Corrosion Behavior of Hastelloy XR Alloy in O2 and SO2 Atmosphere

Corrosion Behavior of Hastelloy-XR Alloy in O

and SO

Atmosphere

Rong Tu and Takashi Goto

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

As Hastelloy-XR alloy is a candidate structural material for the IS (Iodine-Sulfur) process in hydrogen production, oxidation and sulfidation of Hastelloy-XR alloy in Ar–O2and Ar–SO2atmospheres were studied by thermogravimetry at temperatures from 1000 to 1300 K. In Ar–O2atmosphere, the mass change obeyed a linear-parabolic law at oxygen partial pressures (PO2) from 0.01 to 10 kPa. The oxidation scales

consisted of inner Cr2O3layer and outer Mn1:5Cr1:5O4spinel layer. The surface morphology of the oxide scales changed from island-like to buckled and to porous texture with decreasingPO2. In Ar–SO2 atmosphere, the mass change obeyed a linear-parabolic law at SO2partial

pressures (PSO2) from 0.05 to 5 kPa. The morphology of corrosion scales changed mainly with corrosion temperature. While oxidation was

dominant at 1073 and 1173 K forming double-layer scales of inner Cr2O3and outer Mn1:5Cr1:5O4spinel, sulfidation was accompanied with oxidation at 1273 K andPSO2<0:5kPa with scales consisting of Fe3O4, FeCr2O4and Cr2O3layers and Ni3S2dispersed particles together with

CrS particles segregating at the grain boundary of Hastelloy-XR alloy.

(Received May 10, 2005; Accepted June 9, 2005; Published August 15, 2005)

Keywords: Hastelloy-XR alloy, SO2gas, oxidation, sulfidation, corrosion, Iodine-Sulfur process

1. Introduction

Since hydrogen can be a new clean energy source to reduce CO2emission and the green house warming, it is a key issue

to develop a mass production process of hydrogen. Hydrogen can be produced by a thermochemical decomposition of water, and recently IS (Iodine-Sulfur) process has succeeded to produce hydrogen from water in a continuosly operated closed cycle. IS process is fundamentally composed of three processes,i.e., sulfuric acid (H2SO4) decomposition around

1123 K, Bunsen reaction at 373 K and hydrogen iodide decomposition at 473 to 573 K.1,2)_{In the process of H}

2SO4

decomposition, SO3 and H2O are produced, and then SO3

decomposed into SO2 and O2. Such mixing atmosphere of

SO2 and O2 is significantly corrosive to structural metallic

materials.3)_{Thus, in bench-scaled IS process, the tubes and}

vessels have been made of quartz glass and polymers (mainly

Teflon).4)_{However, in order to develop large-scaled}

indus-trial chemical plants, commercially available heat-resistant alloys should be employed to construct the facility. The high-temperature mechanical properties and corrosion resistance of many candidate alloys have been investigated in such

harsh environment,5,6) and then Hastelloy-X alloy (49Ni–

21Cr–18Fe–9Mo–0.6Mn–0.8Si, in mass %) has been expect-ed as the structural material because of its high-temperature strength and creep resistance.7)However, this alloy had not enough corrosion resistance in IS process. On the other hand, to improve the oxidation resistance of Hastelloy-X alloy in a helium cooled very high temperature nuclear reactor (VHTR), the contents of Mn and Si were optimized and Hastelloy-XR alloy (49.47Ni–21.99Cr–17.80Fe–8.73Mo– 0.88Mn–0.33Si, in mass %) has been developed. The oxidation behavior of Hastelloy-X alloy has been reported by Wlodeket al.,8)_Charlot9)_{and Shindoet al.}7,10)_However,

the oxidation behavior of Hastelloy-XR alloy has not been investigated in wide-ranged temperatures and oxygen partial pressures. Moreover, no corrosion resistance of

Hastelloy-XR alloy in SO2atmospheres at high temperatures has been

studied. In the present study, the corrosion behavior of Hastelloy-XR alloy as a candidate structural material for IS process was investigated in O2and SO2atmospheres at high

temperatures. Since SO2would cause oxidation or sulfidation

depending on temperature and PSO₂, the oxidation in O2

atmosphere was separately examined to understand the

corrosion (oxidation and/or sulfidation) in SO2 atmosphere.

