The corrosion behavior of BK-7 glasses for use in non-hermetic electro-optic devices

We have examined the stability, corrosion resistance, of four different commonly used optical glasses, including BK-7 made by three different manufacturers in hot humid ambients. We found that although all three BK-7´s are considered industrial equivalents in terms of optical and physical properties, they have slightly different chemical compositions, and exhibit significantly different corrosion behaviors. The corrosion rates varied by at least a factor of two, this improvement appears to be related to the ZnO Content in the glass. We found one borosilicate optical glass with similar optical properties to BK-7, that has at least a factor of 3 times better corrosion resistance than the best BK-7. Additionally, we developed a technique which alters the surface structure and chemistry of the BK-7 lenses such that they can be used in non-hermetic applications and provide sufficient reliability for telecommunication devices. We propose a six stage corrosion model to explain the corrosion behavior of borosilicate glasses in hot humid environments: i) Adsorption of “bulk” like water at defective sites on the surface of the glass; ii) ion exchange between the alkali ions in the glass and protons in solution; iii) growth of the adsorbed water droplet followed by more ion exchange until the entire surface of the glass is covered with an aqueous film; iv) dissolution of the silicate network by the basic solution and formation ionic SiO^- species; v) reaction between the ionic SiO^- species and the hydrolized alkali in solution; and vi) precipitation and growth of the alkali silicate reaction product. Finally, we determine the acceleration factor for device failure due to corrosion of one type of BK-7 glass. The temperature (T) dependence of failure is assumed to take an exponential form with a fitting constant, activation energy, between 0.64 eV and 0.81 eV. The relative humidity (RH) dependence of failure is estimated to be of the form Rate ∝ exp {constant(RH²)}. The fitting constant is estimated to fall between 4.8×10^-4 %² and 6.0×10^-4/%²