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

Examines the stability and 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, 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 which has at least a factor of 3 times better corrosion resistance than the best BK-7. We propose a six stage corrosion model to explain the corrosion behavior of borosilicate glasses in hot humid environments: 1) adsorption of “bulk” like water at defective sites on the surface of the glass; 2) ion exchange between the alkali ions in the glass and protons in the “bulk” like water 3) 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; 4) dissolution of the silicate network by the basic “bulk” like water film and formation ionic SiO^$species in the “bulk” like water; 5) reaction between the ionic SiO^$ species and the hydrolized alkali in solution; 6) 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 an activation energy between 0.64-0.81 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