Hi Don,


While nitric acid is the historical method of passivation, citric acid
had been the standard for *well* over a decade - not new technology at
all.

Well, my use of the term "new" is relative. A decade takes us back to
1997, and I still think of that as relatively "new".

Notice I said well over a decade. At least a couple of the larger
passivation firms (Calchem and Astropak) began using citric acid based
passivation solutions in the later '80s (I'm just not sure of the dates
- it's that old age thing :-)

For any given temperature and contact time, nitric works better.


I'm not sure that this is true...

Yeah, I worded that poorly. What I meant was that Nitric works faster
for any given temperature.

I read an independent technical lab report on the quality of passivation
films formed by nitric acid and citric acid when I was researching the
Citrisurf. It compared the CrO2? films at a microscopic level for
different passivation techniques, and concluded that the quality of
passivation using citric acid was slightly better than nitric acid.

IIRC the thickness of the passivation film using any method is limited
to some maximum because eventually the reaction stops due to complete
oxidation of the surface.

Other processes come into play as well. When you're talking about
wiping on versus dipping/soaking, you're at a significant disadvantage.
With a dip, or circulation process for piping, you can add
chelants/sequestrants like EDTA and suspension agents like polysorbate
so that all (virtually) of the iron removed is
chelated/sequestered/suspended and can't get redeposited on the surface
being passivated.

The report is available on the web somewhere. I did not bookmark it
because I was only trying to decide what technique to use for myself.

Citric acid is used since it's far less dangerous and environmentally
unfriendly. Phosphoric acid is also frequently used for ambient temp
passivation. Hydrochloric acid is NOT used for passivation of
stainless - ever. Neither is HF, unless you're pickling (i.e.
removing significant material - etching).


I use Phosphoric for pre-cleaning, rust removal and etch of regular
(non-stainless) steel before painting. It works well.

Hydrochloric acid (Battery acid) can be used with a DC power supply for
surface passivation of aluminum. The process is called anodizing ;-)

The Citrisurf material looks OK, but I have little faith in
combination products that both clean and passivate. Far better to
remove all oils *first* with a heavy duty surfactant (e.g. TSP), then
passivate with citric (or other) acid.

AFAIK Citrisurf does not claim to clean. You are supposed to do a
cleaning step (cleaner/degreaser) before passivation. At least that is
what I do.

I just briefly glanced at their site, and it appeared that their
products were mostly one-step clean/passivate products. I could
certainly be mistaken about that though.

Keith Hughes