Oxidation of Metals https://doi.org/10.1007/s11085-019-09944-8 ORIGINAL PAPER Oxidation of 60Si2MnA Steel in Atmospheres Containing Different Levels of Oxygen, Water Vapour and Carbon Dioxide at 700–1000 °C Yisheng R. Chen1 · Yu Liu2 · Xuanxuan Xu2 Received: 30 September 2019 / Revised: 30 September 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract The oxidation behaviour of the spring steel 60Si2MnA in atmospheres containing 0–21% (volume per cent) ­O2, < 20 ppm (part per million) to 17%H2O, with some containing 8%CO2, at 700–1000 °C was investigated. The oxide scale thicknesses formed in both 17%H2O–N2 and dry O ­ 2-containing atmospheres were less than 6 μm after 20 min of oxidation, significantly smaller than those formed in atmospheres containing both oxygen and water vapour, and the scale structures developed in the three different scenarios were also very different. The scale formed in 17%H2O–N2 contained wustite only, the scale formed in dry ­O2-containing atmospheres comprised primarily hematite and some magnetite, and that in O ­ 2–H2O mixtures developed a multi-layered structure, generally with an innermost F ­ e2SiO4 + FeO layer, followed by FeO/Fe3O4/Fe2O3 layers towards the scale surface. A preformed oxide scale and the presence of 8%CO2 in the atmosphere had little effects on further steel oxidation. The mechanisms of forming different scale structures are discussed. Keywords 60Si2MnA · Oxidation · Si in steel · O2 · Water vapour * Yisheng R. Chen [email protected] Yu Liu [email protected] Xuanxuan Xu [email protected] 1 Steel Research Australia, Melbourne, Australia 2 Zenith Steel Group Co., Limited, Changzhou, China 13 Vol.:(0123456789) Oxidation of Metals Introduction 60Si2MnA is one of the commonly used high-strength spring steels in China [1]. While the steel possessed more superior sag resistance than its counterparts that contained other alloying elements (such as Cr and V) [2] and plain carbon spring steels [2, 3], it was susceptible to decarburization, which greatly compromised its fatigue properties and service life [4]. There have been many studies conducted recently to deepen the understanding of the decarburization behaviour of spring steels [5–16], but none of the studies had examined the steel oxidation behaviour in detail and very few studies related the oxidation behaviour, particularly the scale structure developed to the decarburization behaviour observed. Traditionally, the role of an oxide scale layer formed on steel surface was thought to be multi-fold [17, 18]. First, the formation of a scale layer could provide some protection to the steel and slowed down the rate of decarburization [17]. However, when the atmosphere was very aggressive, leading to rapid steel oxidation, excessive steel oxidation could actually consume the decarburization layer or part of it, thus in effect, decreasing the decarburization layer thickness [18–20]. When the atmosphere was less oxidizing, then the decarburization depth was generally increased, as a result of less consumption of the steel by oxidation [19]. The presence of moisture and carbon dioxide in an oxygen-containing atmosphere was found in some previous studies to increase the oxidation rate of Fe–Si alloys [21–25], but most of the recent studies on the oxidation and decarburization behaviour of spring steels [5–16] were conducted in ambient air. There have been no studies conducted to examine the effects of gas composition on the oxidation and decarburization behaviour of spring steels. In view of the above, the current study was designed to examine the simultaneous oxidation and decarburization behaviour of 60Si2MnA at 700–1000 °C in atmospheres containing different levels of oxygen, water vapour and carbon dioxide with a focus on the relationship between the scale