Transgranular Stress-Corrosion Cracking

Abstract

The rates of dissolution of atoms from steps of different widths on a metal surface are calculated. It is shown that there is a critical spacing, below which the rate of dissolution possible from a step falls rapidly. This critical spacing is related to the solution viscosity and hence the solution composition and temperature by the relationship:

l _crit = 8 παb³η10¹³ √2V₂(ε_s − ε_∞)/ε_s ²KT³ For a stainless steel immersed in 42% MgCl₂ solution at 150°, the calculated critical value of the slip line spacing is 6 × 10⁻⁵cm. This agrees well With the experimental observations, as a steel which has a spacing of 4 × 10⁻⁵ cm. takes fifty times longer to fail in a stress-corrosion test under these conditions than does a steel with a spacing of 7 × 10⁻⁵ cm. The reactivities of steps of different widths on copper alloys are also calculated and it is shown that slip line spacings found in cold worked copper-zinc alloys containing between 10–30% Zn lie in the critical range where the reactivity of the step changes rapidly with step width.