Oxidation and Corrosion of SiC(particulate)/Al/Al2O3 Composites in Sodium Silicate at Elevated Temperatures

Ceramic-matrix composites (CMCs) fabricated by the directed metal oxidation process (Dimox^TM) may have applications in heat exchangers in high-temperature corrosive environments such as those in the glass industry. The oxidation and corrosion properties of such CMCs with and without preformed metal-free surface layers have been investigated in the temperature range of 1000–1300°C. The untreated CMCs experienced rapid oxidation in air leading to mass increases of 100 to 140 mg/cm² in less than 1 h. This occurred by oxidation of residual metal in the composite to form Al/Al₂O₃ deposits on the surface. After the initial formation of the oxidation product, there is little further reaction during up to 300-h exposures to oxidizing atmospheres. Experimental composite coupons with metal-free surfaces were resistant to oxidation except for localized events associated with flaws. Small amounts of sodium silicate (2 to 40 mg/cm²) painted on the surfaces produced no corrosive effects on any of the specimens. Dynamic corrosion experiments, in which a continuous mist of sodium silicate was sprayed onto the surfaces, produced corrosion at 1300°C.     

Lead-Ion Stability in Soda–Lime Lead Silicate Glasses

Sodium-calcium-lead silicate glass compositions were prepared over a wide compositional range by conventional glass-melting methods. The acid chemical stability of the glass structure was studied by corroding samples of glass in 4% acetic acid solution at 22°C for periods up to 24 h. Lead corrosion stability was evaluated by measuring lead concentrations in the corrosion solution. At short times, parabolic time dependence was observed and the parabolic time coefficients were regressed against composition, yielding a simple additive relationship. A similar model was fitted to 24-h release data, which showed compositional effects similar to the 2-h data. Of the oxides studied, sodium was the most detrimental to durability, and the coefficients of the oxides decreased in the series: Na₂O, PbO, CaO, SiO₂. The effects of the oxides could be partially explained by the number of nonbridging oxygens expected in the glass structure, and the residual effect was attributed to cation characteristics. Small phosphate additions to the glass greatly increased the lead-ion stability under nearly all experimental conditions examined.     

Hot Corrosion of Silica

The high-temperature corrosion of bulk silica glass was studied in pure oxygen and in SO₃-containing oxygen atmospheres in the presence of liquid sulfate deposits at temperatures of 700° and 1000°C. No reaction and devitrification were observed without Na₂SO₄ on the surface. The wetting of the silica by the sulfate, the tendency toward basic fluxing, and the crystallization of the silica incrased with the activity of Na₂O. The most extensive degradation of vitreous silica occurred by crystallization, and the resulting spalling under basic conditions and thermal cycling at basic conditions were parabolic. This behavior is explained by a model in which the crystallization is controlled by sodium at the glass-crystal interface and its diffusion into the glass. This sodium diffuses into the glass before crystallization and is swept ahead of the crystallization front.     

Development of a Self-Forming Ytterbium Silicate Skin on Silicon Nitride by Controlled Oxidation

A dense and uniform polycrystalline ytterbium silicate skin on silicon nitride ceramics was developed by a controlled oxidation process to improve the hot corrosion resistance of silicon nitride. The process consists of purposely oxidizing the silicon nitride by heating it at high temperatures. It was found that the ytterbium silicate phase was formed as an oxidation product on the surface of the silicon nitride when it was exposed to air at temperatures above 1250°C. The volume fraction of ytterbium silicate compared with that of SiO₂ on the silicon nitride surface increased with increasing oxidation time and temperature. The formation and growth of ytterbium silicate on the surface of silicon nitride is attributed to a nucleation and growth mechanism. Ultimately, a dense and uniform ytterbium silicate skin with 3–4 μm of skin thickness was obtained by oxidation at 1450°C for 24 h. The ytterbium silicate layer, formed by oxidation of the silicon nitride, is associated with the reaction of SiO₂ on the surface of silicon nitride with Yb₂O₃ introduced in the silicon nitride as a sintering additive. Preliminary tests showed that the ytterbium silicate skin appears to protect silicon nitride from hot corrosion. No observable evidence of a reaction between the skin and molten Na₂SO₄ was found when it was exposed to molten Na₂SO₄ at 1000°C for 30 min.     

