Pitting Corosion Data 1

[NavArcJim]

I have a question/problem i hpoe someone out there can help with.

Due two many breakdowns in comunication a 316 ball has been installed in a ball valve which is in a sea connected system where a Ti ball is specified.(seawater is not continuiously in the system but there is usually some amount of traped water in contact with the ball)

Due to opperational requirments tha ball cannot be removed any time soon.

Im trying to find data that correlates the extent of pitting (coverage and depth of pits) to time immersed in seawater at standard temp. I would like to use this data to evaluate the risk of leaving the ball in place for a period of up to two years.

Thanks for your help.

 

[unclesyd]

I don't have any data but based on personal experience with SS ball valves in sea water I would make a concerted effort to get the ball changed out. This is especially true if it is in a critical position.

I have seen numerous failures of 316 SS balls in stagnate sea water service in time frames of days to months. Stagnate anaerobic conditions and the crevice that exist in ball valves accelerates the corrosion of 316 SS. The only 316 SS valves that had any service life were these that were operated frequently and used in high flow systems.

 

[NavArcJim]

Thanks for your input unclesyd,

What do you think about imposing an operational requirment, like cycling the valve after system operation to release the seawater upstreem of the valve?
Would that improve the situation much?

 

[unclesyd]

It would certainly help but the question is again how much it would mitigate the corrosion to the ball as I still see extended periods of stagnation.

Can you comeback with more information about the valve materials, body, stem seals and the environment, bay water, open ocean, etc., it sees.

 

[NavArcJim]

sure,

open ocean water
valve body - CUNI
valve stem - NICU
valve seat and stem seals- PTFE

Thanks again for your help,

I've seen some older threads that stated corrosion rates for pitting at 1 mil depth per three months does anyone know were that might have come from.

 

[EdStainless]

I would expect crevice corrosion where the seal meets the ball withing a couple of months at the longest. Then every time the valve cycles the seals will be scuffed or torn by the corroded ball.
If sealing is impprtant I would try to plan replacement within a month or so.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

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