Stress corrosion cracking of quartz: A note on the influence of chemical environment

Abstract

Tensile (mode I) stress corrosion growth of cracks on the a-plane of quartz in a direction normal to z has been studied in de-ionized water, in 2 N HCl and in 2 N NaOH solutions at 20° C. The double torsion testing method was used to obtain crack velocity (v) stress intensity factor ( K I ) curves. The stress corrosion index, n (where v = α K I n ) was 19.3 for 2 N HCl, 12.8 for deionized water, and 9.5 for 2 N NaOH. The degree of influence of reagent chemistry on crack velocity was most marked at low K I values (at 0.5 K Ic 3 orders of magnitude increase in crack velocity results from substituting 2 N NaOH for 2 N HCl as the corrosive medium), but decreased monotonically on raising K I so that close to K Ic there was no clearly discernible influence on crack growth rates (ca. 10 −3 m s −1). These observations are explained in terms of a model of crack propagation in which at low K I values the chemistry of the bulk fluid environment controls the corrosion reaction. The rate of crack growth is enhanced by increasing environmental OH − concentration. At high K I (or velocity), however, the chemical composition of the quartz surface may control crack propagation rates through its influence on the chemistry of the crack tip fluid. As the pH of natural pore fluids is likely to vary widely in different geological environments these results should be noted when invoking stress corrosion to explain geophysical phenomena.