Aqueous Corrosion of the GeSe4 Chalcogenide Glass: Surface Properties and Corrosion Mechanism

The aqueous corrosion behavior of the GeSe₄ glass composition has been studied over time under various conditions (temperature and pH). The evolution of the surface topography by atomic force microscopy and properties such as surface hardness and reduced modulus, as well as the optical transmission in the 1–16 μm window, have been measured as a function of time spent in the corrosive solution. It was found that even if the glass reacts at room temperature, its optical transparency was barely affected. Nevertheless, the durability of GeSe₄ was found to be drastically affected by an increase of both temperature and pH. Furthermore, pure selenium nanoparticles were formed during the corrosion process, and the nature of these nanoparticles—amorphous or crystallized (hexagonal phase)—depends on temperature. A reaction mechanism was proposed, and the activation energy of the reaction of corrosion in deionized water (47 kJ/mol) was determined from an original technique that relies on the temporal optical loss variation of a GeSe₄ optical fiber placed in water at different temperatures.     

Thermally Stimulated Polarization and Depolarization in Halogen-Containing Lead Silicate Glasses

Thermally stimulated polarization and depolarization current (TSPC/TSDC) measurements were made on lead halide silicate glasses having compositions (65 − x )PbO· x PbX₂·35SiO₂ where x = 0, 0.1, 2, 10 and X = F, CI, Br, and I. The addition of halogen ions to lead silicate glasses gives rise to a new high-temperature TSDC peak in the vicinity of the peak previously observed in binary lead silicate glasses. The integrated area of the new peak is dependent on the amount and type of halogen ion present in the glass and does not saturate in the temperature range of our measurements. This new peak is attributed to space charge polarization of halogen ions.     

Electrochemical Corrosion and Protection of Molybdenum and Molybdenum Disilicide in a Molten Soda-Lime Silicate Glass Environment

Electrochemical corrosion of Mo and MoSi₂ in a molten soda-lime silicate glass environment was studied using a two-electrode cell with Pt as the counterelectrode. XRD, SEM, and EDS were used to characterize the corroded specimens. When Mo was cathodically polarized (-1 V), no corrosion and a Mo surface were observed after 12 h of electrochemical corrosion. Under anodic polarization (+1 V), higher corrosion (0.42 mm) was observed due to formation of MoO₂, which did not form a coherent passive layer. Cathodic protection occurred by electrostatic repulsion of oxygen ions, thus discouraging oxidation of Mo. Anodically polarized (+1 V) MoSi₂ showed a significant decrease in recession at the glass line due to formation of SiO₂ product which attenuated convective flow currents. Accelerated corrosion was observed under cathodic polarization (-1 V), where streams of Mo particles were observed flowing downward in the melt. MoSi₂ was interpreted to decompose into neutral molybdenum and silicon anions, which were rejected into the melt.     

Mechanical Behavior of a Borosilicate Glass Under Aqueous Corrosion

In France, fission products are being vitrified for a possible final geological disposal. Under disposal conditions, corrosion of the glass by groundwater as well as stress corrosion because of stresses occurring at surface flaws cannot be excluded. Within this framework, the mechanical behavior of the French simulated nuclear waste glass SON68 was studied by Vickers indentation and fracture experiments in air and in a corrosive solution. The glass was corroded at 90°C in a solution enriched with Si, B, and Na. The results showed that the glass corrosion enhances the cracks propagation relative to experiments in air. The indentation fracture toughness ( K _{I C} ) obtained using a four-point bending test showed that the K _{I C} of the glass decreased with increasing corrosion time.     

Hot Corrosion of Sintered α-Sic at 1000°C

The hot corrosion of sintered α-Sic by thin films of Na₂SO₄ and Na₂CO₃ was studied at 1000°C in controlled gas atmospheres. Under all conditions corrosion led to 10 to 20 times the amount of SiO₂ formed in pure oxidation after a 48-h exposure. In addition, small amounts of sodium silicate formed. Melts of Na₂SO₄/SO₃ caused uniform pitting of the Sic substrate; Na₂CO₃/CO₂ melts caused localized pitting and grain-boundary attack. In all cases the protective SiO₂ layer dissolved to form silicate, leading to corrosion. In the sulfate case, free carbon in the Sic promotes this process. In all cases the presence of liquid films is responsible for rapid transport rates and the subsequent rapid reaction.     