2. Experimental

Hastelloy-XR alloy disks (10 mm in diameter by 1 mm

thickness) were polished with an alumina paste (1mm) and

supersonically cleaned in acetone. The specimens were

exposed to Ar–O2and Ar–SO2atmospheres at oxygen partial

pressure (PO2) and SO2 partial pressure (PSO2) from 0.01 to

10 kPa at temperatures from 1000 to 1300 K for 43 ks. Ar–O2

and Ar–SO2mixture gases were introduced from the bottom

of a reaction tube at a flow rate of 6:7106_m3_s1_{. The}

total pressure in the reaction tube was fixed at 0.1 MPa. The mass changes were continuously measured by

thermo-gravimetry (A&T: HA202M, sensitivity: 10mg). Crystal

structure, microstructure and composition of corrosion scales were analyzed by X-ray diffraction (XRD, Rigaku: RAD-C), scanning electron microscopy (SEM, Hitachi: S-3100H), transmission electron microscopy (TEM, JEOL: EX-II) and electron probe microanalysis (EPMA, JEOL: JXA-8621MX). The potential diagrams of Fe–S–O, Ni–S–O and Cr–S–O system were calculated by using the thermodynamic database

to estimate the corrosion behavior in Ar–SO2atmosphere at

high temperatures.11,12)

3. Results and Discussion

3.1 Oxidation behavior in Ar–O2 atmosphere

Figure 1 presents the XRD pattern of Hastelloy-XR alloy after the oxidation for 43 ks at 1273 K andPO₂¼1kPa. The oxide scales consisted of Cr2O3 and Mn1:5Cr1:5O4 spinel

phases, and such two-phase oxide scales were observed independent of oxidation conditions.

Figure 2 depicts the cross-sectional back-scattering SEM image and EPMA analyses for the scales formed at 1273 K

and PO₂ ¼10kPa. The oxide scale of 4mm in thickness

consisted of double layers with inner Cr2O3 and outer

Figures 3(a) to (c) demonstrate the surface morphology of oxide scales on the Hastelloy-XR alloy after the oxidation at

1273 K and PO2 from 0.01 to 10 kPa. Although the scales

always consisted of Cr2O3 and Mn1:5Cr1:5O4 spinel layers,

the surface morphology significantly changed with PO₂.

Island-like scales were observed atPO₂ ¼10kPa [Fig. 3(a)]. It was confirmed by XRD that the islands in Fig. 3(a) was Mn1:5Cr1:5O4 spinel and the flat area was Cr2O3 by

mechanically removing the islands. During the cooling process, sudden mass drops were often observed, implying the island-like scale was resulted from the partial delamina-tion of spinel outer layer due to thermal expansion mismatch between Cr2O3and spinel layers. Buckled oxide scales were

observed atPO2 ¼0:1kPa [Fig. 3(b)], which could be also

caused of the thermal expansion mismatch between two layers with the spinel layer remaining on the Cr2O3 layer.

Well-adhered uniform scales with a large amount of micro-pores were formed atPO2¼0:01kPa [Fig. 3(c)]. The

micro-pores could relax the thermal stress between two layers. Eceret al.conducted a marker test for the oxidation of Ni– Cr alloy at 1073 to 1373 K.13)_{The platinum marker stayed at}

the interface between alloy and oxide scales. This implies that the outward diffusion of cations could dominate the oxidation of Ni–Cr alloys. Since Ni and Cr are major components in Hastelloy-XR alloy, the similar oxidation

mechanism (i.e., outward diffusion of cations) can be

dominant in the present study.

Figure 4 presents the cross-sectional back-scattering SEM

image after the oxidation for 432 ks at 1273 K and PO2¼

10kPa. The thickness of Cr2O3 inner layer increased

10° 20° 30° 40° 50° 60° 70° Mn1.5Cr1.5O4

Cr2O3

Hastelloy-XR

2θ / (Cu kα)

Intensity (a. u.)

Fig. 1 X-ray diffraction pattern of the scale on the Hastelloy-XR alloy after the oxidation for 43 ks atPO2¼1kPa and at 1273 K.

O Fe

Cr Mn Ni

10 µm

Mn_1.5Cr_1.5O₄ Cr₂O₃ Hastelloy-XR

Fig. 2 Cross-sectional back-scattering SEM image and EPMA analyses of the scale on the Hastelloy-XR alloy after the oxidation for 43 ks at 1273 K andPO2¼10kPa.

