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Corrosion behavior of alumina ceramics in aqueous HCl and H2SO4 solutions
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Corrosion Science 50 (2008) 872–878 www.elsevier.com/locate/corsci
Corrosion behavior of alumina ceramics in aqueous HCl and H2SO4 solutions ´ urkovic´ a,*, Mirjana Fuduric´ Jelacˇa b, Stanislav Kurajica c Lidija C a
Faculty of Mechanical Engineering and Naval Architecture, Ivana Lucˇic´a 5, Zagreb, Croatia b College for Safety, Ivana Lucˇic´a 5, Croatia c Faculty of Chemical Engineering and Technology, Marulic´ev trg 19, Zagreb, Croatia Received 1 August 2007; accepted 10 October 2007 Available online 17 November 2007
Abstract The corrosion behavior of cold isostatically pressed (CIP) high purity alumina ceramics in aqueous HCl and H2SO4 solutions with various concentrations has been studied simultaneously at room temperature (25 °C). Corrosion tests were also performed with 0.65 mol/ l HCl and 0.37 mol/l H2SO4 solutions at 40, 55 and 70 °C for 48 h. Chemical stability was monitored by determining the amount of Al3+, Mg2+, Ca2+, Na+ Si4+ and Fe3+ ions eluted in different concentrations of HCl and H2SO4 solutions by means of atomic absorption spectrometry (AAS). By increasing the concentration from 0.37 to 6.5 mol/l, it was notified that the corrosion susceptibility in HCl and H2SO4 solutions for the CIP alumina specimens at room temperature decreases. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramic; B. Acid corrosion; B. Kinetic parameters
1. Introduction Aluminium oxide, Al2O3, more often referred to as alumina, is an exceptionally important ceramic material, which has many technological applications [1,2]. It has several special properties like high hardness, chemical inertness, wear resistance and a high melting point. Alumina ceramics can retain up to 90% of their strength even at 1100 °C. Because of the excellent properties of alumina ceramics, they are widely used in many refractory materials, grinding media, cutting tools, high temperature bearings, a wide variety of mechanical parts, and critical components in chemical process environments, where materials are subjected to aggressive chemical attack, increasingly higher temperatures, and pressures. Alumina has different phases, of which a-alumina is a stable phase. Besides the thermodynamically stable a-
*
Corresponding author. Tel.: +385 1 61 68 313; fax: +385 1 61 57 126. ´ urkovic´). E-mail address: [email protected] (L. C
0010-938X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2007.10.008
Al2O3 or corundum, there are many metastable structures of alumina, like c-Al2O3, which is widely used as a catalyst. Corundum or a-alumina is described by a hexagonal unit cell (Fig. 1) [1,2]. Aluminium oxide, i.e., alumina, is the main component of bauxite, the principal ore of aluminium. The largest manufacturers in the world of alumina are Alcoa, Alcan and Rusal. Companies which specialise in the production of speciality aluminium oxides and aluminium hydroxides include Alcan and Almatis. Modern technical ceramics offer promising alternatives to many existing materials of construction (e.g., metals and polymers) in chemical contacting applications, where corrosion properties are paramount. The corrosion behavior in various environments is a limiting factor for the lifetime of such ceramic components. Two main types of chemical resistance of ceramics are: acid resistance and alkali resistance. The study of corrosion resistance of ceramics in acids and alkalis makes it possible to predict their service properties in these media. Although ceramics are generally more stable in corrosive environments than
L. C´urkovic´ et al. / Corrosion Science 50 (2008) 872–878
Fig. 1. Corundum structure of alpha-alumina (a-Al2O3) [4].
common metallic materials, it is important to investigate the chemical resistance under highly corrosive conditions, such as strong acids, long-term exposure and elevated temperatures. In comparison to metals, ceramic materials may be considered resistant to corrosion because their corrosion rates are a great deal lower than those of metals. There are significant differences in the types of corrosion and the corrosion resistance between different groups of ceramic materials (silicate, oxide and non-oxide ceramic). While corrosion in metals is an electrochemical process, the solubility of the material is the determining factor in the level of ceramic corrosion. The chemical composition and the microstructure are key factors in low corrosion rates [3]. Many papers have been published on the corrosion of alumina ceramics under various conditions [3–14]. In hydrothermal condition, [6] the corrosion behavior of alumina ceramics with 99%, 99.9% and 99.99% Al2O3 is attacked by the dissolution of grain boundary impurities (Si, Ca and Na), and the corrosion weight loss for Al2O3 after corrosion decreased linearly with decreasing impurity level. Schachat et al. [7] reported the corrosion resistance of alumina ceramics in aqueous acidic solution (HCl, H2SO4, H3PO4) under hydrothermal conditions. The authors found that the corrosive effect of the acids on alumina ceramics decreases in the order H3PO4 > HCl > H2SO4. In this work, the dominant corrosion mechanisms were identified as intergranular attack and dissolution of Al2O3. Mikeska et al. [8] investigated the corrosion resistance of ceramics in aqueous HF acid. The authors found that corrosion in the case of polycrystalline alumina ceramics with 99.9% Al2O3 occurred primarily at the grain boundaries. Conversely, single-crystal sapphire (Al2O3) did not corrode and showed good resistance to the HF acid. Tuurna et al. [9] investigated corrosion resistance of porous alumina ceramics in acetic acid solution, Genthe and Hausner [11] investigated corrosion of alumina in acids and caustic solutions. These studies show that the major cause of the weight changes in corroded alumina ceramics is additives or impurities such as MgO, CaO, SiO2 and
873
