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#1
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Orval Fairbairn wrote:
NOT ON ALUMINUM! The graphite from the #2 pencil will galvanically corrode aluminum. Use a Sharpie and clean with either lacquer thinner or denatured alcohol after you are done. 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? |
#2
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![]() "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. The pencil works it's way into the molecules, and can not be cleaned off, completely. It is an accepted fact, known to materials engineers as a unacceptable practice. I don't know if it is really a galvanic reaction, or something else, but people *way* smarter than you and me have proven the problem. The metal will become brittle at the pencil line, and with enough stress, *will* cause a crack to start. Why risk it? Why argue? Use something else to mark your aluminum. Period. -- Jim in NC |
#3
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In article ,
"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. The pencil works it's way into the molecules, and can not be cleaned off, completely. It is an accepted fact, known to materials engineers as a unacceptable practice. I don't know if it is really a galvanic reaction, or something else, but people *way* smarter than you and me have proven the problem. The metal will become brittle at the pencil line, and with enough stress, *will* cause a crack to start. Why risk it? Why argue? Use something else to mark your aluminum. Period. And --- DO NOT USE A SCRIBE!!! Scribes, no matter how careful you are, will damage the aluminum and create stress raisers. They can also damage the Alclad coating and provide a path for corrosion. It is not a matter of IF a piece will fail at a scribe line, but WHEN -- due to fatigue. The admonition against pencils in aluminum shops also holds for engine shops. The easiest way to fail a hot steel part is to mark it with a lead pencil. The steel glows red hot and absorbs the carbon from the mark, creating an instant stress concentration. An old friend related how he saw a Lockheed Constellation exhaust manifold with a crack in the shape of "OK", due to lead pencil. |
#4
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![]() Orval Fairbairn wrote: The admonition against pencils in aluminum shops also holds for engine shops. The easiest way to fail a hot steel part is to mark it with a lead pencil. The steel glows red hot and absorbs the carbon from the mark, creating an instant stress concentration. An old friend related how he saw a Lockheed Constellation exhaust manifold with a crack in the shape of "OK", due to lead pencil. The steel absorbs the carbon and becomes brittle, whereupon it cracks after being flexed while in use. The carbon doesn't cause a stress concentration; that's caused by scratches or nicks that interfere with the lines of stress in a metal part. A nick in a metal propeller is an excellent example of a stress riser, as is a scratch in an aluminum skin. For a steel to be hardenable, it needs a carbon content of at least 0.4% carbon. That's not much. Few steels have as much as 1%. 0.6% is commonly found in hand tools and spring steel, and a steel having 1% might be found in ball or roller bearings. A pencil mark could raise the local carbon content a lot, probably to much more that 1% ,especially on a hot item like an exhaust manifold where the heat allows rapid and deep absorption of the carbon. Aircraft tech schools will teach you to leave the pencils at home when working on aluminum. Dan |
#5
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"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 |
#6
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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. The pencil works it's way into the molecules, and can not be cleaned off, completely. It is an accepted fact, known to materials engineers as a unacceptable practice. I don't know if it is really a galvanic reaction, or something else, but people *way* smarter than you and me have proven the problem. The metal will become brittle at the pencil line, and with enough stress, *will* cause a crack to start. Why risk it? Why argue? Use something else to mark your aluminum. Period. I've always puzzled over why John Thorp called for using a pencil to mark lines in his "building the T-18" articles from the mid 60s. Surely they know about it then? John |
#7
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Because he knows what in the hell he is talking about?
Jim "J.Kahn" wrote in message .. . I've always puzzled over why John Thorp called for using a pencil to mark lines in his "building the T-18" articles from the mid 60s. Surely they know about it then? John |
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