Cyclic oxidation of 304L and 316L stainless steel coated and uncoated with Cr3C2–NiCr at elevated temperatures
Kategoria artykułu: Research Article
Data publikacji: 31 mar 2025
Zakres stron: 103 - 114
Otrzymano: 15 sty 2025
Przyjęty: 16 kwi 2025
DOI: https://doi.org/10.2478/msp-2025-0010
Słowa kluczowe
© 2025 Naveen Kumar, published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
The factors primarily responsible for the deterioration of parts of boilers, industrial waste incinerators, and hot areas of gas turbines include oxidation at high temperatures, hot corrosion, and erosion. WC–Co and HVOF sprayed Cr3C2–NiCr coatings have been the subject of extensive research regarding their wear-resistance behaviour. However, there is a growing interest in using these coatings to shield against corrosion caused by high temperatures [1]. Thermal spray coatings, like those made of tungsten or chromium carbide, offer a different option to chromium plating when strong wear resistance is needed. Superior corrosion and oxidation resistance are demonstrated by Cr3C2–NiCr coatings at 900°C [2]. These coatings also have outstanding hardness, strength, and wear resistance, as well as a high melting point [3]. The thermal spraying technique known as detonation gun (D-gun) spray coating offers surfaces with compressive residual stresses, very high adhesive strength, and low porosity [4]. Chemical spraying is another name for this procedure.
In particular, carbide-type cermet coatings like Cr3C2–NiCr have performed well in several industrial applications [5]. In various applications, coatings based on carbide are used extensively under abrasive, erosive, and oxidizing conditions. However, no comprehensive study has been conducted on using these coatings in boiler tubes in power plants. These coatings offer a range of wear performances and a high degree of hardness due to the high percentage of carbide that is preserved during the spraying process [6]. These environments may be mimicked in the laboratory or real industrial situations. When the metal or alloys are oxidized above 500°C in an oxidizing environment like an atmosphere during combustion that has too much air or oxygen, this phenomenon is referred to as high-temperature oxidation of material [7,8]. One of the primary causes of equipment failure is exposure to high temperatures [9]. Using thermal spraying techniques like plasma, HVOF, and D-gun, coatings may be sprayed quickly and effectively on a range of surfaces [10]. Among all of the previously listed thermal spraying techniques, D-gun is one of the most widely used techniques for producing low-porosity and well-adherent coatings [11,12,13]. Studies have shown that thermal spray coatings significantly impact the protection of machine components that are subjected to high temperatures, such as turbine engines and boiler tubes [14,15,16].
These coatings protect against the corrosion attack that occurs at high temperatures. Since it does not change any other characteristics of the substrate material, thermal spraying is an economical and effective way to create the coating [8,17,18]. A thermal spray coating or surface treatment has as its primary objective the ability to make an oxide layer on the surface that is stable and slowly increasing. The coated alloy and the surroundings are separated by this scale [9,12,13,19]. Cr3C2–NiCr coatings have a high melting point, which also provide superior resistance to corrosion and oxidation. Even when subjected to temperatures as high as 900°C, they exhibit remarkable hardness, strength, and resistance to wear. As the foundation of the majority of high-temperature alloys, nickel and iron have thermal expansion coefficients that are almost identical to that of Cr3C2 (10.3 × 10−6°C−1). The least amount of stress is produced during thermal cycles as a result of thermal expansion mismatch because the differences in thermal expansion coefficients are so tiny [14].
Kuruba et al. [20] tried to make NiCr–Cr3C2 coatings stronger by adding multi-walled carbon nanotubes to improve their stickiness and hardness. The NiCr–Cr3C2/7% CNT composite coating had the highest microscopic hardness (9563.8 HV), lowest porosity (1.17%), and best adhesion (55.8 MPa). The NiCr–Cr3C2/7% CNT composite coating showed the best results with the highest average microhardness (9563.8 HV), the least porosity (1.17%), and the strongest adhesion (55.8 MPa).
Ni3Ti + (NiCr–Cr3C2 + 20NiCr) and Ni3Ti coatings by HVOF were applied on titanium (Ti-15) substrate materials based on gas turbines and AISI 420 by Reddy et al. [21]. About 50 cycles of cyclic hot corrosion tests at 650°C were conducted in a molten salt environment with Na2SO4−40%V2O5. The results indicate that coated substrates are mostly covered by protective oxide (NiCr2O4) layers. On coated surfaces, ternary NiCr2O4 protective oxides inhibit corrosive species diffusion. Shivalingaiah et al. [22] deposited an Inconel 718-VCN composite coating on SS304 substrates using HVOF.
