Keywords

1 Introduction

In the process of metal evaporation recovery and stainless steel plate reuse, problems such as incomplete recovery and large amount of iron impurities are encountered. As a result, stainless steel plate can only be used once and cannot be reused for many times due to the residue of radioactive materials. Although in the verification phase, it also caused a large accumulation of radioactive material. Therefore, it is necessary to research on the reuse technology to realize the complete recovery of the metal and multiple reuse of the plate [1]. The active metal atoms have high thermal conductivity, high specific heat capacity, low vapor pressure and other characteristics. The metal atoms and the substrate are very easy to diffuse in the process of evaporation, resulting in the coating metal is difficult to completely desorption. Therefore, the development and preparation of high temperature corrosion resistant coating against active metal atomic steam is the factor to restrict the reuse of plate [2, 3]. The traditional high temperature anti-corrosion protective coating can be generally divided into ceramic coating [4, 5] and metal coating [6, 7]. According to the composition, mainly includes metal oxide coating and other coating. Through the composition design of ceramic, enamel coating can significantly reduce the corrosion rate. Multiple service will not affect the surface state after passivation. So in various professional fields have broad application prospects.

Enamel coating is a kind of amorphous glass coating. In the process of high temperature firing, it is easy to form strong combination with metal or alloy to improve the corrosion resistance of the whole plate [8–10]. Chen et al. [11] prepared 0.6 (mass fraction) borosilicate ratio enamel coating. The softening temperature of the enamel is higher than 750 ℃. Thermal shock results show that the performance of metal enamel composite coating with nickel particles is significantly improved. Zhu et al. [4] coated Ti60 with enamel coating to study its high temperature oxidation and sulfate hot corrosion behavior at 700 ℃ for 1000 h. The enamel coating forms a thin oxide layer composed of α-Al2O3, TiO2, Al2SiO5 and Al2TiO5, thus providing high temperature stability and avoiding high temperature corrosion.

Most of the existing research focuses on the high temperature oxidation corrosion resistance of enamel coating. In view of the high requirement of coating recovery on substrate surface state in atomic vapor laser technology [12–16], the resistance of enamel coating to active metal vapor corrosion in high-temperature evaporation is studied.

2 Enamel Coating Composition Design

Based on the actual service situation, SiO2, Al2O3, Na2O, K2O, CaF2 and CoO are chosen as the main components of special enamel coating. The composition ensure that free oxygen makes all Al2O3 intermediate in the form of four coordination into the silica oxide grid. Enamel coating with high temperature resistance, acid corrosion resistance and strong bonding force with stainless steel is prepared. The addition of CoO mainly considers that the Fe-Co-rich phase generated by firing can self-repair cracks in the service process and improve the thermal shock performance of enamel coating [17]. The composition of enamel coating directly affects the compactness, adhesion, acid resistance, hardness and other properties of the coating, and then affects its service behavior.

2.1 Preparation Technology of Enamel Coating

In this study, 316L stainless steel is used as the base material. Metal flakes of 50 mm × 50 mm × 1 mm are cut by electric discharge wire. In order to ensure more physical and mechanical teeth forming between enamel powder and stainless steel substrate during firing, sand blasting is required to increase roughness. The sand blasting device used is shown in Fig. 1.

Fig. 1.
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Sandblasting equipment.

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Choose 200–220 mesh corundum sand, sand blasting pressure control is less than 1 kg/cm2, the smooth surface with matte surface state is obtained, as shown in Fig. 2.

Fig. 2.
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Macro appearance of sandblasting stainless steel plate.

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Main components of enamel glaze (wt%) is 64.6 SiO2, 14.2 Al2O3, 9.61 Na2O, 4.65 K2O, 4.96 CaF2, 1.98 CoO. The raw material with 500 g weight is placed in the agate tank and agate balls are added. Then mechanical mixing is carried out in the planetary ball mill with the speed controlled at 300 r /min and the mixing time about 30 min. After the mixing, the raw material is put into a corundum crucible into a muffle furnace and heated to 1300 ℃ for 4 h. All the enamel melts are quenched at room temperature to obtain the enamel glaze. A planetary ball mill is used for dry ball grinding of enamel glaze and agate ball again. The rotating speed is set at 300 r/min, and the ball grinding time is about 120 h. Finally, the standard sieve (200 mesh) is used to screen the enamel powder ground by the ball to remove the large particles that are not ground.

