KINETICS AND THERMODYNAMIC APPROACH OF CHROMIA SOLUBILITY IN SILICATE MELTS 

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Pack cementation technique

The NiAl (rod) alloy was not prepared by high frequency induction melting since the reaction between Ni and Al is very exothermic and the heat of the reaction is difficult to control, thereby making it difficult to have a rod shaped alloy. As a consequence, pack cementation process which is a type of chemical vapour deposition (CVD) was used to produce the NiAl coating on the Ni rod. The fundamentals and parameters for this aluminising process were discussed precisely by several authors106–108. The Ni rod (which was produced by high frequency induction melting) with Ø = 5 mm and length ~ 2.5 cm was immersed in a mixture of powders which is called cement (Figure 3.2). The cement consists of 40% of master alloy (Ni2Al3) which provides the element (or elements) to deposit, 60% of inert filler (Al2O3) to avoid sintering of the cement and ~ 20 mg (this quantity is sufficient to limit the pressure to 1 bar inside the silica tube with 15 cm3 of volume at high temperature during the deposition process) of halide activator (CrCl3) which transports the master alloy in gas phase. All the elements were sealed in a silica tube and were subjected to the heat treatment at T = 1000oC for 16 h. The characterisation of the prepared NiAl alloy by using this technique is shown in Appendix B.

Preparation of electrodes

Three different types of electrodes were used in this technique which are the reference, counter and working electrodes. All electrodes were fabricated by using specific materials and particular methods in order to withstand high temperatures for a long run duration.

Reference electrode

Reference electrode is an electrode which has a constant electrochemical potential. In this work, a self-constructed yttria stabilised zirconia (YSZ) was used as a reference electrode due to its stability at high temperatures. It has been used and established in this laboratory since many years ago. The zirconia (ZrO2) with an addition of yttria (Y2O3) possesses a good resistance at a wide range of temperatures. It is chemically inert in the molten glass and also a good ionic conductor of O2- ions. The ability to conduct O2- ions makes yttria stabilised zirconia well suited to be used as a reference electrode based on the redox couple O2/O2-. The electrode (Figure 3.3) consists of a twin holes mullite tube (6) with a platinum wire which was introduced into one of the holes. The platinum wire (7) was connected with the molten glass by the YSZ stick (1) (Ø = 5 mm, length ~ 2.5 cm). Platinum ink (4) was used in order to enhance the ceramic-platinum contact. All the different elements were sealed by zirconia-based cement (2) (supplied by Final Advanced Materials) and were mechanically fixed to the outer mullite tube (5) by introducing a small alumina stick (3) as shown in Figure 3.3(b). This alumina stick could avoid the YSZ stick from dropping into the molten glass if the cement failed. The electrode was flushed with air ( O2 P = 0.21 atm) as a reference gas which was introduced with a syringe needle into another hole of the inner mullite tube.

Electrochemical characterisation of the molten glasses

The working electrode which was used for the characterisation of electroactivity domain of the studied molten glasses has to be chemically inert. Therefore it was composed of a platinum wire with diameter of 1 mm and length of ~2.5 cm (1) (Figure 3.6). This platinum wire was welded to a smaller platinum wire (5) (Ø = 0.5 mm) by Soudax Equipments SD100. Finally the elements were sealed with silico-aluminous cement (2) in order to avoid the contact of the platinum wire with the air/glass interface.

Elelctrochemical measurements

The experiments on the electrochemistry were performed by using an apparatus as shown in the Figure 3.8. The measurements were realised by a Potentiostat/Galvanostat model 263A which is equipped with the software of corrosion EG&G Perkin Elmer M352 and M270. A quantity of 1.3 kg of glass (6) was placed in a clay crucible (5). A clay crucible was used in this study instead of a platinum crucible due to the fact that the reduction of SiO44- to Si0 will lead to a formation of eutectic Pt-Si with low melting point which is responsible for the platinum degradation. In order to avoid any leak from the molten glass that could damage the furnace, the crucible was put in another clay crucible which has a bigger dimension. These crucibles were supported in Carbolite HTF 17/10 furnace (4) by a refractory cement which is known as ‘Zircoram’ (80% Cr2O3, 20% ZrO2) (7). Before starting the electrochemical measurements, the glass was annealed at 1200oC (10oC/min) for 2 h in order to ensure a complete melting of the glass and also to remove the bubbles that could interfere with the electrochemical signals.

