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letter 50005
Passivation Problems With Forgings
cleaned with a Whellabrator
September 17, 2008
We have about a 50% failure rate in the Humidity test on some 304
Stainless Steel Forgings. The parts have both raw forging material
exposed and machined surfaces. The machined surfaces always come out
fine but the un-machined area fail about half the time.
I did find out these parts are "cleaned" with a Wheelabrator system
using steel shot at the manufacturer, so I can see where
contamination is possible, but we grit blast the parts with aluminum
oxide after machining as kind of a poor man's cleaning process. (yes
we change that media frequently) After the grit blast we test the
parts with a Koslow Passivation Tester 2026 and the parts generally
pass that test without Passivation.
So my questions are
1) Would a vibratory polishing process before machining clean the
surfaces as apposed to the Wheelabrator or would that just
contaminate the parts further
2) Is the copper sulfate test a little easier to pass than the high
humidity test? Maybe we are asking for too much
3) This may seem silly but we are on the west coast a couple of
blocks from the beach and the supplier is in Texas. Is it possible
the transfer from a salt air to a dry climate could be causing the
problem?
4) Where can I find a copy of QQ-P-35C. I know it has been replaced
but the customer insists on using it.
5) How common is it for a Passivation supplier to give up and say 304
CRES parts cant be passivated?
Thanks in advance
Allen
Allen prophet
Contract Machining - Tustin Ca USA
September 18, 2008
You can find a copy of QQ-P-35 here
assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=50793
You are correct that it is an obsolete spec though (ASTM
A967 [link is to spec at TechStreet] is the current spec in
place)
Miles KB
- Charlton, MA
First of two simultaneous responses -- September 21, 2008
Allen, I have seen this problem before. If A Wheelabrator machine
can be dedicated to stainless media, say Cut wire Shot, and only used
for Stainless alloys,... then the impinged iron to the surface is
greatly minimized. Trying to blast off an impinged "invisible free
iron is difficult" because it usually shows up as rust days or weeks
later after part is exposed to humidity "water". By the way, the
stainless cut wire shot will last at least 20x longer than cast shot
and produce less dust "free iron" in the Wheelabrator and on the
blasted forgings. The passivation and scrap forgings may be
eliminated too. So running a sample batch of forgings this way may
work well and prove cost effective. There are blast shops set up this
way to handle stainless products and using Cut Wire Shot.
Tim Deakin
- North Tonawanda, NY
Second of two simultaneous responses -- September 22, 2008
Mike is correct that QQ-P-35 has been superseded, it was
originally superseded by SAE-AMS-QQ-P-35 and then by SAE
AMS-2700 [link is to spec at TechStreet].
Brian Terry
Aerospace - Yeovil, Somerset, UK
September 26, 2008
You need something more agressive to remove the surface iron.
Longer time and/or higher temp may do the trick. If not, then first
pickle in nitric 25% - HF 5% ambient temp for 15 minutes, followed by
normal passivation.
Electropolishing will likely also work.
I have not found Koslow results to be meaningful. Copper sulfate will
be better.
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Jeffrey Holmes,
CEF
- Spartanburg, SC, USA
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September 30, 2008
Using steel shot in the Wheelabrator will definitely be a problem,
as everyone has said, and you know. Grit blasting afterward is
problematic, as you will tend to "peel over" some of the stainless on
top of some of the steel. This will initially look okay, but when
exposed to moisture the rust will bleed out from under.
You are better to remove all of the steel from the surface by
passivation. Because there is so much ground into the surface it may
take a hot citric product to remove it easily and safely.
304 passivates very well, although it will NOT hold up to salt spray
on the ocean. Properly passivated it will pass both copper sulfate
and high humidity testing.
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