Corrosion of Lead Glasses in Acid Media: II, Concentration Profile Measurements

Lead concentration profiles were measured in a binary lead silicate glass corroded by acid using ahd a-backscattering technique. These profiles were characteristic of those predicted for an interdiffusion process and revealed a surface layer deficient in lead. Diffusion coefficients for Pb²⁺ were calculated, together with values of the total amount of lead removed at the various experimental temperatures and times. Good agreement was found between these data and those previously obtained experimentally. All diffusion coefficient values were high and, correspondingly, activation energy values were low compared with those predicted by the literature for similar glass compositions. This difference was justified from consideration of the leaching process.     

Chemical Durability of Na2O-K2O-CaO-SiO2 Glasses

The mixed-alkali effect on chemical durability was evaluated for bulk and powdered Na₂O-K₂O-CaO-SiO₂ glasses. Two contributions to this effect are discussed, i.e. those of glass structure and corrosion solution. The magnitude of the mixed-alkali effect depends on the pH of the corroding solution. For pH<9, alkali-ion leaching is the major mode of glass corrosion, and the mixed-alkali effect is large. However, for pH≥9 congruent glass dissolution is the rate-controlling corrosion mechanism, and the effect is diminished. When ion exchange is the rate-controlling corrosion mechanism the magnitude of the mixed-alkali effect depends on the extent of reaction. Since the kinetics of corrosion are influenced by the ratio of glass surface area to solution volume ( SA/V ), results obtained when this ratio is large may be misleading if extrapolated to smaller ratios. Physical evidence of the mixed-alkali effect in bulk glasses was obtained using scanning electron microscopy and ir reflection spectroscopy.     

A Novel Sodium Silicate Fluoride Solution and a H2 Gas Formed by a Reaction Between Si and an Aqueous Solution of NaOH and NaF

Oxidation of Si in an alkali solution has attracted much attention to produce hydrogen gas from Si wastes. It was found that hydrogen gas was generated from Si in NaOH and NaF solution with more than 10 times reaction rate than in the NaOH solution. The reaction by-product was a sodium silicate fluoride, Na _{x + z} Si _y O_{2 y + x /2}F _z solution, and it proved to transform to a glass foam by boiling. The Raman spectrum suggested that the silicon and oxygen atoms in the silicate anions were arranged in a sheet-like structure. This may affect the behavior to form the glass foam.     

X-ray Photoelectron Spectroscopy Study on the Process of Apatite Formation on a Sodium Silicate Glass in Simulated Body Fluid

The process of apatite formation on the surface of Na₂O–SiO₂ glass in a body environment was investigated, mainly by X-ray photoelectron spectroscopy, as a function of soaking time in a simulated body fluid (SBF). The glass was found to release Na⁺ ions via exchange with H₃O⁺ ions in the SBF to form Si—OH groups on its surface. These Si—OH groups induced apatite formation indirectly, by forming calcium silicate and amorphous calcium phosphate. The formation of the calcium silicate and amorphous calcium phosphate is attributed to electrostatic interactions between the Si—OH groups on the glass surface and the calcium and phosphate ions in the SBF.     

Interdiffusion and Phase-Boundary Reaction Between Silicate Glass Surface and Aqueous CsCl Solution

Technical silicate glass samples with fresh fracture surfaces or HF-etched surfaces were treated in aqueous CsCl at 90°C for 10 to 85 min. Reaction profiles were determined quantitatively using a SIMS technique. The Cs₂O concentration at the glass surface increased with increasing time, followed by penetration of Cs⁺ ions into the glass. The profiles can be described assuming a model which contains both a surface phase-boundary process and an interdiffusion process in the glass for which D=5.10⁻¹⁷ cm² s⁻¹.     