Electrochemical characterisation of the glass

Before the corrosion behaviour of the pure chromium and Ni-based alloys could be characterised, it is necessary to study the electrochemical behaviour of the molten glass itself. The platinum working electrode which was used in this work is inert, so the electrochemical signals gave the direct indicative of the characteristic of the molten glass at the given experimental temperature. The polarisation curve of the Pt working electrode was plotted by first polarising the anodic field from free potential to +600 mV. After that the cathodic field was polarised from the free potential to -1300 mV. These two polarisations were carried out at a scan rate of 1 mV/s.

Corrosion of pure chromium and Ni-based alloys by molten glass

The characterisation of different materials was performed by the conventional electrochemical methods which have been established for aqueous corrosion i.e.
Mullite tube Mold Resin measurement of polarisation resistance (Rp) by the Stern-Geary method and tracing of the potential-current curve.
 Measurement of the polarisation resistance and calculation of the corrosion rate The measurement of the polarisation resistance was performed by polarising the working electrode from -10 mV to +10 mV around the corrosion potential (Ecorr) with a scan rate of 600 mV/h. The corresponding current allows the potential-current curve of this domain to be plotted, hence leading to the determination of the slope (1/Rp) according to the following equation: 1 di Rp dE  at E = Ecorr (3.9).

‘Raw immersion’ technique

A ‘raw immersion’ experiment was always coupled with the electrochemical measurement since the latter technique will modify the spontaneous behaviour of the surface of the studied material when the potential is polarised. This technique was performed in order to predict the real corrosion mechanism at the interface of thealloys without the interference from any destructive method. A raw immersion technique was performed on the Ni-30Cr rod, Ni-8Al-28Cr rod and NiAl plate (bulk) which were prepared by using high induction melting. The schematic of the device was illustrated in Figure 3.10. The alloys with rod/plate shape were sealed in a mullite tube (Ø = 10 mm) with silico-aluminuos cement. The alloy which was  embedded in the mullite tube was suspended in the molten glass thank to a smaller mullite tube (Ø = 4 mm) which was joined horizontally to the system with silico-aluminous cement.

Thickness loss measurement

The diameter or the thickness of every sample before and after the corrosion tests was determined by using micrometer (type Palmer) and also by stereo microscope ZEISS Stemi 2000-C (SIP 50147) which is equipped with a digital camera AxioCam ICc 1. This microscope is operated with AxioVision 4 software.

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Solubility of chromia (Cr2O3) in molten glasses

The method for the glass balls preparation was already optimised by Khedim99 in binary melts. Chromium-containing glasses were obtained by mixing 5-10 wt.% Cr2O3 with the soda-lime silicate glasses. This sufficient amount of Cr2O3 has been chosen in order to be higher than the solubility limit of Cr2O3 in the melts. These values have been predicted based on the work which has been performed by Khedim et al.11,99,103,104 on the binary melts. The mixtures were finely grounded, heat-treated at 1200°C in a Pt plate for about 2 minutes and rapidly quenched in air. This procedure was performed two times in order to ensure a good homogeneity of the Cr2O3-rich glass. Then, approximately 100 mg of the homogeneous Cr2O3-rich glass were melted again in a graphite crucible (Ø = 17 mm, length = 18 mm) at 1200 °C for about 1 minute and rapidly quenched in air. Due to the low wettability between the melt and crucible, the sample formed a spherical shape with an average diameter of 4 mm.

