Sodium Dichromate Seal Tank Liner Materials and a compatible filter!

A. Hi Rachel. I once designed a chromate mixing/manufacturing facility for one of the chemical suppliers to the plating industry, and we used rigid Kynar/PVDF tanks with the intention that they would last essentially forever even in the face of every concentration and every temperature. But unfortunately they cost about 10X what polypropylene tanks of the same size cost, so it's probably not realistic for a plating shop. But you might investigate whether kynar/PVDF or teflon/PTFE liners are available for your tanks and check out the cost. Unless mechanically damaged they should last forever, as should a kynar pump or teflon-lined pump.

Luck & Regards,

A. Hi Rachel
I would pay some attention to an additional point: if the liner is compromised in the immediate vicinity of the heating element (in-tank steam line) - it may be beneficial to try to remove the heat from this steam pipe as efficiently as possible. Pump outlet and/or eductors located close to this area and directed towards the steam line may reduce risk to the liner and reduce energy consumption through better heat dissipation.
Good luck,

Hi Leon and Ted!

Thank you both for your help.

Leon, yes the heat right behind the steam line does concern me. I have not found any evidence of melting, per se; however I do believe that the localized hot spot is speeding up chemical degradation. We don't currently have in-tank recirc lines, filter discharge ports, or jets of any sort, for the same reason I'm fighting with the liner haha. I would publicly arm-wrestle the entire Accounting department in the cafeteria for Carte Blanche to spend what it takes to get this tank set up with a proper filtration/mixing system combo!!! Mixing is currently done with a mechanical shaft/prop mixer, and it's angled to drive flow behind the steam lines- how efficiently, I couldn't say. We ARE putting in a simple 316L sheet between the steam lines and the rubber for additional shielding (including from potential mixer-related accidents!).

I've reached out to my new fabricator about quoting flexible PVDF, and am still waiting for a response as to whether this guy even works with that stuff. We shall see if I end up having to post an RFQ in a few months... :)

Cheers!

A. Hello Rachel. The use of Kynar (PVDF) in metal finishing tanks (especially for chrome plating or any hot acid) has come a long way in the past couple of decades.

I now work for GOAD Company that has lined several hundred tanks with Kynar since 1996 and has Kynar jobs in the shop continuously these days. While it would be horrifically expensive and also not a good practice for various reasons to fabricate a tank from thick Kynar sheets, it is not so outrageous to line a steel or stainless tank especially when you consider the cost of downtime.

Liners could be a 1/4" thick rigid drop-in box-type made from a Kynar copolymer or they could be 90-mil thick semi-flexible copolymer that is bonded to the tank in place. I like box liners the best in your case, because the adhesive system used for bonding will not hold up to temperatures above 180 °F. Thinner bag-type liners have numerous problems and I would avoid them. I would also avoid tank fabricators that do not have a great deal of knowledge/experience with Kynar copolymers and their benefits over homopolymer.

All of that being said, I think a regular 316L stainless tank can be good for dichromate seals, but the welds have to be passivated/pickled completely or you will have the failure in the weld heat affected zone that you described. If a replacement tank is not feasible, Kynar liners are the best option that and will prove cost effective over time.