Effects of oxygen concentration on the passivation of Si-containing steel during high-temperature oxidation

1 Introduction

Silicon, a well-established solution strengthening element, is usually added to steels in order to improve its overall strength (Hu, 2013, 2015; Zhou, 2017). During the reheating or hot-rolling process, silicon-containing steels are oxidized due to their exposure to high-temperature atmosphere (Okada, 1994; He, 2016; Yuan, 2016a,b, 2017). This results in red scale, a severe surface defect that often forms on the silicon-containing steels when the Si content exceeds 0.5 wt.% (Liu, 2013). In order to prevent red scale, the oxidizing behavior of Si-containing steels has been studied (Fukagawa, 1994; Okada, 1995; Taniguchi, 2001; Chen, 2003; Yang, 2005; Kusabiraki, 2007; Suarez, 2008; Yang, 2008; Cao et al., 2014), and two mechanisms explaining their oxidizing behavior during these oxidation stages have been proposed (Abuluwefa, 1996). The initial stage is a mass-transport-controlled stage, during which the oxidation rate remains constant for a short time until a certain oxide thickness appears. This results in a linear increase of mass gain and is normally interpreted in terms of the adsorption rate of oxidizing species (transport of oxygen) on a scale surface and the incorporation of atomic oxygen.

Initial oxidation rates depend on oxygen and can be controlled either by the rate of chemical reaction at the metal surface or by the transport rate of oxygen atom through the gas mixture to the reaction surface. When the oxidizing layer reaches a certain thickness, the mechanism of oxidation is controlled by the rate-controlling stage. The rate-controlling stage is a relatively slow process compared to mass-transport control stage, which results in the parabolic oxidation process. It may be either the mass-transport process or the (electro) chemical reaction. Furthermore, a passivation period is observed before the linear oxidation reaction reaches the above-mentioned oxidation stage (Mouayd, 2014). During this passivation period, a passivation film is formed on the surface and the oxidation reaction remains inactive (Yang, 2008; Mouayd, 2014). Mouayd et al. (2014) reported that the passivation period is caused by the formation of a thin silica layer as Si is more prone to oxidation than iron. Al, Cr, Mo, and Ni are also well-known passivation elements (Anna, 2016; Pedrazzini, 2016), but their passivation effect is often ignored when their amount is relatively small in silicon-containing steels.

A passivation course is a significant event, occurring during the passivation period in the entire oxidation process of silicon-containing steels, although it is not clear if oxidation behavior changes subsequently, after the conclusion of the passivation period. It is thus necessary to study the mechanism of passivation period in order to understand the entire oxidation process. It is well-established that the oxidation process is highly influenced by factors such as silicon content, oxygen concentration, and oxidation temperature. Several reports describing the effects of oxidizing temperature and silicon content on the passivation period of Si-containing steels existed in the literature (Mouayd, 2014). It suggested that the scale growth on the surface of steels (especially those comprising higher Si concentrations of 1.6–3.2 wt.%) can be drastically slowed down when the temperature is lower than 1177°C due to the formation of an SiO₂ layer during the initial oxidation stage. Furthermore, the passivation period concluded when the oxidizing temperature reaches 1200°C. However, there is almost no report on the relationship between the oxygen concentration and the passivation period of silicon-containing steels. On the one hand, in all of the reported oxidation mechanisms, oxidizing gas was not introduced until the temperature reached a pre-determined value in the isothermal holding experiments. In industrial production, on the other hand, the oxidizing atmosphere is usually present right from the beginning of the heating process. Thus literature reports pertaining to the oxidation behavior or the effects of oxygen concentration on passivation period of the oxidation reaction in which the oxidizing atmosphere is already present at the beginning of heating process are very scarce.

In order to establish the relationship between the passivation period of silicon-containing steel and the oxygen concentration, we employed four different oxygen concentrations and conducted high-temperature oxidation experiments on a simultaneous thermal analyzer (STA) at 1260°C. This study will provide a valuable contribution to the oxidation theory of steels containing Si.

