Cracking of titanium alloys under cathodic applied potential

The slow-strain-rate test technique was used to evaluate the susceptibility of Ti Gr-7 and Ti Gr-12 to hydrogen-induced-cracking and/or stress corrosi on cracking. Ti Gr-7 and Ti Gr-12 are two candidate container materials for the multi-barrier package for nuclear waste. The tests were done in a deae rated 90 degrees C acidic brine (pH approximate to 2.7) containing 5 weight percent (wt.%) NaCl using a strain rate of 3.3 x 10(-6) s(-1) Before being tested in the acidic brine, specimens of each alloy were pulled inside the test chamber in the dry condition at ambient temperature. Then, while in t he test solution, specimens were strained under different cathodic controll ed potentials. These controlled potentials were selected based on the corro sion potential measured in the test solution before the specimens were stra ined. Results indicate that the times to failure (TTF) for Ti Gr-12 were mu ch shorter than those for Ti Gr-7. Furthermore, as the applied potential be came more cathodic, Ti Gr-12 showed reduced ductility in terms of percent r eduction in area and true fracture stress (sigma(f)) In addition, the TTF a nd percent elongation reached the minimum values when Ti Gr-12 was tested u nder an impressed potential of -1162 mV. However, for Ti Gr-7, all these du ctility parameters were not significantly influenced by the changes in appl ied potential. In general, the results of hydrogen analysis by secondary io n mass spectrometry showed increased hydrogen concentration at more cathodi c controlled potentials. Optical microscopy and scanning electron microscop y were used to evaluate the morphology of cracking both at the primary frac ture face and the secondary cracks along the gage section of the broken ten sile specimen. Transgranular secondary cracks were observed in both alloys possibly resulting from the formation of brittle titanium hydrides due to c athodic charging. The primary fracture face was characterized by dimpled mi crostructure indicating ductile failure. Published by Elsevier Science.