Thermal-fluctuation mechanism of long-term corrosion of glass caused by internal stress: processes of depolymerization, impurity migration and fracturing on an atomic scale
Long-term corrosion of glass is studied using polarising light microscopy, FTIR and SEM including EDX and elemental mapping. Early 19th century beads made of lead-potassium glass are the object of the study. Non-uniformly distributed internal stresses were introduced into the studied glass during bead manufacturing. Corrosion of 19th century bead glass has been shown to comprise several mutually connected processes developing in parallel and intensifying one another. These processes are the depolymerization of glass, the directed migration of alkali-metal impurities and local glass leaching, and the nucleation of micro discontinuities followed by the formation of micro cracks and their gradual growth. In course of time, their cumulative and apparently synergistic effect results in glass fracturing terminated with bead crumbling into tiny discoloured particles. All these phenomena are controlled by the internal stress originated from the bead production process. Thus, the phenomenon of stressed bead glass corrosion is the process of the internal stress relaxation in glass through the nucleation and growth of cracks, which lasts for many decades at room temperature. It is dramatically accelerated when beads are heat treated. Annealing at 300°C for 15 min. is sufficient for artificial ageing of initially intact beads. The corrosion has been observed to occur in glass even if beads are kept in well-controlled museum environment. The local thermal-fluctuation mechanism is proposed to account for the corrosion phenomenon on an atomic scale. The approach based on the thermal-fluctuation mechanism is applicable to describe degradation and destruction processes in various solid substances and composite materials under stress; this approach enables understanding the phenomena of formation and degradation of nanostructures, e. g., heterostructures with quantum wells or quantum dots.