60Si2MnA Steel Oxidation: Effects of O2, H2O, and CO2

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
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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]
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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 oxidaon 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
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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:
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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.
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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)
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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
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Oxidation of Metals
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