structure developed and the decarburization behaviour observed. This paper presents the results of oxidation. The results of decarburization are presented elsewhere [26]. Experimental Procedures A ϕ14 mm wire sample supplied by Zenith Steel, China, was used for the experiments. The steel composition expressed as weight per cent (wt%) is shown in Table 1. Crosssectional examination revealed a scale layer of about 5 μm comprising F ­ e2O3 + Fe3O4 on the surface, with a complete decarburization zone of less than 20 μm and a partial decarburization zone of 20–40 μm in the steel, as shown in Fig. 1. Steel discs of 1.0–3.0 mm thick were cut from the wire sample for experiments. The wire sample Table 1 Steel composition, wt% (weight per cent) C Si Mn P S Cr Ni Cu Al As 0.583 1.688 0.772 0.011 0.008 0.183 0.004 0.026 0.0068 0.007 13 Oxidation of Metals Oxide scale Fe2O3+Fe3O4 Complete decarburization zone 20 µm Partial decarburization zone Fig. 1 Original scale and decarburization zone structures on the steel sample received was not pickled and therefore a typical steel disc comprised two different surfaces for comparison, with two freshly cut surfaces and a circular edge covered with a preformed scale layer. The discs were sectioned using a cut-off wheel and then ground to 1000 grit finish. Prior to each experiment, the specimen was cleaned ultrasonically in ethanol and then wipe-cleaned using a facial tissue and blown dry using pressurized air. The experiments were conducted using a horizontal tube furnace. The steel disc for experiment was suspended on a stand secured in an alumina boat. To start an experiment, the alumina boat was pushed to the hot zone of the furnace, allowing it to be preheated in a 2–3%H2–N2 gas (volume per cent, which will be used to describe gas compositions throughout this paper) for 6 min to reach the hot zone temperature, and then the protective gas was switched to pure nitrogen flowing at 5 L/min (litre per minute) for 2 min for temperature equalization and to purge away the remaining ­H2 gas before an oxidizing gas was switched on. The compositions and their equilibrium oxygen partial pressures of the reaction gases used to assess the atmospheric effects are shown in Table 2. Thermodynamic data given by Richardson and Jeffes [27] were used to calculate the equilibrium oxygen partial pressures of the reaction gases. The gases selected were to investigate the effects of oxygen content, water vapour content and the presence or otherwise of 6–8% carbon Table 2 Equilibrium oxygen partial pressure (atm) under different experimental conditions Atmosphere (in volume per cent) 700 °C 800 °C 900 °C 1000 °C 17%H2O (57 °C)–Na2 2.54 × 10−8 1.69 × 10−7 8.13 × 10−7 3.06 × 10−6 1%O2–8%CO2–N2 – 0.01 0.01 – 1%O2–8%CO2–17%H2O (57 °C)–N2 – 0.01 0.01 – 3%O2–N2 0.03 0.03 0.03 0.03 3%O2–17%H2O (57 °C)–N2 0.03 0.03 0.03 0.03 Air–industry purity (­ H2O < 100 ppm) 0.21 0.21 0.21 0.21 Air–2%H2O (23 °C) – 0.21 – – Air–3.5%H2O (26–27 °C) – – – 0.21 Air–8%H2O (34–36 °C) – 0.21 – – Air–17%H2O (57 °C) 0.21 0.21 0.21 0.21 a Calculated using data given by Richardson and Jeffes [27] 13 Oxidation of Metals dioxide. 3%O2–N2, 1%O2–8%CO2–N2 and 17%H2O–N2 were selected as the base gases for simulating the furnace atmospheres where 15%, 5% and 0% excess airs were used for combustion using natural gas. Industry-purity air was used to examine the situation where steel was heated in a muffle furnace. The experimental gases were supplied by BOC Limited, Australia. Based on the specifications given [28], the 3%O2–N2 and 1%O2–8%CO2–N2 gases contained less than 20 ppm of ­H2O with a typical level of 10 ppm, whereas the industry-purity air contained less than 100 ppm H ­ 2O with a typical level of about 25 ppm. For higher moisture contents, a heated water bath was used, allowing