electronic balance with an accuracy of 0.01 mg. The per unit area mass gains and test duration were used to plot the hot corrosion kinetic curves.
Prior to the further analysis, the corroded coatings underwent sequential washing with water and acetone, followed by dried at 120 ℃ for 2 hours. The phase compositions were identified by XRD performed with CuKα radiation. Morphologies of surface and cross section and chemical compositions were analyzed by SEM and energy dispersive spectrometer (EDS, Oxford Xplore).
The XRD results (Fig. 1) showed that solid solution NiCr existed in all as-sprayed NiCr coatings. In addition, the coatings exhibited the presence of oxides such as Cr2O3 and NiO, along with the spinel NiCr2O4. The peaks intensities of oxides and spinel were significantly lower in comparison to those of NiCr solid solution. This phenomenon indicated that the oxidation reaction occurred during the spraying process.
The cross-sectional morphologies of the as-sprayed NiCr coatings are shown in Fig. 2. These coatings exhibited typically lamellar structures, comprising light-colored and dark gray lamellae as well as particles. Comprehensive chemical analysis of these lamellae and particles was conducted via EDS. The light-colored areas exhibited notable enrichment in both nickel and chromium, while the dark gray areas demonstrated a substantial concentration of chromium and oxygen, with a small amount of nickel. By integrating these results with the XRD data, it could be deduced that the light-colored areas were primarily governed by the presence of NiCr solid solution, while dark gray areas were predominantly composed of Cr2O3, accompanied by a small amount of NiO and NiCr2O4. Furthermore, a slight increase in the abundance of oxides was observed with an increasing in chromium content.
Fig. 3 shows the hot corrosion kinetic curves of the NiCr coatings, along with substrate SA213-T2. Notably, the mass gains observed in the NiCr coatings were significantly lower in comparison to the SA213-T2 substrate, indicating superior hot corrosion resistance of the NiCr coatings over the substrate.
The mass gains of all the coatings exhibited a rapid increase during the initial 20 hours, followed by a gradual rise thereafter. However, distinct variations were observed in the mass gains among the different NiCr coatings. Among them, NiCr13 coating exhibited the highest mass gains, followed by NiCr41 and NiCr27 coatings. After 200 hour of hot corrosion test samles, their mass gains were measured to be 12.12 mg/cm², 3.47 mg/cm², and 6.24 mg/cm², respectively.
The corrosion kinetic curves were assumed to follow a logarithmic law.
| m=a+klln(t+b) | (1) |
The equation (1) was chosen to fit the curves, where m is mass gain, a and b are constants, kl represents the hot corrosion rate, t is hot corrosion duration. The kl values of NiCr13, NiCr27 and NiCr41 coatings were 1.97, 0.39 and 1.49, respectively. NiCr13 coating showed the highest kl value followed by NiCr41 and NiCr27 coatings.
Fig. 4 shows the XRD patterns of the NiCr coatings after hot corrosion. It was evident that the peaks corresponding to the NiCr solid solution were no longer detectable, suggesting that the reaction products in each coating were thick. The obstruction of X-rays by these reaction products prevents the detection of the underlying NiCr solid solution. The presence of NiO, Cr2O3 and NiCr2O4 was detected in all coatings. Specifically, NiO and Cr2O3 were the domination phases in NiCr13 coating and NiCr41 coating, respectively. In the NiCr27 coating, the peaks intensities of NiCr2O4 were more prominent than those of the other phases.
The surface morphologies of the NiCr coatings, along with the EDS analysis of specific reaction products, are shown in Fig. 5. It revealed that the notable impact of chromium on the microstructure and chemical composition of hot corrosion products. As the chromium content in the coating increased, the chromium content in the resulting products also exhibited a corresponding increase. Simultaneously, it was observed that the porosity of the products displayed a significant increase.
High magnification images of cross-section morphologies of NiCr coatings after hot corrosion test are shown in Fig. 6. The images were captured in the back-scattered electron (BSE) mode by SEM. Notably, the scale formed on the NiCr13 coating consisted of mixed oxides, with dark gray oxides predominantly enriched at the lower region of the scale and light-colored oxides enriched in the upper region. With an increase in chromium content of coating, the proportion of dark gray oxides exhibited a significant rise, ultimately resulting in the formation of predominantly dark gray oxides in the NiCr27 and NiCr41 coatings. However, within the NiCr27 coating, several nickel-rich islands were observed within the oxide scale. The findings from the EDS analysis presented in Table 3 corroborate these observations, indicating that the light-colored oxides were nickel-rich, while the dark gray oxides were chromium-rich. Furthermore, a minor presence of sulfur was detected near the interface between the coating and the oxide scale in the NiCr13 coating. Additionally, the formation of pores within the oxide scale was observed, as indicated by white arrows. The areas marked by white dashed ellipse were porous too.
