The Effect of Environmental Sulfur on the Establishment and Structural ...

Oxid Met (2013) 80:517–527 DOI 10.1007/s11085-013-9444-5 ORIGINAL PAPER

The Effect of Environmental Sulfur on the Establishment and Structural Stability of Alumina Scales Xu Liu • Brian Gleeson

Received: 25 May 2012 / Published online: 6 September 2013 Ó Springer Science+Business Media New York 2013

Abstract Single-phase c-Ni of composition (in at.%) Ni–6.3Al–5.4Cr is borderline between forming Al2O3 internally or externally. Oxidation of this alloy in air and O2 ? 1 %SO2 was carried out at 1,000 °C for 20 h. In air, the alloy oxidized in a mixed mode, with regions forming a non-protective product of internal Al2O3/ NiAl2O4 and external NiO. When oxidized in O2 ? 1 %SO2, the alloy formed a continuous Al2O3 scale. Thus, a small amount of sulfur in the atmosphere promoted the transition from internal to external Al2O3-scale formation. In a parallel study, single-phase c0 -Ni3Al of composition (in at.%) Ni–5Cr–20Al–3Pt–0.1Hf–0.05Y was oxidized at 900 °C for 20 h in air, O2 ? 0.1 %SO2 and in air with an Na2SO4 deposit. For all conditions, external alumina scales were established. Metastable –-Al2O3 formed when oxidation took place in air alone, whereas the stable a-Al2O3 O formed during oxidation in O2 ? 0.1 %SO2 and in air with an Na2SO4 deposit. Thus, sulfur from the salt deposit or gas atmosphere promoted the O –-Al2O3 ? a-Al2O3 transformation. Keywords Ni–Cr–Al alloys  Sulfur  Alumina scale  Scale establishment  Phase transformation

Introduction Sulfur is found in a number of applications in which alloys and metallic coatings are exposed to high temperatures. In the case of hot corrosion, sulfur is primarily found X. Liu (&)  B. Gleeson Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, USA e-mail: [email protected] B. Gleeson e-mail: [email protected]

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in the Na2SO4-based deposit. Environmental sources of sulfur may be at elevated levels due to the fuel source being of a low grade or the service environment having a relatively high SO2 and/or sulfate content. A great deal of research has focused on the high-temperature corrosion of metals and alloys in oxidizing-sulfidizing environments [1–3]. Much of this research has been reviewed by Gleeson [4], Gesmundo et al. [5], Stroosnijder and Quadakkers [6], Stringer [7], Grabke et al. [8], and others [9–11]. Most of these studies were done under high Pso2 conditions, in which sulfide usually formed during the reaction process. Very limited work has dealt with oxidation in low Pso2 conditions [12, 13]. Moreover, past research was concerned primarily with the formation and behavior of Cr2O3 in sulfur-containing gases [1]. Only a few studies investigated the formation and growth behavior of Al2O3 in environments containing sulfur. The main focus of those studies was the effect of sulfur on scale adhesion—a detrimental effect that is well-known when the alloy contains sulfur as an impurity [14]. Kubena et al. [15] studied the effect of environmental sulfur on the cyclic oxidation resistance of alumina-scale forming nickel-base superalloys. Their results showed that the presence of sulfur decreases oxide-scale adhesion on alloys having no reactive elements. However, the detrimental effects of sulfur on scale adhesion could be mitigated by adding reactive elements to the alloy, particularly yttrium. A survey of the open literature suggests that the effects of low SO2 on the formation and performance of Al2O3 scales have not been systematically investigated. Usually, an alloy or coating exposed to a high-sulfur service environment relies on the formation of a continuous and intact oxide scale to provide protection. Therefore, understanding the impact of sulfur on the establishment and maintenance of an alumina scale is of considerable importance. In this study, two sets of experiments were conducted with a focus on two different applications. One application represented oxidation in SO2-polluted air while the other was prototypical of Type I hot corrosion. The effects of environmental sulfur on the establishment and structural stabilities associated with alumina-scale formation were assessed and elucidated. Representative results stemming from a more detailed study are presented in this paper.

