Pergamon

Corrosion Science, Vol. 40, No. 2/3, pp. 191-200, 1998 0 1998 Elsevier Science Ltd. All rights reserved.

Printed in Great Britain. 0010-938X/98 %19.00+0.00

PII: s0010-938x(97)00113-3

THE EFFECT OF THE OXIDATION ATMOSPHERE ON THE INITIAL OXIDATION OF TYPE 430 STAINLESS STEEL AT 1273 K

I. SAEKI,a* H. KONNO,” R. FURUICHI,” T. NAKAMURA,b K. MABUCHI” and M. ITOHd

“Division of Material Science and Engineering, Graduate School of Engineering, Hokkaido University, Kita 13 Nishi 8 Sapporo, 060 Japan

bHitachi Works, Hitachi Ltd, Saiwai 3, Hitachi, 319-12 Japan ‘Hitachi Research Laboratory, Hitachi Ltd, l-l, Omika 7, Hitachi, 319-12 Japan

dHitachi Kyowa Engineering Ltd, Benten 3, Hitachi, 319-12 Japan

Abstract-Type 430 stainless steel with 0.9 wt% manganese was oxidized in 02/H20/N2 atmospheres at 1273 K. Iron-rich corundum-type oxide formed in the very initial oxidation stage changed to a chromium-rich oxide with time in all atmospheres. The chromium content in a spinel-type oxide, which appeared after formation of the the corundum-type oxide, increased with increasing oxygen partial pressure. It is considered that the spinel-type oxide forms through a two-step reaction: first, reduction of Mn,O, to MnO; second, solid-state reaction of MnO with corundum oxide. The rate-determining step appears to be the first reaction. 0 1998 Elsevier Science Ltd. All rights reserved

Keywords: A. stainless steel, B. SEM, B. XPS, B. X-ray diffraction, C. oxidation.

INTRODUCTION

A number of studies have been made on the initial oxidation of Fe-Cr alloys and the effect of oxygen partial pressure on the properties of the oxides, since the high-temperature oxidation resistance of these alloys depends on the oxide properties. These studies were mostly done in vacuum and at relatively low temperatures.‘-’ Water vapor is an accelerator of so-called breakaway oxidation and extensive work has been reported at temperatures around 1173 K, where abnormal whisker-like oxide formation occurs with oxidation rates higher than those at higher temperatures.8m’3

Knowledge of the initial oxidation behavior and properties of the oxide formed on Fe- Cr alloys and stainless steels at higher temperatures at atmospheric pressure is important to alloy manufacturing processes such as annealing, acid pickling and cold working.

The authors have reported the effects of chromium and manganese contents, oxidation temperature, and atmosphere on the initial oxidation of Fe-Cr alloys.‘“” Properties such as the morphology and composition of oxides initially formed on type 430 stainless steel

*To whom correspondence should be addressed Manuscript received 20 May 1997

191

192 I. Saeki cl trl

with a low manganese content were not changed with oxygen and water vapor pressure at 1273 K.15

This paper reports the effect of atmosphere on the initial oxidation of type 430 stainless

steel with relatively higher manganese contents at 1273 K, particularly the effect of oxygen

partial pressure on the oxide composition and crystal structure. The oxidation mechanism of the alloy in the initial stage is also discussed through the dependence of oxide properties

on oxygen pressure.

EXPERIMENTAL METHOD

Commercial type 430 stainless steels were used as specimens (Table 1). A 10 mm x 10 mm x 0.5 mm coupon was abraded with silicon carbide paper down to # 1000

in distilled water, and then mirror finished with 1 pm diamond paste. Degreased specimens were kept in a desiccator containing silica gel for more than 24 h before oxidation tests.

Details of the oxidation test apparatus have been reported elsewhere.lh The specimen was

heated from ambient temperature to 1273 K for 20 s and this time was set as the oxidation

time t,, = 0 s. The atmosphere was adjusted to 0-O. 165 atm Oz. GO.2 atm H,O and the rest N?. The oxide thickness was estimated by cross-sectional observations of the specimens in a

field-emission-type scanning electron microscope (SEM; JEOL-6300F) with an acceleration voltage of 3 kV after breaking the specimen in liquid nitrogen. The chemical composition

of the surface and composition profile in the depth direction were determined by X-ray

photoelectron spectroscopy (XPS; VG ESCALAB Mk2, Al Ka, 15 kV, 20 mA) with a 5 kV Ar+-ion beam. An X-ray diffractometer (XRD; JEOL JDX-3500, 30 kV, 400mA, H-28 scan) was used to determine the oxide composition and crystalline structure. Diffraction

angles and intensities were corrected by using diffraction angles of silicon powder attached

to the specimens as a standard.

