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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