Corrosion Behavior of Reaction Bonded Si3n4–Sic and Sialon– Sic Composites in Simulated Aluminum Smelting Conditions

Journal of the Ceramic Society of Japan 116 [6] 712-716 2008 Paper

Corrosion behavior of reaction bonded Si3N4–SiC and SiAlON– SiC composites in simulated aluminum smelting conditions

Mark I. JONES,† Ron ETZION,* Jim METSON,* You ZHOU,** Hideki HYUGA,** Yu-ichi YOSHIZAWA** and Kiyoshi HIRAO**

Department of Chemical & Materials Engineering, University of Auckland, 20 Symonds St, Auckland, New Zealand *Light Metals Research Centre, University of Auckland, 23 Symonds St, Auckland, New Zealand **National Institute of Advanced Industrial Science & Technology, AIST, 2266-98, Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan

Si3N4–SiC composites, which are widely used as sidewall refractories in aluminum smelting cells, have been produced by reaction bonding and their corrosion performance assessed in simulated aluminum electrochemical cell conditions. Additional samples were produced with the silicon nitride bonding phase replaced by β SiAlON with compositions ranging from Z = 1–4 in Si6–zAlzOzN8–z. The formation of the Si3N4 and SiAlON bonding phases were studied by reaction bonding of silicon powders in a nitrogen atmosphere at low temperatures to promote the formation of silicon nitride, followed by a higher heating step to produce β SiAlON composites of different composition. The corrosion performance was studied in a laboratory scale aluminum electrolysis cell where samples were exposed to both liquid attack from molten salt bath and corrosive gas attack. For the Si3N4 bonded samples, the corrosion resistance was shown to be strongly dependent on the environment during corrosion testing, with samples in the gas phase showing higher corrosion than those immersed in the bath. Samples that had been pre-soaked in the bath and then tested in the gas phase showed the highest corrosion due to the combined effects of bath penetration and gas attack. For the SiAlON bonded samples, the corrosion results showed similar trends but were complicated by the presence of a strongly adherent layer on the samples which influenced volume measurements. This layer is believed to be alumina and appears to form through an interaction with the SiAlON and the bath components, since no such layer was observed for the silicon nitride samples. ©2008 The Ceramic Society of Japan. All rights reserved.

Key-words : Silicon nitride, SiAlON, Silicon carbide, Composites, Corrosion, Aluminum smelting

[Received February 15, 2008; Accepted April 17, 2008]

markedly.7) One of the most widespread uses of RBSN in molten 1. Introduction metal handling is in the Hall–Heroult cells used for the produc- Reaction bonding is a low cost, low temperature alternative tion of Al metal. In this process, alumina is electrolytically heating technique for producing silicon nitride ceramic materials reduced into molted aluminum, whereby the bound oxygen in the where the nitride phase is produced by reacting metallic silicon alumina reacts with carbon electrodes to form carbon-dioxide in a nitrogen atmosphere at temperatures of around 1400°C. SiA- gas and metallic aluminum. The electrolyte is mainly cryolite, lON materials can be produced in the same manner by first pro- Na3AlF6, with dissolved alumina and additions of other fluorides ducing the nitride phase and then subsequently increasing the such as AlF3 and CaF2 which serve to reduce the melting point temperature to where the nitride reacts with the Al and O usually of pure cryolite (1009°C) down to a cell operating temperature provided through additions of alumina and aluminum nitride. of between 920 and 980°C.8) The electrolytic cells are lined with Compared with conventional sintering of silicon nitride powders, a refractory material to protect the cell and maintain the correct Reaction Bonded Silicon Nitride (RBSN) has several advanta- heat balance. Originally made from carbon, these sidewall lin- geous features including low sintering shrinkage, low raw mate- ings have now been widely replaced by Si3N4–SiC composites rial cost and high strength retention at elevated temperature.1)–3) produced by reaction bonding. In these composite materials the However, the fact there is little or no shrinkage associated with RBSN, typically 20–30 mass% acts as the bonding phase holding reaction bonding means that unless sintering aids are used the the silicon carbide particles together. The use of these materials resultant materials tend to have porosity of around 20–30%.4) allows thinner sidewalls therefore giving increased cell capacity Whilst therefore not suitable for high performance applications, and the ability to accommodate larger anodes therefore improv- RBSN ceramics find widespread use as a refractory material in ing productivity.9) During cell operation, the sidewall is protected metal handling applications due to properties such as good ther- by the presence of a layer of solidified bath, but if this “frozen mal stability and thermal shock resistance, good chemical corro- ledge” becomes disrupted by anode effects or instability, the sion resistance and lack of wetting by many metals and alloys.5),6) refractory is directly exposed to aggressive chemical environ- Typical applications include blast furnace refractories, where ments both from liquid metal and the cryolitic electrolyte bath, RBSN materials have increased lifetimes of furnace linings and from the gas phase above the bath.10) In these environments, materials show different levels of degradation depending on † Corresponding author: M. I. Jones; E-mail: mark.jones@auckland. manufacturer but also on location within the cell.9) The objec- ac.nz tives of this work are to produce controlled compositions of both

