Titanium - Crevice Corrosion

I have a Grade 2 Ti tube (2mm wall) which has been lying in a stainless tank containing hot orthophosphoric acid. The contacting face of the tube now exhibits a line of perfectly round pits. As Ti is pretty resistant to pitting, crevice and galvanic corrosion, does anyone have any ideas why this had occurred?

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Dave:
It sounds like crevice corrosion. Whether you should have significant corrosion or not depends on some details like the acid concentration and temperature, neither of which you stated.
Timet has some isocorrosion data (5mpy) that you can look up. Go to www.timet.com/coresistframe.html and download the book in pdf format. Look on page 20 of the Acrobat file for Figure 12. You'd expect 5 mpy for 30% acid at room temperature.
Pittsburgh Pete
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in article snipped-for-privacy@posting.google.com, Pittsburgh Pete at snipped-for-privacy@hotmail.com wrote on 28/10/03 2:17 pm:
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Cheers for that Pete.
What i have are perfectly round pits, like that seen in Fig. 18, so i am wondering now if the Ti tube has been rubbing on the stainless and lost it's protective coating? Hmmm. More examination of the job, I think.