2 Materials and methods

2.1 Oxidizing experiment

In this study, a commercially produced 3-mm-thick hot strip (Baosteel, Shanghai, China) containing ferrite and martensite with a composition of Fe-0.069C-1.21Si-1.40Mn-0.035Al-0.016Cr-0.002Nb (mass percent, wt.%) was used for experimentation. The steel was refined in a vacuum induction furnace, and then the slabs (230-mm thick) were hot rolled to 3-mm strips on a seven-stand tandem mill (Baosteel, Shanghai, China). After rolling, the steels were water cooled to room temperature. The oxidizing experiments were conducted on a Setaram Setsys Evo Simultaneous Thermal Analyzer (Setaram, Lyon, France), as shown in Figure 1. The mass gain during the oxidation period was monitored continuously by collecting mass gain signals at every 0.3 s. The influence of the thermal drifts on the measurement of mass gain was usually ignored because of the high precision of the thermobalance equipment (error±0.1 μg). A B-type thermocouple was installed for furnace temperature measurement and control. As the quantity of sample was small, it quickly attained the temperature of the furnace. The temperature control precision was up to ±0.5°C. The specimens for the oxidation test were prepared by drawing into a wire by electrical discharge machining (EDM, HFang, Taizhou, China) from the hot strip. The dimensions of the specimens were 15 mm×10 mm×3 mm. Before the tests, a hole of 4-mm diameter was drilled near the margin of each specimen in order to suspend it in STA chamber during the oxidation. The surface of each specimen was ground with a series of SiC papers up to grade 1000 and then polished to produce a mirror-like surface in Al₂O₃ slurry by employing metallographic polishing machine (YMP-2, Wale, Shenzhen, China). Thereafter, all specimens were cleaned by ethanol and dried in air, thereby ensuring consistent surface conditions free from the surface impurities before the oxidation experimentation.

Figure 1:

The schematic illustration of STA showing the sample position in the furnace, type of thermo-couple, and flow pattern of gases.

Figure 2 shows the heating procedure. The specimens were reheated to 1260°C with a heating rate of 10°C/min and then held for 60 min followed by cooling to ambient temperature with a cooling rate of 50°C/min. A binary gas mixture of oxygen and nitrogen with varying oxygen concentrations was separately introduced into the furnace chamber at a flow rate of 30 ml/min along the weight direction during the entire heating process. In the industrial reheating furnace, the equilibrium oxygen concentration is usually lower than 4.0 vol.%. In order to investigate the relationship between the concentration of oxygen and passivation period, four different oxidizing atmospheres were set up with the following concentrations: (1.0% O₂+99.0% N₂), (1.5% O₂+98.5% N₂), (2.0% O₂+98.0% N₂), and (3.0% O₂+97.0% N₂) (vol.%). Weight changes (mass gain) during the entire heating process were digitally recorded. Duplicate tests were made for each experimental condition to ensure reproducibility.

Figure 2:

The heating procedure.

3 Results and discussion

Figure 3 shows the mass gains and oxidation rates versus time for all the experimental conditions during the entire oxidation process (ambient temperature-1260°C-ambient temperature). As reported by the authors previously (He, 2016; Yuan, 2016a,b), there were three stages in the oxidation process: slow oxidation, intense oxidation, and finishing oxidation stages. Three different critical temperatures were determined (Table 1): nominal oxidizing temperature (NOT, point a), intense oxidizing temperature (IOT, point b), and finishing oxidizing temperature (FOT, point c). In order to establish a measuring standard, the temperature at which the mass gain was 0.1 mg was defined as the NOT. The mass gain did not show any apparent change due to the low oxidation temperature before point a. This is because the diffusion and reaction rates of ions are slow at relatively lower temperature. As illustrated in Figure3B, during the heating process, the oxidation rates prior to 75 min were nearly equal to zero, which means that the sample remained almost unoxidized. The determination of IOT was carried out over a narrow temperature range during which the oxidation rate increased rapidly. The concluding temperature of this interval was selected as the IOT in this study (He, 2016). FOT corresponded to the temperature where there is no more mass gain. In conclusion, it can be inferred that the mass gain increases with the increase of oxygen concentration.

Figure 3:

Oxidation mass gain (A) and oxidation rate (B) versus time at different atmospheres during the entire oxidation process (ambient temperature-1260°C- ambient temperature).

