Metal finishing Q&As since 1989

Passivation sludge

Hi,

I've got what seems to be a big problem with our passivation system.

I'll start by telling you we are passivating stainless steel.
Our passivate tank contains 20-25% Nitric Acid and 2-3% Sodium Dichromate.
All the moving parts are deteriorating, such as the lift and the rollers. The tank itself is holding up well.
We emptied the tank and found sludge at the bottom. My iron concentration seems to be high at a very fast pace. 100 gallon tank needing to be emptied every 6 weeks. Not a lot of parts being passivated.

I'm very suspicious that steel was used in some of the parts rather then stainless steel.

The manufacturer is claiming that perhaps it's pur parts contaminating the system and causing their stainless steel to deteriate.
I'm going to send a part and the sludge to be analyzed by an independent lab. Does anyone have any thoughts on this.

Thanks,
Michy

Ed. note: See also Micheline's letter 45942 which is basically the same subject in different words and may help clarify the inquiry.

Hi,


Upon closer evaluation it does appear that the tank is also deteriorating.

First of two simultaneous responses --

Chloride is the most probable suspect. It could be from using tap water and not DI or it could be from drag in.
ferric chloride [on eBay or Amazon] in an acid solution is also a powerful etchant. Possibly someone dropped an iron part in it a while back?

Second of two simultaneous responses -- 2007

Re tank: A common workaround is dropping in a molded plastic tank (natural polypropylene or crosslinked polyethylene (e.g., Nalgene®), using the existing tank as a heated water jacket. The plastic tank should fit fairly closely (to prevent tipping) and extend above the SS (to prevent flooding). This allows using a fitted plastic lid (use PP; PE may sag from heat).

If space doesn't allow for a drop-in plastic tank, use a vinyl liner (PVDF is probably better than PVC). Before use, soak in hot caustic cleaner for a day to leach out plasticizers.

Are the moving parts cleaned and only immersed in passivation solution during parts processing? Possibly, heavy cold working or welding w/o annealing may contribute. 316 or 316L? Examination by a corrosion expert is perhaps needed.

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