Si3N4–SiC and β SiAlON–SiC composites by reaction bonding and study their corrosion resistance in an environment that simulates that observed in Al smelting. 2. Experimental procedure 2.1 Sample preparation The starting materials for the Si3N4–SiC composites were an abrasive grade silicon carbide (< 40 μ m GC #400, Fujimi Kenmazai Kogyo Co., Ltd., Japan) and a fine, high purity silicon powder (< 1 μ m, > 99% Kojundo Chemical Lab. Co., Ltd., Japan). For the SiAlON–SiC composites, Al2O3 (AKP–50, Sumitomo Chemical Co., Ltd., Japan) and AlN (Grade E, Tokuyama, Corp., Japan) were added in appropriate amounts to produce Si6–zAlzOzN8–z with Z values ranging from 1 to 4. In these materials the oxygen content of the nitride powders was taken in to consideration when designing the compositions, and in all cases the compositions were designed so as to give com- Fig. 1. Experimental rig used for corrosion testing. posites with 25% by weight of the bonding phase (Si3N4 or SiAlON) assuming complete nitridation of the Si powder. The powders were ball milled in methanol using a silicon nitride pot and balls, dried in a vacuum evaporator and passed through a 250 2.3 Characterization μ m sieve. The powders were uniaxially pressed in to plates with The density of the reaction bonded samples was determined dimensions of 47 × 42 × 12 mm under a pressure of 40 MPa and using the Archimedes method in distilled water. The degree of then isostatically pressed at 200 MPa. Heating was carried out in nitridation was determined by changes in weight measured a graphite resistance furnace (High Multi–5000, Fujidempa before and after heating. Phase structure was assessed by X-ray Kogyo Co., Ltd., Japan) under a 0.5 MPa nitrogen atmosphere. diffraction (XRD) and microstructure was observed using both Pressed samples were located inside a double walled crucible optical microscopy and scanning electron microscopy (SEM). arrangement with an inner crucible of boron nitride and an outer Following corrosion tests, the samples were ultrasonically one of graphite. Nitridation was carried out by slowly heating the cleaned in an AlCl3 solution to remove loosely adherent bath and samples through the temperature range 1200–1450°C at 0.25°C the degree of corrosion was assessed as a change in volume, min–1. The SiAlON formation was studied at temperatures rang- determined by measurement of density and mass. ing from 1700–1900°C by subsequently raising the temperature 3. Results and discussion at 10°C min–1 to the desired temperature and then holding for a period of 2 h. 3.1 Characterization The phases present at different sintering temperatures, the 2.2 Corrosion testing degree of conversion of Si to Si3N4 and the relative density of the For the corrosion testing, samples of approximately 30 × 10 × sintered samples are all given in Table 1. The development of the 10 mm were cut from the sintered materials and subjected to final microstructure can be determined by following the change electrolysis conditions in a purpose built experimental cell. A in crystalline phases as a function of temperature. At 1380°C, α schematic of the experimental cell is shown in Fig. 1 and consists Si3N4 was observed for all samples but the remaining presence of a graphite crucible surrounded by a can made of inconel. The of crystalline Si indicates that the transformation was incom- electrolysis tests were carried out at a temperature of 1000°C for plete. At 1450°C the transformation was complete and the silicon 48 hours at a voltage of 4V and current of 15A. The molten bath nitride was present in both α and β crystalline forms. The relative was typical of that observed in industrial practice with a compo- amounts of the two Si3N4 polymorphs, determined from XRD 11) sition of 78% Na3AlF6, 10% AlF3, 7% Al2O3 and 5% CaF2. peak intensities using the method of Gazzara and Messier, Three different sets of sample were tested for each experiment; showed that the α phase was predominant with values typically the first set of samples were immersed directly in the bath and around 70% of the total Si3N4. For the Z = 1–4 samples at this subjected to attack by molten liquid. A second set was suspended temperature the Al2O3 and AlN were still observed indicating in a graphite basket in the area of hot gas above the bath level. that SiAlON had not yet formed. At the final sintering tempera- The final set was presoaked in molten bath for 24 h prior to being ture of 1900°C, all of the α had transformed and the additives located in the graphite basket and tested in the gas phase. had been incorporated into the structure leaving a bonding phase

