Stress Corrosion on Titanium Tubes?

[reactorshell]

I have a titanium tube bundle which has been in service in a cooling water heat exchanger for the past 6 years. Process side contains LPG service with several ppm of chlorides. Tube side contains seawater. The exchanger is a trim condenser. Process temperature and pressure is ~55C and 1950 kPag at the inlet.

Recently, a leak developed in the tube bundle and the several tubes were found to have collapsed. The tubes were found to have uneven thinning on the OD, suggesting corrosion wastage at the surface. The mechanism of the corrosion is still unclear but the main suspect is HCl formation in the process side.

What surprises me is that I always have the impression that Titanium is corrosion resistant, even to acid attacks. Does anyone have any similar experiences? Can anyone suggest how the HCl could have formed under the normal operating conditions?

 

[Metalguy]

Ti metal is very reactive--it's the tough TiO2 (oxide) that protects it. Acidic conditions can disolve the oxide layer, and the metal can corrode rapidly, especially in HCL.