Reddy et al. [23] coated AISI 420 steel and titanium alloy with Ni–Ti. NiO and Cr2O3 phases stabilized weight increase in Ni3Ti⁺(Cr3C2 + 20NiCr) coatings, even in slow-oxidation coatings with compact NiO phases.
N-HEA was deposited on Ti–6Al–4V by twin glow plasma nitriding and HVOF spraying. The CoCrFeNiMn high-entropy alloy coating consists of a single face-centred cube (fcc) phase and a small MnCr2O4 spinel phase, being roughly 200 μm thick. HEA holds a higher microhardness than Ti–6Al–4V [24].
In this study, zinc powder coated with varying phosphoric acid concentrations was used to create polyurethane cold galvanizing coatings. Different phosphate concentrations (wt%; 1, 3, and 5%) were examined for their effects on the morphology of modified zinc powder. The impact of these alterations on cold galvanizing coating strength and rust resistance was also examined [25].
The microstructural characteristics of high-carbon steel have a direct bearing on its mechanical properties and fracture behaviour. An artificial model called a representative volume element (RVE) was created in this study to investigate how the tiny structure of high-carbon steel with many carbides influences its strength and how it breaks when stretched. The RVE simulation’s anticipated mechanical properties and fracture strain matched the actual findings [26].
According to the available literature, the authors found no published articles on the oxidation behaviour of a Cr3C2–NiCr coating deposited by a D-gun on AISI stainless steel (SS) 304L and 316L substrates at temperatures of 750 and 850°C. As a result, this work aims to investigate the D-gun-sprayed Cr3C2–NiCr coatings on 304L and 316L SS that exhibit high-temperature behaviour. The thermogravimetry approach evaluated the kinetics of oxidation of D-gun sprayed Cr3C2–NiCr coating and bare 304L and 316L SS substrates. Both the coated and bare samples’ oxidation products were characterized using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM).
The substrate alloys, i.e. SS 304L and 316L, were procured from M/s R. K. Engineering Pvt. Ltd, Lucknow (India). Table 1 displays the chemical constitution of stainless-steel grade 304L and 316L.
304L and 316L SS chemical composition (%).
| Substrate | C | Mn | P | S | Si | Cr | Ni | Mo | Fe% |
|---|---|---|---|---|---|---|---|---|---|
| SS 304L | 0.030 | 2.00 | 0.45 | 0.03 | 1.00 | 18.0–20.0 | 8.00–12.00 | — | Bal. |
| SS 316L | 0.030 | 2.00 | 0.45 | 0.03 | 1.00 | 16.0–18.0 | 10.00–14.00 | 2.00–3.00 | Bal. |
Commercially available 8 mm thick hot rolled and annealed AISI 304L and AISI 316L sheets were used. Specimens of about 20 mm × 15 mm size were cut from the sheets. With the help of a surface grinder, the sheets’ thickness was reduced to 5 mm. The specimens were polished with an emery paper with grit sizes of 80, 120, 220, 320, 400, and 600, respectively, and then cloth polished to a lustrous, mirror-like surface finish. After cleaning with acetone, the AISI 304L and AISI 316L steel surfaces were sandblasted with Al2O3 at a pressure of 5 bar (200–300 μm) to create a rough surface with a texture of Ra 5–10 μm before applying the coatings. An angle of 90° and a distance of 150 mm were used for blasting.
The substrate used in the D-gun technique of coating deposition measured 20 mm × 15 mm × 5 mm. The macrograph of specimens before and after coating is shown in Figure 1. The Cr₃C2–25% NiCr coatings were applied at M/s SVX Powder M Surface Engineering Pvt. Ltd. in Greater Noida, India, using a TS-50M detonation spray cannon. After several trial tests and selection of the best one with ten duplicates for both substrate types, the coating deposition was finished. Table 2 shows the stable parameters that yield the best outcome. Coating thickness was maintained to 400–500 μm. Eight samples were considered for this study. Four samples were taken for cyclic oxidation behaviour study at 750°C, namely (i) one uncoated AISI 304L steel substrate, (ii) one uncoated AISI 316L steel substrate, (iii) one Cr3C2–25% NiCr-coated AISI 304L steel substrate and (iv) one Cr3C2–25% NiCr-coated AISI 316L steel substrate.