Na2O which exist at grain boundaries. Sato et al. [12] reported the corrosion behavior of alumina ceramics in caustic alkaline solution at 150 and 200 °C. It was shown that the fracture strength of corroded alumina ceramics decreased as the degree of dissolution of alumina increased. Yue et al. [13] found that after the exposure of alumina single crystal at 1700–2000 °C to corrosive atmospheres (argon, argon/water vapour, air, and air/water vapour), there were no obvious weight and volume changes. In these papers, [3–14] the monitoring of corrosion in aqueous solutions is performed based on the measurement of the total weight loss specimens and/or measurement of eluted ions and/or strength change after immersion into a fixed volume of corrosive media. The measurement of eluted ions can determine the chemical stability more precisely than the measurement of the total weight loss specimens. Therefore, we did not measure the weight loss but the elution of elements after immersion in different concentrations of HCl and H2SO4 acids. In the present work, the corrosion behavior of high purity alumina was investigated at room temperature for 10 days in HCl and H2SO4 solutions of different concentrations. The corrosion of alumina ceramics in HCl and H2SO4 solutions is determined by the solubility of alumina and the solubility of grain-boundary impurities. This study shows that the major cause of eluate chemistry in corroded alumina ceramics is additives or impurities, such as MgO, CaO, SiO2 and Na2O, which exist at grain boundaries. 2. Materials and methods The material used in corrosion tests in HCl and H2SO4 solutions was a cold isostatically pressed (CIP)-Al2O3 with 99.8% purity. The chemical composition of investigated alumina ceramics is shown in Table 1. The CIP-Al2O3 specimens were supplied by Applied Ceramics, Inc., Fremont, California, USA. The Al2O3 ceramic contains MgO as sintering aid and the usual impurities, i.e., SiO2, CaO, Na2O and Fe2O3. The specimens were rectangular coupons; their size was 0.8 cm 1.0 cm 2.0 cm. Archimedes density of the CIP-Al2O3 was 3.91 g/cm3. Each surface of Al2O3 specimens was polished to 1.5 lm. After polishing and before corrosion tests, samples were thoroughly cleaned with alcohol and dried in the sterilizer at 150 ± 5 °C for 4 h. The specimens were first put into a sealed polypropylene (PP) tube with 5 ml of HCl and then with 5 ml of H2SO4 in two separate tests. The HCl solution concentrations were 0.65 mol/l (2 wt%), 3.25 mol/l (10 wt%) and 6.5 mol/l (20 wt%). The H2SO4 solution concentrations were 0.37 mol/l (2 wt%), 1.85 mol/l (10 wt%) Table 1 Chemical composition of the Al2O3 ceramics Sample
Alumina ceramics
wt% MgO
Fe2O3
SiO2
Na2O
CaO
Al2O3
0.066
0.015
0.02
0.05
0.013
Rest
L. C´urkovic´ et al. / Corrosion Science 50 (2008) 872–878
874
and 3.7 mol/l (20 wt%). The static corrosion tests were carried out simultaneously on five samples at room temperature (25 °C) from 24 to 240 h. The measurements were conducted after 24, 48, 60, 72, 84 and 240 h of immersion. Few tests were performed with 0.65 mol/l (2 wt%) HCl and 0.37 mol/l (2 wt%) H2SO4 at 40, 55 and 70 °C for 48 h. After the planned exposure time, the specimens were removed from the tubes, rinsed with boiling distilled water, and dried in a warm oven heated at 150 °C. The determination of the amount of Al3+, Mg2+, Ca2+, Na+ Si4+ and Fe3+ ions eluted in the corrosive solutions was carried out by means of atomic absorption spectrophotometry (AAS, AA-6800, Shimadzu). Obtained results are expressed as the amount of eluted ions (Mn+) in lg per
square centimetre of test alumina area (lg Mn+/cm2). All data were an average of five values. 3. Results and discussion The correlation between the amount of eluted ions of Al2O3 ceramics after immersion in different concentrations of HCl and H2SO4 solutions and the immersion time are plotted in Figs. 2 and 3. These results show that eluates after the corrosion process contain ions of Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+. These results indicate that impurities played an important role in the corrosion process of alumina ceramics [6–12]. On the basis of the literature data, the segregation of Si4+, Na+, Ca2+ and Mg2+ to the grain boundaries will
4 3 2
0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
1
μg Mg2+/ cm2
μg Al3+/ cm2
0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
0.6
5
0
0.4 0.2 0
0
50
100
150 t, h
200
250
0
2.5
4
0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
1
μg Na+/ cm2
μg Ca2+/ cm2
2
100
150 t, h
200
250
200
250
0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
2 3
50
1.5 1 0.5 0
0 0
50
100
150 t, h
200
0
250
2
50
100
150 t, h
0.3
μg Fe3+/ cm2
μg Si4+/ cm2
0.25 1.5 1 0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
0.5
0.2 0.15 0.1 0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
0.05 0
0 0
50
100
150 t, h
200
250
0
50
100
150 t, h
200
250
Fig. 2. The amount of eluted ions of Al3+ (A), Mg2+ (B), Ca2+ (C), Na+ (D), Si4+ (E) and Fe3+ ions (F) from Al2O3 ceramics in different concentrations of HCl solution.
L. C´urkovic´ et al. / Corrosion Science 50 (2008) 872–878
0.35
7
5 4 3 2
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
1 0
1.5
0.14 0.07
50
100
150 t, h
200
250
0
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
1.2
2 0.9 0.6 0.3
50
100
150 t, h
200
250
200
250
200
250
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
2.5
μg Na+/ cm2
μg Ca2+/ cm2
0.21
0
0
1.5 1 0.5 0
0 0
50
100
150 t, h
200
0
250
1.5
0.4
1.2
0.3
0.9
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
0.6
μg Fe3+/ cm2
μg Si4+/ cm2
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
0.28 μg Mg2+/ cm2
μg Al3+/ cm2
6
875
50
100
150 t, h
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
0.2 0.1
0.3 0
0 0
50
100
150
200
0
250
50
t, h 3+
Fig. 3. The amount of eluted ions of Al H2SO4 solution.
100 150 t, h
(A), Mg2+ (B), Ca2+ (C), Na+ (D), Si4+ (E) and Fe3+ ions (F) from Al2O3 ceramics in different concentrations of
take place, but their concentrations at the grain boundaries are not sufficient for the formation of the grain boundary phase [15–19]. The distribution of impurities is specified by the solubility of cations in the alumina lattice. The solubility depends on the differences in the charge and the ionic radius of impurities (Mg2+, Ca2+, Si4+, Fe3+ and Na+) and Al3+. Table 2 presents the ionic radius of Mg2+, Ca2+, Si4+, Fe3+, Na+ and Al3+ [7]. By comparing the charge and the ionic radius of impurities and Al3+, it is clear that Ca2+ possesses the smallest solubility. If the solubility limits of the cations in Al2O3 are exceeded, they segregate to the grain boundaries of the ceramic materials [15–19].
