Reactions Between Metallic Iron and Cobalt Oxide‐Bearing Sodium Disilicate Glass

Electrochemical reactions are initiated at a glass‐metal interface if thermodynamic equilibrium, represented by saturation of the glass with the low‐valent oxide of the substrate metal, is not present at the interface. On heating the system iron‐sodium disilicate glass (with or without cobalt oxide) in platinum crucibles, the metal oxidized with the addition of atmospheric oxygen to the glass. Such weight gains did not occur on heating the system in alumina crucibles. With glasses containing cobalt ions, whose potential for oxidation is lower than that of iron, electrochemical reactions resulted in oxidation of iron at the interface and reduction of metallic ions in the glass with the formation of dendrites. The composition of a precipitating dendrite in all cases was in thermodynamic equilibrium with the adjoining glass. A galvanic cell of iron | glass | glass with cobalt oxide | cobalt illustrated the electrochemical nature of the reactions by oxidation of iron at the iron‐glass interface and reduction of cobalt ions at the cobalt‐glass interface to form dendrites. In all cases fulfillment of local electroneutrality was largely satisfied by movement of the highly mobile sodium ions, which generally resulted in an uphill composition gradient for sodium because of the establishment of an electrochemical potential gradient.