Glass corrosion : Towards a Unifying Mechanistic Model

Borosilicate glasses are currently used for the immobilization of highly radioactive waste and are materials of choice for many biomedical and research industries. They are metastable materials that corrode in aqueous solutions, reflected by the formation of silica-rich corrosion rims. Until now, there is no consensus in the scientific community about the reaction and transport mechanism(s) and the rate-limiting steps involved in the corrosion of silicate glasses. Most models have the basic assumption in common that ion release from the glass network is occurring via interdiffusion and that the glass network itself is not being disrupted, only modified. On the contrary stands the interface-coupled dissolution-precipitation (ICDP) model, which first was developed for mineral replacement reactions and was recently adapted to glass corrosion. It is based on the congruent dissolution of the glass network that is spatially and temporally coupled to the precipitation and polymerization of silica, forming the amorphous corrosion rim. The dissolution of a radionuclide-binding glass matrix is naturally a sensitive issue for the safe disposal of vitrified high-level nuclear waste. A sound description of the reaction mechanisms and the identification of the rate-limiting steps is essential to predict the long-term corrosion of silicate glasses, particularly when time scales must reach several thousand to millions of years as required by safety regulations for a nuclear repository. In three project studies, the broad spectrum of borosilicate glass corrosion was investigated from the first surface precipitates at an inward-moving solution-glass interface, over the dynamic development of the corrosion rim itself, and the tracing of individual species within the corrosion rim and across the rim-glass interface. Results of atomic force microscopy and single-pass flow-through experiments deliver strong evidence for a significant compositional difference between the surfacial and bulk solution. Hence, local supersaturation of the interfacial solution with respect to amorphous silica at the glass surface can explain how precipitation of silica can occur when the bulk solution is still undersaturated. To study the dynamic development of the corrosion rim in space and time, a novel fluid cell-based in situ Raman spectroscopy method devised. This method allows monitoring the congruent dissolution of the glass and simultaneously the precipitation and polymerization of the silica-based corrosion rim at elevated temperatures in space and time without the need to terminate the running experiment. For the first time, the formation of a silica- and water-rich zone at the interface between glass and corrosion rim could be observed in operando. Commonly such zones were identified post mortem as gaps or cracks between pristine glass and corrosion rim, and, hence, referred to as result of sample drying. However, these results show that these discontinuities are a primary feature of the corrosion process itself and that the dissolution process must proceed within the therein present interfacial fluid. Lastly, multi-isotope tracer (2H, 18O, 10B, 30Si, 44Ca) experiments were performed on pristine and already corroded glass monoliths of different glass compositions. Results of transmission electron microscopy and analyses by nanoscale secondary ion mass spectrometry reveal a nanometre-sharp interface between the silica-based corrosion rim and the glass, where decoupling of isotope tracer occurs, while proton diffusion and ion exchange can be observed within the glass. Moreover, a dense layer was observed between the corrosion rim and glass, which appears to the quenched silica-rich interfacial (pore) solution, which was observed in operando in the in situ Raman experiment. As these new findings cannot be explained by solid-state diffusion processes, nor the classical ICDP process accounts for ion exchange in the glass, a unifying mechanistic model is proposed, which accounts for all critical observations so far made on naturally and experimentally corroded glasses. The main corrosion rim forming process is based on the interface-coupled glass dissolution-silica precipitation reaction. However, a diffusion front over several tens of nanometres may evolve in the glass ahead of the dissolution interface once transport limitations cause the dissolution rate to become slower than the diffusion rate of individual species (DH = 1.3 × 10-23 m2 s-1)

Corrosion of ternary borosilicate glass in acidic solution studied in operando by fluid-cell Raman spectroscopy

Fluid-cell Raman spectroscopy is a space and time-resolving application allowing in operando studies of dynamic processes during solution-solid interactions. A currently heavily debated example is the corrosion mechanism of borosilicate glasses, which are the favoured material for the immobilization of high-level nuclear waste. With an upgraded fluid-cell lid design made entirely from the glass sample itself, we present the polymerization of the surface alteration layer over time in an initially acidic environment, including the differentiation between pore and surface-adsorbed water within it. Our results support an interface-coupled dissolution-precipitation model, which opposes traditional ion-exchange models for the corrosion mechanism. A sound description of the corrosion mechanism is essential for reliable numerical models to predict the corrosion rate of nuclear waste glasses during long-term storage in a geological repository

An unusual compound object in Yamato 793408 (H3.2-an): The missing link between compound chondrules and macrochondrules?

