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

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.

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

Keith Hughes

Don W wrote:
Bruce wrote:

snip--Eric's good primer on pickling and passivation of austentetic
stainless steels
Pickling and passivation


Thanks for quote. I knew that stainless reacted with oxygen to form a
non-corrosive surface and that unless you had a flow of water the lack
of dissolved oxygen might result in crevice corrosion but somehow had
never related it to pasivating. I'll now use at least come nitric acid
as an oxidizing agent.

Bruce in Bangkok
(brucepaigeatgmaildotcom)


Bruce,

I'll try to steer you to looking into stainless steel passivation using
strong citric acid (ie Citrisurf, etc.) once more. I've not time to
provide the relevant links at the moment.

Use nitric acid if you like. It works also, although not as well as
citric acid according to various studies. It is also much more
hazardous to transport and dispose of. Citric acid can be diluted and
legally flushed down the drain.

Citric acid passivation for SS is fairly new technology, and Nitric acid
is the traditional way of doing it. Citric acid can also be used with a
power supply to electropolish stainless steel.

The people who turned me on to the citric acid passivation do a lot of
industrial and food grade stainless welding and have switched over.

Good luck,

Don W.