Tantalum and galvanic corrosion 1

[moltenmetal]

Where is tantalum in the galvanic series? Is it significantly less noble than graphite?

From what I've read, tantalum suffers from severe hydrogen embrittlement from pick-up of nascent hydrogen in aqueous services when in a galvanic couple with other metals, and hence needs to be electrically isolated from other metals when wetted in a common electrolyte.

We've got a service where we're using tantalum as a cladding material at temperatures beyond which I'm comfortable with teflon as a gasket material. Though we're experimenting with Flexitallic's Thermiculite etc. as an alternative, are there any known problems with the use of expanded graphite as a packing/gasketing material in contact with tantalum? Temperatures are definitely high enough to exclude concerns with electrolytes being present.

 

[MechanicalChef]

While my chemistry is pretty rusty, based on Ta's location on the periodic table (and free electrons), I would suspect it would react similarly to materials like Ti when coupled with graphite. Hence, quite well.

 

[mshimko]

In flowing seawater, Tantalum is between inconel 825 and titanium, but there is quite a range in the data.

 

[strider6]

sorry i don't understand you last statement, "Temperatures are definitely high enough to exclude concerns with electrolytes being present." This means that there is not electrolyte due to the high operating temperature?

S.

http://www.corrosionist.com

 

[moltenmetal]

I guess I should have been clearer with my question, but thanks to mshimko for answering at least half of it.

strider6: there will be no liquid water or other electrolyte present during operation (temperatures exceed 450 F, hence the worry about using teflon as a gasket material). The unit handles corrosive vapours on the Ta-clad side. The only time I can imagine liquid water being present is perhaps if the insulation cladding leaks AND the heat tracing is left off for an extended period- that would be pretty bad operational practice, and if that led to premature corrosion I wouldn't be too concerned.

I was wondering if there were any other contraindications for using expanded graphite in contact with tantalum other than galvanic corrosion- you never know what bogeymen are hiding out there waiting to bite you when you use an unfamiliar material, and this board is full of experienced people who've seen it all- it never hurts to ask!

I was also wondering if tantalum was sufficiently separated from graphite on the galvanic series to be of concern in aqueous environments, as a side question to satisfy my own curiosity. Tantalum is listed on none of the galvanic charts I've seen. Given what I've heard, it's pretty noble but not noble enough that I'd ever consider graphite on aqueous service tantalum components- the risk of hydrogen embrittlement is too great.

 

[mjcook]

Moltenmetal, I may be responding too late to your inquiry, but I have lots of experience with Tantalum and may be able to help. Some literature sources warn about galvanic corrosion caused by coupling Tantalum and graphite. I have seen this phenomenon occur in Zirconium 702 when graphite gaskets were used, but never with Tantalum. I use grapite gaskets with Tantalum in many places, and have for several years.

I've trialed the thermiculite gasket in Tantalum-linned piping. It performed satisfactorily, although I didn't like the low crush strength of the material, and it appeared to delaminate, so I have not used it since the trial.

If temps are high enough you are concerned about using PTFE as a gasket material, you may want to double-check the suitability of Tantalum in your service. Tantalum can start having serious problems with embrittlement above 150 deg C.

If you use graphite gaskets, ensure they do not have a metal foil in the middle - spewcify them as homogeneous graphite.

Lastly, if you're even remotely worried about hydrogen absorption in your service (you probably should be if your temps are high), you should have platinum electroplated onto the Tantalum surface. If you're spending the money for Ta, you ought to take the time to read some tech papers on it.

 

[moltenmetal]

mjcook: hydrogen absorption as a result of the galvanic couple with graphite is specifically what I was worried about. It's good to know that in your services anyway, graphite gasketing hasn't resulted in rapid hydrogen embrittlement of the tantalum it was in contact with.

As I said we don't have any situations where we have graphite in contact with electrolyte-wetted tantalum- the service in question should be entirely vapour phase at the point where the graphite gaskets are used. I was concerned that there might be yet another failure mode due to the graphite contact that I was unaware of.

I have indeed read a bit about tantalum, but wanted to supplement that with the practical experience of people here too. Thanks for the suggestion of platinum plating- that's not something I'd come across before.

 

[unclesyd]

Here is a galvanic series table for a whole bunch of metals thanks to laner5{/b] in a previous post.

This is for Sea Water and it has Ta. Look on page 88 of the pdf.

http://ammtiac.alionscience.com/pdf/Corrosion_Hdbk_S2.pdf