Surface Deterioration of some Optical Glasses by Acidic Vapor

The mechanism for producing the white stain on the surface of some optical glasses possessing low chemical durability was investigated. The compositions of the optical glasses used are given in Table 1. All the glasses were ground and polished in the same manner so as to present a similar type of surface. The samples were kept in the humidity cell (Fig. 1) with varying relative humidity from 100, 80.2 to 68.6%, and atmospheres were controlled by adding small content of CO2, H2S or other acidic gases. Weathering was once a time stopped afer 5hrs, the grade of deterioration of glass surface was measured and then the samples were provided for the repeated exposures under 5hrs, cycle. The surface deterioration was observed by optical microscope, ellipsometer (Figs. 3 and 4), electron diffractometer and electron microprobe X-ray analyzer (EMX). The refractive index and the thickness of anomalous surface film on the glass were measured by means of ellipsometry. The quality and quantity of anomalous surface film were determined by means of electron diffraction and EMX. The relations between the relative humidity and the thickness of anomalous film on the glass SK-16, exposed to the atmosphere containing 50% CO2 gas at 50°C during the period of 10 and 20hrs, are shown in Fig. 7. It can be found that the growth of anomalous film increases rapidly with increasing relative humidity and water vapor plays a most important role in these reactions. The changes of film thickness in the case of air atmosphere are shown in Fig. 8. HCI and SO2 show very violent attack, so the film thickness can not be measured. When acidic gas was added to the atmosphere, the water adsorbed on the surface of glass becomes acidic. Then the exchange reaction takes place between Na+, Ba2+ in glass and H+ in adsorbed acid solution. In the case of CO2 gas attacking on the glass SK-16, the growth of stain increases straightly with increasing ionic concentration of H+ (Fig. 19). In the case of glass BaF-10, the curves are shown in Fig. 20. Clearly the diffusion of Ba2+ will carry out the important role in this exchange reaction. The changes of refractive index of the films produced on the glass samples under various conditions are shown in Table 2. It can be found from Table 2 that in the first stage of attacking, the refractive index of anomalous films are 1.45-1.47 in spite of different glasses and different conditions. But in severe attackng by strong acidic vapor, the values of refractive index