Corrosion for Polished Surfaces of Optical Glass by Small Droplet of Etchant Solution

Abstract

By putting a small droplet of etchant solution on the surface of optical glass were studied topographic marks produced thereon with the aid of a microscope and a multiple-beam interferometer. Small droplets (about 0.01ml) of water, acid and alkaline solutions, and alkaline solutions containing some quantity of dissolved SiO2 were put on two samples of polished surfaces. One of them was kept in air and another was kept in mineral oil, for example gasoline, during various times at room temperature.Several types of corrosion were observed (see Table 2. and Fig. 3). In the case of water corrosion, the dissolution was uniform (Type B) (see Fig. 8). Thereafter the type of corrosion turned from B to C (see Fig. 9) with increasing time of attack. The reason for this phenomena is explained by the change of pH values. In alkaline solution, the degree of corrosion became deeper and that type was observed to be C (see Fig. 12). In acid solution, it was deepest and that type was A (see Fig. 4, 5). And in alkaline solutions with different quantities of dissolved SiO2, there appeared peculiar corrosion marks of type F (see Fig. 14, 15, 17) caused by precipitating supersaturated SiO2.When gasoline was used for immersion liquid, the interference color appeared on glass surface in all etchant solutions used. On the other hand, when the glass was kept in air, the depth of corrosion was smaller than that in gasoline and large number of microcrystals were observed on the corroded surface (see Fig. 7).The cause for producing the stain on the surface of glass during the process of polishing was cleared up and the method for protecting the glass against stain was found; from practical point of view, it is very important to evaporate up traces of water droplet condensed on the surface of glass (lens) before dipping the glass into gasoline (or other organic solvents) for removing pitch or similar adhesives.