Effect of Antimony on Wetting Behavior and Interfacial Reaction between Zinc Liquid and X80 Steel

Abstract

The wetting behavior of molten Zn and Zn-Sb alloy and X80 steel in a high vacuum environment was studied by the modified sessile drop method. The wetting morphology and interface structure were analyzed by scanning electron microscope and energy dispersive spectrometer. The results show when the content of Sb in Zn-Sb alloy increases from 0.0 wt. % to 1.0 wt. %, the initial contact angle between the droplet and the substrate decreases from 102.8° to 82.5°, and the equilibrium contact angle also decreases from 57.4° to 41.4°. Sb element in the Zn-Sb alloy can reduce the contact angle and improve the wettability due to its smaller surface tension. The spreading process of Zn-Sb alloys on X80 steel can be divided into rapid adsorption, reaction control, steady-state equilibrium stages, and Zn-Sb alloys with different mass fractions have the same spreading kinetics. The volatilized Zn element in the Zn-Sb alloy will reduce the oxide film on the surface of the substrate, making it easier for the Zn-Sb droplet to wet the steel plate and induce the formation of a precursor film. The formation mechanism of the precursor film is the subcutaneous penetration mechanism.

1. Introduction

X80 steel is widely used in oil and gas pipelines because of its high strength and high corrosion resistance [1]. However, with the extension of pipeline service time, the pipeline will be corroded to a certain extent, resulting in oil leakage. This will cause irreversible losses to safe production, so the requirements for corrosion resistance of pipelines are getting higher and higher. Hot-dip galvanizing is widely used in various industries due to its dual functions of physical protection and chemical protection. However, in the process of hot-dip galvanizing, the leakage phenomenon caused by the poor fluidity of the zinc solution will damage the quality of the coating. Therefore, improving the fluidity of the zinc solution and the substrate is of great significance to improve the hot-dip galvanized coating [2,3].

At present, most of the research focuses on adding alloying elements to the zinc liquid to improve the fluidity of the zinc liquid while there are less research studies on the wettability and interfacial reaction between the zinc liquid and the substrate. Kong [4] and other studies found that the addition of Ni element in the hot-dip galvanizing process can improve the fluidity of zinc liquid, and can also inhibit the Sandelin effect to a certain extent. The research of Guo [5] shows that the addition of Sn in the hot-dip galvanizing bath can also improve the fluidity of the zinc solution, but it will form spangles on the surface of the coating, which is easy cause intergranular corrosion of the coating and reduce the corrosion resistance. There are also studies [6,7] finding that adding rare earth elements to the zinc solution can improve the fluidity of the zinc solution, but it has no obvious effect on the Zn–Fe interface reaction, but it can improve the corrosion resistance of the coating. The research of Wu [7] showed that adding Mg to the molten zinc pool can improve the fluidity of molten zinc, but when the content of Mg exceeds 1.0 wt. %, the anti-stripping performance of the coating will be reduced.

The wetting behavior of zinc melt and steel plate is essentially a reaction system between metal and metal under high temperature conditions, but the formation of interfacial reaction products and the change of interfacial energy are involved in the high temperature wetting system, so the wetting process is an interfacial reaction. There is still no unified conclusion on whether interface adsorption is controlled [8,9,10] or influenced by the interface reaction products [11,12,13,14,15,16]. Ref. [14] has found that wettability is related to the surface roughness of the substrate, and the two are inversely proportional. However, when the surface roughness of the substrate is less than 200 nm, this effect could be ignored [15]. Zhu et al. [17] believed that, during high temperature wetting, the droplets spread due to unbalanced forces and used calculus to establish the variation equation of the change of the solid–liquid interface energy with the atomic reaction rate. Lin et al. [18] studied the wetting of Sn on Al substrates at 350–450 °C, and found that the precursor film that appeared during the wetting process could improve the wettability between droplets and substrates. Jin [19] studied the wettability of 6061 aluminum alloy on Q235 steel, and found that the change of the interface reaction free energy has little effect on the final wettability. Kondoh [20] studied the wettability of pure magnesium and pure titanium, and found that the volatilized Mg vapor during the wetting process will reduce the oxide film on the substrate, thereby improving the wettability. Studies have found that zinc also vaporizes in the same environment [21,22]. Chung [23] studied the wettability of pure zinc and 440P steel at 470 °C and found that the initial contact angle of the droplet with the substrate was greater than 90°. Giorgi [24] studied the wettability of pure zinc and low alloy steel at different temperatures and found that the initial contact angle of zinc with the substrate was 120° at 475 °C, and the equilibrium contact angle was 35°.