EXPERIMENTAL RESULTS

The thickness of the oxide formed in a 0. I65 atm Oz/Nz atmosphere is estimated to be

about 150 nm at t,, = 30 s, then increases with to about 280 nm at t,, = 180 s. The thickness of the oxides formed in a 0.2atm H,O/N, atmosphere is similar to that obtained in the 0.165 atm Oz/Nz atmosphere.

Figure 1 shows changes in the atomic fraction of elements, .f&,en,, on the specimen

surfaces by XPS. For the oxide formed in the 0.165 atm 0,/N* atmosphere, Fig. 1 (a),,f& is 0.25 at t,, = 0 s and then decreases with oxidation time; ,f& and ,fMn are initially low and increase with t,,,. The .f;, and .fi, values are about 0.6 and 0.02, respectively, and nearly constant up to t,, = 180 s. The surface atomic fractions of iron, chromium and manganese in the oxide formed in the 0.2 atm H,0/N2 atmosphere, Fig. I(b), are similar to those in the oxide formed under the 0.165 atm 02/N2 up to t,,, = 30 s but thereafter they differ: the

Table I, Composition of type 430 stainless steel (wt%)

c Si Mn P s Ni Cr Al Fe ~~. ~~ -

0.070 0.30 0.90 0.028 0.00 I 0.08 15.97 0.14 bal.

Effect of oxidation atmosphere on type 430 SS 193

Oxidation time, t, I s

Fig. 1. Change in the atomic fraction of Fe (-- 0 -), Cr (. 0 ), Mn (- - - A - - -), Si (- -V-. -) and 0 (---O ---) at the oxide surface in (a) 0.165atm Oz/NZ and (b) 0.2atm

H,O/N, atmospheres.

values of fre and fMn are higher and fcr is lower. The amount of oxygen and silicon are similar to those shown in Fig. l(a). The oxidation number of the elements is considered to be the same for both atmospheres by the appearance of the specific satellite peaks in XPS spectra; the oxidation number of iron and chromium is + 3, independent of oxidation time, while that of manganese changes from + 3 to a + 2 and + 3 mixture after t,, = 15 s. Similar results were obtained for other type 430 stainless steels with various manganese contents.17

Figure 2 shows composition changes in the depth direction for the oxide formed in the 0.165 atm 02/N2 atmosphere. The abscissa of the figure indicates the amount of sputtering charge of Ar+ ions, since the sputtering current was changed in the course of sputtering and sputtering time was not an indication of the depth. At t,, = 0 s, iron forms a peak near the surface and chromium is observed in the oxide side at the interface. The fM,, and fsi values are low throughout the oxide. At t,, = 15 s, the peak height of fre decreases in

Fig. 2. Depth profiles of oxide films formed on type 430 stainless steel in 0. sphere (-- Fe,---Cr,---Mn, S&-O).

165 atm WN, atmo-

194 I. Saekl e/ trl

Fig. 3. Depth

Amount of Ar+ ions, QAr / 1 OT3 C cm‘*

profiles of oxide films formed on type 430 stainless steel in 0

sphere ( Fe. Cr. -- Mn. Si. 0).

.2 atm HLO:NZ atmo-

comparison with that at t,,, = 0 s and the position appears to move towards the interior of the oxide. The distribution of chromium in the oxide spreads outwards to show increased

,f& at the surface. The amount of manganese and silicon increase; manganese is rich at the surface and silicon accumulates at the oxide/alloy interface. At t,, = 30s the iron profile

does not show a peak: the content is high at the surface and decreases with depth to QAli = 25 x lo- ‘C cm-‘. The value of,fh,, increases at t,,, = 15 s. At t,,, > 60 s, the shapes of the

curves are similar except that the oxide thickness increases.