Table 2 Ionic radius of Mg2+, Ca2+, Si4+, Na+, Fe3+ and Al3+[7] Ionic radius
Al3+
Mg2+
Ca2+
Na+
Si4+
Fe3+
pm
57
79
106
98
26
67
The corrosion of alumina ceramics in HCl and H2SO4 solutions is determined by the solubility of alumina and the solubility of grain-boundary impurities. Obtained results (Figs. 2 and 3) indicate that the corrosion of alumina ceramics was mainly attributed to the dissolution of MgO, SiO2, CaO, Na2O and Fe2O3 grain-boundary impu-
L. C´urkovic´ et al. / Corrosion Science 50 (2008) 872–878
876
rities. The dissolution of Al3+ in bulk material (Al2O3) is negligible. Total amount of eluted ions from high purity Al2O3 ceramics after immersion in different concentrations of HCl and H2SO4 solutions at room temperature are shown in Figs. 4A and B. By comparing the results presented in Fig 4A and B, it was noted that with increasing concentrations of acids, the dissolution of alumina ceramic decreases. The highest corrosion rate is noted at the lowest acid concentration (0.65 mol/l% of HCl and 0.37 mol/l H2SO4). Also, these tests indicated that the corrosion rate is the highest in the first 24 h, and after that it decreases with the exposure time. In general, the corrosion susceptibility (total amount of eluted ions) of the Al2O3 ceramics increased by increasing the corrosion time. For this reason, a curve which presents the amount of eluted ions vs. time (which is a measure of corroded material) has a parabolic shape. Therefore, CIP-Al2O3 ceramics corrosion kinetics can be put in form [3] X
lg Mnþ =cm2
2
¼ Kp t
16 0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
14
240
10
0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
200 (∑μg Mn+ / cm2)2
+
Σ μg Mn / cm2
12
where Kp is the parabolic corrosion rate constant in lg2 cm4 s1, t, s is the time of immersion in seconds, P lgMnþ =cm2 is the sum of the amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+) per square centimetre in lg cm2. The square of the total amount of eluted ions vs. the corrosion time for different concentrations of HCl and H2SO4 solutions are plotted in Fig. 5A and B. This correlation is linear with the correlation coefficient R2 = 0.98 for 0.65 mol/l HCl, R2 = 0.98 for 3.25 mol/l HCl and R2 = 0.97 for 6.50 mol/l HCl, R2 = 0.98 for 0.37 mol/l H2SO4, R2 = 0.96 for 1.85 mol/l H2SO4 and R2 = 0.96 for 3.70 mol/l H2SO4. The parabolic corrosion rate constants were evaluated from the slopes. The corresponding values of the parabolic corrosion rate constants are given in Table 3. The values of the parabolic corrosion rate constants are higher for all concentrations of HCl solutions in comparison with the same concentrations of H2SO4 solutions. Also, these values are higher for 0.65 mol/l HCl and 0.37 mol/l H2SO4 solutions. It indicated that the maximum corrosion susceptibility of Al2O3 ceramics is in the lowest concentration of investigated corrosive media. Total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, 4+ Si and Fe3+) from CIP-Al2O3 ceramics in different concentrations of HCl and H2SO4 solutions after 240 h of
8 6 4 2
160 120 80 40
0 0
50
100
150
200
250
0
t, h
0
0.1
0.2
0.3
14 0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
10
6 4 2 0 50
0.7 0.8
0.9
0.4 0.5 t,Ms
0.6
0.7
0.9
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/lH2SO4
160
8
0
0.6
200
(∑μg Mn+ / cm2)2
Σ μg Mn+/cm2
12
0.4 0.5 t,Ms
100
150
200
250
t, h Fig. 4. Total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+) from Al2O3 ceramics in different concentrations of HCl solution (A) and different concentrations of H2SO4 solution (B).
120 80 40 0 0
0.1
0.2
0.3
0.8
Fig. 5. The square of the total amount of eluted ions vs. the corrosion time for different concentrations of HCl solution (A) and different concentrations of H2SO4 solution (B).
L. C´urkovic´ et al. / Corrosion Science 50 (2008) 872–878 Table 3 The parabolic (Kp) corrosion rate constant for Al2O3 ceramics after immersion in the HCl and H2SO4 solution with different concentrations Kp (lg2 cm4 s1)
0.98 0.98 0.97 0.98 0.96 0.96
2
23.6 10 15.7 105 7.1 105 19.9 105 10.9 105 2.3 105
HCl HCl HCl H2SO4 H2SO4 H2SO4
y = -1.2852x + 6.2177 R2 = 0.9961; 0.65 mol/l HCl
2.30
n+
0.65 mol/l 3.25 mol/l 6.50 mol/l 0.37 mol/l 1.85 mol/l 3.70 mol/l
y = -1.1718x + 5.6994 R2 = 0.9999; 0.35 mol/l H2SO4
R2
5
ln ΣμgM /cm
Concentration of solution (mol/l)
2.50
877
2.10
1.90
corrosion testing are plotted in Fig. 6. The correlation between the total amount of eluted ions showed that the dissolution of CIP-alumina ceramic decreases with increased concentrations of solutions. It can be noticed from the graph that the total amount of eluted ions at all tested concentrations is higher in hydrochloride acid than in sulphuric acid, which is in agreement with the calculated values of the corrosion rate constants (Table 3). Total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, 4+ Si and Fe3+) from Al2O3 ceramics vs. different tempera16 HCl H2SO4
∑μg Mn+ / cm2
14 12 10 8 6 4 0
1
2
3
4
5
6
7
c (HCl, H2SO4), mol/l Fig. 6. Comparison of total amount of eluted ions after 240 h immersion in different concentrations of HCl and H2SO4 solutions.
12.5
2
9.5
Σ μgM /cm
10.5
n+
11.5
0.37 mol/l H2SO4 0.65 mol/l HCl
8.5 7.5 6.5 5.5 290
300
310
320
330
340
350
T, K Fig. 7. Total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+) from Al2O3 ceramics vs. different temperature in 0.65 mol/l HCl and 0.37 mol/l H2SO4 solutions.
