Fermenters: polishing vs acid passivation

[jurisok]

I got proposal for the internal surface of fermenters to have not polishing of all surface (polishing to 0.4~0.6 μm ) but to have pickle passivation with nitric-hydroflouric acid (surface RA 0.3 μm), produced from 2B stainless steel plates (RA 0.22 μm).

Any experience in usage of only passivated tanks?

Also interested in experience with only passivated HLT, MLT, KWT ...

[dick murton]

The big guys never have the tanks polished. Can you imagine the cost of polishing the whole surface of a 500, let alone a 10,000 hl tank? The only time they use internally polished tanks, and not always even then, is for early stage yeast culture tanks - but since the pipework is not polished, tank polishing is almost meaningless here as well.

As long as the welds are full penetration and are smooth finish, normally they are simply polished over and around the weld, but there is no problem with grinding back to 240 grit standard.

Passivation is normally carried out by using hot nitric acid or citric acid. Lots of discussions about this topic on this site. Personally I have always used nitric, but lots of people use citric quite happily.

Hydrofluoric acid is spectacularly nasty stuff to use even when externally pickling individual welds on pipework, so given any choice whatsoever - don't use it. I strongly suspect nitric or citric is considerably cheaper as well, though I have never checked pricing of hydrofluoric acid.

Polishing the main platework looks nice, I agree, but is not essential.

[arcangel]

arcangel

Hi,

Having got the surface finish you are happy with, perhaps a citric acid treatment might be your answer.

Citric acid is cheap and non toxic - it is used in food. Disposal of used solutions will/should not be a problem.

See ASTM A967 that allows for any concentration of citric acid, temperature and time of treatment, provided the qualifying tests can be met.

Hope this is of use.

Best regards,
Michael George.
mike@michaelgeorgeassociates.com