Experimental Procedures Sample Preparation Two model alloys and three commercial alloys were tested in this investigation. The nominal compositions of the alloys (in wt%) are listed in Table 1. For completeness, the compositions of the model alloys in atomic percent are given in Table 2. For the Ni–Cr–Al model alloy A, a 40 g ingot was prepared by Ar arc-melting followed by drop-casting to form a 25 mm diameter rod. The cast alloy was cut into 5 mm thick disks and then given a preliminary annealing in vacuum at 1,000 °C for 48 h. Each disk was cold rolled to a thickness of 1 mm and subsequently annealed in vacuum at 1,000 °C for 48 h to arrive at a stabilized structure. The disks were

123

Bal.

Bal.

Bal.

Bal.

Commercial C

PWA 1487

602CAa

2.1

5.6

4.9

9.6

3

Al

25.3

5

8.1

4.6

5

Cr

–

–

–

10.4

–

Pt

–

10

9.1

–

–

Co

–

2

0.03

–

–

Mo

a

Wrought alloy

All compositions are nominal except for alloy C, which is measured

Bal.

Model B

Ni

Commercial alloys (wt%)

Model A

Alloy

Table 1 Composition of the alloys studied

–

6

11.7

–

–

W

–

8.7

0.3

–

–

Ta

0.2

0

2.8

–

–

Ti

–

0.1

1.2

0.3

–

Hf

–

3

–

–

Re

0.07

0.01

0.08

–

Y

0.008

–

–

–

–

P

9.6

–

0.09

–

–

Fe

0.2

–

–

–

–

C

0.003

–

–

–

–

S

0.08

–

0.06

–

–

Zr

0.9

–

–

Nb

Oxid Met (2013) 80:517–527 519

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Table 2 Nominal composition of the model alloys studied Alloy

Composition (at.%) Ni

Al

Cr

Model A

Bal.

6.3

5.4

Model B

Bal.

20

5

Pt

Hf

Y

3.0

0.1

0.05

polished to a 600-grit finish. This sample preparation procedure was the same as that conducted by Giggins and Pettit [16] in their study to determine an Ni–Cr–Al oxidation map. For model alloy B, a 40 g ingot was prepared by Ar-arc melting followed by drop-casting to form a 10 mm diameter rod. The As-cast rod was homogenized in vacuum at 1,200 °C for 48 h and then cut into 1 mm thick disks. Before testing, the sample disks were polished to a 1,200-grit finish and then ultrasonically cleaned in acetone. Commercial alloy C is cast and polycrystalline, and it is comparable in composition to MAR-M200. Single-crystal bar of PWA 1487 was provided by Pratt and Whitney, while a wrought plate of 602CA was received from Krupp VDM Technology Corp. The commercial alloys were polished to a 1,200-grit finish prior to testing.

Oxidation Tests Oxidation tests in air were carried out in either a horizontal resistance-heated tube furnace or in a TAG 24 thermobalance (SETARAM instrumentation) with a mass resolution of 0.01 mg. In the TAG, a given test sample was hung from a sapphire hook in the furnace. Oxidation tests in air with Na2SO4 deposit were carried out in a box furnace. The samples were pre-deposited with about 2 mg/cm2 Na2SO4 and then exposed at 900 °C for 20 h. Salt deposition was done by heating the samples to around 150 °C and then spraying a saturated solution of Na2SO4 onto one side of each sample. The water evaporated to leave a uniform Na2SO4 deposit on the sample surface. Exposures commenced by placing the salt-deposited samples (deposited side facing up) into a furnace that was pre-heated to 900 °C. Exposure tests in the O2–SO2 atmosphere were carried out in a horizontal resistance-heated tube furnace. Before testing, the samples were kept near the end of the furnace tube while the reaction was heated to the desired test temperature. The temperature of the samples during this heating period never exceeded *170 °C. The furnace tube was first flushed with pure Ar for 2 h and then overnight with the O2 ? 0.1 %SO2 or O2 ? 1 %SO2 reactant gas. The reactant gas passed through a platinized catalyst prior to reaching the hot zone. The calculated equilibrium gas compositions are shown in Table 3. After following this procedure to establish the reacting atmosphere, the samples were pushed into the hot zone of the furnace and continuously exposed for a fixed amount of time. During oxidation, the gas flow rate