0.37 mol/l H2SO4 0.65 mol/l HCl
1.70 2.88
2.98
3.08
3.18
1000/T, K
3.28
3.38
-1
Fig. 8. Arrhenius plot of the total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+) from Al2O3 vs. the inverse temperature for 0.65 mol/l HCl and 0.37 mol/l H2SO4 solutions.
tures in 0.65 mol/l HCl and of 0.37 mol/l H2SO4 are shown in Fig. 7. Corrosion tests of alumina ceramics carried out at 25–70 °C (298–343 K) showed that the dissolution of Al2O3 specimens in HCl and H2SO4 solutions increased by increasing the temperature. Also, dissolution is faster in 0.65 mol/l HCl. The Arrhenius plots of the total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+) vs. the inverse temperature for a 0.37 mol/l H2SO4 solution (shown in Fig. 8) were used for the determination of the activation energies for the corrosion process. The value of the activation energies for the alumina specimens corroded in 0.65 mol/l HCl solution was 10.68 KJ/mol and in 0.37 mol/l H2SO4 solution was 9.74 KJ/mol. 4. Conclusion The corrosion susceptibility in HCl and H2SO4 solutions for CIP-Al2O3 ceramics at room temperature decreases on increasing the concentration from 0.37 to 6.5 mol/l. The obtained results indicate that the strongest corrosive attack occurs in the first stage. After the first 24 h, the amount of eluted Al3+, Mg2+, Ca2+, Na+ Si4+ and Fe3+ ions is considerably lower. Values of corrosion rate constants are higher in HCl solutions of all concentrations than in the relevant concentrations of H2SO4 solutions. Also, values of corrosion rate constants are the highest in both HCl and H2SO4 solutions of the lowest concentration. It proves that the Al2O3 ceramics is more stable in HCl solutions than in H2SO4 solutions and that its stability is lower at lower concentrations of both acids. The activation energy has been determined on the basis of the results obtained at higher temperatures in solutions of 0.65 mol/l hydrochloride and of 0.37 mol/l sulphuric acid, and the determined values were higher in 0.65 mol/l HCl solution. This confirms
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the lower stability of the Al2O3 ceramics in HCl at higher temperatures. According to the determined mass concentration of eluted ions, which belong to the grain boundaries (Mg2+, Ca2+, Na+, Si4+ and Fe3+), and Al3+, which belongs to the grain, the corrosion of alumina ceramics is mainly attributed to the dissolution of MgO, SiO2, CaO, Na2O and Fe2O3 grain-boundary impurities. The dissolution of Al3+ in bulk material (Al2O3) is negligible. Acknowledgements The presented research results were achieved within the scientific project ‘‘Structure and properties of engineering ceramics and ceramic coatings” (Agreement No. 1201201833-1789; 10106-797300-1) supported by the Croatian Ministry of Science, Education and Sport. We thank Matt Sertic from Applied Ceramics, Inc. for providing alumina ceramics samples. References [1] J.S. Reed, Principles of Ceramics Processing, John Wiley & Sons, Inc.,, New York, 1995. [2] W.H. Gitzen, Alumina as a Ceramic Material, The American Ceramic Society, Westerville, OH, USA, 1970. [3] D.E. Clark, K. Zoitos, Corrosion of Glass Ceramics and Ceramic Superconductors, Noyes Publications, NJ, USA, 1992. [4] D.R. Askeland, P.P. Phule´, The Science and Engineering of Materials, fourth ed., Books/Cole-Thomson Learning, USA, 2003. [5] Q. Fang, P.S. Sidky, M.G. Hocking, The effect of corrosion and erosion on ceramic materials, Corros. Sci. 39 (3) (1997) 511–527.
[6] K. Oda, T. Yoshio, Hydrothermal corrosion of alumina ceramics, J. Am. Ceram. Soc. 80 (12) (1997) 3233–3236. [7] M. Schacht, N. Boukis, E. Dinjus, Corrosion of alumina ceramics in acidic aqueous solutions at high temperatures and pressures, J. Mater. Sci. 35 (2000) 6251–6258. [8] K.R. Mikeska, S.J. Bennison, S.L. Grise, Corrosion of ceramics in aqueous hydrofluoric acid, J. Am. Ceram. Soc. 83 (5) (2000) 1160– 1164. [9] S. Tuurna, A.P. Nikkila, T.A. Mantyla, Corrosion resistance of porous alumina ceramics in acetic acid solution, Key Eng. Mater. 206 (2002) 1923–1926. [10] W.P. Tai, T. Watanabe, N.S. Jacobson, High-temperature stability of alumina in argon and argon/water–vapor environments, J. Am. Ceram. Soc. 82 (1999) 245–248. [11] W. Genthe, H. Hausner, Influence of chemical composition on the corrosion of alumina in acids and caustic solutions, J. Eur. Ceram. Soc. 9 (1992) 417–425. [12] T. Sato, S. Sato, A. Okuwaki, S. Tanaka, Corrosion behavior of alumina ceramics in caustic alkaline solutions at high temperatures, J. Am. Ceram. Soc. 74 (12) (1991) 3081–3084. [13] X.M. Yue, G.J. Zhang, T. Watanabe, W.P. Tai, Corrosion behavior of single-crystal alumina in argon, air, and water vapor atmospheres at 1700–2000 °C, J. Am. Ceram. Soc. 82 (9) (1999) 2560–2562. [14] J. Bennison, M.P. Harmer, Swelling of hot-pressed Al2 O3, J. Am. Ceram. Soc. 68 (1985) 591–597. [15] W.D. Kaplan, H. Mullejans, M. Ruhle, Ca segregation to basal surfaces in a-alumina, J. Am. Ceram. Soc. 78 (1995) 2841–2844. [16] S.K. Roy, R.L. Cobel, Solubilities of magnesia, titania, and magnesium titanate in aluminium oxide, J. Am. Ceram. Soc. 1 (1968) 1–6. [17] K.L. Gavrilov, S.J. Bennison, K.R. Mikeska, R. Levi-Setti, Grain boundary chemistry of alumina by high-resolution imaging sims, Acta Mater. 47 (1999) 4031–4039. [18] L. Miller, A. Avishai, W.D. Kaplan, Solubility limit of MgO in Al2 O3 at 1600 °C, J. Am. Ceram. Soc. 89 (1) (2006) 2841–2844. [19] C. Greskovich, J.A. Brewer, Solubility of magnesia in polycrystalline alumina at high temperatures, J. Am. Ceram. Soc. 84 (2) (2001) 420– 425.
