corrosion

derek

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What anti corrosion methods were used internally in early steel tube frames, I remember reading many years ago that fish oil was used by some manufactures! What if anything did HRD and Vincent use? Also what prevention is taken be modern manufactures.
 

Magnetoman

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What anti corrosion methods were used internally in early steel tube frames,

I got off the plane from Russia last night after 27 hours in transit, followed by a bad night's sleep, but I think what follows should be correct.

Assume your motorcycle has a 1 ft. section of 1"-dia. tubing somewhere in it, it is brazed shut at both ends, and the factory didn't coat the inside with anything before sealing in the humid British air. The total volume of air inside that tube is 9.4 cu.in. (warning: I'm going to jump back and forth between proper units and metric units). As a rough rule of thumb, gasses condense by a factor of ~700 when cooled, and the density of something in liquid form is essentially the same as it is in form solid. So, our 9.4 cu.in. of gaseous oxygen is equivalent to 0.22 cu.cm. of solid. Another note is that for the purposes of this calculation I'm not going to worry about pesky factors of 2 or 3, so our humid British air is going to be all oxygen and no nitrogen.

The surface area of the tube is pi x 1" x 12" = 243 sq.cm. Corrosion will take place because there's no oil to protect the steel, but since the tube is sealed once all the oxygen has been used up in forming iron oxide no more rusting of the inner wall will be possible. For all 0.22 cu.cm. of oxygen to combine with the iron of the tube to form rust (Fe2O3) it will result in a layer only 9 micrometers (350 micro-inches) thick. That is, only a negligible amount of the inner wall will corrode even in the absence of a rust-inhibiting coating.
 

Magnetoman

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Of course, just one pin hole........
But, air moves very slowly through pinholes even when there is a pressure difference across them, which there (mostly) wouldn't be. Why I say "mostly" is that if the tube cooled to a lower temperature than the ambient air the pressure inside would be slightly lower than outside so a bit of air would be slowly sucked in until the pressure equalized.

Given that unpainted patches of steel take quite a while to rust away even in humid environments, it's safe to say the rusting of our motorcycles from the inside out would take a lot longer than from outside in.
 

davidd

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What anti corrosion methods were used internally in early steel tube frames, I remember reading many years ago that fish oil was used by some manufactures! What if anything did HRD and Vincent use? Also what prevention is taken be modern manufactures.

I think Magnetoman has done a nice job of explaining why corrosion does not pop up as a chronic problem with Vincent frames. Additionally, if you ever replace a tube, they are quite substantial.

I have never had corrosion problems with frames except I once owned a Griffith. It was number 24 and it was a TVR chassis with a 289 V-8 in it. I had to replace the MG differential with a Corvette. In doing so, there were so many rusty tubes I had a difficult time stopping. I noticed that TVR started advertising their tubes as being oil sprayed and sealed, so it must have been a chronic problem. I have never had such a problem with another frame.

David
 

derek

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Why then do aircraft tube fuselage's often corrode from the inside, also old motorcycle frame and I think series A frame are open at top and bottom of seat frame tubes which internal space is common to all other tubes. Many frame structures have vent holes to prevent problems when welding.
 

Magnetoman

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Why then do aircraft tube fuselage's often corrode from the inside,
There are several significant differences. Most aircraft are constructed from aluminum alloys. These suffer the phenomenon of stress corrosion, and aircraft are subjected to considerable cyclical stress in operation (you might remember the Hawaiian Airlines jet that had part of its fuselage break ~25 years ago suck at least one person out). In addition, typically at least a few times a day those Al tubes are cycled between the low pressure and temperature of 30,000 feet, and the room pressure and humidity of earth. Each time they fly much of the air is forced out as they gain altitude, and replaced with new (moist) air as they descend to land.

I'm not saying that steel tubing can't rust from the inside if there's a way for bulk water to get in and sit there. It's just that, for the reasons mentioned, under normal circumstances you should see your bike turn to rust from the outside in, not vice versa.
 
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