Cracking of titanium alloys under cathodic applied potential

Area of Science:

• Materials Science
• Corrosion Engineering
• Nuclear Engineering

Background:

• Titanium Grades 7 and 12 are candidate materials for nuclear waste multi-barrier packages.
• Understanding their susceptibility to hydrogen-induced and stress corrosion cracking is crucial for long-term safety.

Purpose of the Study:

• To evaluate the susceptibility of Ti Gr-7 and Ti Gr-12 to hydrogen-induced cracking and stress corrosion cracking.
• To compare the performance of Ti Gr-7 and Ti Gr-12 under simulated repository conditions.

Main Methods:

• Slow-strain-rate testing in deaerated 90°C acidic brine (pH ~2.7, 5 wt.% NaCl).
• Application of controlled cathodic potentials ranging from corrosion potential to -1162mV.
• Hydrogen analysis using secondary ion mass spectrometry.
• Microstructural evaluation using optical and scanning electron microscopy.

Main Results:

• Ti Gr-12 exhibited significantly shorter times to failure (TTF) than Ti Gr-7.
• Cathodic potentials reduced ductility (reduction in area, true fracture stress) in Ti Gr-12, with minimums at -1162mV.
• Ti Gr-7's ductility parameters were not significantly affected by applied potential.
• Increased hydrogen concentration was observed at more cathodic potentials for both alloys.
• Transgranular secondary cracks, possibly due to titanium hydride formation, were observed in both alloys.

Conclusions:

• Ti Gr-12 is more susceptible to hydrogen-induced and stress corrosion cracking than Ti Gr-7 in the tested environment.
• Cathodic charging significantly degrades the mechanical properties of Ti Gr-12.
• Further investigation into hydride formation and its impact on Ti Gr-7 and Ti Gr-12 is warranted for nuclear waste containment applications.