Control of the experimental parameters

In order to control the experimental parameters i.e. temperature (T), basicity (melt compositions) and oxygen fugacity (fO2), the experiments were performed in a closed system (Figure 3.11) which was developed in a previous work105. The system consists of a sealed silica tube containing several components that impose the thermochemical parameters of this study.

Metallographic preparation

Some of the alloys (i.e. samples of isothermal oxidation test) were subjected to the Ni electroplating before being embedded in the cold resin type Epoxy (Escil) in order to preserve the oxide scale at the interface. The samples for corrosion test (i.e. alloys in molten glasses) were subjected to the mechanical polishing starting with SiC abrasive paper grit 120 followed by 240, 400, 400, 600, 800, 1200, 2400 and 4000. The finishing process was performed by using colloidal silica on a polishing disk type Magnet Politex. For the alloys which were immersed in soda silicate melt (hygroscopic glass), the dry polishing must be performed by using an oil based lubricant (Kerdane).
The samples containing just soda-lime silicate glasses were polished with the SiC abrasive paper grit 240 until 4000 before completing with diamond suspension (diameter of 1 μm) on a polishing disk type Magnet ST. The hygroscopic glasses (i.e. soda silicate) were subjected to the dry polishing (without water).
The non-hygroscopic samples were rinsed with water, cleaned with alcohol in the ultrasonic tank and finally dried in air. For the hygroscopic samples, the water was replaced with absolute ethanol.

Thermogravimetric analysis (TGA)

The isothermal oxidation behaviour of the alloys was characterised by Thermogravimetry (SETARAM SETSYS). The thermogravimetric analysis of the alloys led to the knowledge of the nature and thickness of the oxides formed during high temperature treatment in air. The cylinder shape samples (Ø = 5 mm, length ~ 5 mm – 7 mm) were suspended in a furnace with a platinum system which was connected to a beam balance. The oxidation tests were performed in aerated atmosphere. All the samples were subjected to the constant heating and cooling rates of 20oC/min and 5oC/min respectively. The characterisation of the oxidised samples as well as the method used for the treatment of the thermogravimetric data were explained in details in Appendix D.

Table of contents :