Molten-Salt Corrosion of Silicon Nitride: I, Sodium Carbonate

Corrosion of Si₃N₄ under thin films of Na₂CO₃ was investigated at 1000°C. Pure Si₃N₄ and Si₃N₄ with various additives were examined. Thermogravimetric analysis and morphology observations lead to the following detailed reaction mechanism: (I) decomposition of Na₂CO₃ and formation of Na₂SiO₃, (II) rapid oxidation, and (III) formation of a protective silica layer below the silicate and a slowing of the reaction. For Si₃N₄ with Y₂O₃ additions, preferential attack of the grain-boundary phase occurred. The corrosion of pure Si and SiC was also studied for comparison to Si₃N₄. The corrosion mechanism generally applies to all three materials. Silicon reacted substantially faster than Si₃N₄ and SiC.     

Leaching of Lead and Connectivity of Plumbate Networks in Lead Silicate Glasses

The formation of a plumbate network in binary lead silicate glasses was examined based on the leaching behavior of Pb²⁺ in lead silicate glasses over a wide composition region. The effective diffusion coefficient of Pb²⁺ at 40°C was on the order of 10⁻¹⁷ m²/s for PbO<35 mol% glasses, and increased three orders of magnitude for 35–50 mol% PbO contents. Such a steep composition dependence is considered to be because of changes in the medium or longer range structure. That is, it is proposed that the plumbate network forms a percolative 3D network in the composition region to form diffusion paths for the lead ions. In addition, the present results indicate that the lead ion exists as a network former over the entire glass forming composition range of the binary silicate glasses.     

Kinetics and Mechanism of Corrosion of SiC by Molten Salts

Corrosion of sintered α-SiC under thin films of Na₂CO₃/CO₂, Na₂SO₄/O₂, and Na₂SO₄/SO₃ was investigated at 1000°C. Chemical analysis was used to follow silicate and silica evolution as a function of time. This information couples with morphology observations leads to a detailed corrosion mechanism. In all cases the corrosion reactions occur primarily in the first few hours. In Na₂CO₃/CO₂ case, rapid oxidation and dissolution lead to a thick layer of silicate melt in ∼0.25 h. After this, silica forms a protective layer on the carbide. In the Na₂SO₄/O₂ case, a similar mechanism occurs. In the Na₂SO₄/SO₃ case, a porous nonprotective layer of SiO₂ grows directly on the carbidem and a silicate melt forms above this. In addition due to reaction of silicate with SO₃ and SO₂+O₂. The reaction slows when the lower silica layer becomes nonporous.     

High-Temperature Hydroxylation and Surface Corrosion of 2/1-Mullite Single Crystals in Water Vapor Environments

2/1-mullite single crystal (001) plates with thicknesses between 0.9 and 1.9 mm were exposed for 1.5, 3, 6, and 12 h at 1670°C to a slowly flowing (100 mL/min) water-rich gas mixture (O₂/H₂O 80/20). Under the given experimental conditions, 2/1-mullite yielded significant amounts of structurally bound OH groups across the bulk and decomposition of the crystal surface on a micrometer scale. Decomposition products are (i) sodium-containing silicon-rich alumino silicate glass formed from melt and (ii) α-alumina, which crystallizes within melt cavities. The crystal plates that are free of any OH absorption before the corrosion experiments show a steep increase in OH absorption intensity up to 3 h of corrosion and a flattening toward longer times of exposure. The evaluation of OH intensity profiles implies an effective diffusion coefficient D _H in the range between 1.5 and 2.5 × 10⁻⁷ cm²/s.     

Reaction of Glasses with Hydrofluoric Acid Solution

The gravimetric method was used to study the reaction between fused silica and silicate glasses with HF acid solution. The reaction was found to be transport-controlled. Additions of Al₂O₃, CaO, or both to fused silica caused a reduction in corrosion resistance of the resulting glasses.     

Compound-Glass Waveguides Fabricated by a Metal Evaporation Technique

Glass fiber optical waveguides consisting of a potassium silicate glass core and SiO₂ cladding were produced by a potassium metal evaporation technique. An evacuated fused-silica tube containing an evaporated deposit of K was heated to temperatures sufficient to form a uniform potassium silicide layer on its inner surface. This layer was oxidized and reacted with the SiO₂tubing at high temperatures to form a potassium silicate glass. The tube was then collapsed to give a solid rod preform with a potassium silicate core. Fibers drawn from such preforms exhibit moderately low optical loss. The fabrication technique is described, and a representative loss spectrum is presented.