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Table 3 Equilibrium gas compositions in the furnace Flowing gas

Temperature (°C)

Equilibrium gas composition

O2 ? 0.1 %SO2

900

PS2 ¼ 3:8  1011 Pa;

PO2 ¼ 1:0  106 Pa;

Pso3 ¼ 25 Pa;

Pso2 ¼ 76 Pa O2 ? 1 %SO2

1,000

PS2 ¼ 7:9  109 Pa;

PO2 ¼ 1:0  106 Pa;

Pso3 ¼ 130 Pa;

Pso2 ¼ 875 Pa

of the reactant gases was 12.5 cm3/min, which corresponded to a linear velocity of 0.62 cm/min. Characterizations The exposed samples were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The SEM was used for both surface and crosssectional analyses. The SEM used was a Philips XL-30 Field Emission Gun microscope equipped with secondary electron (SE), backscatter electron (BSE), and X-ray detectors for energy dispersive spectroscopy (EDS) analysis. A Philips X’Pert diffractometer capable of grazing incidence XRD (GIXRD) analysis was used for phase identification. The incident angle was fixed at 5° for the GIXRD. For the sample oxidized with an Na2SO4 deposit, residual Na2SO4 was carefully removed by dissolving in water. Oxide scales were maintained intact for characterization.

Results and Discussion Alumina-Scale Establishment in a Sulfur-Bearing Oxidizing Environment The model A alloy (single-phase c-Ni) is borderline between forming alumina internally or externally [17]. Oxidation of this alloy in air and O2 ? 1 %SO2 was carried out at 1,000 °C for 20 h. Resulting cross-sectional SEM images are shown in Fig. 1. When exposed to air, the alloy oxidized in a mixed mode, with the majority of regions forming a non-protective product of internal Al2O3/NiAl2O4 and

Al2O3 scale NiO

Internal oxide in air

50μm

in O2+1%SO2

50μm

Fig. 1 Ni–Cr–Al alloy after 20 h oxidation at 1,000 °C in air [17] and in O2 ? 1 %SO2

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external NiO. The remaining regions formed Al2O3 scale. However, when exposed to O2 ? 1 %SO2, the alloy formed a continuous scale layer of Al2O3. Thus, a small amount of sulfur in the atmosphere promoted the transition from internal to external Al2O3 formation. Strictly speaking, three differences exist between exposures to air and O2 ? 1 %SO2: the presence of N2 in air, the oxygen concentration, and the presence or absence of SO2. Nitrogen was not considered to be the reason for the observed differences in oxidation behavior because nitrides had not formed and so oxygen was the only oxidant during exposure to air. The very small difference in oxygen concentration is also not considered to be the reason because the oxygen supply in both environments was sufficient to allow for the formation and growth of all possible oxides. Thus, it is inferred that the presence of sulfur in the environment was the reason for the observed differences. Specifically, sulfur promoted the transition from internal to external Al2O3 formation. A similar behavior was observed for the commercial alloys (5–8 wt% Cr) assessed in this study. Figure 2 shows cross-sectional SEM images of commercial alloy C after 20 h oxidation at 1,000 °C. When oxidized in air, an internal zone of discrete Al2O3 particles had formed; while oxidation in O2 ? 1 %SO2 caused a continuous inner scale of Al2O3 to develop. A further example is provided in Fig. 3, which shows cross-sectional SEM images of PWA1487 after 100 h oxidation at 900 °C. When oxidized in air, an internal Al2O3 layer had formed; while oxidation in O2 ? 1 %SO2 led to the eventual establishment of a continuous Al2O3 scale. These results confirm that a small amount of sulfur in the atmosphere can promote the transition from internal to external Al2O3 formation. A similar beneficial effect of trace environmental sulfur has been reported by others [18, 19]. Those authors concluded that sulfur in the atmosphere chemisorbs on the alloy surfaces and blocks the sites required for decomposition of the oxygen molecules responsible for oxidation. The chemisorbed sulfur effectively decreases oxygen entry into the alloy and thus reduces the permeability of oxygen into the alloy. This decrease in oxygen permeability would have the effect of decreasing the critical aluminum concentration required for forming an external scale. An in-depth fundamental study on this effect was recently carried out and the results will be reported soon [20]. This apparent beneficial sulfur effect seems to depend on the relative amounts of Cr and Al in the alloy. For commercial wrought alloys with a relatively large Cr Cr2 O3

