Development and growth of corrosion features on protected silver mirrors during accelerated environmental exposure

Various layer schemes have been developed to protect thin film silver mirrors from tarnish and corrosion. However, the mechanisms by which these additional layers improve mirror durability are not fully understood. Accelerated environmental exposure testing of protected silver mirrors was used to investigate the effects of layer composition on the mechanisms of corrosion feature development and growth. Two model mirror coatings were analyzed in which the composition of the base layer below the silver and the adhesion layer above the silver were varied. Large circular corrosion features formed preferentially along the silver-chromium interfaces, regardless of where this interface was located within the layered structure of the mirror. The corrosion features originated at coating defects in the dielectric protection layer, and their growth was likely driven by the oxidation and chloridation of both silver and chromium at the Ag-Cr interfaces.