Redox profile of the glass surface

The redox profiles of tin, iron and sulfur at the float glass surface were determined on purposely cut samples by Electron Probe Micro Analysis (EPMA), X-ray Fluorescence mapping and X-ray Absorption Spectroscopy (XAS) at the micron scale. Going inward from the surface to the bulk, it was observed that (features do not depend on the glass thickness (holding time) though they extend over depths that vary from ca. 25 to 50 μm): (i) after a diffusion-driven decrease and prior to vanishing, the tin concentration passes through a local maximum (the tin hump), where stannous ions, which dominate the shallow layers, switch to stannic ions; (ii) the iron concentration decreases, passes through a minimum at the tin hump where iron is in the more reduced form (lowest Fe3+/Fe2+ ratio); it then increases and, after a hump which appears as a chemical echo of the tin hump, it reaches the bulk value; (iii) the concentration of sulfur increases up to reach the bulk concentration beyond the tin hump region. In a mixed S6+/S2− form at the surface, sulfur is only in sulfate form in the bulk.In the case under study, the iron concentration is much too low to balance the redox reaction Sn2+ → Sn4+ that occurs at the tin hump. Sulfur is shown to play the role usually attributed to iron, according to the reaction4Sn2+ + S6+ → 4Sn4+ + S2−The occurrence of that reaction is supported by the appearance of sulfide S2− in the tin hump region with an appropriate concentration profile of a much stronger S2−/S6+ ratio on the tin side than on the atmosphere side of the float glass. The conclusions drawn herein likely apply to the many cases in which the glass composition is similar as that encountered herein.

Research highlights
► Profiles of Fe, S and Sn, and their valence have been obtained at the micron scale below the surface of a float glass.
► The role of sulfur in the change of the valence state of tin is evident.
► These X-ray spectroscopy experiments allows a full description of the chemical state of the first tens of micrometers of a glass surface.