The high-temperature contact angle measurement device includes the traditional seat drop method, through the tube titration method, etc. The initial contact angle and wetting start time are difficult to determine with the traditional seat-drop method. The customs clearance titration method can easily cause oxidation of the melt when dripping, which affects the accuracy of the results. In this experiment, the modified seat drop method was adopted, which could not only accurately obtain the initial time of wetting, but also use the small hole under the alumina tube to remove the oxide film on the melt surface to improve the accuracy of the experiment.

In this research, the interface reaction between Zn-Sb alloy and X80 steel substrate was studied. The spreading kinetics of Zn-Sb alloy with different contents in X80 steel were studied by the modified seat drop method. The relationship between contact angle change, interfacial reaction and interfacial structure, solid–liquid interface energy, precursor film, and wettability at different times was analyzed, and the wetting mechanism of Zn-Sb alloy and X80 steel was further studied. This experimental study lays a theoretical foundation for the development of galvanized layers of high-quality X80 steel in industrial production. It also provides new ideas for the study of wetting behavior and wetting mechanism of other high-temperature metal melts. In this paper, the wetting behavior and interface reaction between Zn-Sb alloy and X80 steel were studied, and the mechanism of Sb element on improving the wetting behavior of Zn melt and X80 steel was explored. The other elements have not been explored individually, and we will continue to investigate them in subsequent work.

2. Materials and Methods

The substrate selected in this experiment is X80 steel of 20 mm × 20 mm × 3 mm, and the composition of the substrate is shown in

Table 1. Chemical composition of X80 steel substrate.

Composition	C	Si	Mn	P	S	Cr	Ni	Mo	Nb
Mass Fraction (wt. %)	0.054	0.30	1.84	0.011	0.0041	0.33	0.10	0.091	0.075

. The Zn–Sb alloy required for wetting is smelted with 99.99% pure Zn and Sb, and the mass fraction of Sb in the Zn–Sb alloy is 0.25 wt. %, 0.5 wt. %, 0.75 wt. %, and 1.0 wt. %, respectively. Before the start of the experiment, the substrates were polished with 400-mesh, 800-mesh, and 2000-mesh sandpapers respectively, and then polished with diamond polishing liquid to a roughness R_a of about 30 nm. Therefore, there is no roughness effect. Our research group has described the surface roughness of the sample in literature [24]. The steel selected for this experiment is X80 steel, and the carbon content of steel is 0.054%. At the same time, Ref. [25] found that the carbon content had a greater effect on the iron–zinc interface reaction when the carbon content was 0.2–0.25%. The smelted Zn–Sb alloy is cut into small cubes, and then ground into a spherical shape, and the mass of each small cube is 0.2–0.3 g. Before the experiment, the substrate and alloy samples were put into acetone, and ultrasonically cleaned 3 times (3 min each time) to remove the oil stains on the substrate and alloy surface.