The method of preparing special enamel coating on the surface of stainless steel substrate is to mix alcohol and enamel powder into suspension solution at a ratio of 2 g per 30 ml, and then vibrate with high frequency ultrasound until dispersing into slurry. The slurry is sprayed onto the surface of stainless steel by atmospheric spraying. The spraying pressure is about 0.3 MPa, and the distance between the nozzle and the stainless steel plate is controlled to 200 mm. From left to right and top to bottom, repeated spraying sequence to ensure uniform and smooth enamel coating. Each spray is about 2 μm thick as a spray cycle. Control about 20 passes to finish spraying. The coated stainless steel plate is put into the oven and adjusted to 70 ℃ for 10 min to obtain the powder. Then it is adjusted to about 900 ℃ in the muffle furnace for firing, and the firing is completed about 3 min later.

2.2 Interface Between Enamel Coating and Stainless Steel

Figure 3 shows the SEM morphology at the interface between enamel coating and stainless steel matrix.

Fig. 3.
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SEM morphology of enamel coating/matrix interface.

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The thickness of the enamel coating formed by firing is about 40 μm, and the coating is dense without holes (Fig. 3). The enamel coating is closely bonded to the substrate, and the interface is serrated. The presence of these serrated interfaces increases the physical bonding between the coating and the substrate on the one hand. The reaction area is between the coating and the substrate on the other hand, which promotes the chemical bonding and makes the coating and the substrate bond well. In order to further analyze the bonding mode between enamel coating and matrix, EDS line scanning analysis is carried out on the interface (Fig. 4).

Fig. 4.
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EDS line scan of enamel coating/matrix interface

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As shown in the Fig. 4, the main component of enamel coating is Si. Through the observation of the interface, it is found that there is a transition layer at the interface, as shown in the middle of the two yellow dotted lines. In this transition layer, Fe element decreases gradually from substrate to coating, indicating that Fe atom diffusion occurs during the preparation of coating. Cobalt is also found in the transition layer, which confirmed the diffusion of Co from the coating to the interface.

3 Enamel Coating Properties

3.1 Phase Characterization of Enamel Coating

The phase composition of enamel coating is analyzed by X-ray diffraction (XRD) with a sample of 10 × 10 mm cut by linear cutting method. The radiation used is Cu Kα (λ = 0.1548 nm), the working voltage is 50 kV, and the scanning speed is 2°/min. XRD results are analyzed with Jade 6.0 software.

Fig. 5.
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XRD pattern of enamel coating.

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As can be seen in the Fig. 5, steamed bun peak appears in the XRD pattern at 25°, which is a typical morphology of amorphous structure. Therefore, the enamel coating is mainly composed of short-range ordered and long-range disordered glassy phase. The peak of substrate austenite can also be seen.

3.2 Hardness of Enamel Coating

The hardness of enamel coating and substrate is measured by automatic micro hardness tester. The test pressure is 50 g and the holding time is 10 s. The average measured results are 874.8 HV for enamel coating and 195.7 HV for stainless steel substrate. The hardness of enamel coating on stainless steel surface is about 4.5 times that of stainless steel substrate. The improvement of hardness can greatly improve the wear resistance of the surface. So as to ensure that the surface of the work piece is not damaged when it is scratched externally, and maintain a smooth surface.

