It's absolutely true, any cleaning od that peens or scratches the surface
can compromise a penetrant test. It's not likely that for most it will
matter (the penetrant will work it's way around the peening), but it can.
Plastic media does not peen or scratch the surface, that's why I use it.
If I used beads or sand I would acid etch, but I rarely use them because
they get places where they shouldn't.
Acid etching steel parts is not difficult BTW and is worth knowing if yoy
want to do high strength brazing or test a part that's been in rubbing
contact (which smears metal over ). The simplest approach is dilute
muriatic acid like you use for concrete cleaning. And remember, do what
you otta, add acid to watta. A few minutes turns the part dark, which is
what you want. Rinse well and wash with detergent and hot water. If you
want to be more technical about it, there are etching instructions in most
books about knifemaking (David Boyes excellent book comes to mind). You
don't want to remove serious amounts of metal, just a little of the
surface.
-----Original Message-----
From: Gerald Van Vlack
To: John Herrera; fot@autox.team.net
Sent: 6/4/2003 3:57 AM
Subject: Re: hub removal TR3A-6
John and FOT, Years ago when I worked for the Navy Nuclear Program I was
trained in various Non-Destructive Testing ods and Penetrant Testing
was
one of them. It is true that improper cleaning will mask a small
and
therefore invalidate the test. He was not pulling your leg.
It should also be noted that Mag Particle testing can only be done on
materials that can be magnetized. Since aluminum can not be magnetized
it
won't work and Penetrant Testing is your only economical choice.
Jerry Van Vlack
----- Original Message -----
From: "John Herrera" <jrherrera90@hotmail.com>
To: <fot@autox.team.net>
Sent: Tuesday, June 03, 2003 10:19 PM
Subject: RE: hub removal TR3A-6
> My $.02:
>
> Bill Babcock wrote:
>
> >I clean the parts well after (bead blaster with plastic media) and
dye
> >penetrant test. Sometimes I magnaflux as well, though I can do the
> >penetrant testing here. I should set up a magnaflux system--it's not
that
> >tough.
>
> Years ago, when I worked at Fairchild, I found a small ding in an A-10
> aluminum alloy skin. It had to be dye-penetrant inspected for .
> Instead of waiting for someone from the paint shop to remove the
primer in
> the area of the dent with paint , I used a Scotchbrite pad
soaked
in
> MEK. The penetrant inspector then said, "Now it's got to be acid
etched."
I
> learned from that.
>
> Scotchbrite or other abrasive cleaning ods, such as abrasive
blast,
can
> either fill the with material or burnish material over the
crack,
> possibly preventing the penetrant from seeping into it. Since
dye-penetrant
> inspection can only find irregularities that reach the surface (unlike
> Magnetic-particle inspection, which can detect flaws under the
surface),
> such a would not show up. Unless the part was acid-etched after
> cleaning.
>
> Maybe plastic media won't do this, I don't know. Maybe steel is
different
> from aluminum in this regard, I don't know. But then you could use
> magnaflux.
>
> Nowadays, we always teach our students to use a non-abrasive cleaning
method
> if the part is to be dye-penetrant inspected. The inspection kit comes
with
> a good cleaner, Trichlorethylene, I think.
>
> I think there is a question on the FAA mechanic written test about
this
> subject.
>
> Anyone else out there ever hear of this? Or was the penetrant
inspector
just
> pulling my leg?
>
> John Herrera
>
> _________________________________________________________________
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