Table 1. Characteristics of the Silicon Nitride-silicon Carbide and SiAlON–SiC Composites Produced by Reaction Bonded Sintering Sample Starting Powder Phases present at different temperature in addition to silicon carbide Transformation Density

(Z in Si6–zAlzOzN8–z) 1380°C 1450°C 1900°C Si to Si3N4 (%) (%TD)

0 SiC, Si Si, α Si3N4 α Si3N4, (69%) β Si3N4 (31%) β Si3N4 80.7 73.9

1 SiC, Si, Al2O3, AlN Si, α Si3N4, Al2O3, AlN α Si3N4, β Si3N4, Al2O3, AlN β SiAlON 94.7 74.9

2 SiC, Si, Al2O3, AlN Si, α Si3N4, Al2O3, AlN α Si3N4, β Si3N4, Al2O3, AlN β SiAlON 94.9 72.8

3 SiC, Si, Al2O3, AlN Si, α Si3N4, Al2O3, AlN α Si3N4, β Si3N4, Al2O3, AlN β SiAlON 83.1 71.9

4 SiC, Si, Al2O3, AlN Si, α Si3N4, Al2O3, AlN α Si3N4, β Si3N4, Al2O3, AlN β SiAlON 74.5 74.6

713 JCS-Japan Jones et al.: Corrosion behavior of reaction bonded Si3N4–SiC and SiAlON–SiC composites in simulated aluminum smelting conditions

Fig. 4. Changes in volume for the Si3N4 bonded SiC composites following corrosion testing.

Fig. 2. Typical microstructure of the reaction bonded samples, with SiC grains embedded in a silicon nitride bonding phase.

Fig. 3. XRD spectra of the composites following sintering. The formation of the SiAlON phase for samples Z = 1– 4 is indicated by a shift in the Si3N4 peaks as indicated.

of β Si3N4 in the case of the Z = 0 sample and β SiAlON for Z = 1– 4. The degree of transformation, determined by changes in Fig. 5. Cross section ptical microscopy images of the corroded sam- mass, was less than 100%, but the absence of any trace of Si in ples. Samples subjected to liquid bath (b and c) show a dense, dark, outer either XRD spectra or polished SEM images implies that full region. transformation had been achieved and the lower calculated value is likely due to evaporation. Figure 2 shows a typical microstructure of the final reaction in volume, whilst the sample tested directly in the gas phase bonded composite with the bonding phase and SiC particles indi- showed a decrease in volume, i.e corrosion of the brick had cated. This image is taken from the fracture surface close to the occurred. The largest volume change was observed for the sam- center of a Z = 2 sample. The fact that the bonding phase was ple that had been pre-soaked in bath and then tested in the gas SiAlON, and not Si3N4, in the case of the Z = 1– 4 samples is inti- phase. Figure 5 shows cross section optical microscopy images mated from the XRD spectra shown in Fig. 3. These samples of the samples following testing and it can be seen that the sam- show an increasing lattice parameter as a result of the incorpo- ples that had been subjected to molten bath, either through pre- ration of the Al2O3 and AlN into the Si3N4 structure, which soaking or through direct immersion during electrolysis, showed appears on the XRD spectra as a shift to lower angles of the sil- a dark outer layer that even in optical images appeared more icon nitride peaks. This is most obvious for the β (101) peak as dense than the interior. SEM images of this outer region for all indicated in the figure, where it can be seen that the magnitude samples are shown in Fig. 6. The original sample (5a and 6a) of the shift increases with increasing Z value. shows significant porosity. For the samples that had been tested in the bath (5b and 6b), and those that had been pre-soaked in 3.2 Corrosion tests bath then tested in the gas phase (5c and 6c), the areas of the i) Si3N4–SiC composites bonding phase appeared denser. The sample tested only in the The change in volume for the Si3N4–SiC composites following gas phase did not show such densification and the porosity was the corrosion tests is shown in Fig. 4. The sample that was sub- similar to the original sample, although the pores appeared merged in the bath during electrolysis showed a positive change slightly larger and more “open.” EDS analysis of the bonding

