Hi Keith,

Keith Hughes wrote:
Don, Bruce,

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".

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

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

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.

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.

As has been previously discussed, mechanical polishing of non-orbital
welds (prior to passivation) is still a pre-requisite for prevention of
oxidation.

Agreed.

Don W.

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

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.

Hey Don,


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 ;-)

That should be fine. Most folks don't have temp baths to work with, so
I assumed you would be working at lower temps. There are also
off-the-shelf ferroxyl test kits available for testing the surface
passivity if you want.


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.

A buddy of mine has a business specializing in sanitary piping and
equipment, custom s/s fab work, and mechanical / electropolishing /
passivation. I can hook you up with him if you ever need a larger piece
polished and passivated. He's in Tempe AZ (a suburb of Phoenix - but
don't tell them that).

Keith