Corrosion Science 50 (2008) 872–878 www.elsevier.com/locate/corsci
Corrosion behavior of alumina ceramics in aqueous HCl and H2SO4 solutions ´ urkovic´ a,*, Mirjana Fuduric´ Jelacˇa b, Stanislav Kurajica c Lidija C a
Faculty of Mechanical Engineering and Naval Architecture, Ivana Lucˇic´a 5, Zagreb, Croatia b College for Safety, Ivana Lucˇic´a 5, Croatia c Faculty of Chemical Engineering and Technology, Marulic´ev trg 19, Zagreb, Croatia Received 1 August 2007; accepted 10 October 2007 Available online 17 November 2007
Abstract The corrosion behavior of cold isostatically pressed (CIP) high purity alumina ceramics in aqueous HCl and H2SO4 solutions with various concentrations has been studied simultaneously at room temperature (25 °C). Corrosion tests were also performed with 0.65 mol/ l HCl and 0.37 mol/l H2SO4 solutions at 40, 55 and 70 °C for 48 h. Chemical stability was monitored by determining the amount of Al3+, Mg2+, Ca2+, Na+ Si4+ and Fe3+ ions eluted in different concentrations of HCl and H2SO4 solutions by means of atomic absorption spectrometry (AAS). By increasing the concentration from 0.37 to 6.5 mol/l, it was notified that the corrosion susceptibility in HCl and H2SO4 solutions for the CIP alumina specimens at room temperature decreases. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramic; B. Acid corrosion; B. Kinetic parameters
1. Introduction Aluminium oxide, Al2O3, more often referred to as alumina, is an exceptionally important ceramic material, which has many technological applications [1,2]. It has several special properties like high hardness, chemical inertness, wear resistance and a high melting point. Alumina ceramics can retain up to 90% of their strength even at 1100 °C. Because of the excellent properties of alumina ceramics, they are widely used in many refractory materials, grinding media, cutting tools, high temperature bearings, a wide variety of mechanical parts, and critical components in chemical process environments, where materials are subjected to aggressive chemical attack, increasingly higher temperatures, and pressures. Alumina has different phases, of which a-alumina is a stable phase. Besides the thermodynamically stable a-
*
Corresponding author. Tel.: +385 1 61 68 313; fax: +385 1 61 57 126. ´ urkovic´). E-mail address: [email protected] (L. C
0010-938X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2007.10.008
Al2O3 or corundum, there are many metastable structures of alumina, like c-Al2O3, which is widely used as a catalyst. Corundum or a-alumina is described by a hexagonal unit cell (Fig. 1) [1,2]. Aluminium oxide, i.e., alumina, is the main component of bauxite, the principal ore of aluminium. The largest manufacturers in the world of alumina are Alcoa, Alcan and Rusal. Companies which specialise in the production of speciality aluminium oxides and aluminium hydroxides include Alcan and Almatis. Modern technical ceramics offer promising alternatives to many existing materials of construction (e.g., metals and polymers) in chemical contacting applications, where corrosion properties are paramount. The corrosion behavior in various environments is a limiting factor for the lifetime of such ceramic components. Two main types of chemical resistance of ceramics are: acid resistance and alkali resistance. The study of corrosion resistance of ceramics in acids and alkalis makes it possible to predict their service properties in these media. Although ceramics are generally more stable in corrosive environments than
L. C´urkovic´ et al. / Corrosion Science 50 (2008) 872–878
Fig. 1. Corundum structure of alpha-alumina (a-Al2O3) [4].
common metallic materials, it is important to investigate the chemical resistance under highly corrosive conditions, such as strong acids, long-term exposure and elevated temperatures. In comparison to metals, ceramic materials may be considered resistant to corrosion because their corrosion rates are a great deal lower than those of metals. There are significant differences in the types of corrosion and the corrosion resistance between different groups of ceramic materials (silicate, oxide and non-oxide ceramic). While corrosion in metals is an electrochemical process, the solubility of the material is the determining factor in the level of ceramic corrosion. The chemical composition and the microstructure are key factors in low corrosion rates [3]. Many papers have been published on the corrosion of alumina ceramics under various conditions [3–14]. In hydrothermal condition, [6] the corrosion behavior of alumina ceramics with 99%, 99.9% and 99.99% Al2O3 is attacked by the dissolution of grain boundary impurities (Si, Ca and Na), and the corrosion weight loss for Al2O3 after corrosion decreased linearly with decreasing impurity level. Schachat et al. [7] reported the corrosion resistance of alumina ceramics in aqueous acidic solution (HCl, H2SO4, H3PO4) under hydrothermal conditions. The authors found that the corrosive effect of the acids on alumina ceramics decreases in the order H3PO4 > HCl > H2SO4. In this work, the dominant corrosion mechanisms were identified as intergranular attack and dissolution of Al2O3. Mikeska et al. [8] investigated the corrosion resistance of ceramics in aqueous HF acid. The authors found that corrosion in the case of polycrystalline alumina ceramics with 99.9% Al2O3 occurred primarily at the grain boundaries. Conversely, single-crystal sapphire (Al2O3) did not corrode and showed good resistance to the HF acid. Tuurna et al. [9] investigated corrosion resistance of porous alumina ceramics in acetic acid solution, Genthe and Hausner [11] investigated corrosion of alumina in acids and caustic solutions. These studies show that the major cause of the weight changes in corroded alumina ceramics is additives or impurities such as MgO, CaO, SiO2 and
873