714 Journal of the Ceramic Society of Japan 116 [6] 712-716 2008 JCS-Japan

rig. For the samples that were tested in the bath, penetration of the bath and solidification on subsequent cooling leads to the positive volume changes measured and these samples did not experience any noticeable corrosion over the period of the test. It appears that the presence of a gas phase attack leads more readily to corrosion, and this is enhanced in those samples that had been pre-soaked and penetrated by the liquid bath. These samples would have been subjected to infiltration not only of liquid bath and the NaAlF4 vapor but also other gases such as HF and CO2. The corrosion takes place primarily in the matrix region since the Si3N4 bonding phase is more prone to oxidation 12) than the SiC grains, and CO2 is known to be an oxidant, resulting in the formation of silica. This oxidation is enhanced in the presence of Na which has been reported to result in Na2O–SiO2 silicate species which are then more readily oxidized than the 13) Si3N4. Gas phase corrosion has also been described in terms of the effects of Na together with HF.14) Enhanced degradation in the gas phase is reported to be due to the presence of Na species resulting in the formation of volatile SiF4 through the following Eqs.10) Fig. 6. SEM images of the cross sections of Si3N4 bonded SiC compos- 2SiO + NaAlF → SiF + NaAlSiO (1) ites following corrosion. Samples were (a) original, (b) tested in the bath, 2 4(g) 4(g) 4(s)

(c) presoaked in bath and tested in the gas phase and (d) tested directly in 2Si3N4 + 3NaAlF4(g) + 6CO2 → 3SiF4(g) the gas phase. + 3NaAlSiO4(s) + 6C + 4N2 (2)

2SiC + NaAlF4(g) + 2CO2 → SiF4(g) + NaAlSiO4(s) + 4C (3) The higher corrosion rates for the samples that had been pre- soaked compared to the ones directly in the gas phase is described in terms of both the aggressive gas attack and the fact that the amount of NaAlF4 is increased in these samples due to the penetration during the pre-soaking stage. ii) SiAlON–SiC composites The volume change for all the SiAlON samples is shown in Fig. 8. There are similarities with the data given for Si3N4 (Fig. 4), in that all samples tested in the bath showed positive volume changes due to bath penetration, and all samples tested in the gas phase showed corrosion indicated by negative volume changes. The levels of volume change tended to be higher than those for the corresponding silicon nitride samples and there was no direct correlation with the composition, i.e. with Z value. It is more likely that differences are due to the characteristics of the samples, particularly porosity.12) The Z = 3 sample shows the highest volume changes, both positive for the sample in the bath and negative for the sample in the gas phase. This sample had slightly

Fig. 7. EDS analysis of the Si3N4 bonded SiC composites. The original lower density than the other samples and so would have been sample (a) shows only Si and N whilst the sample tested in the bath (b) shows elements of the bath composition. The sample tested in the gas phase (c) shows Al and Na from the cryolitic vapours (NaAlF4). phase region of the samples (Fig. 7) showed the presence only of Si and N in the case of the original material (7a) whilst for the sample tested in the bath (7b) the presence of Al, F, Na, O and Ca was also observed. These are all elements of the bath composition and indicate penetration into the sample during testing. It is this penetration of the liquid bath and subsequent cooling on solidification that leads to an increase in weight. For the sample tested in the gas phase (7c) the EDS analysis showed the presence of Na and Al which comes from the main cryolitic vapour NaAlF4. The corrosion behavior of these samples then can be understood in terms of the processes occurring during Fig. 8. Changes in volume for the SiAlON bonded SiC samples fol- electrolysis and also on the location of the samples in the testing lowing corrosion tests.

715 JCS-Japan Jones et al.: Corrosion behavior of reaction bonded Si3N4–SiC and SiAlON–SiC composites in simulated aluminum smelting conditions

used as the sidewall refractory and rely on maintaining a solidified layer of bath, or “frozen ledge”, it may be that this increased interaction between the ceramic and the molten metal could pos- sibly improve the performance of the bricks in service. They cor- rode and fail when the frozen ledge is disturbed due to anode effects, and the strong adhesion seen in the current work between the SiAlON samples and the alumina layer may help to mitigate such failure. Work is ongoing to assess the nature of the adherent layer and its interaction with the SiAlON refractories and will be reported separately. 4. Conclusions