Cr2O3

Al 2O3

Internal Al2O3

in air

10μm

in O2+1%SO2

Fig. 2 Commercial alloy C after 20 h oxidation at 1,000 °C in air and in O2 ? 1 %SO2

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10μm

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523

Cr2O3+NiO

NiO

Al2O3 internal oxide

in air

Al2O3 internal scale

10μm

in O2+1%SO2

10μm

Fig. 3 PWA1487 after 100 h oxidation at 900 °C in air and in O2 ? 1 %SO2

Cr2O3 Cr2O3 NiAl2O4 Internal Al2O3

a

10μm

b

Internal Al2O3

10μm

Fig. 4 602CA after 20 h oxidation at 1,000 °C in air (a) and in O2 ? 1 %SO2 (b)

content ([20 wt%), a small amount of sulfur in the atmosphere tended to promote the penetration of internal oxide. Figure 4 provides an example of this, showing cross-sectional SEM images of 602CA after 20 h oxidation at 1,000 °C. When oxidized in air and in O2 ? 1 %SO2, the alloy formed a product of internal Al2O3/ NiAl2O4 and external Cr2O3. However, the internal oxide formed far deeper into the alloy exposed to O2 ? 1 %SO2 than to air. The reason for this was initially conjectured to be that sulfur penetrates through the scale to form internal sulfides. These internal sulfides were considered to act as fast diffusion paths for oxygen, thus allowing for the relatively deep formation of internal oxides. However, internal sulfides were not detected in the 602CA exposed to O2 ? 1 %SO2. The reason for the enhanced internal oxidation in 602CA alloys during oxidation in O2 ? 1 %SO2 is still not clear. Alumina-Scale Stabilities in the Presence of Sulfur Oxidation in Air Figure 5 shows the 900 °C oxidation kinetics of the model alloy B (single-phase c0 Ni3Al) exposed to air for up to 100 h. A fitting model [17, 21] of the weight gain as a function of time (Fig. 5a) was used to determine the nature of the measured

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a

b

c

Fig. 5 Kinetics analyses for the determination of kp

kinetics. Specifically, the instantaneous time exponent was determined using the following equation: n¼

oðlogDmÞ oðlog tÞ

ð1Þ

When the oxide scale growth rate is parabolic, n = 0.5. The calculated instantaneous n-values are plotted as function of time in Fig. 5b, where it is seen that n attained a value of 0.5 after a certain initial transient. Due to noise effects associated with the weight-change measurements, fluctuations around n = 0.5 were observed, but these fluctuations were not significant, i.e., n = 0.5 ± 0.03. From the time at which instantaneous n reaches 0.5, the growth rate is considered to be steady-state and parabolic [10]. Accordingly, the instantaneous parabolic rate constants kp were calculated for this period based on the following basic expression: Dm ¼ ðkp tÞ0:5

ð2Þ

Figure 5c plots the instantaneous kp values as a function of the square root of time. An average value of 3.5 ± 0.4 9 10-13 g2/cm4s is calculated from this plot. As shown in Fig. 6, this value is in good agreement with what has been reported for –-Al2O3-scale growth [22]. O The scale phases formed on each oxidized alloy was determined using GIXRD. The diffraction patterns obtained from samples after 20 h oxidation are shown in Fig. 7a. Although some a-Al2O3 peaks can be seen, the predominant peaks are from –-Al2O3, which is in agreement with the inference from the measured kinetics that O

