Hi Kurt - apparently, during WWII, some lift struts on planes were marked for
welding
with pencils. This caused local hardening (like carburizing), and resulted in
fatigue cracks, which in turn resulted in lift strut failure and crashes. This
had
to do with steel that was to be welded. I've never met an aircraft welder who
did
not know of this.
Regards, Ken
Kurt Andress wrote:
> Bill Aycock wrote:
>
> > several people have written me to say they have heard about pencil marks as
> > a no-no, also. It may be significant that the words "sharp insrtument"
> > and "scratch" were also used. One example was also about 0.020 sheet metal,
> > having corroded under paint where it had been marked with a pencil. He
> > didn't say he had seen it, just that it could happen.
> >
> > I will certainly concede that scratching a sheet metal panel, particularly
> > one of alclad metal, is bad. exposing the substrate of alclad by going
> > through the protective layer is to be avoided. However, I was considering
> > only the application of a pencil mark- which involves leaving a trace of
> > graphite on the surface, not cutting through anything. In this regard, ANY
> > marking method that uses enough force in a concentrated spot to leave a
> > mark consisting of moved metal is bad.
> >
> > The original thread mentioned that the pencil mark would cause corrosion---
> > I have yet to see anything that substantiates that a mark alone will do
> > this-
> >
> > I am open to any explanation that does not involve cutting into the metal-
> > ie, corrosion caused by leaving a pencil mark- not a scratch- on the metal.
> > I really want to know, and to have non-anecdotal references.
> >
> > Bill- W4BSG
> >
>
> Hi Bill and all,
>
> Happy New Year!
> I hope everyone experiences a great one and more aluminum goes up than goes
> down.
>
> I'm in agreement about all the other comments about scratching, scoring and
> the
> like. A small defect like a scribed line forms a small stress riser,
> specially in
> thin materials. As the material undergoes load cycles the thin line always
> sees
> higher stress levels and exhibits more rapid work hardening. When the life
> cycle
> of the part gets near its end, this area will be the first to go.
>
> Here's a little concept to chew on about the pencil line on aluminum.
>
> As you said in a previous post pencil lead is usually made from a graphite
> clay
> composition.
> I think that refined clay is a relatively inert akaline material and shouldn't
> participate in deterioration, other than it does readily retain moisture.
>
> So, I looked again at the galvanic series.
> The American Boat and Yacht Council (ABYC) series lists graphite as the most
> passive (cathodic) material. Aluminum alloys are 4th from the most active
> (anodic).
> The series from the Corrosion Engineering Handbook at work lists graphite in
> 3rd
> place next to gold and platinum as the most passive, and aluminum alloys in
> 5th
> place as the most active.
>
> So, when we put the pencil line on the aluminum we have put two of the most
> incompatible materials in the series together!
>
> If we accept that putting these two materials together constitutes a galvanic
> cell, we can also imagine that the subsequent galvanic corrosion will result
> in a
> small fault line, just like the scribe mark and lead to the same accelerated
> failure.
>
> Putting some paint over the thing doesn't do a heck of a lot to slow it down.
> Water migrates thru most paints. This doesn't cause many problems when the
> parts
> are left in open air to breathe. Wrap them up and let them sit for long
> periods
> of time and the paint will blister.
> We just had that happen to a dozen spars shipped to England last August. They
> got
> wet along the way and sat till November. All of them had blisters due to
> moisture
> trapped in the coatings. This was really expensive high grade polyurethane
> paint,
> similar to that used on aircraft.
>
> I remember having the same problem 15 years ago with a bunch of radomes on the
> F16, with really expensive polyurethane rain erosion paint, that were
> imroperly
> stored. Had to go to N.A.S, Jacksonville, Fl to explain how water migrates
> thru
> just about anything.
>
> I think that in the case of the gentleman that posted the comments about
> seeing
> the problem on aircraft, the ability of the moisture to migrate through the
> coating and be held in place probably exacerbated the problem. Probably would
> have been better to have it all bare in the open to allow someone to see the
> problem before it failed.
>
> At my real job, we manufacture carbon fiber/epoxy spars for the marine
> industry.
> The chemical composition of the actual carbon (or graphite) fibers are very
> nearly identical to the the stuff in pencil lead. The big difference is that
> the
> carbon filaments have a more highly oriented molecular structure to exhibit
> better physical properties, but carbon is still carbon on the galvanic series.
> Every time we put a piece of aluminum in contact with the carbon we get a
> frothing mess in less than 6 months on the sea. The aluminum is more active
> so,
> it always loses the battle. The aluminum just dissolves and the carbon
> composite
> remains just fine.
> We take heroic measures to isolate these materials when they must be put
> together. Sometimes, it can't be avoided, we know it should be avoided
> whenever
> possible.
>
> So, there is input #1 for your request for anecdotal information.
>
> Maybe there is someone interested in this topic that lives on the Gulf Coast
> or
> So. Florida that can put some pencil lines on some aluminum and leave it out
> for
> a while to provide some more anecdotal (I call it empirical) evidence.
>
> I guess my opinion on the topic is that putting pencil lead on aluminum
> violates
> all of the basic principles of corrosion engineering and should not be done.
> A simple remedy would be to remove the pencil marks before putting the thing
> in
> service. Maybe that's why they put an eraser at the other end of the stick? I
> think the eraser was designed to correct mistakes.
>
> 73, Kurt
>
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