Analysis of hydrogen-induced subcritical intergranular crack growth of iron and nickel

Abstract

Hydrogen-induced subcritical crack growth of iron and nickel alloys has been shown to occur in both gaseous and aqueous environments and is a concern in a variety of applications. Identification of the mechanisms by which hydrogen induces crack growth in these materials remains an unsettled issue, with hydrogen-enhanced shear and hydrogen-induced decohesion the primary mechanisms under consideration. The purpose of this paper is to report on subcritical crack growth tests of iron and nickel conducted at catholic potentials under conditions in which the crack growth mode was predominantly intergranular. Subcritical crack growth tests were conducted using compact tension samples with thicknesses of 2, 5, and 10 mm at cathodic potentials ranging from −0.6 to −1.25 V for iron and −0.3 to −1.0 V for nickel in 1 N H 2SO 4. Crack growth thresholds and crack velocities were determined as a function of sample thickness and test potential. Plastic zone sizes for iron and nickel were determined using electron channeling patterns, microhardness, and transmission electron microscopy. Comparisons between plastic zone size and crack growth behaviour were made. The hydrogen-induced intergranular subcritical crack growth behavior of iron and nickel were found to exhibit several similarities including K th and stage I crack growth kinetics. Nickel exhibited stage II behavior with a limiting velocity of about 10 −4 mm/s; iron did not. This limiting velocity was consistent with the rate limiting process being hydrogen diffusion in nickel.