Influence of annealing treatment on wetting of steels by zinc alloys

Abstract

In continuous hot-dip galvanizing process, steel strip is immersed in a molten zinc bath containing small amounts of aluminium (less than 1 wt%). Before immersion in the molten metal, the steel strip is annealed at 800 °C in an atmosphere of nitrogen and hydrogen (5 vol.%) with low partial water pressure (20–50 Pa). Under these conditions, the most oxidizable alloying elements in the steel (e.g. Si, Mn) segregate to the surface where they form oxide particles. The presence on the steel surface of such oxide particles sometimes results in problems of wettability by liquid zinc–aluminium and may lead to the formation of surface defects. In order to gain a better understanding of this problem, the wetting by liquid zinc–aluminium of heterogeneous surfaces composed of metallic iron and silica particles or films was investigated at 450 °C using the dispensed drop technique. The liquid zinc–aluminium wets oxidised surfaces to a lesser extent than the iron substrates. With the operating conditions used in the laboratory experiments, the transition from wetting to non-wetting was found to occur when the surface area fraction covered by silica was equal to 34 ± 6 %. Mechanisms are proposed to explain the formation of the surface defects sometimes observed in galvanized coatings, based on the wettability experiments and characterization of surface defects from industrial coatings.

Introduction

Steel strip is often coated with a zinc layer in order to protect it against corrosion. One classical way to produce such coatings is the zinc electro-deposition process which is performed on dedicated continuous lines, processing coils of already annealed steel sheet. A more cost-efficient way to produce galvanized steel sheet is the hot-dip galvanizing process, which combines two metallurgical operations in a single processing line: the annealing of the steel and the coating deposition [1, 2].

In this process, before immersion in a bath of molten zinc or zinc alloy, the steel sheet is annealed in an atmosphere of N₂ and H₂ (5–15 vol.%), containing only traces of water (approximately −40 °C dew point¹). The main purposes of the heat treatment are to (1) recrystallize the steel substrate after cold rolling, and (2) reduce iron oxides in order to improve wettability by liquid zinc. At the same time, the less noble alloying elements of the steel such as Al, Mn, Si, Cr, Ti, and P segregate to the surface where they form oxide particles or films, poorly wetted by liquid zinc. This phenomenon is called selective oxidation [2].

Nowadays, steel sheet is increasingly alloyed with elements such as Mn, Si, Al, and Cr to increase its strength. This means that thinner sheet can be used, for example, in the car manufacturing industry, and as a result, the weight of car bodies can be reduced. These steels with high levels of alloying elements are called high strength steels. The alloying elements are more prone to oxidation than iron, and it is generally impossible to avoid their oxidation on the steel surface [3]. Traditional annealing treatment works well for steels with low levels of alloying additions. In such cases, few oxide particles are formed on the surface and the metallic bonding remains continuous across the steel/coating interface. When steels with higher levels of alloying additions are annealed in the conventional manner, many more oxides are formed. If the amount of oxides on the steel surface at immersion is too high, the galvanizing operation will be impeded, giving rise to a complete lack of or local breaks in the coating (bare spots).

The aim of this article is to explain the formation of bare spots and wetting defects that may occur during the galvanizing of alloyed steel sheet. It is, therefore, essential to understand the mechanisms involved in the wetting by liquid zinc alloy of heterogeneous surfaces composed of metallic iron and oxide particles.

This article is divided into three parts:

• (1) In the first part, the available data for the wettability of liquid zinc alloys on heterogeneous surfaces of metallic iron partly covered by oxides is critically assessed in the field of hot-dip coatings.

• (2) In the second part, the experimental protocol and results obtained using the dispensed drop technique is presented and discussed. In these experiments, the spreading of zinc alloy droplets with very short contact times was recorded using a high-speed camera for three types of surfaces, namely chemically homogeneous surfaces of pure iron and silica and heterogeneous surfaces made of iron and silica.

• (3) In the last part of the article, mechanisms are proposed to explain the formation of bare spots in a galvanizing line, based on the wettability experiments and characterization of defects from industrial coatings.

Wetting of iron, steels, and silica by liquid zinc alloys (literature survey)

Wettability studies to simulate the continuous hot-dip galvanizing process

Immersion of steel strip in a zinc bath leads to interfacial reactions between Fe and liquid Zn which depend on the bath composition and the solutes found in the steel. In the case of pure Zn, a layer (some tens of μm thick) of Fe–Zn intermetallic compounds is formed at the interface [2]. In continuous hot-dip galvanizing, small amounts of Al (0.16–0.2 wt%) are currently added to the zinc bath. The addition of Al provides better control of chemical reactivity due to the formation of a 100 nm thick layer of the Fe₂Al₅Zn _x (0 < x < 1) intermetallic compound at the interface between solid steel and liquid zinc alloy [2]. The galvanizing reactions proceed by iron dissolution in the liquid zinc alloy followed by the nucleation and growth of the Fe₂Al₅Zn _x solid phase in equilibrium with the Fe-saturated liquid phase.

To simulate the wetting and reactivity during hot-dip galvanizing, wettability tests are generally employed. Wetting between steel and liquid zinc can be studied using the sessile or dispensed drop technique in which the contact angle can be measured directly [4–7], as well as the meniscograph method in which the wetting angle can be evaluated from measurements of the wetting force [7–13].

