Development of Alumina-Titania Composite Layers on Stainless Steel through the Detonation Spray Method and Investigation of Salt Spray Corrosion Behavior along with Surface Examination

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Introduction
Stainless steel is widely used for domestic and industrial applications due to its high corrosion resistance in a variety of harsh environments [1]. However, the usage of stainless steel for its applications is limited by the intermittent nature of high-temperature corrosion. Corrosion in SS components causes adverse efects and premature failure of the components. To avoid the corrosion problem in steel, various coating materials are used. However, a ceramic coating is efciently used to combat this problem [2][3][4][5]. Tough ceramic coating has many advantages, it has not entered the commercial market successfully due to its typical coating process. Te most signifcant disadvantage of ceramic coatings, in general, is their lack of self-healing properties. It is an intimidating problem, especially since in-core components such as fuel cladding are subjected to extreme temperatures [6].
To improve the efciency of ceramic material application, ceramic composite coatings have been successfully used. Te corrosion resistance of the substrate was signifcantly infuenced by the various coatings and materials used during the coating process, as evidenced by their performance in a salt spray corrosion test environment under diferent conditions. Te choice of materials and application techniques played a crucial role in determining the level of protection against corrosion. Chromium 20 nickel [7,8], FeCoCrAlNiTix (x: molar ratio, x 1/4 0.5, 1.0, 1.5, and 2.0) [9], tungsten carbide and chromium carbide [10], TiNbMoMnFe high entropy alloy coating [11], sample 316L austenitic stainless steel [12], nanomodifed silicon-based composite coating [13], tube-ftting nuts made of AISI [14], Mg-X alloys (X = Mn, Sn, Ca, Zn, Al, Zr, Si, Sr) [15], titanium nitride [16], arc sprayed aluminum coating [17], Fe 66 Cr 10 Nb 5 B 19 metallic glass coating [18], and titania coated α-Al 2 O 3 platelets [19] are some of the examples. Based on the discussion, it is found that ceramic composite coatings are good for use against corrosion. One of the ceramic composite coatings that can be used for the abovesaid purpose is alumina-titania coatings. Tese coatings are used in textile manufacturing components, tooling, chemical industry components, and electrical insulation due to their high wear resistance, toughness, and good grinding ability [20]. Also, the aluminatitania coating has not been used for improving the corrosion resistance of SS. According to Djendel et al., aluminatitania coatings with an intermediate bond coat of Ni 20 Cr 6 Al, deposited on stainless steel substrates using atmospheric plasma spraying, exhibit desirable coating characteristics, including adhesion strength [21].
Tere are various coating methods for steel; however, thermal spraying is one of the best methods for hightemperature applications. Te substrates (base material), coating materials, coating processes, and conclusions are summarized in Table 1. It is observed from Table 1 that detonation gun thermal spraying is one of the compelling methods to coat steel, which may give good corrosion resistance and adhesive strength to steel.
Te specifc novelty of this study is to investigate the corrosion behavior of 304 stainless steel using an advanced thermal spraying technology called detonation gun spraying. A unique aspect of this research involves applying a 1 : 1 ratio (i.e., 50% wt. of each material) of Al 2 O 3 -TiO 2 on the stainless steel surface, whereas most of the studies involved a maximum of 40% wt. of TiO 2 .
Tis study uses the detonation gun spray coating technique to coat Al 2 O 3 -TiO 2 on stainless steel. OM and SEM have found the surface morphologies, while XRD is used to confrm the alumina-titania coating on the SS. Te corrosion resistance of SS has been tested under a 5.2% NaCl spray condition. Te hardness of the coated and uncoated steels is also investigated in this study.

Materials, Instruments, and Methodology
In this study, 304 grade stainless steel was used as a substrate. Te percentages of various elements in 304 grade stainless steel are tabulated in Table 2. Te coating of alumina-titania powder (50 : 50 by weight) on stainless steel was carried out using the detonation gun spray method. Alumina-titania powder of analytical grade with a purity of 99.5% was purchased for this thermal spray coating method. Te experimental arrangement of the detonation gun spray coating method (D-gun spraying) is shown in Figure 1. Te coating was carried out by Lotus Surface Technologies Pvt. Ltd., Chennai. Before coating, stainless steel was machined to the required dimensions (80 mm × 60 mm × 10 mm). Te selected powder was sprayed on the material with the help of three inert gases. Process parameters for detonation spraying are tabulated in Table 3.
SEM of VEGA3-TESCAN for surface, EV018 (CARL ZEISS) for cross section, and PANalytical X'pert powder XRD system were used to examine the surface. Te optical microscope images were recorded with the Dewinter Optical, Inc. microscope. Te coated steel underwent the salt spray corrosion test to visually examine the corrosion rate of the coated and uncoated steel surfaces. Te salt spray test was conducted according to the ASTM B117-16 standard. Te coated and uncoated steel samples were placed in the chamber, and NaCl salt was sprayed rapidly on the steel. Te corrosion was tested for two-time intervals: 12 hours and 24 hours. Te test parameters are tabulated in Table 4.
Te hardness testing of the surfaces was performed as per the IS 1501-13 (P1) standard by using Vicker's hardness tester, Wolpert.