We found a large (~2 mm) compound object in the primitive Yamato 793408 (H3.2-an) chondrite. It consists mostly of microcrystalline material, similar to chondrule mesostasis, that hosts an intact barred olivine (BO) chondrule. The object contains euhedral pyroxene and large individual olivine grains. Some olivine cores are indicative of refractory forsterites with very low Fe- and high Ca, Al-concentrations, although no 16O enrichment. The entire object is most likely a new and unique type, as no similar compound object has been described so far. We propose that it represents an intermediate stage between compound chondrules and macrochondrules, and formed from the collision between chondrules at low velocities (below 1 m s−1) at high temperatures (around 1550 °C). The macrochondrule also trapped and preserved a smaller BO chondrule. This object appears to be the first direct evidence for a genetic link between compound chondrules and macrochondrules. In accordance with previous suggestions and studies, compound chondrules and macrochondrules likely formed by the same mechanism of chondrule collisions, and each represents different formation conditions, such as ambient temperature and collision speed

Towards a unifying mechanistic model for silicate glass corrosion

Borosilicate glasses are currently used for the immobilization of highly radioactive waste and are materials of choice for many biomedical and research industries. They are metastable materials that corrode in aqueous solutions, reflected by the formation of silica-rich surface alteration layers (SAL). Until now, there is no consensus in the scientific community about the reaction and transport mechanism(s) and the rate-limiting steps involved in the formation of SALs. Here we report the results of multi-isotope tracer (2H,18O,10B, 30Si, 44Ca) corrosion experiments that were performed with precorroded and pristine glass monoliths prepared from the six-component international simple glass and a quaternary aluminum borosilicate glass. Results of transmission electron microscopy and nanoscale analyses by secondary ion mass spectrometry reveal a nanometer-sharp interface between the SAL and the glass, where decoupling of isotope tracer occurs, while proton diffusion and ion exchange can be observed within the glass. We propose a unifying mechanistic model that accounts for all critical observations so far made on naturally and experimentally corroded glasses. It is based on an interface-coupled glass dissolution-silica precipitation reaction as the main SAL forming process. However, a diffusion-controlled ion exchange front may evolve in the glass ahead of the dissolution front if SAL formation at the reaction interface significantly slows down due to transport limitations

The Effect of Heavy Ion Irradiation on the Forward Dissolution Rate of Borosilicate Glasses Studied In Situ and Real Time by Fluid-Cell Raman Spectroscopy

Borosilicate glasses are the favored material for immobilization of high-level nuclear waste (HLW) from the reprocessing of spent fuel used in nuclear power plants. To assess the long-term stability of nuclear waste glasses, it is crucial to understand how self-irradiation affects the structural state of the glass and influences its dissolution behavior. In this study, we focus on the effect of heavy ion irradiation on the forward dissolution rate of a non-radioactive ternary borosilicate glass. To create extended radiation defects, the glass was subjected to heavy ion irradiation using 197Au ions that penetrated ~50 µm deep into the glass. The structural damage was characterized by Raman spectroscopy, revealing a significant depolymerization of the silicate and borate network in the irradiated glass and a reduction of the average boron coordination number. Real time, in situ fluid-cell Raman spectroscopic corrosion experiments were performed with the irradiated glass in a silica-undersaturated, 0.5 M NaHCO3 solution at temperatures between 80 and 85 °C (initial pH = 7.1). The time- and space-resolved in situ Raman data revealed a 3.7 ± 0.5 times increased forward dissolution rate for the irradiated glass compared to the non-irradiated glass, demonstrating a significant impact of irradiation-induced structural damage on the dissolution kinetics