Na2O which exist at grain boundaries. Sato et al. [12] reported the corrosion behavior of alumina ceramics in caustic alkaline solution at 150 and 200 °C. It was shown that the fracture strength of corroded alumina ceramics decreased as the degree of dissolution of alumina increased. Yue et al. [13] found that after the exposure of alumina single crystal at 1700–2000 °C to corrosive atmospheres (argon, argon/water vapour, air, and air/water vapour), there were no obvious weight and volume changes. In these papers, [3–14] the monitoring of corrosion in aqueous solutions is performed based on the measurement of the total weight loss specimens and/or measurement of eluted ions and/or strength change after immersion into a fixed volume of corrosive media. The measurement of eluted ions can determine the chemical stability more precisely than the measurement of the total weight loss specimens. Therefore, we did not measure the weight loss but the elution of elements after immersion in different concentrations of HCl and H2SO4 acids. In the present work, the corrosion behavior of high purity alumina was investigated at room temperature for 10 days in HCl and H2SO4 solutions of different concentrations. The corrosion of alumina ceramics in HCl and H2SO4 solutions is determined by the solubility of alumina and the solubility of grain-boundary impurities. This study shows that the major cause of eluate chemistry in corroded alumina ceramics is additives or impurities, such as MgO, CaO, SiO2 and Na2O, which exist at grain boundaries. 2. Materials and methods The material used in corrosion tests in HCl and H2SO4 solutions was a cold isostatically pressed (CIP)-Al2O3 with 99.8% purity. The chemical composition of investigated alumina ceramics is shown in Table 1. The CIP-Al2O3 specimens were supplied by Applied Ceramics, Inc., Fremont, California, USA. The Al2O3 ceramic contains MgO as sintering aid and the usual impurities, i.e., SiO2, CaO, Na2O and Fe2O3. The specimens were rectangular coupons; their size was 0.8 cm 1.0 cm 2.0 cm. Archimedes density of the CIP-Al2O3 was 3.91 g/cm3. Each surface of Al2O3 specimens was polished to 1.5 lm. After polishing and before corrosion tests, samples were thoroughly cleaned with alcohol and dried in the sterilizer at 150 ± 5 °C for 4 h. The specimens were first put into a sealed polypropylene (PP) tube with 5 ml of HCl and then with 5 ml of H2SO4 in two separate tests. The HCl solution concentrations were 0.65 mol/l (2 wt%), 3.25 mol/l (10 wt%) and 6.5 mol/l (20 wt%). The H2SO4 solution concentrations were 0.37 mol/l (2 wt%), 1.85 mol/l (10 wt%) Table 1 Chemical composition of the Al2O3 ceramics Sample
Alumina ceramics
wt% MgO
Fe2O3
SiO2
Na2O
CaO
Al2O3
0.066
0.015
0.02
0.05
0.013
Rest
L. C´urkovic´ et al. / Corrosion Science 50 (2008) 872–878
874
and 3.7 mol/l (20 wt%). The static corrosion tests were carried out simultaneously on five samples at room temperature (25 °C) from 24 to 240 h. The measurements were conducted after 24, 48, 60, 72, 84 and 240 h of immersion. Few tests were performed with 0.65 mol/l (2 wt%) HCl and 0.37 mol/l (2 wt%) H2SO4 at 40, 55 and 70 °C for 48 h. After the planned exposure time, the specimens were removed from the tubes, rinsed with boiling distilled water, and dried in a warm oven heated at 150 °C. The determination of the amount of Al3+, Mg2+, Ca2+, Na+ Si4+ and Fe3+ ions eluted in the corrosive solutions was carried out by means of atomic absorption spectrophotometry (AAS, AA-6800, Shimadzu). Obtained results are expressed as the amount of eluted ions (Mn+) in lg per
square centimetre of test alumina area (lg Mn+/cm2). All data were an average of five values. 3. Results and discussion The correlation between the amount of eluted ions of Al2O3 ceramics after immersion in different concentrations of HCl and H2SO4 solutions and the immersion time are plotted in Figs. 2 and 3. These results show that eluates after the corrosion process contain ions of Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+. These results indicate that impurities played an important role in the corrosion process of alumina ceramics [6–12]. On the basis of the literature data, the segregation of Si4+, Na+, Ca2+ and Mg2+ to the grain boundaries will
4 3 2
0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
1
μg Mg2+/ cm2
μg Al3+/ cm2
0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
0.6
5
0
0.4 0.2 0
0
50
100
150 t, h
200
250
0
2.5
4
0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
1
μg Na+/ cm2
μg Ca2+/ cm2
2
100
150 t, h
200
250
200
250
0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
2 3
50
1.5 1 0.5 0
0 0
50
100
150 t, h
200
0
250
2
50
100
150 t, h
0.3
μg Fe3+/ cm2
μg Si4+/ cm2
0.25 1.5 1 0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
0.5
0.2 0.15 0.1 0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
0.05 0
0 0
50
100
150 t, h
200
250
0
50
100
150 t, h
200
250
Fig. 2. The amount of eluted ions of Al3+ (A), Mg2+ (B), Ca2+ (C), Na+ (D), Si4+ (E) and Fe3+ ions (F) from Al2O3 ceramics in different concentrations of HCl solution.
L. C´urkovic´ et al. / Corrosion Science 50 (2008) 872–878
0.35
7
5 4 3 2
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
1 0
1.5
0.14 0.07
50
100
150 t, h
200
250
0
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
1.2
2 0.9 0.6 0.3
50
100
150 t, h
200
250
200
250
200
250
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
2.5
μg Na+/ cm2
μg Ca2+/ cm2
0.21
0
0
1.5 1 0.5 0
0 0
50
100
150 t, h
200
0
250
1.5
0.4
1.2
0.3
0.9
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
0.6
μg Fe3+/ cm2
μg Si4+/ cm2
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
0.28 μg Mg2+/ cm2
μg Al3+/ cm2
6
875
50
100
150 t, h
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
0.2 0.1
0.3 0
0 0
50
100
150
200
0
250
50
t, h 3+
Fig. 3. The amount of eluted ions of Al H2SO4 solution.
100 150 t, h
(A), Mg2+ (B), Ca2+ (C), Na+ (D), Si4+ (E) and Fe3+ ions (F) from Al2O3 ceramics in different concentrations of
take place, but their concentrations at the grain boundaries are not sufficient for the formation of the grain boundary phase [15–19]. The distribution of impurities is specified by the solubility of cations in the alumina lattice. The solubility depends on the differences in the charge and the ionic radius of impurities (Mg2+, Ca2+, Si4+, Fe3+ and Na+) and Al3+. Table 2 presents the ionic radius of Mg2+, Ca2+, Si4+, Fe3+, Na+ and Al3+ [7]. By comparing the charge and the ionic radius of impurities and Al3+, it is clear that Ca2+ possesses the smallest solubility. If the solubility limits of the cations in Al2O3 are exceeded, they segregate to the grain boundaries of the ceramic materials [15–19].