the passing gas to absorb the desired moisture content before it was led to the furnace through a heated gas line. 2.0–3.5%H2O in air was used to simulate ambient air oxidation. A gas flow rate of 2 L/min at room temperature was used. Some experiments were also conducted in laboratory ambient air for comparison. The oxidation duration was set at 10–20 min at 700 °C and mostly 20 min for 800–1000 °C. Limited experiments were conducted for 1–20 min at 700°–900° in 3%O2–17%H2O–N2 to examine the oxidation kinetics in atmospheres containing both free oxygen and water vapour. Most of the experiments were conducted once only with their reliabilities judged by comparison with the results obtained under similar conditions. For example, the results obtained in 17%H2O–N2 were judged by comparing results obtained at different temperatures, and those in 1%O2–8%CO2–N2 and in industry-purity air were judged by comparing the results with those obtained in 3%O2–N2 where free oxygen was present but water vapour was essentially absent. Repeated experiments were also conducted for certain oxidation conditions, e.g. in 3%O2–N2 at 800 °C and 1000 °C and in ambient air at 800–900 °C. The scale thickness obtained during oxidation was calculated from the total weight gain, divided by the total surface area of the sample, assuming that the scale comprised wustite (FeO) only with a density of 5.65 g/cm3. Clearly, this was the average scale thickness of the sample. No attempt was made to differentiate the scale thickness on the freshly cut surfaces and that at the circular edge in obtaining the average scale thickness. The effects of different densities of magnetite and hematite on the calculated scale thicknesses were considered to be negligible. A small weight loss of the steel sample was observed during preheating. This weight loss was taken into account in calculating the scale thickness. When the scale thickness was thick, the weight loss as a result of decarburization from the surface layer was negligible. However, when the scale thickness was small and the decarburization was severe, the effect of decarburization became significant and will be considered. The scale structure was determined primarily by the contrasts of different phases under the optical microscope, as established previously [29], and confirmed using SEM on selected samples. The images presented were mostly optical images, unless otherwise indicated. Other details are the same as those described elsewhere [30]. 13 Oxidation of Metals Results Scale Thickness and Kinetic The scale thicknesses obtained after oxidation in different atmospheres at 700–1000 °C are compared in Fig. 2. The effect of temperature in different atmospheres and the effect of water vapour content at different temperatures are compared in Figs. 3 and 4, respectively. It can be seen that the scales obtained in 17%H2O–N2 and in 3%O2–N2 and 1%O2–8%CO2–N2 were similarly thin (≤ 6 μm) at all temperatures, Fig. 3 and more clearly seen in Fig. 4a, much thinner than those formed in atmospheres containing both oxygen and water vapour. Fig. 2 Average scale thicknesses obtained in different atmospheres at a 700 °C for 10 min, b 800 °C for 20 min, c 900 °C for 20 min and d 1000 °C for 20 min 13 Oxidation of Metals Fig. 2 (continued) When the steel was exposed to industry pure air, containing a slightly higher level of moisture (typically 25 ppm), the scales were noticeable thicker than those formed in 3%O2–N2 and 1%O2–8%CO2–N2 (containing typically 10 ppm ­H2O), and the difference was much greater at higher temperatures. The magnitude of increase in scale thickness as a result of adding 17%H2O in 1%O2–8%CO2–N2 and 3%O2–N2 and air was 4.5 times (from 2 μm to more than 9 μm) at 700 °C and more than 10 times at 800–1000 °C. The effect of water