26 Nov
2001
26 Nov
'01
3:50 p.m.
I'm also a metallurgist and we have gone overboard on passivation.
The key to all of this is a clean surface. You need to clean the surface to
remove all the organic residue and metallic residue on the surface. This
would be true for citric or nitric acid passivation, or just letting the
atmospheric oxygen do it's job. You need a clean surface. If you don't have
rust, stay with your method.
If you have rust you need to passivate the surface to remove the free iron
that caused the rust. You can passivate with either a nitric acid solution
or a citric acid solution, but the surface needs to be clean.
A solution with 4-10% by weight citric acid should only need to be in
contact with the cleaned surface for 30 minutes at room temperature to
passivate. This is about the same time for a warm nitric acid solution.
Either solution will remove iron oxide and free iron from the surface. Allow
the surface to air dry after the acid rinse.
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From: Steve Piatz[SMTP:piatz@cray.com]
Sent: Monday, November 26, 2001 2:09 PM
To: bergbrew(a)juno.com
Cc: brew987(a)yahoo.com; mba(a)thebarn.com
Subject: Re: Re-Passifying Stainless Steel
Here is a message from the HBD May 15, 2000 from John Palmer who is a
metallurgist. I think we may be going overboard in Will's kegs here.
Since I don't brew daily I can just get by with letting atmospheric
oxygen do the job for me when I need to scrub the kettle.
+ Date: Mon, 15 May 2000 08:33:14 -0700
+ From: "John Palmer" <jjpalmer(a)gte.net>
+ Subject: Stainless passivation
+
+ Passivating Stainless Steel
+ A situation that often comes up is, "Hey, my stainless steel is
+ rusting! Why? What can I do to fix it?"
+
+ Stainless steel is stainless because of the protective chromium oxides
+ on the surface. If those oxides are removed by scouring, or by
+ reaction with bleach, then the iron in the steel is exposed and can be
+ rusted. Stainless steel is also vulnerable to contamination by plain
+ carbon steel, the kind found in tools, food cans, and steel wool. This
+ non-stainless steel tends to rub off on the surface (due to
+ iron-to-iron affinity), and readily rusts. Once rust has breached the
+ chromium oxides, the iron in the stainless steel can also rust. Fixing
+ this condition calls for re-passivation.
+
+ Passivating stainless steel is normally accomplished in industry by
+ dipping the part in a bath of nitric acid. Nitric acid dissolves any
+ free iron or other contaminants from the surface, which cleans the
+ metal, and it re-oxidizes the chromium; all in about 20 minutes. But
+ you don't need a nitric acid bath to passivate. The key is to clean
+ the stainless steel to bare metal. Once the metal is clean (and dry),
+ the oxygen in the atmosphere will form the protective chromium oxides.
+ The steel will be every bit as passivated as that which was dipped in
+ acid. The only catch is that it takes longer-- about a week or two.
+
+ To passivate stainless steel at home without using a nitric acid bath,
+ you need to clean the surface of all dirt, oils and oxides. The best
+ way to do this is to use an oxalic acid based cleanser like Bar
+ Keepers Friend, RevereWare Cleanser, or Kleen King, and a non-metallic
+ green scrubby pad. Don't use steel wool, or any metal pad, even
+ stainless steel, because this will actually promote rust. Scour the
+ surface thoroughly and then rinse and dry it with a towel. Leave it
+ alone for a week or two and it will re-passivate itself. You should
+ not have to do this procedure more than once, but it can be repeated
+ as often as necessary.
+
+ John Palmer
+ www.realbeer.com/jjpalmer/
David H Berg writes:
I've never used citric acid, but for nitric you wash it with the nitric
and let it air dry (ie, do not rinse).
By the way, the reason you need to passivate stainless is so it remains
stainless--without the protective coat it will stain and corrode
On Mon, 26 Nov 2001 10:27:49 -0800 (PST) Will Holway <brew987(a)yahoo.com>
writes:
> I actually have some citric acid in powdered form.
> what is the method? Just let it soak overnight or
> something? Do I need to first try to clean the
> stainless steel as best I can then let it soak?
>
> Thanks,
> WH
--
Steve Piatz piatz(a)cray.com
Cray Inc. 651-605-9049
1340 Mendota Heights Road
Mendota Heights, MN 55120