"Morgans" wrote:
"Jim Logajan" wrote

Why can't you just clean off the pencil marks?

Also, doesn't a galvanic reaction require an electrolyte between the
metals, so if you didn't erase or clean off the pencil mark but
primed and/or painted over the aluminum surface, no salt water or
other electrolyte could get in there to produce the galvanic
reaction?

The danger of using pencil for aluminum is not a "maybe" kind of
problem.

I'm not doubting galvanic reaction involved at all. Nor the inadvisability
of drawing a pencil line and leaving it there.

The pencil works it's way into the molecules, and can not be
cleaned off, completely.

This is the claim that bothers me. But assume it is true. I would expect
the rate of corrosion to be roughly proportional to the ratio of the
size of the exposed cathodic material (graphite) relative to the size
anodic material (aluminum). This ratio is usually expressed C/A, (where
C is the cathode surface area and A is the anode surface area). And I
did find a source on the net that verifies that [1]. So even if one
couldn't clean it all off, the corrosion rate would be greatly reduced.
And if the surface were sealed before any electrolyte (e.g. salt
water) could be introduced, no reaction would take place.

Why risk it?

Not asking anyone to risk anything.

Why argue?

Because rules of thumb are okay when deeper understanding takes too long
to impart to the practitioner, but understanding the deeper nature of
the problem allows so much more to be accomplished.

[1] http://www.ocean.udel.edu/mas/masnotes/corrosion.pdf