Depth profiles of the oxide formed in the 0.2 atm H,O/N, atmosphere are shown in Fig. 3. The profiles are similar to those shown in Fig. 2 but,frc andf,, are higher and,fc, is lower

in the outer part of the oxide. The manganese content of the oxide decreases with t,, in accordance with the change in surface composition shown in Fig. 1 (b).

From the XRD results, the oxides are found to be composed of only corundum-type oxide until t,, = 15 s, after which spinel-type oxide appeared. This result is similar to that

previously reported for type 430 stainless steels with various manganese contents.” The intensity fraction of spinel-type oxide, I,,, calculated in the same manner as in the previous report, is plotted against t,, in Fig. 4. The value of IsI, is small at t,, = 15 s, then increases

with t,, and becomes constant between f,,, = 30 and 120 s. There is a decrease in ISI, at

t,, > 120 s. The shape of the two curves is similar, but ISP is larger in the 0.2 atm H,O/N, atmosphere. The previous paper showed that the relative XRD intensity of spinel-type

oxide is proportional to the atomic manganese fraction at the surface, agreeing with the

result obtained in this study.‘” Lattice constants of (a) corundum-type oxide, u,. and (b) spine1 type oxide, u,,,, assumed

to be rhombohedral and cubic-closed packed respectively, are plotted against t,,, in Fig. 5.

The values of a, are high at t,,, = 0-l 5 s and decrease sharply with t,,; after t,, = 30 s they reach a constant value of about U, = 5.38 A in both atmospheres. It has been reported that

3.g o- 0 30 60 90 120 150 180

Oxidation time, tax / s

Fig. 4. Time variation m the XRD intensity fraction of spin&type oxide formed in 0.165atm

O,.‘N,( 0 ) and 0.2 atm H20/N7 (- A - - -) atmospheres.

Effect of oxidation atmosphere on type 430 SS 195

0 30 60 90 120 150 180 Oxidation time, t,, I s

Fig. 5. Time variation in the lattice constants of (a) corundum- and (b) spinel-type oxide formed in 0.165 atm 0Z/N2 (- 0 -) and 0.2 atm H20/NZ (- -- A - - -) atmospheres.

Fe203 and Cr,03 form a continuous series of solid solutions, that up to 12 wt% of Mn203 dissolves in Fe-Cr oxide at 1273 K and that the lattice constant of solid solutions changes with composition.‘9,20 These suggest that the lattice constants shown in Fig. 5 should correspond to the average composition of corundum-type oxides and that the compositions do not change with atmosphere. Lattice constants for the same oxidation period at 1073 and 1173 K were higher than that at 1273 K.‘* It was supposed that the composition of corundum-type oxide on type 430 stainless steels is determined only by the oxidation temperature except for the initial few tens of seconds, if oxygen pressure is sufficiently high to form corundum-type oxide. As the diffraction intensity of spinel-type oxide at t,, < 30 s was too weak to calculate asp, the results at r,, > 30 s are shown in Fig. 5(b). The lattice constant for the oxide formed in 0.165 atm 0,/N, at t,, = 30 s corresponds to the value for MnFe,O,. As the oxidation proceeds, the measured lattice constant decreases and reaches the value for MnCr20d. For the oxide formed in the 0.2atm H20/N2 atmosphere, the lattice constant is nearly equal to that of MnFe,O,. These results show that the oxidation atmosphere affects the oxide properties after t,, = 30s and details of the effects on the surface and average composition of the oxide were investigated at t,, = 180 s.

Figure 6 shows the effect of oxygen partial pressure, poz, on the surface composition of the oxides. The atomic fraction of chromium at the oxide surface increases and those of the other elements decreases with po, independent of water vapor pressure, pHzO. Relation- ships between po, and the lattice constants of (a) corundum-type and (b) spinel-type oxides are shown in Fig. 7. The lattice tonstant of corundum-type oxide does not change with po, and the value is around 5.38 A, whereas that of spinel-type oxide is initially high and decreases with po,. This indicates that the average composition of spinel-type oxide changes from iron-rich to chromium-rich with increasing po,. Figures 8 and 9 show the effect of pHzO on the surface composition (Fig. 8) and lattice constants of the oxides (Fig. 9), showing that pHzO does not significantly affect the composition of the two oxide types.