100 µm (a)

20 µm (b)

10 µm (c)

significantly with time, and the slight increase in the spinel

outer layer was observed. Douglass et al.14)_{and Shindo et}

al.7)_{have obtained the similar results for the oxidation of Ni–}

Cr–Mn alloys. Douglasset al.reported that the double-layer scales of inner Cr2O3and outer MnCr2O4spinel were formed

on a Ni–20Cr–1Mn alloy after the oxidation at 1373 K in air.14)Shindoet al.reported that the increase in Cr2O3inner

layer thickness was much faster than MnCr2O4outer layer in

the oxidation of Hastelloy-X alloys and suggested that the oxidation reaction should proceed mainly at the interface be-tween inner and outer layers.7)_{Present study clearly indicated}

that the growth rate of Cr2O3 inner layer was significantly

greater than that of Mn1:5Cr1:5O4 outer layer, implying that

the oxidation of Hastelloy-XR alloy in the later stage was

mainly dominated by outward diffusion of Cr3þ_ion.

Figure 5 presents the TEM micrograph for the

cross-section of the oxide scale formed at 1273 K and PO₂¼

0:01kPa after 43 ks. The double-layer structure with inner Cr2O3 and outer Mn1:5Cr1:5O4 spinel was observed. The

diffusion rates of metallic ions in oxides including Cr2O3

were reported as Fe3þ_{, Mn}2þ_>Fe2þ_>Ti3þ _>Co2þ_>

Ni2þ >Mn3þ>Cr3þ.15)It is assumed that the spinel layer

formed at the outside of Cr2O3 due to faster diffusion of

Mn2þ_{ion than that of Cr}3þ _{ion particularly in an early stage}

of oxidation.15,16) The thickness of outer layer was thicker than inner layer.

Figure 6 shows the mass change of Hastelloy-XR alloy as a function of oxidation time at 1273 K. The mass change increased with increasing time andPO2. The time dependence

of mass change obeyed a linear-parabolic law at PO2 from

0.01 to 10 kPa. The linear (kl) and parabolic rate constants (kp) for the oxidation of Hastelloy-XR alloy were calculated separately from the time dependence of mass change. Figure 7 depicts the Arrhenius plots of linear rate constants (kl) for the oxidation of Hastelloy-XR alloy. Theklincreased

with increasing the oxidation temperature and PO₂. Wlodek

studied the oxidation of Hastelloy-X alloy in air from 1140 to 1470 K and reported the linear-parabolic mass change behavior at less than 1255 K.8)The transition from linear to parabolic behavior occurred between 12 and 36 ks, which

10 µm

Mn_1.5Cr_1.5O₄ Cr₂O₃

Hastelloy-XR

Fig. 4 Cross-sectional back-scattering SEM image of the scale on Hastelloy-XR alloy after the oxidation for 432 ks at 1273 K andPO2¼ 10kPa.

200 nm

Mn

1.5

Cr

1.5

O4

Cr

2

O3

Hastelloy-XR

Fig. 5 Cross-sectional TEM image of the scale on the Hastelloy-XR alloy after the oxidation for 43 ks at 1273 K andPO2¼0:01kPa.

Time, t / s 102

10-2

10-3

105 104

0.01 0.1 1 10

PO2 / kPa 1

2

Mass change,

∆

M

/ kg m

-2

1 1

103 10-4

10-5

Fig. 6 Relationship between mass change and time for the oxidation of Hastelloy-XR at 1273 K in Ar–O2atmosphere.

-8 -7 -6 -5

7 8 9 10

Temperature, T / K

Reciprocal of temperature, T-1 _{/ 10}-4_K-1

Log(Linear rate constant,

kl

/ kg m

-2 s -1)

0.01 0.1 1 10 20 (air)8) P_O2 / kPa

1000 1100

1200 1300 1400 1500

was longer than that in the present study (2 to 10 ks), and the

klobtained by Wlodek (PO₂ ¼20kPa) were close to that of

PO₂¼0:1kPa in the present study. In the present study, the time for starting the parabolic behavior increased with decreasingPO₂, which coincides the general trend of linear to parabolic transition.17)These suggest that Hastelloy-XR alloy can form protective oxide scale more easily than Hastelloy-X alloy.

