Formation Mechanism of Low Temperature Infiltrated Multilayer Aluminide Coating on 316L Stainless Steel (2023)


The aluminide coating and the aluminum oxide film formed on the surface of the coating can prevent the substrate from being oxidized or damaged in high temperature or corrosive environments [[1], [2], [3]]. At the same time, the iron-aluminum alloy layer can provide the active elements required for the self-healing of the aluminum oxide film. In addition, aluminum oxide films have also been used as tritium permeation barrier coatings due to their high permeation reduction coefficients [[4], [5], [6]]. Commonly used aluminide coating preparation methods include hot-dip aluminum plating [7,8], electroplating aluminum [9,10], coating (PC) [[11], [12], [13]], physical vapor deposition [ 4] and chemical vapor deposition (CVD) [[14], [15], [16]]. CVD and PC processes can be applied to components with complex shapes, and the resulting aluminide coatings are dense and uniform. Both approaches have great advantages.

The cladding process can be considered as a chemical vapor deposition process [3]. The basic mechanism of the two processes to form the aluminum coating is almost the same: low-halogen aluminum diffuses to the surface of the substrate, aluminum atoms are deposited on the surface, and the aluminum coating is formed through a diffusion reaction. However, aluminizing reactions may occur during the heating and cooling stages of the infiltration process. In the CVD process, gaseous halides are generated in the evaporator and only enter the reaction chamber to react when they reach the processing temperature. The equipment cost of the CVD process is high [17]. Therefore, a cost-effective encapsulation cementation process is a better choice.

The preparation of aluminide coatings involves complex chemical reactions, and many researchers have studied the thermodynamics of different preparation processes [14,15,18,19]. Perez et al. [14] studied the equilibrium composition of gaseous precursors during CVD using thermodynamic software. The results showed that AlCl, AlCl2H and AlCl2Possibly the main precursor. In addition, they found that deposition time has an effect on the type of phases in the coating. only iron2Al5phase formed in a short deposition time, and with increasing time, the FeAl3phase appears. Yang et al. [19] calculated the possible reactions during the aluminizing process. they found iron2Al5The phase with the smallest free energy is preferentially formed. The possible formation sequence of iron-aluminum alloys is: Fe2Al5>Iron Aluminum3>Iron Aluminum2> Iron and aluminum. Fe free energy3The Al phase is greater than zero, and the Al phase does not form when the temperature is higher than 400 °C. Furthermore, dynamics are also the focus of attention [3,13,[20],[21],[22],[23],[24],[25]]. Studies have shown that aluminide coatings prepared by hot-dip aluminizing, cladding, and CVD methods follow “parabolic growth kinetics” [3]. Yener et al. [13] carried out low temperature infiltration on Fe-Cr-Ni superalloy. Aluminide coating composed of FeAl and Fe3The activation energy of Al phase is 207kJ/mol. item etc. [20,21] Low carbon steels were aluminized by cladding at temperatures between 600 and 750°C. Coating is single layer iron2Al5or iron14Al86A phase with an activation energy of about 75kJ/mol. Ei-Mahallawy et al. [22] Hot-dip aluminizing of mild steel in a pure aluminum bath with an activation energy of 138kJ/mol. Coating contains FeAl3, iron2Al5and iron and aluminum2stage.

Due to the complexity of the reaction in the low-temperature filling and cementing process, so far there are few related thermodynamic studies. In this paper, we investigate the possible reactions occurring in the low-temperature aluminizing process used in this work by studying a similar CVD process. The formation model of aluminide coating with multilayer phase structure was established. Furthermore, low-temperature aluminizing was performed at different temperatures (600-680 °C) and times (2-4 h), and the growth kinetics were studied. The thermodynamic and kinetic studies provide a theoretical basis for the parameter selection of the aluminizing process.

partial fragment

Materials and methods

316L stainless steel was used as the base material. A sample with a size of φ25×1mm was polished with 150-800 mesh water sandpaper, and then ultrasonically cleaned in acetone. The aluminide coating was prepared by the combination of surface slurry precoating and low temperature aluminizing. Detailed experimental procedures and parameters have been reported in previous literature [12]. In this study, different aluminizing processes were used for low temperature aluminizing

Morphology and Phase Structure

The cross-sectional morphology of the coating at different aluminizing temperatures and aluminizing times is shown in Figure 1. 600 ℃ aluminized 2h coating is discontinuous, and some places even have no coating. The thickness of the discontinuous aluminum coating is defined as zero. The EDS results in Table 1 show that the coating consists of Fe2Al5and iron and aluminum3stage. As the aluminizing time increases to 3h and 4h, the coating becomes continuous with a thickness of approx.

in conclusion

In this study, aluminide coatings were prepared by a combined process at different aluminizing temperatures and times. After aluminizing at 600°C for 2 hours, the aluminized layer is discontinuous. With increasing temperature and time, the coating becomes continuous and increases significantly in thickness. At 600°C, Fe2Al5phase is the main phase of the coating. At 650 and 680°C the coating consists of Fe3aluminum, iron aluminum, iron2Al5and iron and aluminum3stage. In addition, the surface finish

Declaration and verification

Yanhui Sun and Jian Dong designed the experiments, Jian Dong performed the experiments, Feiyu He processed the data and images, and Jian Dong wrote and revised the paper.

All authors endorse the final article

thank you

This work was financially supported byNational Natural Science Foundation of China(No.51774030).

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