Macrographs of specimens. (a) Uncoated AISI 304L steel substrate. (b) Uncoated AISI 316L steel substrate. (c) Cr3C2–25% NiCr-coated AISI 304L steel substrate. (d) Cr3C2–25% NiCr-coated AISI 316L steel substrate.
Powder coating process parameters.
| Parameters | Cr3C2–NiCr coating |
|---|---|
| Proportion | 75% Cr3C2/25% NiCr |
| Oxygen flow rate (O2) | 2720 SLPH |
| Acetylene (C2H2) flow rate | 2320 SLPH |
| Pressure (P) | 0.2 MPa |
| Nitrogen flow rate (N2) | 720 SLPH |
| Pressure (P) | 0.14 MPa |
| Power | 450 VA |
| Spray distance | 165 mm |
| Spray angle | 90o |
| Coating thickness (average) | 450 µm |
| Fire rate | 10 Hz (10 shots per second) |
Similarly, four samples are taken for cyclic oxidation behaviour study at 850°C, namely (v) one uncoated AISI 304L steel substrate, (vi) one uncoated AISI 316L steel substrate, (vii) one Cr3C2–25% NiCr-coated AISI 304L steel substrate, and (viii) one Cr3C2–25% NiCr-coated AISI 316L steel substrate. A schematic representation of D-gun spray coating is shown in Figure 2.
Schematic diagram of the D-gun method.
Figure 3(a) and (b) shows the coating thickness along the cross-section before the oxidation test for the Cr3C2–NiCr coating on 304L and 316L SS samples. The thickness of the 304L sample was 452 µm before oxidation. The average thickness of the 316L sample was 448 µm.
Coating thickness: (a) substrate SS 316L and (b) substrate SS 304L.
The oxidation test was carried out in a cyclic setting for 50 cycles. In a muffle furnace, 50 cycles of cyclic oxidation were carried out at temperatures of 750 and 850°C. The cross-section SEM image after oxidation is shown in Figure 4. It exhibits the degradation of coating surface after oxidation.
Cross-section SEM image after oxidation.
Figure 5 displays the muffle furnace’s schematic diagram. The muffle furnace has a maximum temperature limit of 1,200°C with a control panel. In a single cycle, heating takes 1 h, while cooling takes 20 min at room temperature. Emery paper and cloth were used to polish the uncoated sample before it was subjected to the oxidation test. After thoroughly cleaning the samples with acetone to remove any dirt, they were dried to remove any remaining moisture. The alumina boats that were employed in the research were warmed for 6 h at 1,100°C.
Schematic diagram of the muffle furnace.
Before the alumina boat was subjected to high temperature oxidation, the samples were weighed separately and collectively. A weighing balance with a count of at least 1 mg was used to record weight changes following each cycle. Following varying intervals, visual observations were documented.
Figure 6 illustrates the weight gain per area vs the number of cycles for coated and uncoated 304L SS and 316L SS oxidized at 850°C in air. SS 304L showed rapid weight gain during the initial 10 cycles at 850°C temperature and then increased gradually up to 50 cycles. SS 316L showed approximately parabolic behaviour in weight gain up to 50 cycles. Coated SS 316L showed gradual weight gain up to 24 cycles, then it showed some weight loss for the next two cycles, and after that it increased gradually. Coated SS 304L showed rapid mass gain up to the 4th cycle and, after a gradual increment, up to the 18th cycle. There was a slight decrease in weight from the 18th to the 20th cycle, then progressive weight growth until the 50th cycle.
Weight gain/area vs the number of cycles for bare and coated 304L and SS 316L in dry air at 850°C.
Figure 7 depicts the plot of weight gain/area vs the number of cycles for bare and coated SS 304L and 316L exposed to oxidation at 750°C. The uncoated SS 304L showed gradual weight gain during the initial 23 cycles, rapidly increasing for the subsequent 4 cycles and then gradually increasing up to the 38th cycle. Some weight loss occurred after the 38th cycle, reaching the 40th cycle and then increasing parabolically to the 50th cycle. The 316L uncoated SS exhibited a progressive increase in weight until the 25th cycle, followed by a parabolic increase in weight until the 47th cycle, and then a negligible increase in weight until the 50th cycle. Coated SS 304L showed a parabolic weight gain up to 50 cycles at a temperature of 750°C. Coated 316L showed a small increment in weight up to the initial three cycles after which a sudden increase in weight occurred for the next cycle. Then, it exhibited an approximately parabolic behaviour up to 50 cycles at 750°C.
Weight gain/area vs the number of cycles for bare and coated SS 304L and SS 316L in dry air at 750°C.