The wettability of Zn–Sb alloy with different Sb content and X80 steel in a high vacuum environment was studied by the modified sessile drop method. First, place the cleaned substrate on the sample stage in the furnace, ensure that the position of the substrate is in a horizontal state, and then cover the shielding layer inside the furnace. Then, put the alloy in the stainless-steel tube outside the furnace. After the drip tube and the stainless-steel tube are connected, install the drip tube and the stainless-steel tube, then cover the furnace cover and tighten it symmetrically with screws. After the above steps are completed, the vacuuming experiment is started in the furnace. First, the air pressure in the furnace is evacuated to below 10 Pa with a mechanical pump at room temperature, and then the pre-extraction valve is closed, and the N₂ furnace is quickly passed into the furnace (this reciprocates 3 times), and the furnace is washed. After completion, use a mechanical pump to pump the air pressure in the furnace to below −0.1 Pa, and finally use a molecular pump to pump the air pressure in the furnace to 2.2 × 10⁻³ Pa. After the vacuuming is completed, the high-purity N₂-10%H₂ gas is quickly introduced into the furnace and the heating system is started (the pressure in the furnace is always about 0.12 MPa during the heating process), and the substrate is first heated at a speed of 25 °C/min to 800 °C, then kept at 800 °C for 5 min, and then cooled to 450 °C at a rate of 20 °C/min. After the temperature in the furnace dropped to 450 °C and stabilized, pour the alloy in the stainless-steel tube into the dropper. After the alloy is fully melted, slowly open the air outlet valve and use the pressure difference to drop the molten liquid alloy onto the X80 steel substrate. At the same time, a high-resolution CCD digital camera is used to record the video. When the shape of the droplet on the substrate is basically unchanged, the video is recorded to finish. At least three experiments were performed for each alloy composition to reduce experimental errors.

After the experiment, the images obtained in the wetting experiment were processed by the Axisymmetric Drop Shape Analysis (ADSA) software to obtain the contact angle of the molten metal and the substrate at different times. After the experiment was completed, the alloy was first cut from the middle by a wire cutting machine, and then etched and dried with 4% nitric acid alcohol. Scanning electron microscope and energy dispersive spectrometer were used to observe the microscopic morphology and composition analysis of the interface reaction layer and the three-phase line. The thickness of the alloy layer and the width of the precursor film were measured by Smile view software.

3. Results and Discussion

3.1. The Effect of Sb on the Wettability of Zinc Solution on X80 Steel

When the alloy is melted in the drop tube, the alloy is mainly subjected to the pressure caused by gravity and the pressure difference between the upper and lower drop tubes. Since the quality of the alloy is controlled within a small range in each experiment, and the drop tube is different from the drop tube in each experiment. The distance to the substrate is also different so that the surface profile of the droplet on the substrate is hardly affected by the gravity of the alloy. When the alloy is dropped by the pressure difference between the upper and lower parts, the small hole in the lower part of the drop tube will remove the oxide film on the surface of the alloy melt, ensuring that there is no oxide film on the surface of the droplet just in contact with the substrate, reducing the experimental error.

Figure 1 shows the changes in initial contact angle and equilibrium contact angle of Zn-Sb alloy on X80 steel with different Sb content. It can be seen from Figure 1 that the initial contact angle of pure zinc on X80 steel is 102.8°, although this is different from the results of Giorgi [23]. Giorgi studied the experimental temperature of 475 °C, and the substrate is also different from the substrate used in this experiment. In addition, the wettability will be improved as the temperature increases, so the results of this experiment have certain reliability. It can be seen from Figure 1 that, when the Sb content in the Zn-Sb alloy increases from 0.0 wt. % to 1.0 wt. %, the initial contact angle between the droplet and the substrate decreases from 102.8° to 82.5°, and the equilibrium contact angle also decreases from 57.4° to 41.4°; it can be seen that the Sb element can improve the wettability of zinc liquid and X80 steel. From the binary phase diagram of the Zn–Sb alloy, it can be seen that the 1.0 wt. % Sb element added to Zn at 450 °C exists in the alloy in the form of a liquid phase, and does not form a binary compound with the zinc element. Under the experimental conditions, both Zn and Sb elements exist in the melt in the form of elemental substances. Some studies have found that the relationship between the surface tension of Sb and the temperature is [25]:

$$\mathsf{\sigma }(\mathrm{mN}·{\mathrm{m}}^{-1})=371 - 0.045 \left(\mathrm{T}-630\right)$$