CHAPTER 1: INTRODUCTION 
CHAPTER 2: LITERATURE REVIEW 
Introduction
2.1. General aspects of glass
2.1.1. Formation and structure of silicate network
2.1.1.1. Network formers
2.1.1.2. Network modifiers
2.1.1.3. Intermediates
2.1.1.4. The role of CaO
2.1.1.5. The role of Al2O3
2.1.2. Acid-base properties in glass melts
2.1.2.1. Acid-base concepts in glass melts
2.1.2.2. Evaluation of acid-base properties in molten glass
2.1.2.3. Optical basicity
2.1.2.4. Influence of the acid-base properties of the melts on the solubility of oxides
2.1.3. Redox properties in silicate melts
2.1.3.1. Equilibrium constant
2.1.3.2. Redox properties by electrochemical measurement
2.2. Corrosion of metals and alloys by molten glass
2.2.1. Corrosion of pure metals in molten glasses
2.2.1.1. Case of noble metal
2.2.1.2. Case of metals used in glass and nuclear industries
2.2.2. Corrosion of alloys in molten glasses
2.2.2.1. Chromia forming alloys in molten glasses
2.2.2.2. Alumina forming alloys in molten glasses
2.3. Solubility of important oxides in molten glasses
2.3.1 Physicochemical behaviour of chromium oxide in molten glass
Summary
CHAPTER 3: MATERIALS AND EXPERIMENTAL METHODS 
Introduction
3.1. Raw materials
3.1.1. Metal and alloys
3.1.1.1. Pure chromium
3.1.1.2. Ni-based alloys
3.1.2. Glass synthesis
3.2. Experimental procedures
3.2.1. Corrosion by molten glasses
3.2.1.1. Preparation of electrodes
3.2.1.2. Elelctrochemical measurements
3.2.1.3. ‘Raw immersion’ technique
3.2.1.4. Thickness loss measurement
3.2.2. Solubility of chromia (Cr2O3) in molten glasses
3.2.2.1. Glass balls (samples) preparation
3.2.2.2. Control of the experimental parameters
3.3. Sample characterisation
3.3.1. Metallographic preparation
3.3.2. Technique of analysis
3.3.2.1. Thermogravimetric analysis (TGA)
3.3.2.2. X-ray diffraction (XRD) analysis
3.3.2.3. Differential thermal analysis (DTA)
3.3.2.4. Optical microscope
3.3.2.5. Scanning electron microscope (SEM)
3.3.2.6. Electron probe micro-analysis (EPMA)
CHAPTER 4: CORROSION OF CHROMIA FORMING AND ALUMINA FORMING ALLOYS BY MOLTEN GLASSES 
4.1. Corrosion of pure Cr and Ni-30Cr by molten glass
4.1.1. Electrochemical characterisation of the solvents
4.1.2. Electrochemical measurements of the corrosion of pure Cr and Ni-30Cr alloy in silicate melts
4.1.2.1 Spontaneous behaviour of pure Cr and Ni-30Cr alloy in NC3S at 1100°C
4.1.2.2. Behaviour of preoxidised pure Cr and Ni-30Cr alloy in NC3S at 1100°C
4.1.2.3. Influence of temperature on the stability of the passivity states
4.1.2.4. Influence of melt basicity on the corrosion behaviour of pure Cr and Ni-30Cr alloy
4.1.3. Summary of the behaviour of chromia forming alloys in molten glass media
4.2. Corrosion of NiAl and Ni-8Al-28Cr by molten glass
4.2.1. Electrochemical measurements of the corrosion of NiAl and Ni-8Al-28Cr alloys in silicate melt
4.2.1.1. Spontaneous behaviour of NiAl and Ni-8Al-28Cr alloys in NC3S at 1100°C
4.2.1.2. Behaviour of preoxidised NiAl and Ni-8Al-28Cr alloys in NC3S at 1100°C
4.2.2. Summary of the behaviour of alumina forming alloys in molten glass media
CHAPTER 5: KINETICS AND THERMODYNAMIC APPROACH OF CHROMIA SOLUBILITY IN SILICATE MELTS 
5.1. Dissolution kinetics of Cr2O3 in the Na2O-CaO-xSiO2 (NCxS) system
5.1.1. Influence of oxygen fugacity (fO2)
5.1.1.1. Oxidising condition (air)
5.1.1.2. Reducing condition (Fe/FeO)
5.1.2. Influence of temperature
5.1.2.1. Oxidising condition (air)
5.1.2.2. Reducing condition (Fe/FeO)
5.1.3. Influence of melt compositions
5.1.3.1. Influence of melt basicity in soda-lime silicate melts
5.1.3.2. Influence of oxide modifiers
5.1.4. Diffusion of oxygen in the melt
5.1.5. Summary of the dissolution kinetics of Cr2O3 in melts
5.2. Chromia solublity in silicate melts: Thermodynamic approach
5.2.1. Influence of different experimental parameters on the solubility of chromia in silicate melts
5.2.1.1. Influence of temperature
5.2.1.2. Influence of oxygen fugacity (fO2)
5.2.1.3. Influence of glass compositions
5.2.2. Redox behaviour in silicate melts
5.2.2.1. Determination of redox ratio
5.2.2.2. Influence of oxygen fugacity (fO2) on Cr redox behaviour in silicate melts
5.2.3. Summary of the thermodynamic approach of Cr2O3 in silicate melts
5.3. Chromia solubility in soda-lime silicate glass: Correlation with the corrosion of chromia forming alloy in the melt
CHAPTER 6: CONCLUSIONS AND FUTURE RESEARCH RECOMMENDATIONS 
REFERENCES 

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