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 manufactures 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 higher ZnO content in the glass. We found one borosilicate optical glass with similar optical properties to BK-7, Schott Glass ZKN-11, has at least a factor of 3 times better corrosion resistance than the best BK-7. Furthermore, we found that an anti-reflective (AR) SiO sub 2 based coating can and does increase the durability of the BK-7 lenses such that they can be used in non-hermetic application 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 the "bulk" like water; iii) growth of the "bulk" like water droplet followed by more ion exchange until the entire surface of the glass is covered with a "bulk" like water film; iv) dissolution of the silicate network by the basic "bulk" like water film and formation ionic SiO species in the "bulk" like water; 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 0.64eV and 0.81eV. The relative humidity (RH) dependence of failure is estimated to be of the form Rate prop exp {constant(RH sub 2)}. The fitting constant is estimated to fall between 4.8x10 sup (-4) /% sup 2 and 6.0x10 sup (-4) /% sup 2.