The cracking of titanium in hot aqueous sodium chloride solutions

Abstract

Slow tensile straining of ASTM Grade 2 and Grade 12 titanium in 3.5% aqueous sodium chloride has revealed hydride penetration and associated cracking only in Grade 12 when the testing temperature is above about 150°C, a significant cathodic polarization is applied, and the nominal strain rate is < 10 −6 s −1. Similar embrittling effects are also observed as a result of long-term soaking after straining Grade 12 titanium beyond the yield point. The observations are explained on a basis of the penetration of hydrogen into the metal when passivity is locally broken down during slow straining. Considerable cathodic polarization is necessary to promote the appropriate hydrogen fugacity at exposed metal surfaces and the higher temperatures promote more rapid diffusion of hydrogen into the metal. The hydride channels that form at breaks in the passive film provide ready paths for crack propagation, and loss in ductility is accompanied by secondary cracking, both of which increase with increase in either the temperature or the time available for hydride penetration.