Figure 4 presents the mass gains and oxidation rates versus time during the heating phase from ambient temperature to 1260°C, which is clearly shown in the beginning portion of the graph in Figure 3. It was evident that mass gain increases with the increasing temperature and oxygen concentration. The mass gain increased slowly before showing a sharp increase in the mass gain curve. During this stage, oxygen concentration was sufficient but temperature was relatively low. The rate of O₂ diffusion through the gas phase to the reaction surface played a vital role during the course of oxidation reaction between Fe and the oxygen in oxygen-nitrogen binary mixtures. Unlike the findings in other researches (Yang, 2008; Liu, 2013; Mouayd, 2014), the linear relation of mass gain versus time lasted for about 20 min in this study, even though the same relationship was found to last for 1 min or sometimes even <10 s, for example, in the study of Mouayd et al. (2014). This difference arose due to the introduction of oxygen atmosphere at the beginning of the heating process, which is in sharp contrast with that of the other studies, where atmosphere was introduced into the chamber when the temperature reached a pre-designed value. However, during the industrial reheating process of slabs in hot-rolled strip production, the oxidizing atmosphere is always present throughout the entire heating process. Therefore, the relationship between mass gain and time presented in this study provides a more practical approach. After this slow oxidation stage, the reaction eventually reached a passivation period (Mouayd, 2014), which was depicted by the plateau in the oxidation rate versus time curve during the heating process. During this period, the mass gain was very sluggish and the oxidation rate remained almost constant. From Figure 4B, it can be estimated that the passivation period roughly occurs at the oxygen concentrations of 2.0 and 3.0 vol.%, whereas no passivation period was observed when the oxygen concentration was ≤1.5 vol.%. As depicted in Figure 4B, the oxidation rate curve remained almost horizontal during the passivation period, but when the oxygen concentration was >1.5 vol.%, the oxidation rate started to increase gradually instead of being constant. There was a sharp increase in the oxidation rate when the oxygen concentration was 1.0 vol.%, which is even greater than that at 1.5 vol.% and 3.0 vol.% oxygen concentration. This sharp increase in the oxidation rate was due to a buoyancy effect, caused by turbulent gas streams (He, 2016). As the passivation period is a relatively short process, it finds a very ambiguous mention in the literature reports pertaining to metal oxidation and corrosion. However, our results pertaining to a longer passivation period as depicted by the horizontal portion of the graph will provide some valuable insights in this filed. Figure 5 provides a magnified image of the oxidation rates during the passivation period for each of the specimens oxidized at different oxygen concentrations. The passivation periods lasted for about 4.0 min and 7.5 min when the specimens were oxidized at 2.0 vol.% and 3.0 vol.% oxygen concentrations, respectively, owing to the different heating routes employed. The duration times of the passivation periods in the duplicate tests are basically coincident with each other. The tiny difference between the duration times of the passivation periods in the duplicate tests is in the range of experimental errors. In the study of Mouayd et al. (2014), the oxidation reaction occurred during the high-temperature isothermal stage, whereas in the present study, the oxidation reaction occurred at a lower temperature. In addition, as no passivation was observed at 1.0 and 1.5 vol.% oxygen concentrations, the passivation period could be considered to be nearly 0 min at this stage for these specimens. Therefore, it can be concluded that that the duration of passivation period increases with the increase of oxygen concentration.

Figure 4:

Oxidation mass gain (A) and the oxidation rate (B) versus time during the heating stage from ambient temperature to 1260°C, corresponding to the beginning portion (0–120 min) of the graph in Figure 3.

Figure 5:

The image of oxidation rate curve and passivation time during passivation period for specimens oxidized at different oxygen concentrations.

The SEM images of the evolution process of passivation film during the heating stage are shown in Figure 6. In order to observe the evolution of passivation film, the heating procedures were designed depending on the time of appearance and the disappearance of passivation period as observed in the previous oxidation experiments (Figure 4B). For example, in Figure 4B, the passivation period lasted for about 90–94 min when the specimens were oxidized at 2.0 vol.% oxygen concentration, thus, the two specimens were heated for 93 and 96 min, respectively, at a heating rate of 10°C/min and then cooled to room temperature at 50°C/min in full N₂ atmosphere to avoid any further oxidation. Similarly, in order to observe the formation of passivation film when 3.0 vol.% oxygen concentration was used, three heating time intervals (96, 98, 100 min) were selected because the passivation period lasted for about 7.5 min from 90 min to 97.5 min. Figure 6A shows the morphology of relatively thin and undamaged silica passivation films at 3.0 vol.% oxygen concentration. This silica film acting as a diffusion barrier dramatically retarded the diffusion rate of iron ions by hindering the oxidation reaction between Fe²⁺ and O₂. As a result of this hindrance, which occurred during passivation period, there was no significant mass gain. Figure 6B and C show the microstructures of broken passivation films at the oxygen concentration of 3.0 vol.% and 2.0 vol.%, respectively. Some residual passivation film was still left adhering to the iron matrix. The binary eutectic FeO/Fe₂SiO₄ grains generated by the combination of FeO and SiO₂ due to the increasing temperature caused the disappearance of passivation period (Mouayd, 2014). The formation of binary eutectic FeO/Fe₂SiO₄ means the end of passivation period. With the proceeding oxidation, the silica film was cracked gradually and was replaced by the binary eutectic FeO/Fe₂SiO₄. The disappearance of passivation film as well as the increasing temperature facilitated an increase in the oxidation rate, which also marked the end of passivation period, after which the normal oxidation reaction began. As shown in Figure 6D, most of the passivation films have disappeared and the FeO layer has been formed. Figure 6E presents the cross-sectional image of oxide scale corresponding to Figure 6D, which demonstrates the formation of FeO/Fe₂SiO₄. During the same low-temperature stage, when the oxygen concentration was ≤1.5 vol.%, a decreased rate of reaction between silicon and oxygen was observed. This resulted in the formation of a very thin silica film and an almost unperceivable passivation period. In addition, it can also be inferred that the accelerating effect of binary eutectic FeO/Fe₂SiO₄ bears a greater influence on the oxidation reaction than the impeding effects induced by silica film with an increase in the temperature during the passivation period.

