HYDROGEN ABSORPTION IN CATHODICALLY POLARIZED TITANIUM ALLOYS

摘要

Cathodic polarisation may cause hydrogen absorption and embrittlement of titanium alloys. There is no need to protect titanium in seawater, but polarisation is inevitable when titanium components are connected to steel that is cathodically protected. The risk of hydrogen embrittlement of risers and other heavily loaded components rouse a need to quantify the amount of hydrogen that titanium alloys may absorb as a junction of alloy composition, polarisation potential, temperature, cold working and time. The test program included Ti-3A1-2.5V and Ti-6Al-4V alloys that are candidate materials for risers. The hydrogen uptake was measured over a 3 years period in natural seawater. The results show that titanium absorbs hydrogen when it is polarised to potentials less than -0.8V vs. Ag/AgCl in seawater, but the absorption rate decreased with time. The maximum hydrogen loading measured after three years exposure at -1.04 V vs. Ag/AgCl was 150 ppm. A calcareous deposit built up on all surfaces and limited the hydrogen evolution and thereby the hydrogen uptake. Ti-3Al-2.5V alloys picked up less hydrogen than Ti-6Al-4V alloys, and palladium or ruthenium enhanced hydrogen uptake to some extent. Welding or cold working did not influence hydrogen uptake of the alloys included in the tests. Temperature had little effect, and thermal gradients in the titanium materials had no measurable influence. The effect of hydrogen on the mechanical properties of the alloys was not studied in detail in these experiments, but most specimens were tensioned C-rings with permanent strain. Neither of these developed cracks or other signs of ambrittlement during the tests.