(1)

The unit of T in the formula is K. It can be seen from the calculation that the surface tension of the Sb element is 0.366815$\mathrm{mN}·{\mathrm{m}}^{-1}$ under the experimental conditions, which is much smaller than that of Falke [26]. The surface tension of pure Zn studied by Nogi et al. [27] is 0.78$\mathrm{mN}·{\mathrm{m}}^{-1}$. Since the surface tension of Sb element is less than that of Zn element, the addition of Sb element to zinc liquid can improve the wettability of zinc liquid and X80 steel because Sb elements tend to enrich on the surface of molten droplets, which can better promote the spread of zinc liquid on X80 steel, reduce the contact angle between melt and substrate, and improve wettability.

3.2. Spreading Kinetics of Zn-Sb Alloy on X80 Steel

Although it has been known that the added Sb element can improve the wettability of Zn-Sb alloy and X80 steel, the contact angle between the droplet and the substrate and the spread of the droplet radius show irregularities during the wetting process. Therefore, in this experiment, the contact angle and the spreading radius of the droplet at different times during the wetting of the droplet with the substrate were also calculated and analyzed. Figure 2 is a graph showing the variation trend of the contact angle and normalized contact radius of Zn and Zn-Sb alloy on the surface of X80 steel with time. As shown in Figure 2a, the contact angle decreases rapidly within 4–6 s when the droplet contacts the substrate. In the next 25 s or so, the contact angle decreases linearly with time. When the contact time reaches 30 s, the contact angle is already basically unchanged, to achieve balance. As shown in Figure 2b, the normalized radius first increases rapidly with the contact time and then maintains equilibrium around 30 s. The relationship between the normalized radius and time in the first 30 s can be fitted by an exponential function [28]:

$${\mathrm{R}}_{\mathrm{d}}{/\mathrm{R}}_{\mathrm{o}}{=\mathrm{R}}_{\mathrm{e}}{/\mathrm{R}}_{\mathrm{o}}-\mathrm{aexp}[-{(\mathrm{t}/\mathsf{\tau })}^{\mathrm{m}}]$$

(2)

where a, τ, and m are all fitting parameters and have no units; R_d, R_o, and R_e are the dynamic contact radius, initial contact radius, and initial contact radius, respectively, in millimeters; t is the wetting time of the droplet on the substrate, in seconds. The results show that the fitting effect is good within the first 30 s and R² > 0.995. After wetting equilibrium, the wettability of Zn-Sb alloy and X80 steel is obviously better than that of pure Zn sample, but the spreading kinetics of both are the same, and a precursor film appears in the later stage of spreading.

It can be seen from Figure 2a that the variation trend of the contact angle with time can be divided into three stages:

Stage I: the contact angle decreases rapidly within 6 s when the droplet contacts the substrate. The three-phase line-out equilibrium between the droplet and the substrate has not yet been established. At this time, the droplet and the substrate have not yet undergone interfacial reaction, so the wettability is determined by the initial contact angle, which is mainly determined by the surface tension of the droplet. Therefore, at this time, the wettability will increase with the increase of the Sb content in the Zn-Sb alloy, and this time period has a tendency to prolong with the increase of the Sb content in the Zn-Sb alloy.

Stage II: 6–30 s, when the droplet contacts the substrate. At this time, due to the progress of the Zn–Fe interface reaction, the droplet will gradually spread on the substrate, and the three-phase line will move slowly, and it can be seen from Figure 2a that, although the added Sb element can improve the wettability of zinc liquid and X80 steel, it has little effect on the spreading rate of droplets at this stage, which indicates that the added Sb element is only an active element and does not participate in the interfacial reaction at this stage.