vapour content is shown clearly in Fig. 4. It is seen that a significant increase in scale thickness was obtained by even by just increasing the water vapour content to 2% at 800 °C and 3.5% at 1000 °C, and the scale thickness continued to increase with further increase in the moisture content to 17%. It was found in a later study that, the 13 Oxidation of Metals Fig. 3 Effect of oxidation temperature on scale growth in a 17%H2O–N2 and essentially dry ­O2-containing gases and b ­O2-containing atmospheres added with 17%H2O. The 17%H2O–N2-corrected line was obtained by adding the equivalent scale thicknesses due to decarburization to those calculated from the weight gain data obtained scale thickness continued to increase by a further 10–20% for up to 45%H2O in air [31]. The similarly thick scales obtained in 1%O2–8%CO2–17%H2O–N2, 3%O2–17%H2O–N2 and air–17%H2O suggested that the rate of oxidation was essentially unaffected by oxygen content within the range of 1–21% ­O2 when 17%H2O was added. The thin scales obtained in dry 1%O2–8%CO2–N2 at 800–900 °C indicated that the presence of 8%CO2 did not behave like water vapour in accelerating the oxidation rate. The oxidation kinetics at 700–900 °C in 3%O2–17%H2O–N2 is shown in Fig. 5, plotted using weight gain data, Δm in mg/cm2, against square root of time t, following 13 Oxidation of Metals Average scale thickness, µm 160 140 800⁰C 120 1000⁰C 100 80 60 40 20 0 0 5 10 15 20 H2O content in O2-containing gases, % Fig. 4 Effect of moisture content in ­O2-containing atmospheres on scale growth 7 In 3%O2-17%H2O-N2 Weight gain, mg/cm2 6 5 700°C 4 800°C 3 900°C 2 1 0 0 1 2 3 4 5 Time1/2, min1/2 Fig. 5 Parabolic plot of oxidation kinetics in 3%O2–17%H2O–N2 at 700–900 °C the recommendation of Pieraggi [32]. It is seen that for the duration of 3–20 min, the oxidation kinetics followed the parabolic law. Regression of the data using the following equation, 1∕2 Δm = kP ⋅ t1∕2 + C where C is a constant, yielded the parabolic rate constants, kP , in ­mg2 cm−4 min−1, as shown in Table 3, as compared to those of pure iron in air or oxygen [20]. It is seen that the oxidation rates were lower than those of pure iron in air or oxygen, but interestingly, the oxidation rate at 800 °C was even greater than that at 900 °C. 13 Oxidation of Metals Table 3 Parabolic rate constants at 700–900 °C, kP, ­mg2 cm−4 min−1 T (°C) 60Si2MnA in 3%O2– 17%H2O–N2 Pure iron in oxygen or air [20] 700 0.167 0.639 800 2.863 3.923 900 1.946 17.67 Scale Microstructures 700 °C Consistent with the scale thickness data shown in Figs. 2a and 3a, very thin oxide scales (~ 2 μm) were observed in the samples oxidized in 17%H2O–N2, 3%O2–N2 and industry-purity air as shown in Fig. 6a, b. The scale formed in 17%H2O–N2 was single-phase wustite ­(Fe1−yO) only whereas those formed in 3%O2–N2 and industrypurity air comprised primarily hematite ­(Fe2O3) and some magnetite ­(Fe3O4). The scale structures were differentiated by the contrasts of the scale phases under the optical microscope, with wustite showing a uniform darker colour, whereas hematite appeared lighter grey with some areas attached with a slightly darker magnetite 20 µm FeO (a) In 17%H2O-N2 for 20 min Fe2O3 + Fe3O4 20 µm (b) In industry purity air for 10 min Fe3O4 20 µm Fe2SiO4 + FeO FeO Fe2O3 (c) In 3%O2-17%H2O for 20 min Fig. 6 Oxide scales formed on the freshly cut surfaces after oxidation at 700 °C a in 17%H2O–N2 for 20 min, b in industry-purity air for 10 min and c in 3%O2–17%H2O–N2 for 20 min 13 Oxidation of Metals phase, as well be seen more clearly when higher temperature scale structures are presented. Much thicker scales (> 9 μm) were observed after oxidation in 3%O2–17%H2O–N2 and air–17%H2O and the scale thus obtained had a multi-layered structure, as seen in Figs. 6c and 7, comprising an outer