Figure 8 depicts the Arrhenius plots of parabolic rate constants (kp) for the oxidation of Hastelloy-XR alloy. Thekp

values increased with increasing temperature and PO2.

Wlodek obtained thekp values of Hastelloy-X alloy in air8)

which were almost the same as those of Hastelloy-XR alloy inPO₂¼10kPa. Charlotet al.reported that thekpvalues of Hastelloy-X alloy increased with increasingPO₂from 5 Pa to

3.3 kPa at 1393 K.9) _{Their values are almost in agreement}

with the present values extrapolated to higher temperatures. Shindoet al.obtained slightly lowerkpvalues of Hastelloy-X

alloy than the present results.7) They studied in a helium

atmosphere containing a small amount of impurity (water

vapor and carbon dioxide), in which PO₂ was estimated as

1017to1020Pa. Charlotet al.9)and Shindoet al.7)reported that the activation energy (Ea) for the parabolic oxidation of

Hastelloy-X alloy was 234 kJ mol1_{, which is almost in}

agreement to that for the diffusion of Cr3þ_{in Cr}

2O3reported

by Gigginset al.(255 kJ mol1_).18)_{Charlotet al.and Shindo} et al.implied that the rate-controlling step for the parabolic oxidation of Hastelloy-X alloy could be the diffusion of Cr3þ

in Cr2O3layer. In the present study, theEafor the parabolic

oxidation of Hastelloy-XR alloy was 220 kJ mol1_{, which}

almost agreed with those of Hastelloy-X alloy reported by Shindo et al.7)and Wlodek (238 kJ mol1_).8)_{Therefore, the}

diffusion of Cr3þ_{in Cr}

2O3could be the rate-controlling step

for the parabolic oxidation of Hastelloy-XR alloy.

3.2 Corrosion behavior in Ar–SO2 atmosphere

Oxide scales consisting of Cr2O3and Mn1:5Cr1:5O4spinel

were formed on the Hastelloy-XR alloy after the corrosion in

Ar–SO2 atmosphere between 1073 and 1273 K, as observed

in Ar–O2 atmosphere. Figure 9 depicts the surface,

cross-sectional SEM images and EPMA analyses of the scale on the Hastelloy-XR alloy after the corrosion for 43 ks at 1073 K

and PSO₂¼1kPa. Buckled scales were observed on the

surface, being almost the same as that formed at 1273 K and

PO₂¼0:1kPa in Ar–O2 atmosphere as shown in Fig. 3(b).

The EPMA analysis showed that the buckled scale consisted of inner Cr2O3 layer and outer Mn1:5Cr1:5O4 layer, and a

-12 -11 -10 -9 -8 -7

Temperature, T / K

20 kPa(air) 8)

1393 K 9) 3.3 0.44 0.013 0.005 P_O2 / kPa

Reciprocal of temperature, T-1 _{/ 10}-4_K-1

Log(Parabolic rate constant,

kp

/ kg

2 m -4 s -1)

PO2=10-17~10-20 Pa 7)

0.01 0.1 1 10

PO2 / kPa

7 8 9 10

1000 1100

1200 1300 1400 1500

Fig. 8 Arrhenius plots of parabolic rate constants for the oxidation of Hastelloy-XR alloys.

50

µ

m

2

µ

m

(a)

(b)

Fe

Ni

Mn

Cr

O

S

small amount of sulfur was identified near the surface of

Hastelloy-XR alloy. No CrS or Ni3S2was detected by XRD.

At 1273 K, the corrosion scales showed two kinds of microstructures depending on PSO₂. At PSO₂>0:5kPa, the scale was almost the same as that shown at Fig. 9. At

PSO₂<0:5kPa, on the other hand, the scale had a multi-layer microstructures with dispersions of sulfides as shown in Fig. 10, where the surface SEM (a), cross-sectional

back-scattering SEM image (b) and a schematic of cross section (c) of the scale on the Hastelloy-XR alloy after the corrosion for

43 ks at 1273 K and PSO₂¼0:1kPa were demonstrated.

Partially delaminated scale with many bumps was observed [Fig. 10(a)]. The corrosion scale consisted of Fe3O4 with

Ni3S2 particles (layer A), FeCr2O4 with Ni3S2 particles

(layer B), Cr2O3 (layer C), Ni–Fe–Mo metal with CrS

particles (layer D) and Hastelloy-XR substrate (layer E) as depicted in Fig. 10(c).

