Keywords

1 Introduction

Liquid lead-bismuth alloy (Pb44.5%, Bi55.5%, LBE) as the coolant of LFR, possesses favorable thermophysical property, neutron radiation-resistant, and chemical stability [1,2,3,4]. However, the working environment of LBE is easy to cause the problem of liquid metal embrittlement (LME) of stressed structural materials. The phenomenon of LME occurring under specific load or environmental conditions may lead to rapid and uncontrollable crack growth, which cause the transition of liquid metal from ductile fracture mode to brittle fracture mode under plastic deformation [5, 6]. There are many complicate factors affecting the LME. The “adsorption theory” of the solid-liquid two-phase interface is widely used for explaining the phenomenon of LME [7]. It is considered that the liquid metal atoms adsorbed on the steel surface reduce the binding energy between the main alloy elements (such as Fe, Cr, Ni, etc.) of the steel, thereby reducing the fracture toughness of the steel and leading to early fracture [7, 8]. The wetting behavior between liquid metal and structural materials can qualitatively reflect the adsorption capacity of the steel to liquid metal atoms and has guiding significance for the prediction and mechanism study of brittle fracture of structural materials in liquid metal [9].

At present, there are a large amount of data on the wettability between liquid metals and materials. However, the research data about the wettability between LBE and structural materials are limited. Among them, D. Giuranno et al. [10] studied the wetting behavior of LBE and lead on AISI 316L at different temperatures. They found that the contact angle of LBE on the sample was smaller than that of pure lead. Liu Jing et al. [11] conducted real-time contact home test of liquid lead and LBE at different temperatures on T91 steel surface. It is concluded that the contact angle of T91 in both systems is greater than 90°, indicating that liquid lead and LBE are in a nonwetting state with T91 steel. And the contact angle of the two systems decreased with the increase of temperature. Besides, Zhen Qi et al. [12] studied the contact angle of liquid lead on alumina substrates with different roughness at temperature range from 923K to 1123K. With the increase of surface roughness, the increase of the actual surface area of the substrate leads to the increase of surface free energy, which increases the contact angle. However, there is little research on the influence of different surface roughness of LBE and structural materials on wettability.

In this work, T91 and 316L steels with different roughness were prepared. The surface roughness values and three-dimensional surface morphology were measured. The contact angles of LBE on different samples were measured systematically at temperatures ranging from 200 to 500 ℃. The reasons for the difference of wettability were analyzed by observing the micro morphology of boundary between LBE and sample surfaces.

2 Experimental

2.1 Samples Preparation

Two typical candidate structural materials 316L and T91 are selected. The nominal composition of commercial 316L and T91 steels is shown in Table 1. Before the experiment, the two materials were cut into small pieces of 15 × 5 × 1.5 mm3 in size. To make different surface roughness, the UNIPOL-802 automatic precision grinding and polishing machine of Shenyang Kejing Auto-instrument Co, LTD. (Fig. 1-a) was used to perform surface treatment on the surfaces of the two materials. Four samples are selected for each material, and the treatment method is shown in Table 2. The polishing time on each type of sandpaper was 20 min. The mirror polishing time was 30 min. All the samples were stored in vacuum before the experiment. For the polished samples, the roughness and three-dimensional morphology were measured by atomic force microscope (AFM).

Table 1. Chemical composition of experimental steel (in wt. %).
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Table 2. Sample preparation method.
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Fig. 1.
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Polishing process: (a) UNIPOL-802 automatic precision grinding and polishing machine, (b) Sample loading platform, (c) Prepared sample.

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2.2 Wetting Test

The wetting test was carried out on the contact angle measuring device by using static drop method. The device is shown in Fig. 2.

The LBE droplet was 0.25 g in weight with an error of ± 0.005 g. Before experiments, LBE droplets were heated and melted in glycerol at 300 ℃ and quickly stirred to discharge air after reaching the preset temperature. The purpose is to make the droplet shape more regular. Then cooled in glycerin. Put the cooled sample into acetone for 2 min, removed the residual glycerol on the sample surface, and then stored it in vacuum.

