Hi Keith,
Keith Hughes wrote:
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 :-)
Yeah, I know what you mean. Even twenty year old
stuff still seems pretty new to me now-- and I'm
_not_ that old. Ah well... I guess I mean I'm not
_that_ old ;-)
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.
That may very well be true, however the immersion
times with the citric acid are not excessive.
They recommend 2-5 minutes at 120F-160F, and that
seems to be giving me good results. Ask me in
another two years ;-)
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.
Yes. I always try to dip my parts in a temp
controlled bath with some sort of agitation.
Sometimes this cannot be done because the part is
too large for my available containers.
I'd like to find a process that would work well
for re-passivating SS parts on the boat without
removing them.
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.
I had to think about what you were saying there
for a moment. I don't use any of these additives
at the moment, however they may be included in the
Citrisurf and I just don't know it.
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.
Quoting from the Citrisurf Product Information I
have on file:
"Description - CitriSurf 2050 is a high quality
blend of chemicals formulated for the cleaning and
passivation of stainless steel products.
CitriSurft 2050 is a solution of chemicals
specifically designed to provide low cost and
efficient removal of contaminants and all free
iron from the surface of stainless steel."
"Application Procedure - To assure best results,
the parts to be passivated should be thoroughly
cleaned with an appropriate cleaning solution
prior to treatment with CitriSurf solution..."
So I see where you got the idea that it was a
one-step. The description seems to imply that it
is, however the directions make clear that you are
supposed to clean and degrease the parts before
passivation.
Don W.