Table 2 Ionic radius of Mg2+, Ca2+, Si4+, Na+, Fe3+ and Al3+[7] Ionic radius
Al3+
Mg2+
Ca2+
Na+
Si4+
Fe3+
pm
57
79
106
98
26
67
The corrosion of alumina ceramics in HCl and H2SO4 solutions is determined by the solubility of alumina and the solubility of grain-boundary impurities. Obtained results (Figs. 2 and 3) indicate that the corrosion of alumina ceramics was mainly attributed to the dissolution of MgO, SiO2, CaO, Na2O and Fe2O3 grain-boundary impu-
L. C´urkovic´ et al. / Corrosion Science 50 (2008) 872–878
876
rities. The dissolution of Al3+ in bulk material (Al2O3) is negligible. Total amount of eluted ions from high purity Al2O3 ceramics after immersion in different concentrations of HCl and H2SO4 solutions at room temperature are shown in Figs. 4A and B. By comparing the results presented in Fig 4A and B, it was noted that with increasing concentrations of acids, the dissolution of alumina ceramic decreases. The highest corrosion rate is noted at the lowest acid concentration (0.65 mol/l% of HCl and 0.37 mol/l H2SO4). Also, these tests indicated that the corrosion rate is the highest in the first 24 h, and after that it decreases with the exposure time. In general, the corrosion susceptibility (total amount of eluted ions) of the Al2O3 ceramics increased by increasing the corrosion time. For this reason, a curve which presents the amount of eluted ions vs. time (which is a measure of corroded material) has a parabolic shape. Therefore, CIP-Al2O3 ceramics corrosion kinetics can be put in form [3] X
lg Mnþ =cm2
2
¼ Kp t
16 0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
14
240
10
0.65 mol/l HCl 3.25 mol/l HCl 6.50 mol/l HCl
200 (∑μg Mn+ / cm2)2
+
Σ μg Mn / cm2
12
where Kp is the parabolic corrosion rate constant in lg2 cm4 s1, t, s is the time of immersion in seconds, P lgMnþ =cm2 is the sum of the amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+) per square centimetre in lg cm2. The square of the total amount of eluted ions vs. the corrosion time for different concentrations of HCl and H2SO4 solutions are plotted in Fig. 5A and B. This correlation is linear with the correlation coefficient R2 = 0.98 for 0.65 mol/l HCl, R2 = 0.98 for 3.25 mol/l HCl and R2 = 0.97 for 6.50 mol/l HCl, R2 = 0.98 for 0.37 mol/l H2SO4, R2 = 0.96 for 1.85 mol/l H2SO4 and R2 = 0.96 for 3.70 mol/l H2SO4. The parabolic corrosion rate constants were evaluated from the slopes. The corresponding values of the parabolic corrosion rate constants are given in Table 3. The values of the parabolic corrosion rate constants are higher for all concentrations of HCl solutions in comparison with the same concentrations of H2SO4 solutions. Also, these values are higher for 0.65 mol/l HCl and 0.37 mol/l H2SO4 solutions. It indicated that the maximum corrosion susceptibility of Al2O3 ceramics is in the lowest concentration of investigated corrosive media. Total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, 4+ Si and Fe3+) from CIP-Al2O3 ceramics in different concentrations of HCl and H2SO4 solutions after 240 h of
8 6 4 2
160 120 80 40
0 0
50
100
150
200
250
0
t, h
0
0.1
0.2
0.3
14 0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/l H2SO4
10
6 4 2 0 50
0.7 0.8
0.9
0.4 0.5 t,Ms
0.6
0.7
0.9
0.37 mol/l H2SO4 1.85 mol/l H2SO4 3.70 mol/lH2SO4
160
8
0
0.6
200
(∑μg Mn+ / cm2)2
Σ μg Mn+/cm2
12
0.4 0.5 t,Ms
100
150
200
250
t, h Fig. 4. Total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+) from Al2O3 ceramics in different concentrations of HCl solution (A) and different concentrations of H2SO4 solution (B).
120 80 40 0 0
0.1
0.2
0.3
0.8
Fig. 5. The square of the total amount of eluted ions vs. the corrosion time for different concentrations of HCl solution (A) and different concentrations of H2SO4 solution (B).
L. C´urkovic´ et al. / Corrosion Science 50 (2008) 872–878 Table 3 The parabolic (Kp) corrosion rate constant for Al2O3 ceramics after immersion in the HCl and H2SO4 solution with different concentrations Kp (lg2 cm4 s1)
0.98 0.98 0.97 0.98 0.96 0.96
2
23.6 10 15.7 105 7.1 105 19.9 105 10.9 105 2.3 105
HCl HCl HCl H2SO4 H2SO4 H2SO4
y = -1.2852x + 6.2177 R2 = 0.9961; 0.65 mol/l HCl
2.30
n+
0.65 mol/l 3.25 mol/l 6.50 mol/l 0.37 mol/l 1.85 mol/l 3.70 mol/l
y = -1.1718x + 5.6994 R2 = 0.9999; 0.35 mol/l H2SO4
R2
5
ln ΣμgM /cm
Concentration of solution (mol/l)
2.50
877
2.10
1.90
corrosion testing are plotted in Fig. 6. The correlation between the total amount of eluted ions showed that the dissolution of CIP-alumina ceramic decreases with increased concentrations of solutions. It can be noticed from the graph that the total amount of eluted ions at all tested concentrations is higher in hydrochloride acid than in sulphuric acid, which is in agreement with the calculated values of the corrosion rate constants (Table 3). Total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, 4+ Si and Fe3+) from Al2O3 ceramics vs. different tempera16 HCl H2SO4
∑μg Mn+ / cm2
14 12 10 8 6 4 0
1
2
3
4
5
6
7
c (HCl, H2SO4), mol/l Fig. 6. Comparison of total amount of eluted ions after 240 h immersion in different concentrations of HCl and H2SO4 solutions.
12.5
2
9.5
Σ μgM /cm
10.5
n+
11.5
0.37 mol/l H2SO4 0.65 mol/l HCl
8.5 7.5 6.5 5.5 290
300
310
320
330
340
350
T, K Fig. 7. Total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+) from Al2O3 ceramics vs. different temperature in 0.65 mol/l HCl and 0.37 mol/l H2SO4 solutions.
