Insights into the mechanisms controlling the residual corrosion rate of borosilicate glasses

Abstract

Borosilicate glasses are widely used to confine high-level radioactive wastes. The lifetime of these materials could reach hundreds of thousands of years if leaching of the glass into groundwater enables the formation of a passivating gel layer. Even in this regime, the glass will never stop corroding as thermodynamic equilibrium between glass and solution cannot be achieved. Therefore, accurate predictions of glass durability including passivation, require a deep understanding of the mechanisms controlling the so-called residual rate. However, despite tremendous efforts, these mechanisms remain poorly understood. Here, focusing on the behavior of the soluble elements of the International Simple Glass (B, Na, and Ca), we show that the residual rate is controlled by the behavior of B, a glass former supposed to dissolve instantaneously when in contact with water and thus widely considered as an ideal tracer. We then demonstrate that B release is controlled by multiple processes highly dependent on the pH. At the beginning of the passivating layer formation, the hydrolysis of B-O-Si linkages is rate-limiting and has an activation energy of ∼60 kJ mol−1, a value slightly lower than that for breaking Si-O-Si linkages. Once the fraction of closed pores resulting from gel restructuring is high enough, then diffusion of both reactants (water molecules) and some products (mainly Baq, Caaq) through the growing gel layer becomes rate-limiting. Consequently, B and Ca accumulate in an inner layer referred to as the active zone, with potential feedback on the B-O-Si hydrolysis. A new paradigm, including B as a key element of the system, is proposed to develop a comprehensive model for the corrosion of borosilicate glass.

Atomic Insights into the Events Governing the Borosilicate Glass–Water Interface

“…In the same context, a combination of DFT and NEB calculations indicate that the barrier energy for hydrolysis of B–O–B and B–O–Si bridges is much less than for Si–O–Si hydrolysis under acidic and neutral conditions, but this behavior is less pronounced under basic conditions . From experimental evidence, it was shown that under basic conditions and in dissolved silica, the hydrolysis of the B–O–Si bridges is rate limiting and has a slightly lower activation energy than for Si–O–Si hydrolysis . This may suggest that under acidic and neutral conditions, the dissolution of boron is faster than silicon (the release of boron will be much faster compared to silicon at low pH than at high pH).…”

“…34 From experimental evidence, it was shown that under basic conditions and in dissolved silica, the hydrolysis of the B− O−Si bridges is rate limiting and has a slightly lower activation energy than for Si−O−Si hydrolysis. 89 This may suggest that under acidic and neutral conditions, the dissolution of boron is faster than silicon (the release of boron will be much faster compared to silicon at low pH than at high pH). However, as the previous studies did not take into account, on the one hand, two different kinds of boron units, namely, B III and B IV (see Table S2), and, on the other hand, the difference between the boron atoms of the same unit, understanding the boron release mechanism cannot be so simple (see Table 2).…”

Theoretical Considerations on the Characteristic Timescales of Hydrogen Generation by Serpentinization Reactions on Enceladus

“…Whether such a mechanism also applies in the basic pH range remains an open question. Whereas the formation of ASSLs in the basic pH range has been well documented for the weathering of borosilicate glasses (e.g., Gin et al, 2017Gin et al, , 2020, very few studies have investigated the chemical composition at the nm-scale of the near-surface region of dissolved olivine and pyroxenes in the basic pH range. Worthy of note, the spectroscopic characterizations of the surface composition of altered olivine performed by (Pokrovsky & Schott, 2000) suggested that ∼2 nm-thick ASSLs could be evidenced up to a pH of ∼9.…”

Effects of Al:Si and (Al+Na):Si ratios on the static corrosion of sodium‐boroaluminosilicate glasses

“…The CJ4 composition was chosen for fabrication as the International Simple Glass (ISG) by the international nuclear waste glass community to study glass properties and durability behavior. 1,[55][56][57][58][59][60][61][62][63][64][65] A new series covering an extended Al compositional space based on the composition of ISG would provide valuable information regarding the effect of Al on long-term durability applicable to nuclear waste glasses.…”