Corrosion of Heavy Crown Glass by Alkaline Solutions

Abstract

It has been generally recognized that alkaline solutions attack the silica network in glass, leaving attacked glass surface ready to be exfoliated. As well known, NaOH solution has larger corrosive action than NH4OH solution at an equivalent pH value6, despite that both solutions should have similar corrosive power, if the concentration of OH- is the only factor related to alkali attacking.In the present work, a series of alkaline solutions containing one of various mono- or divalent canons, that is, LiOH, NaOH, KOH, RbOH, NH4OH (weak base and ionic radii of NH4+ is same to that of Rb+) and N (CH3)4OH (strong base) or Ca(OH)2, Sr(OH)2 and Ba(OH)2 were used under controlled conditions of attacking time and pH for studying the mechanism of corrosion of heavy crown glass (SK-16) by alkaline solutions. Interferometric techniques were applied for the determination of corrosion depth taken up as a measure of “degree of corrosion”.From the results obtained, were pointed out the following remarkable features.(1) With the exception of NH4OH and Ca(OH)2, the solutions containing a given monovalent cation were stronger in corrosive action than all those containing any divalent cation (see Fig. 2-10). This can be interpreted in terms of “effective reactivity of OH-” determined by the characteristics of cation being in the solution.(2) It was found that the curves representing the relation between depth of corrosion and time of attacking went parabolically on in the range of pH value less than 10.0 and ran linearly over 10.5 (see Fig. 2-10). It is considered that the diffusion of Ba2+ in the former and the surface reaction between glass and OH- in the latter are the rate-determining process respectively.(3) The curve plotted corrosion depth against pH value had a minimum observed at around 10-10.5 in pH value (see Fig. 11).(4) NH4OH had largest corrosive power in all alkaline solutions. This can be explained by the fact that as the dissolution of glass into alkaline solutions proceed so far, the pH value decrease with increasing B2O3 dissolved in strong bases, while the pH value in NH4OH solution are not so much altered probably owing to the buffer action exerted between weak acid (H3BO3) and weak base (NH4OH) (see Fig. 13).(5) Ca(OH)2 had the smallest corrosive power. This might be accounted for by the consideration that silica has the smallest solubility in Ca(OH)2 solution (see Fig. 12).