Stage III: when the contact time between the droplet and the substrate exceeds 30 s, the three-phase line has basically established equilibrium and the interface reaction has basically stopped. Thus, the contact angle is in a stable state and no longer decreases with the extension of the contact time.

Zhu and others [29,30,31] believed that the wettability and the interfacial reaction rate were positively correlated in the wetting process, and deduced the expression of the solid–liquid interface energy and the liquid surface tension in the reactive wetting process:

$${\gamma }_{sl}\left(t\right)=\frac{{\gamma }_{lg}}{2}\left[\sqrt{1+{\left(\sin \theta \left(t\right)\right)}^2}-\cos \theta \left(t\right)\right]$$

(3)

where ${\gamma }_{sl}\left(t\right)$ is the solid–liquid interface energy, in J/m²; ${\gamma }_l{}_g$ is the liquid surface energy, in J/m²; $\theta$ is the contact angle between the droplet and the substrate, in degree; t is the contact time between the droplet and the substrate, in seconds. Since the surface energy of the droplet is a fixed value, the solid–liquid interface energy is related to the contact angle, and a smaller solid–liquid interface energy tends to improve the wettability. It can be seen from Figure 3a that the contact angle is less than 90° in the second stage of spreading. When the contact angle is in the range of 45° to 90°, the value of $\sqrt{1+{\left(\sin \theta \left(t\right)\right)}^2}$ is larger than $\cos \theta \left(t\right)$, but the contact angle gradually decreases with the extension of wetting time, so $\sqrt{1+{\left(\sin \theta \left(t\right)\right)}^2}$ also decreases gradually, while $\cos \theta \left(t\right)$ increases gradually. Therefore, at this stage, the solid–liquid interface energy between the droplet and the substrate gradually decreases. Although the equilibrium contact angles of Zn-0.75 wt. % Sb and Zn-1.0 wt. % Sb with X80 steel are 44.2° and 41.4°, respectively, they are not very different from the 45° value, so they have the same wetting tendency as other alloys.

3.3. Wetting Morphology and Interface Analysis

In order to further analyze the wetting mechanism of Zn and Zn-Sb alloy and X80 steel, the wetting samples of Zn and X80 steel and the Zn-1.0 wt. % Sb alloy sample with the best wettability were selected for analysis in this experiment. Figure 3a,b are the macro-morphologies of the wetted sample with Zn and X80 steel and the micro-morphology of the precursor film, respectively. Figure 3c,d are the macroscopic image and the microscopic image of the precursor film after wetting of Zn-1.0 wt. % Sb with X80 steel, respectively. As shown in Figure 3a,c, there is a bright ring at the three-phase line between the melt and the substrate. This bright ring is the precursor film that appears during the wetting process. The so-called precursor film is actually the extension of the three-phase line when the droplet spreads on the substrate during the wetting process. The overall morphology of the precursor film is gully-like, and the width is not uniform. The liquid flows quickly in the wider area of the precursor film and slowly in the narrower area. It can be seen from Figure 3a,c that, when 1.0 wt. % Sb is added to the Zn melt, although the macroscopic morphology of the precursor film is groove-like, the width of the precursor film increases by about 20 μm, which is also consistent with the conclusion of Saiz et al. [8] that the wider the precursor film, the better the wettability.

