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Ed. note: before the internet existed, there was a time when "Sysops" (system operators) ran publics forums via dial-up modems and "BBS" (Bulletin Board System) software. Here is one such discussion from 1990.

Passivating stainless steels

Msg. #1041 in ** Ask Plater B**
Posted on 08/14/90 at 10:03:05
To: ALL From: ANNE BAIC - Passivation of 440A Castings  

From time to time we have difficulty passivating a 440A casting. I
suspect our problems are caused by inhomogeneities in the castings and I
am looking for specific ideas as to the cause and cure.
Normally the parts are 440A in the solution heat treated condition. The
passivation process, which was developed specifically for these parts and
usually works very well, is 50% nitric at 140-150F for 30 minutes.
The problem is that every now and then we have to re-passivate parts that
were re-machined for some reason. These parts are in the hardened
condition (quenched and tempered) and the problem is that they rust within a
few hours of passivation. The as-cast surfaces rust worse but the
machined surfaces rust also.

Msg. #1071 in ** Ask Plater B**
Posted on 09/09/90 at 14:52:15

Anne, get ye a copy of ASM's Volume 5 "Surface Cleaning, Finishing & Coating" of the Metals Handbook, from American Society for Metals, Metals
Park, Ohio 44073. It's a great reference book but few shops have it.
From my reading of the relevant paragraphs, the first possibility is
that you haven't properly cleaned the parts after machining and the oil is
preventing proper passivation. Anyway, read the chapter on passivation of
SST and I think you'll find a tremendous amount of useful stuff.

Msg. #1619 in ** Ask Plater B**
Posted on 02/25/91 at 17:01:17
To: ALL From: ANNE BAIC - Reaction in Passivation of CRES  

HELP! I have a problem which has occurred before and has come back to
haunt me. We are doing passivation of stainless steel per QQ-P-35 using
Type II (25% Nitric with 2% dichromate at 120-130F). Once in a while we
come across a batch of parts which for some unknown reason react with the
bath. We have smelly fumes, fizzing of parts and etching, usually
resulting in ruined parts because critical dimensions are lost. It has
happened twice on 416 and last week on 303 CRES, which to me is the
easiest stuff in the world to passivate. What am I doing wrong? People
here are getting sick of me saying "I can't explain it." The thing which
complicates the matter is that the condition is not repeatable: The same
parts which reacted the first time will react again, but the bar stock
they were cut from will react one day and not the next, other parts from
the same bar may or may not react, and the bath operates fine with all
other parts, including parts in the same BASKET as the ones which react!

The steel companies tell me there may be some surface phenomenon going
on, but they are unable to be specific or even to tell me how to prevent
it. So maybe some experienced plater out there can help?

Msg. #1652 in ** Ask Plater B**
Posted on 03/02/91 at 17:28:48

You are describing what appears to be happening, but I'm not sure you are
describing what is actually happening. It just doesn't make much sense.

Nitric acid is a very powerful oxidizing agent and thus will react very
strongly with anything that is oxidizable: which would include organics
still on the part, or a tramp metal such as aluminum.

When you say it fumes and is smelly: do you see mustard color oxides of
nitrogen evolving from the tank surface? Is there any way you can
characterize the smell at all?

Msg. #1876 in ** Ask Plater B**
Posted on 06/04/91 at 10:32:11
To: ALL From: ANNE BAIC - SS Passivation Test for 400 Series  

I hear that the stainless steel passivation industry may be moving away
from mandatory 24 hour testing on 400 series stainless, possibly changing
to 4 hour testing (this according to Nico in Mpls). Is this true? And if
so, when are the military/federal specs going to reflect this? We work to
QQ-P-35, which currently requires 24 hour high humidity or water immersion
testing. It is very difficult--nay, dumb luck--if you can get a 440 or
even 410 past this test.

Msg. #5353 in ** Ask Plater B**
Posted on 07/01/94 at 22:51:18
To: ALL From: DARLA WOFFORD (FAX) - Green smut in SST passivation Periodically in our passivation process we get a green powder residue on the surface of the parts. This is a problem on 300-series stainless steel castings and can only be removed by blasting the surface. It happens at random and may occur on half of a load. Have you had any experience in the past with this? We are at a loss in trying to determine the cause.

Msg. #5355 in ** Ask Plater B**
Posted on 07/05/94 at 22:17:55
To: DARLA WOFFORD (FAX) From: BERL STEIN - Reply To 'Green smut in SST passivation'
Darla: I believe the most common reason for smut or other problems with SST passivation is a diluted passivating bath (through drag-in/drag-out, mostly). Also, don't let the temperature of your passivating solution rise much above 30 Deg.C. Lastly, but not least, make sure your rinses are not too contaminated. Good luck and keep your baths strong (at the upper end of the range). Plater B

Msg. #5362 in ** Ask Plater B**
Posted on 07/08/94 at 07:57:32
To: BERL STEIN From: STEVE RUDY - Reply To 'Green smut in SST passivation'
What type os stainless steel are you processing? What is the passivation bath composition, and what is the pre passivation cycle? SFR

Msg. #5363 in ** Ask Plater B**
Posted on 07/08/94 at 08:00:51
To: DARLA WOFFORD (FAX) From: STEVE RUDY - Reply To 'Green smut in SST passivation'
It seems to be a detrimental effect of poor rinsing. I would assume the green residue is copper nitrate. Review your rinsing process. This might be why the problem does not occur consistently. SFR


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