layer of hematite, followed by a magnetite layer with some wustite (FeO), an inner ­Fe2SiO4 (fayalite) + FeO layer, and an internal oxidation zone. 800 °C The scale formed at 800 °C in ­17H2O–N2 was similar to that formed at 700 °C, Fig. 8a, comprising a single-phase wustite layer. The scale structures formed in essential dried O ­ 2-containing atmospheres were similar to those formed at 700 °C, Fig. 8b–d, comprising a two-layered structure with a top hematite layer and a bottom magnetite layer. The scale structures developed at the edge and freshly cut surfaces were similar although it appeared that the hematite layer was relatively thick at edge regions. In clear contrast, the scales formed in wet O ­ 2-containing atmospheres were multilayered, Fig. 9a, with a surface hematite layer, followed by a thick magnetite layer, a relatively thin wustite layer and an inner ­Fe2SiO4 + FeO layer, as also confirmed by SEM analyses, Fig. 9b. An internal oxidation zone was also observed under the ­Fe2SiO4 + FeO layer, as shown in Fig. 9b. 900 °C The effects of atmosphere on steel oxidation at 900 °C were similar to those observed at 700–800 °C, with the formation of much thinner scales (< 5 μm) in 17%H2O–N2, 3%O2–N2, 1%O2–8%CO2–N2 and industry-purity air and much thicker scales (~ 50 μm) in 3%O2–17%H2O–N2, 1%O2–8%CO2–17%H2O–N2 Fe3O4 FeO Fe2SiO4 + FeO Internal oxidation zone 5 µm Fig. 7 SEM observation of the inner scale layers formed on the freshly cut surface in 3%O2–17%H2O–N2 at 700 °C for 20 min 13 Oxidation of Metals and air–17%H2O, Fig. 10. The scale formed in 17%H2O–N2 comprised FeO only, Fig. 10a, whereas those formed in 3%O2–N2, 1%O2–8%CO2–N2 and industry-purity air comprised primarily hematite (lighter grey colour) and some magnetite (darker grey), Fig. 10b, c. The scales formed in 3%O2–17%H2O–N2, 1%O2–8%CO2–17%H2O–N2 and air–17%H2O were multi-layered, with an outer layer of hematite, followed by an intermediate layer of magnetite, then a FeO + Fe3O4 layer, and an innermost layer of ­Fe2SiO4 + FeO, as shown Fig. 11. 1000 °C Similar to those observed at 700–900 °C, thin scales (< 4 μm) were observed in the samples oxidized in 3%O2–N2, industry-purity air and 17%H2O–N2. The scale formed in 3%O2–N2 was nearly completely occupied by hematite, Fig. 12a, and that formed in industry-purity air was two-layered with an outer hematite layer and an inner magnetite layer, Fig. 12b. The scale formed in 17%H2O–N2 was a single layer of wustite, with a thin ­Fe2SiO4 or ­Fe2SiO4 + FeO layer and an internal oxidation zone formed underneath, Fig. 12c. In contrast, an internal oxidation zone was absent after oxidation in 3%O2–N2 and industry-purity air, Fig. 12a, b. The scale structures formed in air–3.5%H2O, air–17%H2O and 3%O2–17%H2O–N2 were similar, comprising an outer layer of hematite, followed by a magnetite layer, an FeO + Fe3O4 layer and an inner FeO + Fe2SiO4 layer, Fig. 12d, and confirmed by Fig. 12e, f for the inner layers. An relatively deep internal oxidation zone was also seen in some areas, Fig. 13f. The structures of the different phases in Fig. 12f were determined by EDS analyses. Discussion Effects of Atmosphere on Scale Structure Development From the results shown, the oxidation atmosphere had a significant effect on scale growth and scale structure development and the effects are similar at all temperatures. When the steel was exposed to ­17H2O–N2 without the presence of free oxygen, the scale thickness obtained from weight gain data was thin (< 4–5 μm) and nearly independent of the oxidation temperature (Figs. 2, 3), comprising FeO only. When the steel was exposed to 3%O2–N2 and 1%O2–8%CO2–N2, similarly thin scale (< 4 μm) also developed (Figs. 2, 3), but the scale structures were very different, comprising primarily a surface hematite layer and a relatively thin magnetite layer underneath. The presence of water vapour had a significant effect on the scale growth rate and structure development. Even with the presence of a slightly higher moisture level in industry-purity air, the scale thicknesses obtained at 900 and 1000 °C were already significantly greater than those in 3%O2–N2 and 1%O2–8%CO2–N2. Further increasing the water content in the O ­ 2-containing atmospheres led to rapidly increased scale thickness, Fig. 4. 