4 High Temperature Evaporation Performance

Self-designed vacuum chamber evaporation device is adopted, and its limit vacuum is 3 × 10–4 Pa. The water-cooled copper crucible is selected as the electron beam evaporation vessel. The purity of 99.5% cerium metal is used as raw material, and the smooth 316 stainless steel plate of 360 mm × 240 mm × 1 mm and the same stainless steel plate with enamel coating are used as the substrate respectively. The power of the electron gun is 30 kW, and the heating temperature of the material plate is 360 ℃. When the vacuum degree is better than 5 × 10–3, the cerium metal is vaporized by electron beam to form metal atom vapor and the stable evaporation begins. After evaporation, the microstructure and phase composition of the bonding interface between cerium metal and substrate are analyzed by means of SEM and EDS.

4.1 Interface Between Cerium Metal and Stainless Steel After Evaporation

The bonding interface between cerium metal and stainless steel after evaporation is shown in the Fig. 6.

Fig. 6.
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SEM morphology at the bonding interface with (a) 500X and (b) 100X

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The bonding interface between Ce metal coating and stainless steel can be clearly seen from Fig. 6a. The bonding interface is slightly cracked, and the diffusion of Ce metal to stainless steel can be observed on the surface of stainless steel. As the diffusion deepens, the diffusion of Ce metal layer is patterned. The results show that Ce metal diffused first on the surface of stainless steel along the grain boundary of 316L stainless steel. The Ce metal atoms at the grain boundary gradually diffused into the crystal to form a solid solution under the thermodynamic action, and finally formed metal compounds of CeFe2, which is consistent with the XRD analysis results. In the relatively deep region of 316L stainless steel, the diffusion ability of Ce metal atoms is weakened, so Ce metal atoms only diffuse along the grain boundary and rarely diffuse into the crystal interior, resulting in patterned diffusion [21]. As can be seen from Fig. 6b, the diffusion distribution of Ce metal coating is relatively uniform. The distribution of elements at the cerium-stainless steel bonding interface is shown in the Fig. 7.

Fig. 7.
figure 7

Surface scanning spectrum at the interface.

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It can be seen from Fig. 7a that Fe diffuses inside Ce metal. Different from the diffusion of Ce metal, Fe expands into a zigzag shape, which may be related to the crystal structure of Ce metal and the radius of Fe element. In addition, the distribution cross section of Cr element is linear. It indicates that Cr element has not diffused inside Ce metal to form a solid solution. The solid solution is divided into intergap solid solution and replacement solid solution. For intergap solid solution, its formation factors are determined by lattice gap, atomic radius and electronegativity of elements. For replacement solid solutions, the formation factors are only related to atomic radius and electronegativity. According to the table element radius, Fe atomic radius is 64.5 pm, Cr atomic radius to 52 pm, Ce atomic radius of 103.4 pm. This result indicates that the solid solution formed by Ce metal and Fe element at the interface of the coating is substitution solid solution.

Fig. 8.
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Element line scan analysis at interface

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Figure 8 shows the linear scanning analysis results of Ce metal and stainless steel substrate surface. It can be seen from the figure that the composition of Fe and Ce elements changed significantly with the change of scanning area. In the area about 40 μm left and right of the interface, Fe and Ce is a zigzag. This result further proves that Fe and Ce diffused each other rather than one way. The content of Cr element shows a cliff-like change, indicating that Cr element did not participate in diffusion. The above test results show that the stainless steel plate is corroded by cerium atoms after long time evaporation, forming a diffusion layer of tens of microns. Because of the diffusion effect between Ce metal and stainless steel layer, the bonding force between the two is strong, and the Ce metal atoms in the inner layer of stainless steel are difficult to be desorbed by means of steam oxidation treatment, which is the root cause of the material plate can not be reused.

4.2 Interface Between Cerium Metal and Enamel Coating After Evaporation

The macro morphology of cerium metal and stainless steel plate with enamel coating after evaporation is shown in the Fig. 9.

Fig. 9.
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Macromorphology of enamel coated stainless steel plate after evaporation.

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As can be seen from the Fig. 9, the enamel coating combines well with cerium vapor. After weighing, the deposition capacity of left plate, middle plate and right plate is equivalent to that of stainless steel plate. The microstructure of stainless steel plate with enamel coating at the bonding interface after steam plating is shown in the Fig. 10.

