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RE: 316L stainless steel passivation

Passivation is a topic that seems to come up in this forum every fall!

I have long been a proponent of using citric acid rather than the more
common and traditional nitric acid for passivation.  This comes from long
personal experience where we had to process corrosive products that
liberated HF and HCl into the headspace during processing operations.  The
only way to control corrosion and corrosion build up in these 316L vessels
was with monthly passivation.  That fact ruled out using noxious fuming
nitric acid.  HNO3 is simply too nasty to deal with first in handling it,
second in using it and thirdly disposing of the spent acid.  Furthermore, we
happened to have copious amounts of pure clean FCC grade anhydrous citric
acid available for production and therefore available for passivation
whenever we needed it.  We have all experienced passivation with HNO3 in the
past and have had to live with the inevitable leaks that sprung up during or
following use.

Nitric acid leaves the surface a dull gray while citric acid leaves a
brilliant shiny surface. I have found results with passivation using citric
acid better than any work we ever did with HNO3.

Back to the art of passivation:  There are four main steps to follow:
Cleaning, passivation, neutralizing and rinsing.  First is that you can not
passivate SS that has a film of oil, grease or other debris on it which is
often common on new tanks or tubing.  Nor can you passivate SS that is dirty
with product residues.   The first step is a thorough cleaning and
degreasing which is done using an non- or low-foaming detergent that does
not itself leave a film.  The choice of detergent depends on the nature of
residues to be removed.  Again, most plants have a detergent that has a
history and is approved and routinely used to clean the inside of tanks and
equipment before and after production. This could be used to clean the
surface before passivation.

The next step is passivation whose purpose is to remove exposed Fe from the
surface of the SS.  Iron is a component making up of the SS alloy and
oxidation of exposed iron is what causes the appearance of rust or rouge.
Iron is removed using an acid which readily reacts with the iron to form a
salt which is soluble in water.  Once the iron has been removed by the acid,
the exposed surface is then only Cr, Ni, Mo, etc depending on the SS
involved.  This surface then oxidizes and that is what is resistant to
corrosion. Nitric acid is the most traditional method for passivation.  This
stems from the fact that it is one of the most common chemicals around and
it is cheap.  Most people buy it concentrated (60%) which makes it extremely
corrosive and hard to handle.  Do you really want to use one of your pumps
to pump that stuff?  Or do you buy a special pump for HNO3 that is used once
or twice a year?  Or do you use an old pump which starts to leak as soon as
you start trying to pump it.  Orange fumes of N02 are given off from the
moment you open the drum and continue to be given off throughout the
passivation process.  Most people will use a 10-20% solution of what they
purchased which actually amounts to a 6-12% concentration of HNO3.
Passivation is usually done hot but HNO3 breaks down above 75C so
temperatures are usually in the range of 50C for HNO3 passivation. Following
that you then have the unpleasant task of neutralizing the remaining acid.

Passivation with citric acid is far less dramatic.  You do not experience
the extremely low pH seen with mineral acids as citric acid acts as a
buffer. It is far easier to handle and use.  I would recommend a 5% solution
of anhydrous citric acid or a corresponding higher concentration of hydrated
citric acid.  (Anhydrous being often less costly) The acid readily dissolves
in hot water and we would passivate as hot as practical often as high as 80C
but always above 65C.  We would generally pump it through the system using
product pumps which passivated all product contact surfaces.  We would
passivate for a nominal 60 minutes once we had achieved the temperature we
wanted.  Tanks were filled to the top and spray balls were used if available
in order to passivate the headspace.  Citric acid worked well with none of
the challenges of HNO3.

As for frequency of passivation, I would certainly recommend that a system
or surface be passivated following any major work and definitely following
any welding, grinding or polishing which would leave exposed iron.
Passivation would be indicated if there were visible pitting, corrosion,
rusting or rouging.  For tanks, once a year is usually adequate unless you
are dealing with a corrosive product or headspace.  For hot WFI loops, once
or twice a year usually makes sense to control rouging: Christmas and/or
summer shutdowns.  There are specialist companies who can come in with all
that is required, perform the passivation and leave. Some of these are
excellent but very busy when you need them as everyone wants to passivate
when they are shut down.


Jeffrey Brooks
Executive Consultant to the Pharmaceutical Industry
tel: 514.868.9537
mobile: 514.242.1333
email: brooks@jbrooks.ca

-----Original Message-----
From: PharmTech@www2.pharmweb.net [mailto:PharmTech@www2.pharmweb.net]On
Behalf Of Sanfeliu@bcn.boehringer-ingelheim.com
Sent: Monday, December 03, 2001 10:14 PM
To: PharmSciTech
Subject: 316L stainless steel passivation

Dear all,

I'm looking for a passivation method for a 316 L stainless steel reactor to
be used for compounding of parenteral solutions. I've read nitric acid has
been broadly used but it seems as lately is being substituted by the use of
citric acid. Could anyone give me some advice in terms of best agent, agent
concentration, time of passivation and temperature? I would also like to
know whether passivation is generally performed on a regular basis in the
pharmaceutical industry or just once after the installation of the

Thank you very much in advance.

Anna Sanfeliu

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