Figure 6:

SEM images of the evolution process of passivation film during the heating stage (A) the intact passivation film formed at 3.0 vol.% oxygen concentration, (B) the broken passivation film formed at 3.0 vol.% oxygen concentration, (C) the broken passivation film formed at 2.0 vol.% oxygen concentration, and (D) the formation of FeO layer; (E) the cross-sectional image of oxide scale formed at 3.0 vol.% oxygen concentration.

The abovementioned oxidation process of metals belonged to the solid-gas class of reactions. In the kinetic model of solid/gas reaction, unreacted core model (UCM), which assumes gas as fluid to be flowing over a spherical particle, is the most commonly accepted theory (Braun, 2000; Oliveira, 2013). There are some similarities between the oxidation of metals and spherical particles in a gas flow in the view of metallurgy. Firstly, the oxidation of metals also belongs to the class of solid/gas reactions. Secondly, during the passivation period, solid/gas reaction was generally regarded as an external diffusion process, which was accordant with the process in UCM. The reaction process includes many stages, i.e. the oxygen spreads to the metal surface, the reaction occurs in the surface layer of metals, the reaction interface moves towards the UCM, and so on. The mentioned stages are similar to the reaction in the UCM. Therefore, this model has been chosen by us to qualitatively explain how the duration of silica passivation period increases with the increasing oxygen concentration.

Figure 7 illustrates the concentration distribution of gas film via external diffusion control. The concentration of gas reactant on the outer surface of the metal (C_gs) is equal to that at the interface of the unreacted core (C_gi). The diffusion velocity of oxygen in the layer of gas film is equal to the chemical reaction or consumption rate of iron metal at the interface of the unreacted core. The relationship between unreacted core r_i and the chemical reaction time t can be represented by the following equation (Huang, 2011):

Figure 7:

The concentration distributions of the gas film under the external diffusion control. C_gs, the concentration of gas reactant in metal outer surface; C_gi, the concentration of the gas reactant in the interface of the unreacted core; C_gb, the concentration of gas reactant; r_o, the radius of spheroidal particle; r_i, the radius of the unreacted core.

where ρ_S is the density of solid metal, m_s is the molar mass of solid metal, k_g is the mass transfer coefficient in the boundary layer of the gas phase, r₀ is the radius of a spheroidal particle, r_i is the radius of the unreacted core, and C_gb is the concentration of the gas reactant.

In addition, the diffusion of oxygen in the solid metals follows the universal Fick’s first law of diffusion (Cheng, 2002), as illustrated in equation (2).

where R is the diffusion rate, r₀ is the radius of spheroidal particle, and k_g is the mass transfer coefficient. It can be inferred that the diffusion rate of oxygen on the surface of the unreacted core surface increases with the increase in oxygen concentration, demonstrating that the amount of oxygen surrounding the unreacted core increases with the oxygen concentration. In addition, from equation (1), it can be inferred that the chemical reaction time t of the unreacted core is shortened with an increase in the oxygen concentration. Therefore, more amount of silica was generated at an oxygen concentration of 3.0 vol.%, resulting in a relatively thick passivation film, thus prolonging the duration of silica passivation period.

The schematic diagram of the entire oxidation process in silicon-containing steels oxidized at the heating temperature of 1260°C is shown in Figure 8. Initially, iron substrate was not oxidized due to lower temperature conditions (before 970°C), but with an increase in temperature, a weak oxidation process was initiated and a thin silica passivation film started forming. A large amount of FeO grains also started to appear simultaneously. Fayalite grains (Fe₂SiO₄) in Wüstite matrix appeared in the form of net-like distribution by the combination of FeO and SiO₂ between the temperature ranges of 950°C and 1100°C, consequently leading to a fracted film during this severe oxidation. This was followed by the continuous rigorous oxidation with an increasing temperature, resulting in a thick oxide scale.

