Corrosion fatigue of titanium in sea water under cathodic polarisation

Titanium is used in sea water applications because of its good corrosion and fatigue properties. In aggressive conditions, however, environment sensitive cracking caused by hydrogen embrittlement of titanium may be a problem. Hydrogen uptake is possible especially when titanium is connected with steel parts which are cathodically protected in order to prevent corrosion. The possible cracks in titanium parts may absorb hydrogen produced by cathodic reaction. The aim of this study has been to examine whether titanium shows accelerated fatigue crack growth caused by hydrogen embrittlement under cathodic polarisation. The studied materials were unalloyed titanium (Grade 2) and Ti-6A1-4V (Grade 5). The crack growth rate tests were performed in flowing ASTM sea water at room temperature using a servo hydraulic universal testing machine equipped with an environment cell. The applied potentials were -800 mV ... -1500 mV vs. saturated calomel electrode (SCE). For comparison, crack growth rate tests in air for both materials were also performed. Cathodic polarisation in sea water did not accelerate the crack growth in unalloyed Grade 2 type titanium compared with the crack growth rate in air. For titanium Grade 5, the applied cathodic potentials slightly accelerated the crack growth in the potential range of -800 ... -1050 mV(SCE). However, at the lowest applied potentials, -1200 and -1500 mV(SCE), the crack growth rates decreased again to even lower rates than was measured for the same material in air. No effect of hydrogen on the fracture surface morphology of the fatigued specimens was observed in the scanning electron microscopy examination.