Before the experiment, the test bench was leveled to ensure that the sample was in a horizontal state. When the test began, one sample was placed on the metal ceramics heater (MCH), and the prepared LBE droplet was placed on the center of the sample surface. Then, high-purity argon was introduced to remove the air in the chamber, and the argon atmosphere in the chamber was maintained during the experiment. The test temperature was from 200 ℃ to 500 ℃, and every 50 ℃ was a test recording point. When the temperature reached the set temperature point, the image of the sessile droplet was taken by the camera after 0.75h. In order to explore the influence of contact time on contact angle, the contact angle of four samples (1#, 4#, 5# and 8#) were tested after keeping the temperature at 500 ℃ for 6 h. The cross-section of the boundary between droplet and sample surface were analyzed by SEM.

Graphical analysis software JC2000 was used to analyze the droplet photos. Contact angles value was measured by the five-point fitting method, as shown in Fig. 3.

Fig. 2.
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Contact angle measuring device: (a) Device picture, (b) Schematic diagram.

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Fig. 3.
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Five-point fitting example.

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3 Results and Discussions

3.1 Surface Morphology Analysis

Figure 4 shows the 3D morphology of the surface roughness of eight samples by AFM. There are obvious differences between samples polished with different sandpapers. Different colors in the figure represent different heights. The surface roughness characterizing in Sa is shown in the figure. The larger the Sa value, the rougher the sample surface. As the mesh number of sanding paper increases, the Sa value decrease and the sample surface becomes more flat. After mirror polishing, almost no bulge exists on the surface of the sample. The surface roughness of the two steels is different when they are polished under same conditions, which may be caused by the difference of steel hardness.

Fig. 4.
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3D topography of AFM on different surfaces. (a) 316L 400#, (b) 316L 1200#, (c) 316L 2000#, (d) 316L mirror polished, (e) T91 400#, (f) T91 1200#, (g) T91 2000#, (h) T91 mirror polished.

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3.2 Contact Angle

The contact angle measured by the five-point measurement method refers to the angle θc between the tangents of the gas-liquid interface as shown in Fig. 5. The relationship between contact angle and liquid surface tension can be obtained from Young's equation [13].

$$ \cos \theta_{c} = \frac{{\gamma_{SG} - \gamma_{SL} }}{{\gamma_{LG} }} $$
(1)

In the above formula, \(\gamma_{SG}\), \(\gamma_{SL}\) and \(\gamma_{LG}\) represents the free energy of solid-gas, solid-liquid and liquid-solid interfaces respectively. When the surface roughness of the sample increases, it will cause obstacles to the liquid paving, the solid-liquid free energy \(\gamma_{SL}\) decreases, the contact angle increases, and the wettability becomes worse. When θc = 0°, define that the liquid completely wets the solid surface. When 0 < θc ≤ 90°, solid surfaces are defined as wetted surfaces. When 90° < θc ≤ 150°, solid surfaces are defined as nonwetting surfaces. When θc ≥ 150°, Solid surfaces are defined as super nonwetting surfaces.

Fig. 5.
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The contact angle of a liquid drop on the solid surface.

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Fig. 6.
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Contact angle with different roughness. (a) 316L, (b) T91.

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The contact angles of LBE droplet on 316L sample surface with different roughness are shown in Fig. 6.

As shown in Fig. 6 (a). All the contact angles of LBE on 316L samples surfaces are greater than 149°, indicating that they are nonwetting to LBE. With the increase of temperature, the contact angle shows an increasing trend as a whole. For example, when the temperature is 200 ℃, the contact angle of the sample surface polished by 400# sandpaper is 158.32°. With the increase of temperature, the contact angle decreases to 153.36° at 350 ℃. When the temperature rises to 500 ℃, the contact angle continues to decrease to 152.29°. A special temperature point can be seen from the overall trend. When the temperature rises to 450 ℃, the contact angle of the four samples increases. This may be caused by the change of surface morphology caused by carbide precipitation of austenitic stainless steel. Besides, with the decrease of surface roughness, the surface contact angle reduces significantly. This means that the increase in surface roughness helps to reduce the wettability of LBE on 316L surfaces.

