Stress Corrosion of Titanium Alloys In Aqueous Magnesium Chloride Solution at 154 C

Titanium alloys have been found susceptible to slow intergranular stress corrosion cracking in MgCl₂ solutions boiling at 154 C (309 F). Titanium metal suffers from pitting but not from cracking in the same solution. Experiments on Ti-Al alloys have shown that as aluminium is added there is a transition from pits linked by wide fissures to narrow cracking. The effect of chemical polishing of specimens, which creates a surface layer of hydride phase, is to change the mode of cracking in the richer alloys from predominantly intergranular to predominantly transgranular. This is explained by the formation of reactive dislocations in these alloys particularly at the hydride/matrix interfaces where corrosion attack is initiated. Formation of a hydride phase at the bases of pits during corrosion also will create reactive interfaces. Intergranular cracking arises from segregation to the grain boundaries of solute and possibly impurity atoms.