0.37 mol/l H2SO4 0.65 mol/l HCl
1.70 2.88
2.98
3.08
3.18
1000/T, K
3.28
3.38
-1
Fig. 8. Arrhenius plot of the total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+) from Al2O3 vs. the inverse temperature for 0.65 mol/l HCl and 0.37 mol/l H2SO4 solutions.
tures in 0.65 mol/l HCl and of 0.37 mol/l H2SO4 are shown in Fig. 7. Corrosion tests of alumina ceramics carried out at 25–70 °C (298–343 K) showed that the dissolution of Al2O3 specimens in HCl and H2SO4 solutions increased by increasing the temperature. Also, dissolution is faster in 0.65 mol/l HCl. The Arrhenius plots of the total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+) vs. the inverse temperature for a 0.37 mol/l H2SO4 solution (shown in Fig. 8) were used for the determination of the activation energies for the corrosion process. The value of the activation energies for the alumina specimens corroded in 0.65 mol/l HCl solution was 10.68 KJ/mol and in 0.37 mol/l H2SO4 solution was 9.74 KJ/mol. 4. Conclusion The corrosion susceptibility in HCl and H2SO4 solutions for CIP-Al2O3 ceramics at room temperature decreases on increasing the concentration from 0.37 to 6.5 mol/l. The obtained results indicate that the strongest corrosive attack occurs in the first stage. After the first 24 h, the amount of eluted Al3+, Mg2+, Ca2+, Na+ Si4+ and Fe3+ ions is considerably lower. Values of corrosion rate constants are higher in HCl solutions of all concentrations than in the relevant concentrations of H2SO4 solutions. Also, values of corrosion rate constants are the highest in both HCl and H2SO4 solutions of the lowest concentration. It proves that the Al2O3 ceramics is more stable in HCl solutions than in H2SO4 solutions and that its stability is lower at lower concentrations of both acids. The activation energy has been determined on the basis of the results obtained at higher temperatures in solutions of 0.65 mol/l hydrochloride and of 0.37 mol/l sulphuric acid, and the determined values were higher in 0.65 mol/l HCl solution. This confirms
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the lower stability of the Al2O3 ceramics in HCl at higher temperatures. According to the determined mass concentration of eluted ions, which belong to the grain boundaries (Mg2+, Ca2+, Na+, Si4+ and Fe3+), and Al3+, which belongs to the grain, the corrosion of alumina ceramics is mainly attributed to the dissolution of MgO, SiO2, CaO, Na2O and Fe2O3 grain-boundary impurities. The dissolution of Al3+ in bulk material (Al2O3) is negligible. Acknowledgements The presented research results were achieved within the scientific project ‘‘Structure and properties of engineering ceramics and ceramic coatings” (Agreement No. 1201201833-1789; 10106-797300-1) supported by the Croatian Ministry of Science, Education and Sport. We thank Matt Sertic from Applied Ceramics, Inc. for providing alumina ceramics samples. References [1] J.S. Reed, Principles of Ceramics Processing, John Wiley & Sons, Inc.,, New York, 1995. [2] W.H. Gitzen, Alumina as a Ceramic Material, The American Ceramic Society, Westerville, OH, USA, 1970. [3] D.E. Clark, K. Zoitos, Corrosion of Glass Ceramics and Ceramic Superconductors, Noyes Publications, NJ, USA, 1992. [4] D.R. Askeland, P.P. Phule´, The Science and Engineering of Materials, fourth ed., Books/Cole-Thomson Learning, USA, 2003. [5] Q. Fang, P.S. Sidky, M.G. Hocking, The effect of corrosion and erosion on ceramic materials, Corros. Sci. 39 (3) (1997) 511–527.
[6] K. Oda, T. Yoshio, Hydrothermal corrosion of alumina ceramics, J. Am. Ceram. Soc. 80 (12) (1997) 3233–3236. [7] M. Schacht, N. Boukis, E. Dinjus, Corrosion of alumina ceramics in acidic aqueous solutions at high temperatures and pressures, J. Mater. Sci. 35 (2000) 6251–6258. [8] K.R. Mikeska, S.J. Bennison, S.L. Grise, Corrosion of ceramics in aqueous hydrofluoric acid, J. Am. Ceram. Soc. 83 (5) (2000) 1160– 1164. [9] S. Tuurna, A.P. Nikkila, T.A. Mantyla, Corrosion resistance of porous alumina ceramics in acetic acid solution, Key Eng. Mater. 206 (2002) 1923–1926. [10] W.P. Tai, T. Watanabe, N.S. Jacobson, High-temperature stability of alumina in argon and argon/water–vapor environments, J. Am. Ceram. Soc. 82 (1999) 245–248. [11] W. Genthe, H. Hausner, Influence of chemical composition on the corrosion of alumina in acids and caustic solutions, J. Eur. Ceram. Soc. 9 (1992) 417–425. [12] T. Sato, S. Sato, A. Okuwaki, S. Tanaka, Corrosion behavior of alumina ceramics in caustic alkaline solutions at high temperatures, J. Am. Ceram. Soc. 74 (12) (1991) 3081–3084. [13] X.M. Yue, G.J. Zhang, T. Watanabe, W.P. Tai, Corrosion behavior of single-crystal alumina in argon, air, and water vapor atmospheres at 1700–2000 °C, J. Am. Ceram. Soc. 82 (9) (1999) 2560–2562. [14] J. Bennison, M.P. Harmer, Swelling of hot-pressed Al2 O3, J. Am. Ceram. Soc. 68 (1985) 591–597. [15] W.D. Kaplan, H. Mullejans, M. Ruhle, Ca segregation to basal surfaces in a-alumina, J. Am. Ceram. Soc. 78 (1995) 2841–2844. [16] S.K. Roy, R.L. Cobel, Solubilities of magnesia, titania, and magnesium titanate in aluminium oxide, J. Am. Ceram. Soc. 1 (1968) 1–6. [17] K.L. Gavrilov, S.J. Bennison, K.R. Mikeska, R. Levi-Setti, Grain boundary chemistry of alumina by high-resolution imaging sims, Acta Mater. 47 (1999) 4031–4039. [18] L. Miller, A. Avishai, W.D. Kaplan, Solubility limit of MgO in Al2 O3 at 1600 °C, J. Am. Ceram. Soc. 89 (1) (2006) 2841–2844. [19] C. Greskovich, J.A. Brewer, Solubility of magnesia in polycrystalline alumina at high temperatures, J. Am. Ceram. Soc. 84 (2) (2001) 420– 425.