13 Oxidation of Metals Fig. 8 Scale structures developed after oxidation at 800 °C: a in 17%H2O–N2, b in 3%O2–N2, at edge ▸ regions, c in 3%O2–N2 on freshly cut surface and d in industry-purity air, at edge regions Mechanism of Atmospheric Effects Scale Growth in 17%H2O–N2 The scale thicknesses developed in 17%H2O–N2 calculated from weight gains, were very thin (< 4 μm) and did not increase monotonically with increased temperature. It appeared that the thickness formed at 1000 °C was even smaller than that formed at 900 °C and 700 °C and that at 800 °C was similar to that formed at 1000 °C. This was very unusual and required more considerations. Very little information was available in the literature regarding the oxidation behaviour of Si-containing steels in H ­ 2O–N2 mixtures. Gleeson et al. [33] examined the oxidation behaviour of two Si-containing steels (0.51 wt% and 2.2 wt%Si) in 27%H2O, dry air and air–27%H2O at 730–1050 °C and found that the oxidation rates of the silicon-containing steels in 27%H2O–N2 were considerably lower than those of the Si-free steel. A “kinetics inversion” phenomenon was observed with the weight gain after oxidation for 60 min at 935 °C being smaller than those obtained at 730 and 850 °C. A higher volume fraction of fayalite formed in the inner scale layer at 935 °C was thought to be responsible for the greater protection observed. It had been shown [26] that decarburization was the most severe at 800 °C (~ 100 μm), followed by that at 900 °C (~ 40 μm), 1000 (~ 20 μm + partial decarburization) and very little at 700 °C. By calculation, a complete decarburization zone of 100 μm at 800 °C was equivalent to a weight loss of about 0.46 mg/cm2 (assuming that 0.583% of carbon was completed lost) which was equivalent to a wustite scale thickness of 3.5 μm, and those at 900 °C and 1000 °C would account for scale thicknesses of 1.5 and 1.2 μm, respectively. Adding these equivalent scale thicknesses into those calculated from weight gain data measured, it can be seen from Fig. 3a that the corrected scale thickness obtained at 800 °C was the greatest, followed by that at 900 °C and those at 700 and 1000 °C. The smaller scale thickness at 700 °C could be just caused by the lower oxidation temperature. The thinner scales formed at 900 and 1000 °C than that formed at 800 °C can be attributed to the formation of a more continuous and compact fayalite layer at the wustite–steel interface, as observed in Fig. 12c and suggested also by Gleeson et al. [33]. Effect of Moisture Addition in the Oxygen‑Containing Atmospheres The thin scales formed in essentially dry O ­ 2-containing gases with the development of hematite and magnetite only indicated that iron transport through the scale–steel interface had been hindered significantly. Two potential mechanisms were proposed in the literature to interpret the effects of silicon in slowing down steel oxidation. The first mechanism was the development of a continuous ­SiO2 layer on the steel surface when the Si concentration in the steel was high enough, as proposed by 13 Oxidation of Metals Fe1-yO Internal oxidaon zone Complete decarburizaon zone 30 µm (a) In 17%H2O-N2 – freshly cut surface Fe2O3 Fe3O4 10 µm (b) In 3%O2-N2 – edge region 10 µm (c) In 3%O2-N2 – freshly-cut surface 10 µm (d) Industry-purity air – edge region 13 Oxidation of Metals Fe3O4 Fe1-yO Fe1-yO + Fe2SiO4 Internal oxidation zone Complete decarburization zone 20 m (a) Optical image, 500x Fe3O4 Fe1-yO Fe1-yO + Fe2SiO4 Internal oxidation zone 10 µm (b) SEM image Fig. 9 Oxide scale formed in 3%O2–17%H2O–N2 at 800 °C for 20 min: a optical image (× 500) and b SEM image Darken [34], Wagner [35], Atkinson [36] and Hughes [37], but the Si content in the steel examined in the current study was lower than the critical concentration of 5 at.