Alibeigi et al. [13] studied the wettability of Mn-containing steels (0.14–5.1 wt% Mn) by a liquid zinc alloy containing 0.2 wt% Al, using the meniscograph method. Before immersion in the liquid zinc alloy at 460 °C, the steel samples were annealed at a fixed temperature ranging from 640 to 750 °C in an atmosphere of N₂–5 vol.% H₂. Manganese diffuses from the bulk to the surface of the steel, where it contributes to the formation of manganese oxide particles. The surface density of the oxide particles depends on the Mn concentration in the steel and the partial water pressure of the gas atmosphere. The surface coatability by liquid zinc is found to be a function of the distribution of these surface oxides. The experiments of Alibeigi et al. [13] confirmed that the wetting of steel surfaces by liquid zinc alloy is reactive, with the formation of Fe–Al intermetallic compounds at the interface between iron and zinc.

The coatability of Si and Mn–Al alloyed steels after annealing has been studied by Drillet et al. [14], using the meniscograph technique. The liquid metal was composed of zinc with small amounts of aluminium. This study reveals that a very thin layer of amorphous silica forms on the surface of Si-alloyed steels during the annealing step. When the steel surface is fully covered by this oxide layer, the Fe₂Al₅Zn _x intermetallic compound is not formed at the zinc alloy/Fe interface, meaning that the silica layer is not wetted by the liquid zinc alloy and prevents the interaction between iron and liquid zinc (see also “Contact angles of zinc, aluminium and zinc alloy on silica at short contact time” section). The same phenomena occur in the case of Al alloyed steels, with the formation of a continuous layer of alumina. In this case, it was also shown that a slight increase in the atmosphere dew point during annealing from −60 to −55 °C (corresponding to a partial water pressure from 1.9 to 3.6 Pa) leads to the formation of thinner Al₂O₃ layers (6–8 nm), which are not thick enough to inhibit the interfacial reactions and wetting.

The effect of annealing atmosphere on the wetting of high strength steels by liquid zinc has also been investigated in the work of Bordignon and Vanden Eynde [7]. For the Mn- and Si-alloyed steels studied, the highest surface density of selective oxides was observed at a dew point of −30 °C (i.e. partial water pressure of 50 Pa, classically used in industrial production lines), leading to poor wetting by liquid zinc. The wettability of such steels is improved by either a decrease or an increase in the dew point. The formation of selective oxides at the steel surface (external oxidation) is limited at low and high dew point. At high dew point (from −10 to +5 °C), the oxidation front is located beneath the steel surface (internal oxidation).

The results obtained with the meniscograph device [7–13] showed a major influence of the atmosphere partial water pressure and the steel surface chemistry on the surface coatability by liquid zinc alloy. However, the published results usually allow only a qualitative observation of the surface coatability without including a detailed description of the interfacial phenomena taking place at the steel/liquid metal interface (lack of information about the steel surface chemistry or the spreading kinetics of the liquid metal). For a proper understanding of the wetting of the annealed steel surface by liquid zinc alloy, direct measurement of the contact angle is required using the dispensed drop method coupled with characterization of the chemistry and distribution of selective oxides. The following sections describe the published experimental results obtained for the contact angle measurements in the field of hot-dip galvanizing.

Contact angles of zinc and zinc alloy on iron and steel surfaces with short contact time

To the authors’ knowledge, only one study on the dynamic wetting of pure iron by liquid zinc has been conducted to date [5]. The wetting experiments were performed using the dispensed drop method. Iron substrates were annealed at 750 °C in a hydrogen atmosphere, then cooled to 430 °C and brought in contact with liquid zinc. The spreading kinetics of the zinc droplet can be divided into two steps (Fig. 4). In the first step, the contact angle decreases rapidly to reach the value of 50° in 30 ms. In the second step, the contact angle decreases at a much smaller rate than during the first step. At the end of spreading, a contact angle of 36° is obtained after a holding time of about 1 s at 430 °C. The characterization of the interface, carried out in this work, showed that the wetting was reactive: Fe–Zn intermetallic compounds were detected at the Fe/Zn interface.

Tarasova et al. [15] studied the wetting of a low alloyed steel by pure Zn at different temperatures in a hydrogen atmosphere. At 475 °C, the contact angle decreases rapidly from 120° to 35° in 14 ms and then remains almost stable for 1 s. This contact angle value is close to that observed by Popel et al. [5] for pure zinc on pure iron.

Swaminathan et al. [16] measured the contact angle of a zinc–aluminium (0.25 wt%) droplet on an interstitial-free titanium (IFTi) steel annealed in an atmosphere of N₂ and H₂ (5 vol.%) with a dew point of −79 °C. The contact angle value obtained at the end of drop spreading (drop/substrate contact time of 3 s) is equal to 30°, a value close to that observed by Popel et al. [5] for pure zinc. If the dew point of the N₂–H₂ atmosphere (5 vol.%) is increased to −30 °C, a higher contact angle value is obtained for the zinc–aluminium alloy/IFTi steel system [16]. This increase in contact angle is certainly related to changes in the steel surface chemistry after annealing at a higher dew point.