It was reported that continuous Cr2O3layers were formed

in SO2 atmosphere at high temperatures for Ni–Cr alloys

containing high-content Cr.19,20)_{Hancock et al.studied the}

corrosion of Ni–20Cr alloy in SO2atmosphere at 1173 K, and

reported that the Cr2O3scale contained Ni3S2 particles, and

the internal sulfidation caused the segregation of CrS at the grain boundary in the alloy.19)_{Zureket al.reported that the}

mass change of Ni–22Cr–10Al–1Y alloy almost obeyed a parabolic law, and scales consisted of Cr2O3, Al2O3, NiO,

NiCr2O4and a small amount of AlS at 1173 to 1273 K in SO2

atmosphere.20)_{In the present study, continuous Cr}

2O3layers

were formed on Hastelloy-XR alloy because Hastelloy-XR alloy contained enough amount of Cr. The dispersion of Ni3S2particles in the oxide scales and the segregation of CrS

at the grain boundary in the present study were similar to the

results by Hancock19) and Zurek.20) The corrosion of

Hastelloy-XR alloy became more significant at high

temper-ature and low sulfur potential (at 1273 K and PSO2<

0:5kPa), and CrS was observed at the grain boundary of

Hastelloy-XR alloy due to the internal sulfidation of Cr. This behavior was similar to that of Ni–Cr, Fe–Cr and Co–Cr alloys in H2S–H2atmosphere studied by Naritaet al.21)They

reported that Cr-containing alloys were hardly sulfidized at high sulfur potentials, however sulfides were easily formed at low sulfur potentials. At low sulfur potentials, copious internal sulfidation (CrSxlayer) was formed for Ni–Cr alloys,

whereas sulfidation was confined to grain boundary for Fe–Cr alloys. As Hastelloy-XR alloy is a Ni–Cr–Fe based alloy, the internal sulfidation mainly occurred at the grain boundary

and no CrSxlayer was formed.

Figure 11 demonstrates the relationship between the mass change and time for the corrosion of Hastelloy-XR alloy at

CrS

CrS

A B C D

E

Ni3S2

Hastelloy -XR

Ni3S2

.

20 µm (b)

100 µm (a)

(c)

Fig. 10 Surface SEM (a), cross-sectional back-scattering SEM image (b) and a schematic of cross-section of the scale on the Hastelloy-XR alloy (c) after the corrosion for 43 ks at 1273 K and PSO2¼0:1kPa. (A:

Fe3O4(Ni3S2), B: FeCr2O4(Ni3S2), C: Cr2O3, D: Ni–Fe–Mo(CrS), E: Hastelloy-XR)

102 10-2

10-3

105 104

1 2

1 1

103 10-4

Time, t / s

Mass change

,

∆

M

/ kg m

-2

1073 K, PSO2=0.1 kPa

1273 K, PSO2=1 kPa

1273 K, PSO2=0.1 kPa

1073 K, PSO2=1 kPa

1073 to 1273 K andPSO₂¼0:1 to 1 kPa. The mass change obeyed a linear-parabolic law at the whole conditions, and increased with increasing temperature. The linear to para-bolic transition occurred at about 20 ks, which was longer than that of oxidation (2 to 10 ks in Fig. 6) implying that it is more difficult to form protective scales in Ar–SO2rather than

in Ar–O2 atmosphere. At 1073 K, the mass change at

PSO2¼1kPa was higher than that at PSO2¼0:1kPa, but

the trend was opposite at 1273 K. Since the oxidation was dominant at 1073 K as above-mentioned, the mass change

increased with increasing PO₂ calculated from the

decom-position of SO2 as given by eqs. (1) to (4). When a

temperature increases from 1073 to 1273 K at PSO₂¼

0:1kPa, the equilibriumPS₂ increases from 4:31012 _to

2:81010_{kPa andPO}

2increases from8:610

12_to5:6

1010_kPa.