% or 2.6 wt% silicon proposed. The second mechanism proposed was the effect of moisture on scale–steel interface adhesion [33, 38], possibly as a result of reduced plasticity of the scale due to the absence of water vapour in the atmosphere [33]. In the current study, it appeared that both mechanisms were operating. Although the level of silicon in the steel was lower than the theoretical level to 13 Oxidation of Metals FeO Internal oxidaon zone (a) in 17%H2O-N2 – freshly cut surface Fe3O4 Fe2O3 (b) in 1%O2-8%CO2-N2 – freshly cut surface Fe3O4 Fe2O3 10 µm (c) in 1%O2-8%CO2-N2 – edge region Fig. 10 Oxide scales formed after oxidation at 900 °C in a 17%H2O–N2, freshly cut surface, b 1%O2– 8%CO2–N2, freshly cut surface and c 1%O2–8%CO2–N2, edge regions. Magnification × 1000. The scales formed in 3%O2–N2 and industry-purity air are similar to those formed in 1%O2–8%CO2–N2 produce a complete coverage of the steel surface with a ­SiO2 layer, but judging from the absence of an internal oxidation zone and the extremely thin nature of the scale developed, it was likely that within the 20 min of oxidation, a continuous ­SiO2 had formed and provided the protection, and the stability of this layer was maintained by the poor adhesion of the iron oxide scale formed above it, preventing the formation of a wustite layer over the ­SiO2 layer. However, when water vapour was present in the atmosphere, the scale–steel interface adhesion was very much improved possibly as a result of much improved plasticity of the scale and therefore, formation of wustite over the ­SiO2 layer became possible. Once a wustite layer formed, the ­SiO2 scale would then react with FeO, forming a less protective fayalite ­(Fe2SiO4) layer via the following reaction, 13 Oxidation of Metals Fe1-yO + Fe3O4 Fe1-yO + Fe2SiO4 50 µm (a) Optical image, 200X Fe1-yO Fe3O4 Fe1-yO + Fe2SiO4 (b) SEM image Fig. 11 Scale structure formed on the freshly cut surface in air–17%H2O at 900 °C for 20 min: a optical image (× 200) and b SEM image. Scale structures formed in 3%O2–17%H2O–N2 and 1%O2–8%CO2– 17%H2O–N2 are similar to these SiO2 + 2FeO = Fe2 SiO4 (1) or a mixed FeO + Fe2SiO4 layer at the interface, as shown in Figs. 9, 11 and 12d–f. When a FeO + Fe2SiO4 layer formed, instead of ­SiO2, the oxygen potential at the scale-steel interface would be increased and therefore internal oxidation became possible, as observed in Figs. 9b and 12f. The morphologies of the internal oxides were similar to those observed by others [39–42]. Based on the EDS analysis results shown in Fig. 12f and those presented by others [39–42], it can be concluded that the internal oxides formed near the steel surface was primarily ­Fe2SiO4 precipitates and those formed further away from the steel surface was amorphous ­SiO2. Once internal oxides formed, it was then impossible to develop a S ­ iO2 layer as silicon in solution was already consumed. With the formation of a ­Fe2SiO4 or FeO–Fe2SiO4 layer at the interface, iron diffusion now had a continuous path through the scale layers outwards, and 13 Oxidation of Metals therefore, the kinetics would then be controlled by solid-phase diffusion thus having a parabolic nature. However, as the fayalite layer was a slower path for iron diffusion [33], the rate constant was noticeably decreased, particularly when its volume