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Towards understanding the environmental durability and corrosion behavior of protected silver mirrors


High-reflectivity silver mirrors are widely used in optical applications. However, silver tarnishes and corrodes in air, which degrades its reflectivity and optical performance. While various layer schemes have been developed to protect thin film silver mirrors from corrosion and degradation, the mechanisms by which these protective layers improve durability are not fully understood. Accelerated environmental exposure was utilized to investigate the effects of layer composition and nanostructure on the environmental durability and corrosion behavior of Gemini-style protected silver mirrors prepared by plasma beam sputtering. Mixed flowing gas exposures of mirrors with various NiCr- and Cr- based layers revealed that the mechanisms of corrosion and degradation depend largely on the composition of the layers adjacent to the silver.

The corrosion process originates at defects in the dielectric protection layer and proceeds into the coating structure, laterally along various layer interfaces, only sometimes resulting in substantial silver corrosion. Mirrors with one Cr-based layer adjacent to the silver develop characteristic circular corrosion regions along the Ag-Cr interfaces, influenced by oxidation and chloridation of Ag and Cr, reduced adhesion due to dissolution, and stress-induced delamination. Mirrors with NiCr-based layers on either side of the silver develop characteristic corrosion nodules as the interfacial attack is limited but corrosive attack of nickel and silver is locally disruptive to the layered structure. Single flowing gas exposures showed that low concentrations of chlorine significantly influence the extent of corrosion. Investigations on reduced stress in the mirror layers revealed that while stress affects the corrosion kinetics, the composition of the layers has a larger influence on corrosion feature development and growth.

The electrochemical behavior of the layers adjacent to the silver also substantially influences the corrosion process as moisture transports corrosive species into the coating. NiCr, NiCrNx, and freshly-deposited Cr are available to cathodically protect Ag, whereas CrNx is not. However, these materials may be easily passivated, making them less available to provide protection. The oxidation and chloridation of nickel in the NiCr-based layers affects the silver dissolution and redistribution process. A novel application of coulometric reduction showed that optical degradation in these mirrors requires very little corrosion product.

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