As shown in Fig. 6 (b). The contact angle of LBE on T91 samples surfaces is greater than 145°.which is also nonwetting to LBE. The change trend of contact angle with temperature and roughness is the same as that of 316L steel.

Figure 7 compares the contact angle of LBE on two stainless steels with same roughness. The contact angle of 316L is greater than that of T91 on each surface roughness. The difference of surface contact angle between the two steels after mirror polishing is the smallest. As the surface becomes rougher, the difference of contact angle between the two steels becomes larger. This indicates that the wettability of LBE to 316L steel is worse under the same temperature and surface roughness. It can be inferred that T91 steel has a higher tendency of LME under the actual operating conditions of the reactor.

The influence of contact time on contact angle is shown in the Fig. 8. With the increase of holding time, the surface contact angle between smooth surface and rough surface shows obvious difference. For the smooth surface obtained by mirror polishing, with the increase of holding time, the contact angle of 316L steel decreases from 148.69° to 136.04°, and that of T91 steel decreases from 146.76° to 143.82°. For the rough surface obtained by grinding with 400# sandpaper, with the increase of holding time, the contact angle of 316L steel increases from 152.29° to 158.22° and that of T91 steel decreases from 149.45° to 151.68°. In the change of contact angle between the two surfaces, the change degree of 316L steel is much greater than that of T91 steel. At high temperature, the mutual diffusion of elements intensifies, which accelerates the interaction between liquid metal and material surface, which leads to the enhancement of wettability. This phenomenon can be effectively suppressed by using only rough surface treatment. In contrast, the surface roughening treatment has a better inhibition effect on the infiltration behavior of LBE on the surface of austenitic stainless steel.

Fig. 7.
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Comparison of contact angle between 316L and T91 steel at the same roughness.

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Fig. 8.
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Influence of contact time on contact angle.

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3.3 Contact Time Affects

As mentioned above, with the increase of contact time, the change of contact angle of high and low roughness surfaces shows an opposite trend. In order to further analyze the influence of holding time on the infiltration degree of LBE on the surface of the material, SEM was used to analyze the morphology of 316L interface of LBE under different polishing degree and holding time. The cross-sectional morphology is shown in Fig. 9. It can be seen that when the holding time is 0.75h, LBE at the surface of materials with different polishing conditions is incomplete infiltration, and there are certain gaps at the interface. With the increase of contact time, LBE infiltrates into the surface gap. For rough polished samples, the surface gap is completely penetrated by LBE, and the material surface is in close contact with LBE. For the fine polished samples, in addition to the complete penetration of LBE at the surface, the original flat surface was also found to become rough. The matrix elements of the material diffuse to the LBE, and the combination between the two is closer. Combined with the analysis results of contact angle, it can be seen that with the increase of holding time, the existence of initial rough surface increases the time of complete contact of LBE on the material surface, effectively delaying the dissolution of matrix elements to LBE. The smooth surface is more prone to lead bismuth alloy infiltration, which is not conducive to inhibiting the dissolution of matrix elements in LBE. Therefore, in practical engineering applications, rough polishing of the surface of structural materials is more conducive to the inhibition of LME.

Fig. 9.
figure 9

Cross-section morphology of (a) 316L 400#-0.75h, (b) 316L 400#-6h, (c) 316L mirror polished-0.75h, (d) 316L mirror polished-6h, The green dashed line indicates the location of the original surface of the material.

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4 Summary

This paper mainly studied the LBE contact angle of 316L and T91 steels with different surface roughness at 200 ~ 500 ℃ to analyze the change of wettability, combined with the SEM analysis results. The conclusion can be drawn as follows.

  1. (1)

    With the increase of temperature, the contact angle of two kinds of stainless steels on different roughness surfaces decreases gradually.

  2. (2)

    With the increase of roughness, the surface contact angle of the two stainless steels decreases at any temperature.

  3. (3)

    With the increase of holding time, the contact angle of the two steels on the smooth surfaces decreases, while increases on the rough surfaces.

  4. (4)

    In contrast, LBE has better wettability to T91 than 316L, indicating that 316L has a lower tendency of LME in practical applications.