Fig. 10.
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SEM morphology of enamel coated stainless steel plate after evaporation

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SEM observations show that a layer of metal is evaporated above the enamel coating in the service process, and the main component is Ce after EDS analysis. The interface between the enamel coating and the matrix is still well combined, there is no obvious change compared with that before service. The thickness of the enamel coating is basically unchanged, proving that the enamel coating can serve stably in the target environment. In order to further analyse the bonding between enamel coating and autoclaves at the interface layer, EDS line scanning analysis is carried out on the interface layer of enamel coating/autoclaves, and the results are shown in the Fig. 11.

Fig. 11.
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EDS line scanning map of enamel coating/steam plating interface

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EDS line scanning map shows that, a transition region is formed at the interface between Ce and enamel coating. The thickness of this region is about 2 μm surrounded by the double yellow curve in the Fig. 11. In this transition region, the content of Si decreases and the content of Ce increases, and Si increases with the direction of steam gold plating. This phenomenon indicates that chemical reactions took place in the transition region, and the following reactions can be inferred from the distribution results of elements.

$$Sio_{2} + Ce\mathop{\longrightarrow}\limits^{500^\circ{^\text{c}}}Ceo_{2} + Si$$
(1)

The Gibbs free energy change of the reaction is −158.3 kJ/mol less than −40 kJ/mol, it can be spontaneous. The reduced Si atoms diffused into the Ce metal with low Si content, resulting in the enrichment of Si elements towards the Ce metal layer. However, once CeO2 is formed at the transition layer, the CeO2 layer can hinder the further diffusion of Ce to the enamel coating, thus preventing the further reaction between Ce and the enamel coating, resulting in the termination of the reaction. Due to chemical reaction, make the enamel coating and steamed metallized caused by van der Waals force not only the physical union. It is a chemical reaction to form the Si, O and Ce chemical combination. So the enamel coating can well with steamed metallized combination.

4.3 Enamel Coated Stainless Steel Plate Reuse

The method of pickling is used to verify that enamel coated stainless steel plate can be reused. Through the study on the acid resistance of enamel coating, it can be concluded that the enamel coating has a high acid resistance, so pickling has no effect on the enamel coating while removing the steam gold plating.

Fig. 12.
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Macromorphology of stainless steel plate with enamel coating before and after pickling.

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In order to compare the morphology of enamel stainless steel plate with evaporation before and after pickling, only part of pickling is carried out. The pickling part is surrounded by yellow dotted line in the Fig. 12. Seen from the figure, pickling parts show that the smooth surface of the enamel coating, and no local corrosion phenomenon, found that pickling in good remove steaming and plated at the same time, does not damage to enamel basement. The rainbow-like fringe on the left of the pickling part should be different elements or oxides with different valence states of the same element. The specific chemical composition has not been further analyzed here, but the results of local experimental cleaning by dilute hydrochloric acid show that the oxides here can also be removed by pickling. Therefore, the enamel coated material plate has the ability of reuse in theory.

5 Conclusions

By characterizing the phase composition, composition, hardness and microstructure of enamel coating at the interface between coating and substrate, the following conclusions are obtained.

  1. (1)

    The enamel coating is closely bonded to the substrate, and the coating itself has high density and low roughness. The enamel coating is closely bonded with the substrate due to the chemical bonding of Fe-rich Co phase and mechanical tooth bonding;

  2. (2)

    Ce and Fe in stainless steel will interact with each other to form a solid solution, resulting in greater roughness of stainless steel plate, cannot be reused;

  3. (3)

    Enamel coating can be stable in high temperature service. Silicon and cerium metal form a good chemical bond, so that the steam metal does not fall off;

Enamel coating has the ability of acid, alkali and high temperature oxidation corrosion. The bonding mode with cerium metal is non-diffusive and no corrosion occurs at the bonding interface. Therefore, pickling or high temperature oxidation can be used to remove cerium metal without damaging the enamel coating, so as to achieve multiple reuse of the material plate.