Contact angles of zinc, aluminium, and zinc alloy on silica at short contact time

Like a majority of oxides, silica is not wetted by liquid zinc. Kelley and Harris [17] studied the wetting of vitreous silica by liquid Zn in pure He (99.999 vol.%) at a fixed temperature in the range of 500–700 °C. A non-wetting contact angle of 133° was observed at 500 °C for this system. The contact angle decreased when the temperature was increased, its value at 700 °C was 113°. These values are close to the contact angle measured by Lee et al. [18] in similar conditions (130° for the Zn/silica system in an Ar–10 vol.% H₂ atmosphere at 560 °C).

The contact angle of liquid aluminium on SiO₂ measured by the dispensed drop technique is about 60° at 800 °C under vacuum (about 2 × 10⁻⁴–5 × 10⁻⁴ Pa) [19]. In the classical sessile drop configuration, a contact angle of 88° was obtained at 800 °C under high vacuum (5 × 10⁻⁵ Pa) [20]. In both cases, silica is reduced by aluminium, leading to the formation of some Al₂O₃ grains at the SiO₂/Al interface and Si diffusion from the interface into liquid Al. When a higher contact angle is measured for the Al/SiO₂ system, this is due to the presence of a thin alumina layer on the liquid aluminium droplet [19–21]. With the dispensed drop method, the alumina film present on the liquid metal surface is broken during extrusion of the liquid through the capillary system. In the classical sessile drop configuration, under high vacuum, the alumina film is reduced by liquid aluminium to form volatile Al₂O by the reaction Al + Al₂O₃ → 3 Al₂O [20]. In both cases, the real contact angle of aluminium on silica (less than 90°) can then be observed.

Frenznick et al. [22] showed that a small amount of Al in liquid zinc (i.e. hot-dip galvanizing bath composition) does not improve the wetting of silica and even deteriorates it. The contact angle observed in this work was about 150° at 470 °C (higher than 133° for pure zinc [17]). Considering the low value of the contact angle measured for the Al/silica system [19–21], a decrease in the contact angle might have been expected when a small amount of aluminium is added to zinc. The high contact angle value obtained can be explained by the presence on the zinc alloy droplet surface of very thin layers of zinc and aluminium oxides, which represent a barrier to drop spreading. These values are close to the contact angle measured by Lee et al. [18]. They also obtained a high contact angle of 130° for a liquid Zn–Al (0.2 wt%) droplet on a silica substrate at 560 °C in an Ar–10 vol.% H₂ atmosphere.

Experimental results obtained at laboratory scale

Materials and methods

Materials

The liquid metal droplets are produced from shots of zinc containing 0.18 ± 0.05 wt% Al and 0.010 ± 0.001 wt% Fe. The analysis was performed by atomic absorption spectroscopy (AAS, SpectrAA, Varian). The droplet weight m is equal to 80.0 ± 0.2 mg in all experiments.

The wettability experiments were performed on an iron–silicon (0.077 wt%, ArcelorMittal) binary alloy, in the form of 1.0-mm thick strip. The experimental results obtained are compared with those obtained on a 2.1-mm thick high purity iron substrate (99.98 wt%, Goodfellow) and on a 3.2-mm thick silica substrate (96 wt%, Goodfellow).

The composition of both the high purity iron (referred to as Iron) and the iron–silicon alloy (referred to as FeSi0.077) is listed in Table 1. The analysis was performed by means of optical emission spectroscopy (Spectrolab M10) in ArcelorMittal laboratories. Before the wettability experiments, the FeSi0.077 and Iron substrates were polished up to 1 μm. The silica substrates were investigated as-received.

Table 1 Composition (wt%) of the substrates studied

In order to obtain information on the solid surface when the metal droplet is brought in contact with it, some substrates were annealed under the same operating conditions as the wettability experiments. The droplet was not released onto the annealed substrate to prevent contamination by the liquid metal. The particles formed on the solid surfaces were then observed in a field emission gun scanning electron microscope (FEG-SEM, LEO 1530) and analysed by means of energy dispersive spectroscopy (EDS, Princeton Gamma-Tech). The substrate roughness after the annealing treatment was measured either by means of a white light interferometer (ZYGO, NewView 6K) or an atomic force microscope (AFM, Digital Instruments Nanoscope IIIA). The AFM mapping (2 μm × 2 μm) was carried out using the tapping mode with etched silicon tips (TESP, Nanoprobe Digital Instruments).

Description of the sessile drop experiments

The experimental system is described in detail elsewhere [4].

In this study, wetting experiments were performed by means of the dispensed drop technique in a resistance furnace (Pyrox Thermique Matériaux, Rambouillet, France). After establishing a primary vacuum in the quartz chamber of the resistance furnace (about 0.03 mbar), a controlled atmosphere of N₂–5 vol.% H₂ mixture (Air Liquide with less than 3 ppm of H₂O and 2 ppm of O₂) is introduced at a flow rate of 45 l h⁻¹ at standard conditions of temperature and pressure (0 °C, 100 kPa). The frost point² in the furnace chamber is measured using an aluminium oxide moisture probe (GE Sensing Panametrics, Labège, France) located at the gas outlet.

The molten metal droplet generator is composed of a cylindrical alumina crucible, with a hole 2.0 ± 0.1 mm in diameter at its bottom. The liquid metal droplet is released onto the solid substrate by applying an excess pressure of N₂–H₂ gas (about 15 mbar). The fall height was 3.9 ± 0.4 mm.

