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.