fraction was high. Summarizing the results and discussions above, the scenarios of scale structure development in different atmospheres can be schematically shown in Fig. 13. The mechanism of forming a more adherent scale–steel interface when water vapour was present in the atmosphere is still unclear. Improved scale plasticity [33] had been proposed to be a potential mechanism but in what way water vapour would improve the scale plasticity is still not known. Improving the cleanness of the scale–steel interface through reaction of water vapour or hydrogen (as a result of water vapour dissociation) with impurity elements segregated at the interface could be another possible mechanism. Regardless what mechanism was responsible, it was abundantly clear that when water vapour was present, the scale was more adherent and the scale thickness was much greater with the formation of a multi-layer structure generally including a wustite layer and a FeO + Fe2SiO4 layer at the interface, leading to a much greater oxidation rate. Conclusions The oxidation behaviour of 60Si2MnA in different atmospheres at 700–1000 °C was investigated. It was found that: 13 Oxidation of Metals Fig. 12 Oxide scales formed at 1000 °C a in 3%O2–N2, b in industry-purity air, c in 17%H2O–N2; d in ▸ 3%O2–17%H2O–N2; e SEM image of the FeO + Fe2SiO4 layer formed in air–17%H2O and f SEM image of the FeO + Fe2SiO4 mixture and internal oxide particles formed in air–17%H2O 1. When the steel was exposed to 17%H2O–N2, very thin scales (< 6 μm) formed at all temperatures and the scale structure comprised wustite only. 2. When free oxygen was present in the atmosphere, the degree of oxidation was not affected significantly by the oxygen content within the range of 1–21%, but was significantly affected by the concentration of water vapour in the atmosphere. 3. When the steel was exposed to essentially dry 3%O2–N2 and 1%O2–8%CO2, very little oxidation, forming thin scale comprising primarily hematite, was observed at all temperatures with the absence of an internal oxidation zone. Based on these observations, it was proposed that the formation of a S ­ iO2 layer on the steel surface was possible within the duration examined, thus providing good protection to the steel against oxidation. 4. When water vapour was present in the O ­ 2-containing atmospheres, more severe oxidation occurred to the steel and the severity increased with increased moisture content. Multi-layered scale structures, with the formation of an innermost layer of ­Fe2SiO4 + FeO mixture and outer layers of ­Fe1−yO/Fe3O4/Fe2O3, were observed at all temperatures and the scale thickness increased with increased temperature, although the increase from 800 to 900 °C was small due to the formation of a higher volume fraction of ­Fe2SiO4 at 900 °C. 5. The presence of a preformed scale layer of about 4–5 μm thick and the presence of 8%CO2 in the atmosphere had little effect on further oxide scale growth. 13 Oxidation of Metals Fe2O3 10 µm (a) 10 µm Fe2O3 Fe3O4 (b) Fe1-yO Fe2SiO4 Internal oxidaon zone 10 µm (c) Fe2O3 Fe3O4 Fe1-yO+Fe3O4 Fe1-yO+Fe2SiO4 20 µm (d) 13 Oxidation of Metals Pores Fe1-yO Fe2SiO4 (e) (f) Fig. 12 (continued) FeO layer Fe2SiO4 or Fe2SiO4+FeO layer Internal oxidation zone Steel substrate (a) Fe2O3 layer Fe3O4 layer Intermittent interface Possible SiO2 layer Steel substrate (b) Fe2O3 layer Fe3O4 layer Fe1-yO layer Fe2SiO4+Fe1-yO layer Internal oxidation zone Steel substrate (c) Fig. 13 Schematic illustration of the different scenarios of scale formation on 60Si2MnA: a in ­17H2O– N2, b in dry O ­ 2-containing atmospheres, c in wet (e.g. containing 17%H2O) ­O2-containing atmospheres 13 Oxidation of Metals References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 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