The behaviour of Cr3C2–NiCr coatings sprayed on superalloys based on nickel and ferrous metals was examined by Kamal et al. [1]. The better resistance to oxidation in Cr3C2–NiCr-coated superalloys came from a tight and sticky thin layer of Cr2O3 that formed on the surface of the coating when it was oxidized.
Kamal et al. [27] analysed Cr3C2–NiCr coatings in a 75 wt% Na2SO4 + 25 wt% K2SO4 film at 900°C over 100 cycles on Super 75, 718, and superior 800H Fe-based superalloys. Cr3C2–NiCr-coated superalloys that have a steady and protective layer of nickel, chromium, and spinel were found to be more resistant to hot corrosion than those without any coating.
Shukla et al. [15] examined the cyclic oxidation behaviour of the coatings after 50 cycles of Cr3C2–NiCr coatings on 310SS sprayed with HVOF at 700°C under the atmosphere. They found that the Cr3C2–25% NiCr-coated specimen showed very little micro-spalling of the scales upon oxidation. One reason why Cr3C2–NiCr coatings resist oxidation better is the formation of nickel and chromium oxides, along with Ni and Cr spinels.
After completion of 50 cycles, the visual and SEM observations of the specimens were documented. Figure 8 shows the macrograph of Cr3C2–25% NiCr-coated AISI SS 304L and SS 316L at 850°C in dry air.
Macrographs of specimens in dry air at 850°C. (a) Uncoated 304L. (b) Coated 304L. (c) Uncoated 316L. (d) Coated 316L.
The coatings’ microstructure was examined using a SIGMA 500 FEG-SEM instrument (Carl Zeiss Sigma, Germany) before and after an oxidation cycle.
Uncoated SS 304L demonstrated a light brownish colour until the end of the fifth cycle, which gradually transformed into a dark blackish colour (Figure 8a). In contrast, SS 316L displayed a light brownish colour until the ninth cycle, gradually transforming into a dark brown colour (Figure 8b). Initially, the colour of the coated specimen 316L and 304L was grey. Coated SS 304L turned into a dark blackish colour at the end of the sixth cycle, which then gradually converted to a dark blackish colour (Figure 8c). The scale formed on the coated SS 316L was dark brownish in colour up to the 11th cycle, and further it was gradually converted into dark blackish colour up to the 50th cycle. Cracking of scale occurred (spallation) at the end of the 19th cycle (Figure 8d).
Figure 9 shows the visual macrographs of Cr3C2–25% NiCr-coated AISI SS 304L and SS 316L at 750°C in dry air. Until the eighth cycle, the uncoated SS 304L had a light brownish hue. After that, it progressively became dark blackish (Figure 9a). The coated SS 304L turned into light blackish at the end of the eighth cycle. After that, it gradually converted into a dark blackish colour (Figure 9b). The uncoated SS 316L colour was light brownish up to sixth, and it slowly transformed into a dark brownish tint (Figure 9c). The coated SS 316L was initially grey, which gradually converted into a dark blackish colour. Figure 9(d) shows that after the 24th cycle, a few tiny spots were seen on the surface of the specimen.
Macrographs of specimens in dry air at 750°C. (a) Uncoated 304L. (b) Coated 304L. (c) Uncoated 316L. (d) Coated 316L.
Micrographs using SEM and EDS mapping analysis of the Cr3C2–25% NiCr-coated AISI SS 304L and SS 316L sample before oxidation are shown in Figure 10. Figure 10(a–c) exhibits a compact and well-bonded surface of coating with very few pores, indicating the intense energy impact characteristic of the D-gun technique. The SEM micrographs reveal a consistent dispersion of the NiCr phases, with the carbide particles embedded inside the metallic NiCr matrix. This microstructure’s existence improves the coated surface hardness and wear resistance. The hard particles resist abrasive and erosive wear, while the ductile NiCr matrix absorbs impact energy and redistributes stress. Also, Figure 10(d–f) shows how a somewhat thick layer developed and adhered to the substrate. Figure 10(b and e) illustrates the coating’s fused NiCr matrix. Only a few surface gaps and holes are visible. In addition, in micrographs, there are minuscule fractures or microfractures often arising from thermal pressures during the fast-cooling stage. However, these drawbacks are usually insignificant and do not significantly impair the covering’s longevity. Voids are often seen in cermet coating sprayed with a D-gun. The discovered microstructure indicates that most of the holes and voids in the coating are situated in its top layer.
SEM images of Cr3C2–NiCr-coated (a–c) SS 304L and (d–f) SS 316L.
