Observations on the structural degradation of silver during simultaneous exposure to oxidizing and reducing environments

The structural stability of silver (Ag) in dual atmosphere exposure conditions, which are representative of solid oxide fuel cell (SOFC) current collector and gas seals, has been examined in the 600-800 degreesC temperature range. Experiments conducted on Ag tubular sections exposed to flowing H-2-3% H2O (inside the tube) and air (outside the tube) showed extensive porosity formation along the grain boundaries in the bulk metal. Similar tubular sections, when exposed to air only (both inside and outside the tube), showed no bulk porosity or structural changes. It is postulated that the porosity formation in the bulk metal is related to the formation of gaseous H2O bubbles due to simultaneous diffusion of hydrogen and oxygen followed by subsequent interaction resulting in the formation of steam. Thermochernical processes that are responsible for structural degradation are presented and discussed. Based on experimental observations, it is concluded that Ag metal may not provide adequate long-term structural stability under a dual-environment condition that is typical of interconnects or gas seals in intermediate temperature SOFCs.