The furnace is fitted with two viewing ports enabling back illumination with a light source and visualisation of the sessile drop on the substrate. Images of the sessile drop are obtained using a high-speed CMOS camera (pco.1200hs, Photon Lines, Saint-Germain-en-Laye, France) with a recording speed of 1,000 images per second and a 780 × 501 pixel resolution. An image analysis procedure is performed to obtain the experimental contour of the droplet. Contact angles are calculated by means of the Drop Snake Approach [23], based on B-spline snakes (i.e. active contours) to shape the drop, with ImageJ software [24, 25]. The error in the contact angle has been estimated to be less than ±5°.

During the wettability experiments, furnace temperature is measured by means of two type-K thermocouples located a few millimeters below the substrate. Their location corresponds to the middle and one corner of the substrate. In the 350–800 °C temperature range, the temperature error ΔT, given by half the difference between the two temperatures measured by the thermocouples, is estimated to be less than ±5 °C, the maximum error being obtained at low temperature. The scatter in the furnace temperature was also measured between the substrate and the alumina crucible containing the liquid metal. The scatter is less than ±1 °C in the 350–500 °C temperature range.

The experimental procedure is divided into three steps:

In the first step, after establishing a primary vacuum, the furnace chamber is flushed with the controlled atmosphere of N₂–5 vol.% H₂ for about 2 h. The frost point reached is then less than −50 °C.

In the second step, the substrate is heated to 850 ± 2 °C in 1 h and then cooled down to 452 ± 4 °C and maintained at this temperature. At the end of the annealing treatment, the frost point is equal to −54 ± 3 °C (i.e. partial pressure of water of 1.6–3.5 Pa). Assuming that the thermodynamic equilibrium between O₂, H₂, and H₂O is reached, this frost point corresponds to a partial pressure of oxygen in the range 3 × 10⁻³²–7 × 10⁻³³ Pa at 450 °C [26, 27].

In the third step, once the sample temperature has stabilised at about 450 °C (after 280 min), the metal to be melted is introduced into the alumina crucible from a spherical valve located outside the furnace. After 5 min, the liquid metal temperature is estimated to be 451 ± 1 °C. The relative pressure in the crucible is increased to 15 mbar in order to push the molten metal droplet through the hole in the crucible. The initial kinetic energy E _c of the liquid metal droplet is given by:

$$ E_{\text{c}} = \frac{1}{2}\,{mV}_{0}^{2} $$

(1)

where m is the droplet weight, and V ₀ is the impact velocity (=1.11 ± 0.03 m s⁻¹). With the operating conditions used in the laboratory experiments, E _c = (4.9 ± 0.3) × 10⁻⁵ J.

The spreading of the liquid metal droplet on the solid sample is filmed using the high-speed camera. The left and right contact angles are calculated using these images taken from 0 to 600 s.

Characterization of the surface after annealing

Figure 1 shows micrographs of the surface after the annealing treatment for Iron and FeSi0.077. As shown in Fig. 1a, the Iron surface presents strong faceting, varying from one grain to the other, and thermal grooving of grain boundaries. A few rare globular-shaped particles can be observed from time to time. Their diameter is a few tens of nanometres. Clear evidence of external oxidation is visible on the iron–silicon (0.077 wt%) substrate (Fig. 1b): the surface is covered with small particles, containing silicon as analysed by EDS.

Fig. 1

FEG-SEM images of the substrate surface after the annealing treatment for a iron and b FeSi0.077 alloy. The FeSi0.077 alloy is covered with globular silica particles

Thermodynamic calculations show that iron oxides are completely reduced under the annealing conditions chosen [26] and silicon is oxidised to SiO₂ (the Gibbs free energy of formation of SiO₂ is less than 0 [26, 27]). The surfaces studied will then be pure metallic iron for the Iron samples and metallic iron partly covered with silica particles for the iron–silicon substrates.

It is well known that the wetting of solid substrates depends on two particularly important parameters: roughness and surface composition (presence of chemical heterogeneities, silica particles for this study) [21].

The average roughness R _a and maximum roughness R _t measured by AFM (for Iron) or white light interferometer (for FeSi0.077 and Silica) are given in Table 2. For Iron, as grain boundaries are engraved by annealing (Fig. 1a), the maximum roughness R _t for several grain boundaries was determined as well as the mean roughness R _a for the centre of several grains. For FeSi0.077, the grain boundaries are covered with oxides and the two roughness values R _a and R _t are measured on mapping containing grains and grain boundaries. For the three substrates, the average roughness is less than 100 nm. The grain-boundary grooves formed on Iron substrates (given by R _t values) do not cause any distortion of the triple line.

Table 2 Characterization of the substrates after the annealing treatment (average surface roughness R _a, maximum roughness R _t, and surface area fraction covered by silica f _silica)

An image analysis procedure is used to estimate the percentage surface area covered by silica f _silica for the FeSi0.077 substrate. Three FEG–SEM images, sufficiently distant from one another, were used to evaluate the reproducibility of the measurements. The surface area fraction covered by silica particles is equal to 34 ± 6 % (Table 2).

