100% Iodine...Ouch ! ! ! 1

Here are your choices according to Philip A Schweitzer, P.E.
(assumes undiluted iodine)
Hastelloy C/C-276 1 apparently is excellent to 260 deg C.
Note same alloy does not fare as well with diluted iodine
Tantellum apparently is good to 149 deg. C
Excellent = <2 mils /year
Good =<20 mils /year

Vinyl ester apparently resistant to 65 deg. C
PTFE apparently resistant to 238 deg.
UHMWPE & HMW apparently resistant to 60 deg. C.
PET 120 apparently resistant to 120 deg. C.
FEP apparently resistant to 200 deg. C
Chemraz (FPM) apparently resistant to 200 deg C

What thickness did you add 2 years ago? I assume you used the same material for cladding.
Use the same units for measuring (ins/mils/mm),& your loss rate numbers do not seem to compute.
You now appear to be losing 0.300 inches in two years. Whereas you lost .75ins in 10 years previously.

The crux is - what is the actual remaining thickness of the cladding left? From this you can estimate how much time you have left and can make your decisions based on that.

To rework the vessel means taking it out of service (I assume) and loss of production(?). Look at how long it was out of action when you did the re-work.

Initially it does not seem to be too bad to get 10 years out of the vessel. (what was the initial design life intended to be? I would think that it was designed with a corrosion allowance.)

From my long working experience, you will see on these pages that I often bang on that it is my view that we should cure the problem at source where we can and not rely on fixes to get us out of the mire.

I believe you should have looked at planning a replacement when you had 0.5ins (3+ years) left instead of doing the re-work. You would have had time to do so, look at new design and at least another ten years out of the new one. The only loss of production would have been changing one vessel out for the other. Now you are looking for another 'fix'

I suggest that you get back onto a planned maintenance / monitoring routine where you are in control, not the corrosion. Put some planning into place.
Do it now!
Find out the delivery and cost of a new one.
Design and order a new one with the design corrosion allowance / design life with the same materials, (or better, as suggested by Boomerang)if ten years is satisfactory and economic.
Depending on how much cladding you put on / have left, you may have six months or so left.
Monitor the corrosion of the existing one, get the new one delivered and replace it with the new one when most convenient from a production point of view (during a planed shut-down?).
Put into place a monitoring system on the new one, identify a minimum safe thickness (as designed) and when you reach you that (in ten years time say) order another new one.
Regards,
Quadswift

Does anyone have input on my question about sacrifical anoides??? I think Inconel 600 or Nickel 200 is too close to 625 to be affective. Fontana shows a galvanic series where Ni-Resist (hi nickel cast iron) is far from Inconel 625, but I don't think we can take the Fe. The anoides should be much smaller than the vessel (Pourbaix diagram) right???

How good a conductor is liguid Iodine?
Are you just melting the iodine are vaporizing it?

We are melting the pellets... however, like all liquids, there is some vapor pressure associated with the liquid.

You bring up a very good point, liquid iodine has 0 conductivity...so that tells me sacrifical anoide won't work...right. What about impressed current on a vessel???

Neither one will work. And a Pourbaix diagram has nothing to do with the anode/cathode area relationship-it is merely a STATIC potential (voltage) vs. pH indicator.

ok ok so it's not the Pourbaix diagram, but 0 conductivity tells me sacrifical anoides won't work and I think your saying impressed current won't work???

So our only options are a better overlay??? What about coatings???

120F isn't all that hot. What about non-metallic linings or coatings? Granted, they'll mess up your heat transfer a bit, but if the alternative is to throw out and replace the vessel, it might be worth considering. Your vessel probably won't have the right geometry for a welded teflon liner, and a spray-applied liner probably won't last very long due to permeation. What about a trowel-applied flake-reinforced vinylester FRP liner? May be some issues getting it to bond, or it might react with the liquid iodine over time- but it might be worth some investigation.

How big is this vessel?

6'6&quot; diameter, 12-ft t/t.

Maybe I'm a little obtuse, but Unclesyd and Metalguy brought up a point I'm still not 100% clear on... since the iodine (electrolyte) has zero conductance, I've inferred a sacrifical aniode won't work... right???

And I believe Metalguy nix'ed the impressed current idea...right??? Why???

Neither anodes or impressed current can work with “O” conductivity.

The only thing other than a corrosion resistant material of construction is some type of inhibitor to lower the rate.

What happens to the metallic ions(corrosion products)?