SO2 ¼

1

2S2þO2 ð1Þ

G_T ¼RTlnðP

1 2

S2PO2=PSO2Þ ð2Þ

PO2 ¼2PS2 ð3Þ

therefore,

PO2 ¼ 1:414PSO2exp

G_T RT

2

3

ð4Þ

Figure 12(a) shows the relationship between linear rate constant (kl) andPSO2for the corrosion of Hastelloy-XR alloy

in Ar–SO2atmosphere. At 1073 and 1173 K, theklincreased

with increasingPSO₂, but decreased with increasing thePSO₂

at 1273 K [Fig. 12(a)]. As the corrosion of Hastelloy-XR alloy at 1073 and 1173 K was almost the same as the behavior of oxidation as described in 3.1, the relationship between

kl and PO₂ calculated from eq. (4) was demonstrated in

Fig. 12(b). The kl slightly increased with increasing PO₂. Figure 13(a) shows the relationship between parabolic rate

constant (kp) and PSO₂ for the corrosion of Hastelloy-XR

alloy. The trend ofkpwas almost the same as that ofkl. The relationship betweenkp andPO2 calculated from eq. (4) was

1 2 3 4

-8 -7 -6 -5

1273 1073 1173 T / K

Log(Partial pressure of SO2, PSO2 / Pa)

Log(Linear rate constant,

kl

/ kg m

-2 s -1)

(a)

-8 -7 -6 -5

1073 1173

T / K

Log(Partial pressure of O2, PO2 / Pa)

Log(Linear rate constant,

kl

/ kg m

-2 s -1)

-8 -7 -6

(b)

Fig. 12 (a) Relationship between linear rate constant (kl) of Hastelloy-XR alloy andPSO2from 1073 to 1273 K. (b) Relationship between linear rate

constant (kl) of Hastelloy-XR alloy andPO2 decomposed by SO2 from

1073 to 1173 K.

-11

1 2 3 4

-10 -9 -8

1273 1073 1173

T / K

Log(Partial pressure of SO2, PSO2 / Pa)

Log(Parabolic rate constant,

kp

/ kg

2 m -4 s -1)

(a)

-11 -10 -9 -8

1073 1173

T / K

Log(Parabolic rate constant,

kp

/ kg

2 m -4 s -1)

Log(Partial pressure of O2, PO2 / Pa)

-8 -7 -6

(b)

Fig. 13 (a) Relationship between parabolic rate constant (kp) of Hastelloy-XR alloy and PSO2 from 1073 to 1273 K. (b) Relationship between

parabolic rate constant (kp) of Hastelloy-XR alloy andPO2decomposed by

depicted in Fig. 13(b). The linear relation was observed at

1073 and 1173 K wherekp/P

1=n

O2 (n¼5to 6). It is generally

understood that Cr2O3was a p-type semiconductor, and the

defect formation reaction could be expressed as eq. (5).

3

2O2 ¼3OOþ2V 000

Crþ6h

_ð5Þ

The equlibrium constant for reaction (5) is given by eq. (6) because the concentration of vacancyes formed is propor-tional to the concentration of electron holes.

Keq¼ðV 000

CrÞ 2_ðhÞ6

ðPO₂Þ3=2 ¼ ðV000

CrÞ 2_ð3V000

CrÞ 6

ðPO₂Þ3=2 ð6Þ

Therefore,

V_Cr000 /const.P3_O=16

2 ð7Þ

The 3/16-power relationship can be predicted in the oxidation where the Cr2O3 formation is the rate controlling

step. Charlot et al. reported the relationship of kp/P 1=n O2

(n¼5).9)_{Present results of n¼5to 6 can be close to that of}

Charlotet al.9)_{and above-mentioned calculated results.}

The potential diagram can be useful to understand the corrosion behavior and the formation of oxide or sulfide

scales in SO2 atmosphere. The corrosion in SO2 atmosphere

may occur by dissociated O2or S2, thus the oxygen and sulfur

potentials may determine the corrosion process. We have calculated the potential diagrams by using thermodynamic database to compare with the experimental results. Figure 14 represents the potential diagrams for Ni–S–O, Fe–S–O and Cr–S–O systems at 1273 K, where the three diagrams were

superposed in one diagram. The PS2 and PO2 at PSO2¼

0:1kPa are 1:31010 _{and 2:610}10_{kPa, respectively.}

These values correspond to point P in Fig. 14 and located in the stable region of Cr2O3, Fe3O4 and Ni. The outermost

layer of the corrosion scale consisted of Fe3O4 with Ni3S2

particles and no NiO [layer A in Fig. 10(c)]. In the layer A, thePO₂can be in the stable area of Fe3O4and Ni, and thePS2