Wetting by the liquid zinc–aluminium alloy

Results

The variation in contact angle as a function of spreading time is given in Fig. 2 for Iron, FeSi0.077, and Silica substrates. The images of the liquid zinc–aluminium droplet obtained at the end of spreading on iron, iron–silicon binary alloy, and silica are given in Fig. 3. The triple line is located at the border between the sessile drop and its reflection on the polished sample. The images of the liquid metal droplets shown in Fig. 3 were taken at the end of spreading (a few seconds). They can be deformed through the formation of aluminium or zinc oxides on the liquid metal surface after spreading for a few seconds.

Fig. 2

Variation with spreading time of the contact angle θ of liquid zinc alloy droplet on iron, iron–silicon (0.077 wt%) (three different trials) and silica substrates. Two rebounds of the liquid zinc droplet are observed on silica

Fig. 3

Liquid zinc drop shape obtained at the end of spreading on a iron, b FeSi0.077 alloy, and c silica at 450 °C

Three trials for each system were used to evaluate the reproducibility of the contact angles obtained. For Iron and Silica, the contact angle scatter is less than ±8° from one trial to another. The contact angle scatter is greater for the wettability experiments performed with the FeSi0.077 alloy. Figure 3b shows an example of a FeSi0.077 droplet with different left and right contact angles due to this contact angle scatter. For this reason, the final contact angles obtained for the three different trials are presented in Fig. 2 (numbers 1, 2, and 3). For trial number 2, the left and right contact angles were very different (Fig. 3b) and both are given in Fig. 2. At short contact times, the variation in the contact angles of droplets spreading on FeSi0.077 substrates is very similar to the variation obtained for Iron (lowest contact angle) and Silica (highest contact angle). For the sake of clarity, this initial behaviour is not shown in Fig. 2. One or two rebounds of the liquid zinc droplet are observed on silica and on FeSi0.077 (for trials 1 and 2), the initial kinetic energy of the droplet being sufficiently high to cause a first step of forced wetting followed by dewetting.

The mean contact angle measured for Iron and Silica (considering the six left and right contact angles obtained for three trials) and the mean minimum and maximum contact angles obtained for FeSi0.077 at the end of the spreading are given in Table 3. As can be seen, the lowest mean contact angle is obtained for the Iron substrate; the largest mean contact angle is obtained for Silica. The contact angle scatter is much greater for FeSi0.077 samples than for the other samples. The minimum contact angle on this substrate is close to the value for Iron, while the maximum contact angle is close to the value for Silica.

Table 3 Mean contact angles of liquid zinc alloy droplets measured on Iron and Silica, mean minimum and maximum contact angles measured on FeSi0.077. There is a much wider contact angle scatter on FeSi0.077 substrate than on the other substrates

Discussion

In these experiments, pure iron is wetted by liquid zinc–aluminium (0.2 wt%) saturated in iron, with a final contact angle of about 22°. The dynamic contact angle decreases sharply during the spreading of the liquid metal droplet on iron. Wetting is accompanied by (and to a certain extent due to) the reactions occurring at the interface: when the liquid zinc–aluminium alloy is brought in contact with metallic iron, two reactions occur, namely dissolution of iron followed by nucleation and growth of an intermetallic phase rich in iron and aluminium Fe₂Al₅Zn _x [1, 2]. The final contact angle, therefore, corresponds to the contact angle of the liquid metal droplet on the intermetallic compound formed [21].

The results obtained for iron are compared to the experimental data measured by Popel et al. [5] for pure zinc in similar conditions (“Contact angles of zinc and zinc alloy on iron and steel surfaces with short contact time” section, Fig. 4). The final contact angle obtained in this study (22°) is less than the contact angle measured by Popel et al. [5] (36°). This discrepancy can be explained by some differences in the operating conditions between the two studies: temperature, liquid metal composition, and impact velocity.

Fig. 4

Variation of contact angle θ measured on pure iron as a function of spreading time. The results obtained are in good agreement with published experimental results [5] even if the operating conditions are slightly different (Zn–0.18 wt% Al saturated in iron at 450 °C (this work) and pure zinc at 430 °C [5])

Silica is not wetted by liquid zinc–aluminium (0.2 wt%) saturated in iron, with a final contact angle of about 156°. This value is in good agreement with the contact angles published in the literature for the same operating conditions [18, 22]. The initial contact angle is of the same order of magnitude as the final contact angle. With our experimental device, this behaviour is characteristic of non-reactive wetting [4]. Two rebounds of the liquid zinc droplet are observed on silica, caused by the combination of two effects, namely the high initial kinetic energy of the droplet and its very weak adhesion (reflected by the high non-wetting contact angle).

Under our experimental conditions, the transition from wetting to non-wetting is observed for the wettability experiments performed with the FeSi0.077 substrate. In this case, the surface area fraction of metallic iron covered by silica f _silica is about 34 % (Table 2). As shown in detail elsewhere [28], if f _silica is less than 34 %, the heterogeneous surface is wetted by the liquid zinc alloy whereas if f _silica is higher than 38 %, it is not wetted. As shown in Fig. 2, the final contact angle is very different from one trial to another (numbers 1, 2, and 3). This final contact angle can be compared to the contact angle predicted at equilibrium by the Cassie–Baxter model [29, 30] for a liquid Zn–Al droplet in two cases: (1) on metallic iron partly covered by silica (Eq. 2), the Cassie contact angle $\theta_{\text{C1}}$ is calculated using the experimental contact angles obtained for pure iron ($\theta_{\text{Fe}} = 22^\circ$) and silica ($\theta_{\text{silica}} = 156^\circ$), and (2) on a composite surface consisting of silica and air [31] (Eq. 3), the Cassie contact angle $\theta_{{{\text{C}}2}}$ is calculated using a contact angle of 180° for air that could be trapped inside the asperities of the surface.