As shown in Figure 3b,d, there is a difference in the microscopic morphology of the two precursor films. Although both precursor films have a stepped fish scale, the precursor film of Zn melt is not as good as that of the Zn-Sb alloy. The precursor film has an obvious stepped shape, and the more obvious stepped precursor film can increase the capillary force of the zinc solution to a certain extent, promote the spreading of the zinc solution, and improve the wettability. The moment the droplet contacts the substrate, an interface reaction occurs with the oxide film on the surface of the substrate. At this time, the interface reaction product will destroy the oxide film at the three-phase line. When the droplet spreads further, due to the existence of the interface reaction product, at this time, the reaction product between the droplet and the substrate will accumulate on the previous interface reaction product, so the microstructure of the precursor film is stepped. When the compositional analysis was performed on other parts of the steel substrate surface, the presence of zinc was also found on the substrate, indicating that zinc would exist on the substrate in gaseous form during the wetting process; this is mainly because the saturated vapor pressure of zinc is higher, and it is more volatile. When the droplet falls on the substrate and begins to spread, the zinc vapor will reduce the oxide film on the substrate, so that the original dense oxide film on the substrate begins to crack, and the zinc liquid begins to penetrate into the steel substrate along the crack in the middle of the oxide film and the interface with the steel substrate react. At the same time, the phase generated during the interfacial reaction will continue to break the oxide film on the surface of the substrate, promote the movement of the three-phase line, and then induce the formation of the precursor film. Therefore, the precursor membrane is actually a reaction product layer outside the three-phase line, and its formation mechanism is the subcutaneous permeation mechanism.

In order to further analyze the wetting mechanism of Zn and its alloys with the substrate, the interface analysis of the samples after the experiment was carried out in this work. As shown in Figure 4a,b, the products of the interface reaction between Zn and X80 steel substrate are mainly FeZn₁₀ phase and FeZn₁₃ phase. Compared with the FeZn₁₃ phase, the FeZn₁₀ is closer to the steel matrix, and the thickness of the reaction layer at the center of the interface is about 15 μm smaller than that at the three-phase line. This is mainly because, when the droplet just contacts the substrate, the droplet will react with the oxide film first and then with the substrate due to the existence of the oxide film on the substrate. The oxide film at the three-phase wire will be destroyed by zinc vapor reduction and interface reaction products, so the substrate at the three-phase wire will be cleaner and the interface reaction intensity will be more intense. Fe-Zn is an intermetallic compound formed by a solution reaction without the formation of new chemical bonds between the compounds [24]. At the same time, because the Sb element has less surface tension, the wettability of zinc liquid can be improved.

As shown in Figure 5a,b, the interfacial reaction products of Zn-1.0 wt. % Sb and X80 substrate are no different from those shown in Figure 4a,b, and the thickness of the central reaction layer at the interface does not change significantly. Therefore, it can be judged that the Sb element does not participate in the interfacial reaction, but the thickness of the interfacial reaction layer between the Zn-1.0 wt. % Sb and X80 substrates at the three-phase line is approximately 20 μm higher than that in Figure 4b. This indicates that the interface reaction at the three-phase line is stronger at this time. According to the theory that the reaction product determines the wettability [18,19], the violent interfacial reaction between the melt and the substrate will inevitably improve the wettability. Thus, the wettability at the three-phase line is improved at this time; this is mainly because the surface tension of the Sb element is smaller than that of the Zn element. Thus, it tends to concentrate on the surface of the zinc solution, thereby reducing the surface tension of the Zn solution, which is beneficial to the flow of the Zn solution and improves the wettability.

4. Conclusions

(1) Adding a Sb element smaller than Zn surface tension to Zn solution can significantly improve the wettability of Zn solution and X80 steel substrate, and the wettability will improve with the increase of Sb content when the content of Sb in Zn solution is 1.0 wt. %, the equilibrium contact angle is about 41.4°.

(2) The spreading kinetics of Zn and Zn-Sb alloys on the X80 steel substrate are the same, and the change of the contact angle can be divided into three stages, i.e., the rapid reduction stage, the linear change stage, and the equilibrium stage. The normalized radius and wetting time during the first two stages can be fitted with an exponential function, and the reduction of the solid–liquid interfacial energy during spreading can improve the wettability.

(3) During the wetting process, the Zn vapor volatilized from Zn and Zn-Sb alloys will reduce the oxide film on the surface of the substrate and induce the formation of a stepped precursor film, which can significantly improve the wettability. The formation mechanism is subcutaneous penetration, while the strong interfacial reaction at the three-phase line can also improve the wettability.