Results and Discussion
Te actual images of SS and coated SS surfaces are shown in Figures 2(a) and 2(b). Te coating on SS was clearly observed in the images through diferent surface textures. Figure 3 revealed that the marks and pits were present on the surface; however, the surface still looked free from any additional layers of contaminants. Figure 4 represents the after-coating surface of stainless steel. A covering of SS can be observed in Figures 4(a) and 4(b) after comparison with Figure 3(a). It was evident that the SS surface became rougher and more granular after the alumina-titania coating. Te clusters in the coating were more prominently observed in the images shown in Figures 4(c) and 4(d). Tus, it was confrmed through SEM images that an alumina-titania coating was successfully developed on SS through D-gun spray coating.

SEM, X-Ray Difraction, and Hardness Analysis of the Surface. Te SEM of SS shown in
Te cross-sectional SEM morphology with EDX is shown in Figures 5 and 6. It is observed from the fgures that the coating is deposited on the substrate and that the respective elements values are found by the EDX spectra, and the respective elemental values are tabulated in Table 5. Figure 7 shows the XRD difraction peaks for aluminatitania coated SS. An intense peak was recognized around 22°d ue to the alumina-titania coating [33]. Te peaks of 33°and 50°could also relate to the alumina-titania coating. Tis fact indicated that alumina and titania interacted with each other to form an alumina-titania composite. Besides aluminatitania, alumina, titania, and SS could also be detected in the coating through their peaks [34]. Tis fact revealed that some alumina and titania could not react with each other and remain in their original form. Based on XRD peaks, it could be claimed that alumina-titania was successfully coated on SS through D-gun spraying. Te XRD analysis of the coated material, Al 2 O 3 -TiO 2 , which consists of alumina (JCPDS card number: 10-173) and titania (JCPDS card number: 21-1276), confrms the successful deposition of the coating on the SS 304 substrate. Figure 7 illustrates the corresponding HKL values (Miller indices). International Journal of Chemical Engineering Te XRD plot for the corroded sample is analyzed by using a BURKER ECO D8 Advance instrument. Te peaks at 16°, 43°, and 74°could also relate that corrosion occurred on the SS 304 sample. Te XRD pattern of corroded SS 304 is shown in Figure 8. Figure  Te hardness numbers of the coated and uncoated steel are shown in Figure 9. Te values are in VHN (Vickers hardness number), and the Rockwell hardness number denotes the equivalent hardness number. It is evident from the results that the hardness of the coated surface is increased by 43.51% than the uncoated surface. It could indicate that an alumina-titania coating was successfully deposited on SS through D-gun spraying. However, surprisingly, the hardness of the coated surface went down with respect to SS instead of increasing. It is evident in Figure 4 that the coating is applied uniformly by the D-gun spray method and that roughness was also greater with respect to the SS surface. Due to this, the hardness of the coated stainless steel is reasonably increased by the alumina-titania coating.

Salt Spray Corrosion Test.
Before the test, the specimen was cleaned gently before loading. Ten, the sample was washed in a considerate manner with clean running water to remove testing salt deposits from their surfaces and dried immediately. Te obtained results are tabulated in Table 6.
Te observations were conducted at 12 h intervals and 24 h intervals during the salt spray test. Tis selection of intervals allowed for better monitoring of corrosion behavior and quicker evaluation of material performance, as one year in the test is equivalent to one year in normal atmospheric conditions [35,36].
It was important to mention that both coated and uncoated SS did not show corrosion after 12 h. After 24 h, it was observed that uncoated SS showed some corrosion products formed on its surface. On the other side, very few (negligible) corrosion products were observed on the surface of coated SS after 24 h (Figure 8). Tus, it was evident from the salt spray test that the coated SS showed better corrosion resistance than the uncoated SS [37,38].
A simple immersion test was also performed after this. Both coated and uncoated SS were immersed in 0.5 M NaCl solutions for 24 h. Afterward, they were cleaned and dried, and optical images were clicked. Te microscopic images are shown in Figure 9.
It is evident in Figure 10(a) that the uncoated SS had some scratch marks on it. After 24 h immersion, corrosion products formed on the clear surface in Figure 10(b). In the comparison of Figures 10(a) and 10(c), it could be observed that a layer had covered the SS surface. It could indicate that an alumina-titania coating was there. After 24-hour        immersions, some corrosion was observed on the surface; however, the surface of coated SS looked better than that of uncoated SS. Tus, it was predicted based on immersion test that the alumina-titania coating enhanced corrosion resistance of SS [39,40]. Te corrosion rate is calculated using equation (1). Te corrosion rate with respect to the exposure time is represented in Figure 11: where W is weight loss in milligrams, D is stainless steel density (8 g/cm 3 ), A is the area of the sample (48 cm 2 ), and T is the exposure time in hours.

Conclusion
Alumina-titania was deposited on the stainless steel by the detonation spray method. Tis study provides the following conclusions: (i) Te SEM images also confrmed this fact and suggested that a rough coating of alumina-titania was deposited on stainless steel. (ii) Te X-ray difraction peaks suggested that aluminatitania, alumina, and titania were detected on the coated stainless steel surface. (iii) Micro Vickers hardness test results showed that the coated stainless steel has a higher hardness than the uncoated stainless steel. From these results, it is observed that the hardness of coated stainless steel is increased by 43.51% than uncoated stainless steel. (iv) From the salt spray corrosion test, it could be concluded that the uncoated stainless steel had less corrosion resistance than the coated stainless steel. In other words, it could be said that the applied coating was highly resistant to oxide formation caused by salt spraying. (v) Te immersion test also confrmed this fact. As an overall conclusion, it could be stated that the alumina-titania coating increased the corrosion resistance of stainless steel against chloride ions. International standard HVOF: High-velocity oxy-fuel.

Data Availability
Te data used to support the fndings of this study are available on request. International Journal of Chemical Engineering 9