$$ \cos \theta_{\text{C1}} = \left( {1 - f_{\text{silica}} } \right) \cdot \cos \theta_{\text{Fe}} + f_{\text{silica}} \cdot \cos \theta_{\text{silica}} $$

(2)

$$ \cos \theta_{\text{C2}} = - 1 + f_{\text{silica}} + f_{\text{silica}} \cdot \cos \theta_{\text{silica}} $$

(3)

In one case (trial 3, Fig. 2), the final contact angle obtained for FeSi0.077 alloy was close to the contact angle measured on pure iron, i.e., less than the Cassie contact angle calculated for the iron/silica surface ($\theta_{\text{C1}} = 72^\circ$). This means that the measured contact angle should be a receding one. The receding triple line is pinned by iron that is better wetted than silica. The same behaviour was previously observed by de Jonghe and Chatain [32] for liquid Sn droplets in contact with Si/SiO₂ surfaces. In another case (trial 1, Fig. 2), the final contact angle obtained in our experiments was close to the contact angle measured on silica. The Cassie contact angle $\theta_{\text{C2}}$ calculated for a silica/air composite surface is equal to 166°, meaning that the liquid metal droplet probably sits on such a composite surface. For trial number 2, the left and right contact angles were very different. The droplet probably rests on a three-phase surface, metallic iron, silica, and air.

A partly oxidised metallic substrate such as FeSi0.077 can be wetted by the liquid metal droplet if it penetrates between the silica particles, i.e., if the wetting is forced on the vertical sides of the silica particles. The energy barrier can be overcome if the initial kinetic energy E _c of the liquid metal droplet is high enough.

As shown with the wettability experiments performed on the FeSi0.077 alloy, the final contact angle is very different from one trial to another (Fig. 2). This means that a slight modification of the initial kinetic energy of the droplet will entail a great difference in wetting behaviour: with a slight decrease in the initial kinetic energy, the liquid metal drop is placed on a silica/air composite surface and the final contact angle measured is high; with a slight increase in the initial kinetic energy, the liquid metal drop is pushed between the silica particles and can react with metallic iron to form Fe₂Al₅Zn _x which will retain the triple line as the droplet recedes and the final contact angle is low.

Wetting defects in the galvanizing process

Description of bare spots

During continuous annealing, before immersion in the galvanizing bath, oxide particles or films form on the steel surface. This surface oxidation is the result of gas–metal reactions at high temperature. Thermodynamic calculations indicate that the use of a non-oxidising gas such as nitrogen is not sufficient to prevent iron oxidation because the oxygen partial pressure $p_{{{\text{O}}_{ 2} }}$ corresponding to the coexistence of Fe and Fe_0.95O at 800 °C is in fact of the order of 10⁻¹⁹ atm [26, 27], far below the value in the purest nitrogen available industrially for which $p_{{{\text{O}}_{ 2} }}$ is no lower than 10⁻⁶ atm.

The only way to reach such a low $p_{{{\text{O}}_{ 2} }}$ is to add hydrogen to the nitrogen atmosphere. The reaction H₂ + ½O₂ → H₂O then occurs at the steel surface, leading at equilibrium at 800 °C to an oxygen partial pressure $p_{{{\text{O}}_{ 2} }} = 2. 4 \times 10^{ - 2 3}$ atm with 5 vol.% H₂ and a dew point of −30 °C (partial water pressure of 50 Pa). The oxidation of iron is, therefore, prevented by introducing some hydrogen and controlling the dew point in the gas atmosphere.

Nowadays, manufactured steel sheet is increasingly alloyed with elements such as Mn, Si, Al, and Cr. These elements are more prone to oxidation than iron, and it is generally impossible to avoid their oxidation on the steel surface. For instance, in order to avoid silicon oxidation at 800 °C in a steel containing 0.1 wt% Si, the required dew point would be less than −112 °C in N₂–5 vol.% H₂, which is practically impossible in an industrial galvanizing line.

The presence of oxide particles or films can give rise to galvanizing defects, called bare spots, on alloyed steels such as high strength interstitial-free (HS-IF), dual phase (DP), TRansformed Induced Plasticity (TRIP) or TWinning Induced Plasticity (TWIP) steels.³ This is particularly true when the selective oxidation occurs on the steel surface, which happens when the outward flow of alloying elements (from the steel bulk to the steel surface) taking part in the oxidation process is greater than the inward flow of oxygen. This kinetic mode of selective oxidation is designated as external. The reverse, internal, mode occurs when the oxygen inward flow is greater than the alloying element outward flow [33]. Therefore, when galvanizing defects arise because of external selective oxidation, a possibility is to increase the oxygen inward flow to switch the selective oxidation from external to internal mode [34].

Two examples of wettability problems caused by the presence of oxide particles or films at the steel surface are presented in Figs. 5, 6, and 7.