Figure 10(c and f) shows EDS mapping of Cr3C2–NiCr-coated SS 304L and SS 316L, respectively. The concentration of chromium is essential for increasing the alloy’s resistance to oxidation because it creates a protective, sticky, and slowly expanding layer of Cr2O3. The presence of Cr is confirmed by EDS.
Following the oxidation process at two distinct temperatures, FESEM images of the Cr3C2–25% NiCr-coated sample are acquired, which are shown in Figure 11. Figure 11(a) shows an FESEM image of the surface of the oxide scale after it underwent cyclic oxidation at 750°C on a substrate made of SS 304L.This establishes the existence of pores. On the surface of the oxide scale, which was produced by cyclic oxidation at 750°C on a SS 316L substrate, various voids, pores, and microfractures are visible, as shown in Figure 11(b). Developing NiO and Cr2O3 is a possibility in this scenario. As a result of decarburization, cyclic oxidation causes holes and voids to form on the top surface of the coating at 850°C. This is shown in Figure 11(c and d). More significant quantities of Cr and O have been found on the scales, and protective Cr2O3 has likely grown on their surface, further inhibiting oxidation. At a temperature of 850°C, the rapid decarburization causes an increase in the number of voids, as shown in Figure 11(c and d). Some cracks have been discovered at the surface of the coating [28].
SEM images of Cr3C2–NiCr coated (a) SS 304L at 750°C, (b) SS 316L at 750°C, (c) SS 304L at 850°C, and (d) SS 316L at 850°C after the oxidation cycle.
Figure 12 displays the as-sprayed and oxidized coatings’ X-ray diffraction patterns. The primary phases of coating during spraying are NiCr and Cr3C2. It is clear that the peaks of NiCr and Cr3C2 disappeared after cyclic high-temperature oxidation treatment, while separate peaks due to Cr2O3 and NiCr2O4 were seen [29].
XRD patterns of as-sprayed and oxidized coatings.
At high temperature oxidation due to decarburization of Cr3C2, a more stable phase Cr2O3 is developed on the surface of the coatings. The Cr2O3 phase generated on the oxide scale provided significant oxidation resistance. The chromium oxide phase is formed according to the following equation:
In the current research, 304L and 316L SS substrates were subjected to oxidation tests at 750 and 850°C. Experiments using D-gun-sprayed Cr3C2–25% NiCr coating have been undertaken to further enhance the oxidation resistance of SS 304L and 316L.
The following deductions are made: The D-gun spraying method has effectively deposited a Cr3C2–25% NiCr coating on both the substrate materials. The oxidation resistance of both alloys has been significantly enhanced. During the oxidation test, the weight growth per unit surface area graph indicates that uncoated and coated SS 304L gained 6.66 and 4.16 mg/cm2, respectively, up to 50 cycles at 850°C, and for SS 316L, they were found to be 4.76 and 3.44 mg/cm2, respectively, up to 50 cycles at 850°C. According to the graph of weight increase per unit surface area, uncoated and coated SS 304L acquired 4.51 and 3.522 mg/cm², respectively, during the oxidation test. In contrast, uncoated and coated SS 316L gained 3.80 mg/cm2 and 2.73 mg/cm2, respectively, up to 50 cycles at 750°C. SS 304L coating has shown approximately 26.54 and 21.93% improvement in the oxidation resistance at 850 and 750°C, respectively. For SS, 316L coating has shown approximately 27.67 and 25.92% improvement in the oxidation resistance, respectively, at 850 and 750°C. During 50 cycles of cyclic oxidation at 750 and 850°C, the Cr2O3 phase produced on the oxide scale demonstrated notable oxidation resistance. The applications of such coatings at high temperature may reduce the formation of oxide scale, which attacks and corrodes exhaust valves, turbocharger nozzles, and blade.
The authors extend their appreciation to Taif University, Saudi Arabia, for supporting this work through project number (TU-DSPP-2024-34).
This research was funded by Taif University, Saudi Arabia, Project N0. (TU-DSPP-2024-34).
Naveen Kumar: Writing – review & editing, Visualization, Validation, Md Sarfaraz Alam: Methodology, Investigation. Nagendra Kumar Mishra: Writing – Visualization, Resources. Sujeet Kumar: Investigation, Formal analysis, Data curation. Jayant Giri: Investigation, Data curation. Ayman A. Aly: Conceptualization, Funding. Gaurav Kumar Gupta: Data curation.
The authors declare that they do not have any conflicting interest.