Figure 8:

The schematic diagram of the entire oxidation process in silicon-containing steels oxidized at the temperature of 1260°C.

Figure 9 illustrates the mass gain versus time of specimens oxidized at different oxygen concentrations during isothermal holding period at 1260°C, corresponding to the middle portion of Figure 3A. For comparison, the starting points of the mass curves during the holding and subsequent cooling stage were set at zero. It is interesting to note that the relation of mass gain was linear with time at all oxygen concentrations. A linear kinetics clearly indicates a non-protective corrosion product. Combining the results from Figure 5, it can be concluded that the appearance of passivation film has no effect on the oxidation behavior during the isothermal stage in silicon-containing steels. The oxidation reaction was controlled by two opposite factors: the promoting effect of liquefied FeO/Fe₂SiO₄ on the ion diffusion and the inhibiting effect of oxide scale formed on it (He, 2016; Yuan 2016a; Zhou, 2017). The concentration of liquefied FeO/Fe₂SiO₄, which acts as a passageway for the mass transfer process, increases with the increasing oxidation time during the thermal holding stage. Likewise, the thickness of the oxidizing layer increased, resulting in an unfavorable effect on ion diffusion. During the oxidation period, the dynamic equilibrium between the promotion and inhibition effects on the diffusion of ions resulted in a constant oxidation increase. However, Mouayd et al. (2014) and Abuluwefa et al. (1996) reported that mass change versus time obeys a parabolic relationship during the isothermal holding at 1200°C. A parabolic kinetics indicates a protective corrosion product. An oxidizing atmosphere was introduced for the isothermal oxidation at 1200°C in their studies. They explained that the thickness of the oxide scale increases at high temperatures, resulting in a longer diffusion path of Fe ions to the scale surface, so the mass change curve is a parabola. In addition, the parabolic relationship between mass change and time in earlier studies (Abuluwefa, 1996; Mouayd, 2014) may be attributed to high oxygen concentration (about 21 vol.%), which results in a very thick oxide scale. The thin oxide scale at initial stage of oxidation hinders further oxidation, leading to the parabolic relationship between mass change and time. Thicker oxide scale may crack, resulting in the decrease of hindrance effect. However, hindrance effect always exists because of thick oxide scale. Conversely, in the present study, to simulate the industrial reheating process of hot strips, oxidation atmosphere was close to that in an industrial furnace. Moreover, oxygen and nitrogen mixture was introduced at the beginning of the reheating process. The samples underwent a long period of oxidation before isothermal oxidation. During the holding stage at 1260°C, the promotion effects of liquefied FeO/Fe₂SiO₄ and the inhibition effects of oxide scale on ion diffusion reached a dynamic equilibrium more easily, resulting in a constant oxidation rate.

Figure 9:

The oxidation mass gain versus time of specimens oxidized under different oxygen atmospheres during isothermal holding period at 1260°C, corresponding to the middle portion of Figure 3A.

Figure 10 shows the mass gain and oxidation rate versus time during the cooling stage from 1260°C to ambient temperature, corresponding to the end portion of Figure 3. Similarly, mass gain increased with the increasing oxygen concentrations. Moreover, the turning point of oxidation rate (as shown by the direction of arrows) was observed early at higher oxygen concentrations. This is because the thickness of oxide scale increases at higher oxygen concentration, leading to a stronger hindering effect during the following oxidation reaction. Therefore, the finishing oxidation temperature (Table 1) increased with oxygen concentration.

Figure 10:

Oxidation mass gain (A) and the oxidation rate (B) versus time during the cooling stage from 1260°C to ambient temperature.

4 Conclusions

In the present study, the relationship between the oxygen concentration and the passivation period of Si-containing steels was investigated. The oxidizing experiments were conducted at a heating temperature of 1260°C on STA at four different oxygen concentrations. The results indicate that the passivation periods occurred at oxygen concentrations of 2.0 and 3.0 vol.%, whereas no passivation period was observed when the oxygen concentration was ≤1.5 vol.%. Moreover, the duration of passivation period increased from about 4 min in the case of the specimen oxidized at 2.0 vol.% oxygen concentration to about 7.5 min in case of specimen oxidized at 3.0 vol.% oxygen concentration. Furthermore, before passivation period, the linear relationship of mass gain versus time lasted for about 20 min, as the mixed binary atmosphere was introduced into the sample chamber at the beginning of the heating process.