Fig. 5

Example of a TRIP steel sample containing 1.7 wt% Si and 1.7 wt% Mn annealed at 800 °C in N₂ – 5 vol% H₂ with a dew point of −35 °C and galvanized at 460 °C in Zn–Al (0.2 wt%) saturated in iron: a macroscopic image of the sample after annealing and galvanizing, b TEM picture of the oxide particles and films extracted from the annealed steel surface on a thin carbon film (replica technique). The steel sample mainly remains uncoated due to the full coverage of the steel surface by SiO₂ and Mn₂Si0₄ films (originally published in Ref. [34], reproduced with kind permission from Associazione Italiana di Metallurgia)

Fig. 6

SEM image of a 300-μm wide bare spot obtained on a hot-dip galvanized IF steel containing boron (originally published in Ref. [36], reproduced with kind permission from Associazione Italiana di Metallurgia)

Fig. 7

Characterization of the bare spot shown in Fig. 6. Auger mapping at high magnification indicates the presence of patches rich in Mn, B, and O (attributed to MnB₂O₄) and patches rich in Fe and Al (attributed to Fe₂Al₅Zn _x ) (originally published in Ref. [36], reproduced with kind permission from Associazione Italiana di Metallurgia)

Figure 5a presents an example of a galvanized sample of a TRIP steel containing 1.7 wt% Si and 1.7 wt% Mn [34]. The annealed sample was immersed for 3 s at 460 °C in liquid zinc–aluminium (0.2 wt%) saturated in iron. Annealing before dipping was performed at 800 °C under a N₂–5 vol.% H₂ atmosphere in which the dew point was kept equal to −35 °C. Only some Zn droplets could be deposited onto this surface completely covered by a continuous oxide film. This oxide film was extracted from the steel surface on a carbon replica (Fig. 5b) and analysed by EDS and electron diffraction in the transmission electron microscope (TEM). The oxide film is made of SiO₂ and Mn₂SiO₄.

The second example (Figs. 6, 7) concerns the galvanizing of a re-phosphorized IF steel (allowing element content in wt%: C 0.0020, Mn 0.2100, Si 0.0040, Ti 0.0110, P 0.0310, B 0.0005). A small amount of boron (about 5 ppm) can be added to this steel, in order to afford protection against brittleness induced by phosphorus segregation at steel grain boundaries. Such a small addition of a rapidly diffusing element prone to oxidation can have a very strong influence on external selective oxidation and wettability by liquid Zn. Such steels are soaked at 800 °C for 40 s under a N₂–5 vol.% H₂ atmosphere with a dew point of −40 °C then cooled down and immersed in the liquid Zn at 460 °C. Figure 6 presents an example of a large bare spot (300 μm in diameter) caused by boron external oxidation during annealing. The inside of the bare spot is observed by scanning electron microscopy (SEM) and analysed by means of the Auger electron spectroscopy (AES) technique. Two areas are highlighted by the analysis: (1) an area rich in iron and aluminium attributed to the presence of Fe₂Al₅Zn _x , and an area rich in O, Mn, and B, attributed to MnB₂O₄. This strong boron effect has already been reported elsewhere [9, 35].

Proposed mechanisms of bare spot formation

The steel-coating interface can be examined by SEM after chemical selective dissolution of the zinc layer. This is a classical way of checking the quality of the galvanizing reaction by observing the continuity of the Fe₂Al₅Zn _x layer (Fig. 8).

Fig. 8

Fe₂Al₅Zn _x layer observed in a scanning electron microscope after selective dissolution of the Zn coating. Reactive wetting between steel and liquid Zn–Al (0.2 wt%) alloy leads to the formation of a continuous layer of Fe₂Al₅Zn _x

The wetted areas around the bare spots on industrial galvanized steel sheet can be characterised after the dissolution of the zinc layer: some patches of a very thin Al₂O₃ film, exhibiting a puzzle shape, can be observed in some places instead of the continuous Fe₂Al₅Zn _x layer [36] (Fig. 9). Such an observation has also been reported by Kawano et al. [37].

Fig. 9

Patches of Al₂O₃ film, exhibiting a puzzle shape, embedded within the Fe₂Al₅Zn _x layer, in wetted areas around the bare spots, as recorded after selective chemical dissolution of the zinc layer [36]. Figure 9b is a magnification of the area located in the white rectangle shown in Fig. 9a (originally published in Ref. [36], reproduced with kind permission from Associazione Italiana di Metallurgia)

This particular observation supports the following mechanism proposed to understand the occurrence of bare spots in a galvanizing line.

Wetting condition

In the static condition, metallic iron is well wetted by the galvanizing bath and a low wetting angle (about 20°) is obtained, while poor wetting, with a wetting angle higher than 90°, is expected to occur with a partly oxidised steel surface (Table 3).

In a galvanizing line, the steel runs through the metal bath at a fixed speed of about 2 m s⁻¹ and the wetting is no longer static but dynamic. In such a situation, in good wetting conditions, the dynamic wetting angle should increase with line speed [38].

The capillary length (Eq. 4), the capillary number (Eq. 5) and the Weber number (Eq. 6) can be calculated in order to discuss the shape of the meniscus formed on the steel surface as it enters the galvanizing bath.

$$ \lambda = \left( {\frac{\sigma }{\rho \, g}} \right)^{1/2} $$

(4)

$$ {\text{Ca }} = \frac{\mu V}{\sigma } $$

(5)

$$ {\text{We }} = \frac{{\rho V^{2} L}}{\sigma } ,$$

(6)

where σ is the liquid/gas surface tension (N m⁻¹), g the gravity acceleration (=9.81 m s⁻²), ρ the liquid density (kg m⁻³), μ the liquid viscosity (Pa s), V the line speed (m s⁻¹), and L a characteristic length, chosen equal to the capillary length.