can be in the stable area of Ni3S2. Therefore, thePO2andPS2

in the outermost layer (layer A) can be in the area A in Fig. 14. The second layer of the corrosion scale consisted of FeCr2O4 and Ni3S2 particles [layer B in Fig. 10(c)]. In the

layer B, thePO2 can be in the stable area of FeO and Cr2O3

because FeCr2O4may be formed by the reaction of FeO and

Cr2O3. ThePS2can be in the stable area of Ni3S2. Therefore,

thePO2 andPS2 in layer B can be in the stable area of FeO,

Cr2O3and Ni3S2(area B in Fig. 14). ThePO2 andPS2 in the

third layer [layer C in Fig. 10(c)] can be in the stable area of

Ni, Fe and Cr2O3 (area C in Fig. 14) because the layer

consisted of Ni, Fe and Cr2O3. Since the layer D in Fig. 10(c)

consisted of Ni and Fe with CrS particles, thePO₂andPS₂can be in the stable area of Ni, Fe and CrS (area D in Fig. 14). The PO₂ andPS₂ in the layer E of Fig. 10(c) can be in the stable area of Cr, Ni and Fe (area E in Fig. 14) because the

layer was non-corroded Hastelloy-XR alloy. Since the PO₂

and PS₂ in the corrosion scale may continuously decrease

from the outermost to inside, thePO2andPS2can be possibly

changed along the broken line from point A to E in Fig. 14. ThePS2at the outermost of corrosion scale can be higher than

the initial S2 partial pressure. Since O2 may be consumed

significantly at the outermost layer, PS2 may be relatively

increased more than the initialPS2. Therefore, the calculated

potential diagrams could be applicable to understand the

corrosion behavior in Ar–SO2for Hastelloy-XR alloy.

4. Conclusions

The corrosion behavior of Hastelloy-XR alloy in O2 and

SO2atmospheres were investigated in the temperature range

between 1073 and 1273 K. In Ar–O2 atmosphere, the mass

changes mainly obeyed a linear-parabolic law. The corrosion scales had a double-layer structure of inner Cr2O3and outer

Mn1:5Cr1:5O4spinel layers. The surface morphology of scales

changed from island-like to buckled and to smooth porous layers with decreasingPO2 from 10 to 0.01 kPa. In Ar–SO2

atmosphere, the mass changes obeyed a linear-parabolic law. The corrosion scales formed at 1073 and 1173 K were similar to the scales formed in Ar–O2atmosphere at 1073 K, because

the oxidation was dominant at these low temperatures. At

1273 K and PSO₂ <0:5kPa, the corrosion scales were

consisted of multi-layers changing from Fe3O4 with Ni3S2

particles (outmost), FeCr2O4, Cr2O3, and CrS particles

(innermost). The sulfidation became more significant in less

PSO₂at 1273 K. The thermodynamic potential diagrams may

be useful to understand the corrosion behavior of

Hastelloy-XR alloy in Ar–SO2 atmosphere.

Acknowledgments

A part of this study was supported by Tokyu Foundation for Inbound Student. The authors thank Mr. Y. Murakami of Laboratory for Advanced Materials, Institute for Materials Research for EPMA analyses.

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E

D C

B A

P

Fe2(SO4)3(s)

FeSO4(s)

Fe(s)

FeO(s) FeS(s)

Fe

Cr(s)

Cr2O3(s)

Cr2(SO4)3(s)

CrS(s) Cr3S4(s)

Cr

Fe2O3(s)

Fe3O4(s)

NiS(s)

Ni3S2(l)

Ni(s) NiO(s) NiSO4(s)

Ni 2 -3 -8 -13 -18 -23

-33 -28 -23 -18 -13 -8

log(

PS2

/ kPa)

-3 2 log(PO2 / kPa)

5) Y. Imai, S. Mizuta and H. Nakauchi: Boshoku Gijutsu35(1986) 230– 240.

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17) N. Birks and G. H. Meier:Introduction to High Temperature Oxidation of Metals, Edward Arnold (publishers) Ltd., London, 1983, pp. 59–60. 18) C. S. Giggins and F. S. Pettit: Trans. Metall. AIME245(1969) 2495–

2507.

19) P. Hancock:1st Int. Congr. Metall. Corros., Butterworths, London (1961) pp. 193–198.

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