The atomic fractions of iron and chromium at the surface determined by XPS are plotted against the lattice constant of spinel-type oxide by XRD in Fig. 10. The lattice constant increases with increasing iron and decreasing chromium at the surface. This indicates that the average composition of the spinel-type oxide coincides with the surface composition of oxides.

I. Saeki C/ (11.

Oxygen pressure, po2 I atm

Fig. 6. Effect of oxygen pressure on the atomic fraction of Fe (~--Pm 0 ~~ -), Cr ( 0 .). Mn (---A---) and Si ( V -) at oxide surfaces formed in oxygen (a) without H,O and (b)

with 0.2 atm Hz0 atmospheres.

/ -._._ 0 0.05 0.10 0.15 0.20

Oxygen pressure. po2 / atm

Fig. 7. Effect of oxygen pressure on lattice constants of (a) corundum- and (b) spinel-type oxide

formed in 0:/N? with 0 ( 0 ~).O.OS(. -0’ .),().I(---n---)and0.15(~. V ~) atm Hz0 atmospheres.

DISCUSSION

Figures IL3 show that at t,, = 0 s the atmosphere makes no significant difference to the

oxide composition at the surface and in the depth direction. The oxides are composed of

only corundum-type oxide and its average composition seems to be Fe,O, (Figs 4 and 5). The oxide formed at this time may be so thin and porous that iron, which is the major component of stainless steel, is oxidized preferentially. As the oxide becomes compact with

1 ux, the amount of chromium in the corundum-type oxide increases sharply owing to preferential oxidation of chromium and becomes constant after t,, = 30s although the composition does not reach Crz03 (Fig. 5). This suggests that the oxygen activity at the oxide/substrate metal interface remains constant after t,, = 30s independent of the atmosphere, and is higher than the decomposition pressure of iron oxides. The average composition of spinel-type oxides which developed after t,, = 15 s is, however, different withp,?: iron-rich spinel-type oxide is formed independent oft,, in the HzO/Nz atmosphere,

Effect of oxidation atmosphere on type 430 SS 197

0 0.05 0.10 0.15 0.20 Water vapor pressure, pHP I atm

Fig. 8. Effect of water vapor pressure on the atomic fraction of Fe (p O----), Cr (. . 0. .),Mn(---A---)andSi(-.-V-,-)attheoxidesurface.

\ 8 0.25 ‘3 0.20 4 0.15 & _____ f----..-..-.2! __......... fl............

.O tI

0.10 ,o 0.05 * 0

0 0.05 0.10 0.15 0.20

Water vapor pressure, pHfl / atm

Fig. 9. Effect of water vapor pressure on lattice constants of (a) corundum- and (b) spinet-type oxide formed in H,O/N* with 0.02 (-- 0 -), 0.05 (. 17 .), 0.1 (---A - - -) and 0.165

(- -V - -) atm H20 atmospheres.

8.44t , , , , ,

8.52

8.50

8.48

8.46

8.441 , , , , 1 0 0.05 0.10 0.15 0.20 0.25

Atomic fraction,f

Fig. 10. Relationship between atomic fraction of (a) Fe and (b) Cr at the oxide surface and lattice constant of the spinel-type oxide.

198 1. Saekl et cd

while the oxide formed in the 02/N2 atmosphere becomes increasingly chromium-rich with

1,X. Spinel-type oxide can form only at a finite yoZ and the development of a thick oxide

layer is improbable at the oxide/substrate metal interface because of the gradient of oxygen activity across the oxide layer. Moreover, if the interior of the oxide were a mixture of corundum- and spinel-type oxides, the composition of the corundum-type oxide will change

with poz because the equilibrium decomposition pressure of the spinel-type oxide should

change with composition and affect the oxygen activity near the oxide/substrate metal interface. The composition of corundum-type oxide, however, does not change with po, in

this study. This suggests the formation of a single layer of corundum-type oxide in the

interior of the oxide. There is no significant change in oxygen concentration in the depth direction corresponding to the different oxygen contents of corundum- and spinel-type

oxides (Fig. 1). This suggests the formation of a mixed layer of corundum- and spinel-type oxides next to the corundum-type single layer, similar to the oxide formed on Fe-Cr-Nip