In hot-dip galvanizing, with a line speed of 2 m s⁻¹ and liquid zinc at 450 °C (μ = 0.0035 Pa s, ρ = 6,600 kg m⁻³ [39] and σ = 0.810 N m⁻¹ [21, 40]), λ = 3.5 mm, Ca = 0.0084, and We = 115. The high Weber number indicates that inertial forces dominate the capillary force for the considered volume of liquid zinc. Hence, over the capillary length, the meniscus contact angle could be close to 90° or even higher, meaning that the meniscus could curve downwards for dynamic reasons.

However, closer to the steel surface, the contact angle is expected to be smaller than the macroscopic dynamic angle, for a reduced amount of involved liquid so that the inertial force becomes smaller than the capillary force. This distance along the meniscus, for which inertia of the liquid zinc can be neglected compared to the capillary force, corresponding to a Weber number lower than or equal to 1, is of the order of 30 μm. The meniscus can then be expected to curve upwards over a length of 30 μm, while it bends downwards over a length of 3.5 mm.

Non-wetting condition

The question now is to understand what happens when a non-wetting surface is introduced into the galvanizing bath under dynamic conditions. Although a definite answer to this question cannot be given, a tentative explanation can be proposed: if wetting is poor (θ ≫ 90°), a solid/gas interface will be more stable than a solid/liquid interface. The only way to maintain a solid/gas interface when the strip enters the galvanizing bath is to trap gas bubbles or films at the strip surface [41].

When the steel surface is fully covered by non-wetted oxides, the static contact angle is close to 160° (Table 3). The meniscus curves downwards over the capillarity length λ.

In dynamic conditions, such a non-wetting situation should be maintained and gas bubbles or films should be continuously trapped between the steel surface and the liquid zinc as the steel strip enters the galvanizing bath. As the steel strip leaves the galvanizing bath, the liquid zinc dragged out of the bath by the running strip can then be removed easily from the solid surface in front of the wipers.⁴

Transition from non-wetting to wetting conditions

When the steel surface is covered only by patches of non-wetted oxides, the meniscus has to oscillate between its downward and upward positions. Consider a starting configuration with the meniscus curved downwards (the steel surface being covered by a non-wetted oxide). No liquid zinc is dragged by the strip as it enters the bath and a gas bubble is trapped between the strip surface and the liquid zinc (Fig. 10). This gas bubble can lead to the formation of a bare spot in the coating in front of the wipers (see Footnote 4).

Fig. 10

Bubbles trapped between the steel strip and the liquid zinc bath when the steel strip enters the galvanizing bath in a poor wetting situation. a Schematic representation of the strip as it enters galvanizing bath. b Side view of the meniscus and a trapped bubble (originally published in ISIJ International [41], reproduced with kind permission from the Iron and Steel Institute of Japan)

When a subsequent metallic iron area is immersed, the surface is wetted by liquid zinc and the meniscus curves upwards at the steel surface (“Wetting condition” section). Because of this sudden wetting situation, liquid zinc is suddenly dragged again by the strip movement.

As the liquid surface tension cannot withstand an elongation of the meniscus surface, liquid at the bath surface has to be accelerated to compensate for the sudden viscous drag force. As a result, the liquid bath surface, covered by a thin Al₂O₃ film, is pressed and stretched onto the strip surface.

This mechanism can explain the particular aspect presented in Fig. 9, the puzzle shape of the alumina film being created by the stretching caused by the sudden acceleration of the liquid meniscus surface.

Conclusion

In the continuous hot-dip galvanizing process, the zinc coating can contain bare spots due to the presence of oxide particles formed on the steel surface during the continuous annealing performed before immersion in the liquid zinc bath.

In the first part of the article, the available data for the wettability of liquid zinc alloys on heterogeneous surfaces of metallic iron partly covered by oxides were critically assessed. It was shown that wettability is deteriorated (i.e. the contact angle increases) if the concentration of the less noble alloying elements of the steel is increased.

Wetting by liquid zinc–aluminium of heterogeneous surfaces composed of metallic iron and silica particles or films was then investigated at 450 °C using the dispensed drop technique. In the experiments, pure iron is wetted by liquid zinc–aluminium, with a final contact angle of about 22°. Silica is not wetted by liquid zinc-aluminium, with a final contact angle of about 156°. These values are in good agreement with the contact angles published in the literature for the same operating conditions. The transition from wetting to non-wetting was found to occur when the surface area fraction covered by silica is equal to 34 ± 6 %. At this transition, the final contact angle is very different from one trial to another. It can be of the same order of magnitude as the contact angle measured either on pure iron or on silica.

The presence of oxide particles or films is detected inside the bare spots, sometimes observed in industrial zinc coatings. When the steel surface is covered by non-wetted oxides, a gas bubble can be trapped between the strip surface and the liquid zinc as the strip enters the galvanizing bath. This gas bubble can lead to the formation of a bare spot in the coating in front of the wipers.