| Area | Ni | Cr | O | S |
| A | 72.1 | 3.2 | 24.7 | — |
| B | 30.9 | 37.8 | 31.3 | — |
| C | 90.8 | 6.0 | 2.9 | 0.3 |
| D | 0.6 | 64.2 | 35.2 | — |
| E | 91.8 | 8.2 | — | — |
| F | — | 76.5 | 23.5 | — |
| G | 0.6 | 77.2 | 22.2 | — |
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The objective of the present investigation is to provide insights into the influence of chromium on the NiCr coating during the hot corrosion test conducted in this study. It is hypothesized that the observed corrosion mechanism involved the oxidation of nickel and chromium, resulting in the formation of NiO and Cr2O3 compounds. Furthermore, the reaction between NiO and Cr2O3 was responsible for the formation of NiCr2O4. It is noteworthy that the necessary oxygen for these oxidation reactions was not only derived from atmospheric oxygen but also from the molten salts present within the system. This relationship was mathematically expressed by equation (2)[16].
| Na2SO4(l)=Na2O(l)+12S2(g)+32O2(g) | (2) |
Simultaneously with the liberation of oxygen from Na2SO4, the release of sulfur was also observed. Sulfur diffused into the inner regions of the coatings and underwent chemical reactions, leading to the formation of chromium sulfide. This phenomenon could be attributed to the more negative value of the Gibbs’ free energy of formation for chromium sulfide compared to that of nickel sulfide. Subsequently, when the partial pressure of oxygen surpassed a critical threshold, the chromium sulfide species were oxidized to chromium oxide. Notably, the boundaries of lamellae and particles within the coatings served as pathways for the transport of sulfur and oxygen. These regions were prominently associated with the formation of corrosion products in the form of oxides.
In fact, the formation and dissolution of Cr2O3 occurred simultaneously, which was named basic fluxing as shown in equation (3)[17]. Cr2O3 underwent dissolution into ions due to the presence of molten salts and subsequently precipitated from the molten salts, resulting in the formation of flake-like Cr2O3 structures (Fig. 5c). The reformed Cr2O3 exhibited a loose structure characterized by the presence of interconnected pores.
| Cr2O3+2O2−+32O2⇌2CrO42− | (3) |
When equations (2) and equations (3) persisted, porous Cr2O3 was formed continuously. The high chromium content led to the formation of large amounts of porous Cr2O3. Low chromium content and porous Cr2O3 led to the formation of more NiO in NiCr13 coating. When chromium content in the metallic lamellae dropped to a critical level, Ni was oxidized and formed NiO. With the formation of NiCr2O4, NiO was consumed. NiCr lamellae turning into small Ni-rich particles in NiCr27 coating due to the proceeding of those processes[18]. Meanwhile, the amount of Cr2O3 decreased, resulting in the formation of a small amount of porous Cr2O3 in the NiCr27 coating.
(1) The corrosion resistance of the NiCr coatings demonstrated a remarkable enhancement compared to the SA213-T2 steel substrate. Notably, among the three coatings, the NiCr13 coating exhibited the least favorable corrosion resistance, while it should be noted that NiCr27 coating exhibited the highest level of corrosion resistance among the coatings under investigation.
(2) The presence of a higher chromium content within the coatings resulted in the substantial formation of Cr2O3, which consequently led to the generation of a greater quantity of porous Cr2O3 due to the basic dissolution of Cr2O3.
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| 1. | Singh, S., Goyal, K., Bhatia, R. Molten sulphate vanadateinduced hot corrosion behaviour of YSZ-reinforced thermal spray coatings at elevated temperature. Anti-Corrosion Methods and Materials, 2024. DOI:10.1108/ACMM-08-2024-3071 |
Figures(6) / Tables(3)
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| Wires | Cr | C＋Si＋Mn | Ni |
| NiCr13 | 13.0 | <1.0 | Balance |
| NiCr27 | 27.0 | <1.0 | Balance |
| NiCr41 | 41.0 | <1.0 | Balance |
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| Current/A | Voltage/V | Stand-off distance/mm | Gas pressure/MPa | Gas type | Coating thickness/μm |
| 190 − 210 | 29 − 31 | 200 | 0.55 − 0.60 | Air | 300 − 400 |
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| Area | Ni | Cr | O | S |
| A | 72.1 | 3.2 | 24.7 | — |
| B | 30.9 | 37.8 | 31.3 | — |
| C | 90.8 | 6.0 | 2.9 | 0.3 |
| D | 0.6 | 64.2 | 35.2 | — |
| E | 91.8 | 8.2 | — | — |
| F | — | 76.5 | 23.5 | — |
| G | 0.6 | 77.2 | 22.2 | — |
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