The Effect of Heavy Ion Irradiation on the Forward Dissolution Rate of Borosilicate Glasses Studied In Situ and Real Time by Fluid-Cell Raman Spectroscopy

Borosilicate glasses are the favored material for immobilization of high-level nuclear waste (HLW) from the reprocessing of spent fuel used in nuclear power plants. To assess the long-term stability of nuclear waste glasses, it is crucial to understand how self-irradiation affects the structural state of the glass and influences its dissolution behavior. In this study, we focus on the effect of heavy ion irradiation on the forward dissolution rate of a non-radioactive ternary borosilicate glass. To create extended radiation defects, the glass was subjected to heavy ion irradiation using ¹⁹⁷Au ions that penetrated ~50 µm deep into the glass. The structural damage was characterized by Raman spectroscopy, revealing a significant depolymerization of the silicate and borate network in the irradiated glass and a reduction of the average boron coordination number. Real time, in situ fluid-cell Raman spectroscopic corrosion experiments were performed with the irradiated glass in a silica-undersaturated, 0.5 M NaHCO₃ solution at temperatures between 80 and 85 °C (initial pH = 7.1). The time- and space-resolved in situ Raman data revealed a 3.7 ± 0.5 times increased forward dissolution rate for the irradiated glass compared to the non-irradiated glass, demonstrating a significant impact of irradiation-induced structural damage on the dissolution kinetics

Fingerprinting fluid sources in Troodos ophiolite complex orbicular glasses using high spatial resolution isotope and trace element geochemistry

The Troodos igneous complex (Cyprus) is a ca. 90 Ma old, well preserved supra-subduction zone ophiolite. Troodos is unique in that it shows evidence of fluid-saturation throughout the complex, from its base (i.e. podiform chromitites) to its uppermost units-the upper pillow lavas (UPL). However, it is unclear what the source of dissolved water in UPL tholeiites is, with possibilities including shallow seawater infiltration, assimilation of altered Troodos oceanic crust, recycled serpentinized oceanic crust, or subducted pelagic sediments. In order to identify and characterize these components we have carried out a detailed high-resolution study on tholeiitic lavas on orbicular structures and glasses from the UPL in Troodos. Basaltic orbicules were measured for their Sr-Nd-Hf-Pb isotope compositions, and in situ for their B isotopes using LA-MC-ICP-MS. UPL orbicules display a very narrow range in Nd-is an element of and Hf-is an element of (+ 7 to + 8 and + 13 to + 15, respectively) indicating melting of a depleted mantle source. Lead isotopes, specifically Pb-207/(204) Pb vs. Pb-206/Pb-204, form a mixing array with pelagic sediments. Furthermore, high-resolution characterization of individual orbicules revealed that UPL tholeiites display strong variability in Sr-87/Sr-86 (0.7039-0.7060) at the outcrop scale. Samples display delta B-11 between -8.2 (+/- 0.5)% and + 5.9 (+/- 1.1)% with an average B content of ca. 5 mu g/g. Contrary to expectation, altered orbicules and their associated hyaloclastite matrixes display lower delta B-11 (down to -10%) and higher B contents (max. 200 mu g/g) when compared to fresh glass. Furthermore, the orbicules studied here show little or no evidence of interaction with seawater, which is supported by their trace element contents and isotope compositions. When all isotope systems are taken into account, UPL lavas reflect melting of a depleted mantle source that was overprinted by hydrous sediment melts, and potentially, fluid-like subduction components that in part originate from serpentinized oceanic crust. Subsequent low-temperature alteration then drove d delta B-11 to lower values coupled with increased B uptake due to its adsorption into palagonite. (C) 2016 Elsevier Ltd. All rights reserved