Mn alloys.‘,” Since there is good correlation between the surface and average composition of spinel-type oxide (Figs 5 and IO), the outermost part of the oxide (the region within 2 or

3 nm, the escape depth of photoelectrons) may be composed of spinel-type oxide only. As the average oxidation number of spine]-type oxide is smaller than that of corundum-

type oxide, spinel-type oxide should be formed at the oxide/metal interface. Stott et al.

proposed that the formation of spinel-type oxide occurs at the oxide/metal interface and moves to the outside of the oxide.” In this study, however, no such inner spine1 layer is

observed. and it becomes necessary to explain the experimental results that the formation of spinel-type oxide occurs at the corundum-type oxide/gas interface. It is considered that

the formation process may consist of a two-step reaction: (1) reduction or decomposition

of Mn,O, to MnO and (2) solid-state reaction of MnO with Fe,O, (2a) and/or Crz03 (2b).

2Mn,0,+4MnO+O,

MnO + Fe,Ol+ MnFe,O,

MnO+Cr,O,+MnCrzO,

(1)

(2a)

(2b)

It should be noted that only reaction (1) involves gaseous oxygen and proceeds to the right- hand side as po, decreases. The increase in fSP (Fig. 4) and,& (Fig. 6) at the surface with

decreasing pal may be explained if the first step is the rate-determining step. When is po, low, the iron concentration in the spinel-type oxide is high as shown in Fig. 6. Cox et a/. indicated that the order of diffusion rates of trivalent metal ions in the close-packed corundum-type oxide, composed of MnO and MnFe,O, or MnCr?O,, is Fe’ ’ >Cr3+ on the basis of crystal-field stabilization energy theory.” With decreasing po, the reaction rate

of (1) becomes larger than that of (2a) and (2b), the Cr’+ ion activity at the reaction interface may decrease due to a slower supply of Cr3+ than Fe’+ from corundum-type oxide. This situation would lead to the preferential occurrence of reaction (2a). The Gibb’s free energy change of formation for the overall reaction, (1) and (2) was calculated as -30 kJmol_’ and this would allow the reaction to occur spontaneously.24 Further, the development of the convoluted structure by mixing corundum-type oxide with spinel-type oxide in the middle part of the oxide may lead to a decrease in the surface energy and to an enhancement of the overall reaction. Reaction (1) contains gaseous oxygen as a product and its rate of evolution is directly controlled by the po, in the atmosphere. This implies that the outer part of the oxides is very porous and has-no resistance to passing oxygen.

Effect of oxidation atmosphere on type 430 SS 199

Spinel-type oxide is formed after t,, = 15 s (Fig. 4). This may explain the time required to accumulate a certain amount of Mn3+ at the reaction interface as necessary for reaction

(1). It is well known that water vapor promotes breakaway oxidation, but the water vapor

does not affect the oxide properties on type 430 stainless steels with 0.9 wt% manganese (this study) and 0.09 wt% manganese (previous study16). This suggests that water vapor affects other properties, like mechanical stress.

CONCLUSION

Initial oxidation of type 430 stainless steels with 0.9 wt% manganese was studied in 02/H20/N, atmospheres at 1273 K and the following conclusions were obtained.

(1) In the first 15 s of oxidation, only a corundum-type oxide rich in iron was formed and then the chromium concentration in the oxide increased with oxidation time. Oxygen pressure and water vapor pressure did not affect the composition of the oxide.

(2) Spinel-type oxide appeared after formation of the corundum-type oxide. At higher oxygen partial pressures this phase was rich in chromium and at lower pressures rich in iron.

(3) The outer part of the oxide is considered to be composed of spinel-type oxide with a small amount of corundum-type oxide and to be porous to allow the transportation of gaseous oxygen.

(4) Spinel-type oxide was expected to form by reduction or decomposition reaction of Mn,03 to MnO and 0, followed by solid-state reaction of MnO with corundum-type oxide. Because the first step limits the reaction rate, the reaction was suppressed and the relative amount of spinel-type oxide deceased with increasing oxygen partial pressure in the atmosphere.

Acknouledgrmenr-This work was supported by a grant-in-aid for scientific research from the Ministry of

Education, Science, Sports, and Culture, Japan (No. 05750638 and 08555216).

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