Si3N4–SiC and β SiAlON–SiC composites have been produced by reaction bonding and their corrosion properties assessed in a test rig that simulates conditions in an electrolytic Al smelting cell. The corrosion was dependent on the location of the samples in the cell with samples located in the gas phase suf- fering more corrosion than samples below the bath level. Of Fig. 9. SEM images and EDS analysis of the Z = 3 sample pre-soaked those samples tested in the gas phase, corrosion was enhanced in in bath and corroded in the gas phase. those that had been pre-soaked in bath due to the presence of Na species from both the liquid and the gas phase which reacts with the Si species to from volatile SiF4. SiAlON bonded samples more susceptible to bath penetration and to attack by the corro- showed inconsistent values in terms of volume change due to the sive gases. For the samples pre-soaked in bath and then tested in presence of a strongly adherent layer, believed to be alumina, the gas, the trend was much less clear than the case of Si3N4 and which was developed in all samples that had been immersed in the results showed little consistency, with large volume gains in the liquid bath. addition to volume loss. This was due to the fact that these samples had an extremely adherent solidified layer of bath on the sur- References face that could not be removed with the AlCl cleaning, and 3 1) A. J. Moulson, J. Mater. Sci., 14, 1017–1051 (1979). which remained intact through subsequent grinding and polish- 2) B. T. Lee, J. H. Yoo and H. D. Kim, Mater. Sci. Eng. A, 333, ing. Figure 9 shows a cross section SEM image of the Z = 3 sam- 306–313 (2002). ple that was pre-soaked and then tested in the gas phase. Several 3) W. Bunker and W. D. Scott, Am. Ceram. Soc. Bull., 63[8], distinct regions are observable as indicated in the figure. The 1000 (1984). inner region (1) shows a structure similar to the original material 4) P. Edwards, B. C. Muddle, Y.-B. Cheng and R. H. J. Hannink, along with the wide internal cracks or pores where gas attack J. Eur. Ceram. Soc., 15, 415–424 (1995). occurred. Region (2) is a densified area where the bath has pen- 5) N. K. Reddy and J. Mukerji, J. Am. Ceram. Soc., 74[5], 1139– etrated during pre-soaking. The EDS spectra in Fig. 9b is taken 1141 (1991). 6) Yu. V. Naidich, V. S. Zhuravlev, N. I. Frumina, B. D. Kostyuk, from this area and is identical to the same area shown for the sil- N. A. Krasovskaya and V. G. Ostrovskii, Powder Metall. Met. icon nitride samples (7b) with all the elements of the bath being Ceram., 27[11], 888–809 (1988). present. The region labeled as (3) in Fig. 9a is the outer region 7) H. Zhang, B. Han and Z. Liu, Mat. Res. Bull., 41[9], 1681– where the adherent layer can seen. The higher magnification 1689 (2006). image (9c) shows that this layer is strongly adherent and has a 8) http://www.alcoa.com/global/en/about_alcoa/pdf/Smeltingpaper. plate-like morphology. EDS analysis of this area (9d) indicates pdf only Al and O elements, and along with the morphological data 9) E. Skybakmoen, L. I. Stoen, J. H. Kvello and O. Darell, “Light suggests that this is an alumina layer. This layer was not Metals 2005,” Ed. by H. Kvande, The Minerals, Metals & observed on any of the silicon nitride samples but was commonly Materials Society, Philadelphia (2005) pp. 773–778. seen on the SiAlON ones and is responsible for the variations in 10) E. Skybakmoen, H. Gudbrandsen and L. I. Stoen, “Light Met- als 1999,” Ed. by C. E. Eckert, The Minerals, Metals & Mate- volume change seen for these samples. The presence of this layer rials Society, Philadelphia (1999) pp. 215–222. seems to indicate either different wetting characteristics or a 11) C. P. Gazzara and D. R. Messie, Am. Ceram. Soc. Bull., 56, reaction between the alumina in the bath and Al and O substitu- 777–780 (1977). tional elements in the SiAlON. Whilst it is difficult therefore to 12) M. Thorley and R. Banks, J. Therm. Anal., 42, 811–822 quantify the corrosion of the samples using volumetric changes, (1994). it is thought that the mechanism is the same as for the silicon 13) M. I. Mayer and F. L. Riley, J. Mater. Sci., 13, 1319–1328 nitride samples. In molten metal applications, it is usually con- (1978). sidered that non-wetting by liquid metal is a required property 14) F. B. Anderson, G. Dorsam, M. Stam and M. Spreij, “Light of any refractory ceramic to avoid any reaction between the two. Metals 2004,” Ed. by A. T. Tabereaux, The Minerals, Metals In the Al smelting cell, where the RBSN–SiC composites are & Materials Society, Philadelphia (1999) pp. 413–418.

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