Critical Species in the Transgranular Stress Corrosion Cracking of Titanium Alloys in Aqueous Solutions

Stress corrosion tests using a Ti-8Al-1 Mo-1 V alloy in chloride containing solutions of dimethyl sulfoxide (DMSO), water, DMSO-water, and arsenic containing solutions, having been carried out to resolve the question as to whether chloride ions or hydrogen are the critical (i.e., failure inducing) species in the transgranular stress corrosion cracking (SCC) of high strength titanium alloys. No susceptibility could be detected in 3.5% LiCl-DMSO solutions using the notched cantilever beam test, and only very low crack velocities were observed in Instron tests in which the cross head motion was continued during the test. Additions of water to the 3.5% LiCl-DMSO solution in quantities exceeding 35%, increased the velocity of cracking in the Instron tests. The addition of arsenic, which is a hydrogen ion recombination poison, to aqueous chloride solutions, lowered K_ISCC in cantilever beam tests at applied potentials known to prevent the formation of arsine. These and other observations are discussed in terms of the current understanding of the transgranular cracking process, and it is concluded that in aqueous solutions the presence of hydrogen is responsible for the propagation of stress corrosion cracks.