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525

Fig. 6 Arrhenius plots of measured and literature reported kp values for alumina scales [22]

a

γ'-Ni3Al

b

-Al2O3 α-Al2O3

5μm

Fig. 7 a GIXRD and b surface morphology of mode B alloy after 20 h oxidation at 900 °C in air

the scale is primarily O –-Al2O3. Figure 7b shows the surface morphology of a sample after 20 h oxidation. The whisker and needle-like morphologies on the outer surface of the alumina scales are indicative of O –-Al2O3 [22, 23]. Oxidation in Air with Na2SO4 Present A sample of model alloy B with an Na2SO surface deposit was oxidized 20 h at 900 °C in air and the resulting surface morphology and XRD spectrum are shown in Fig. 8. The alumina scale formed is continuous, dense and free of any needles. X-ray analysis identified the scale to be a-Al2O3, as indicated in Fig. 8a, which shows diffraction peaks from c0 -Ni3Al (underlying substrate) and a-Al2O3 (scale). – ? a-Al2O3 transition. Therefore, Na2SO4 deposit apparently promoted the O Oxidation in O2 ? 0.1 %SO2 –?a To assess whether sulfur in the Na2SO4 played a role in accelerating the O transformation, exposure was done in an O2 ? SO2 atmosphere. Specifically, model

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b

a γ'-Ni3 Al α-Al2O3

10μ μm

Fig. 8 a X-ray analyses and b surface morphology of mode B alloy after 20 h oxidation at 900 °C in air with the presence of Na2SO4 deposit

a

γ'-Ni3Al

b

α-Al2O3

10μ μm

Fig. 9 a GIXRD and b surface morphology of mode B alloy after 20 h oxidation at 900 °C in O2 ? 0.1 %SO2 atmosphere

alloy B was oxidized in an O2 ? 0.1 %SO2 atmosphere at 900 °C for 20 h. Figure 9 shows the resulting GIXRD spectrum and surface morphology. The scale surface is rather granular in structure and was identified to be a-Al2O3. Thus, sulfur from the salt deposit or gas atmosphere promoted the O –-Al2O3 ? a-Al2O3 transformation. This could be possibly explained based on a consideration of the energetics associated with a nucleation in preference to O –. The O – ? a transformation is diffusional in nature [24], so that interfacial energy plays a critical role in affecting the competitive nucleation behavior. Specifically, the presence of sulfur may decrease the interfacial energy of a relative to h to the extent that nucleation of the former is favored. Alternatively, or in addition, the preferential manner by which sulfur chemisorbs on an alloy surface will tend to poison reaction sites and, hence, kinetically hinder any oxidation reaction. One manifestation of this effect is a decrease in the critical concentration of Al in an alloy for Al2O3 scale formation, as was observed in this study (Fig. 1). It is proposed that a further manifestation is a hindrance in the formation rate of the normally faster-growing O –-Al2O3 to the extent that it is easier for the thermodynamically more stable a-Al2O3 to establish itself. The overall effect would be an apparent promotion of the O – ? a transformation. Moreover, this interpretation suggests that a relatively low PO2 should favor the O – ? a transformation, an effect that needs to be verified.

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Conclusions A small amount of sulfur in the atmosphere can promote the transition from internal to external Al2O3-scale formation. This was shown for a NiCrAl model alloy and the commercial cast superalloy PWA 1487. However, whether sulfur has a beneficial effect appears to depend on the relative amounts of Cr and Al in the alloy. For the commercial wrought alloy 602CA (*25 wt% Cr), the presence of sulfur in the atmosphere increases the depth of internal Al2O3 formation. Sulfur from the salt –-Al2O3 ? a-Al2O3 transformation. The deposit or gas atmosphere promoted the O mechanistic reason for this effect is still a being assessed, but our current interpretation favors chemisorbed sulfur kinetically hindering the rate of O – formation and, consequently, biasing establishment of the thermodynamically more stable a. Acknowledgments This research was supported by the US Office of Naval Research, award N00001409-1-1127 and managed by Dr. David Shifler. The authors would also like to thank Dr. Wei Zhao for his help with the kinetics analyses.

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.

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