I bought a frame off eBay and it turns out that the seat tube shows some signs of rust: at the rear near the bottom bracket (nothing alarming) but also inside the seat tube (and this alarms me more)...
When I picked up the frame from the box I heard some loose stuff in the seat tube and quite a bit of rust/chunks of rusted metal fell off! This worried me but I took it to a trusted bike shop who told me that it was "not too bad". They knocked on the tube and it sounded ok to them, which reassured me for a bit, but later I did again scratch the inside of the tube with a metal hanger, and more "dust" and small chunks fell off.
So, I know nothing about rust: how it propagates, if it can be neutralised, stopped, etc. Basically the two scenarios I am looking at are: 1) repaint if the rust inside the tube can be treated 2) replacing the seat tube if the current one is compromised (no idea how much this will cost, I am expecting answers from the framebuilder who built the frame and a shop who does this kind of work... - yes, the frame is probably worth it)
On the one hand the rust would be at the most critical point of the frame, where most of the forces are applied, near the bottom bracket. On the other hand the seat tube is tapered, with the tube near the bottom bracket thicker, and giving me hope that there is plenty of metal untouched by the rust which can be stopped and treated.
It's really hard to take photos to show the condition of the tube, I have tried and photos look either way better or way worse than in person. Not to mention that I can't take photos from the inside of the frame.
My question... In the meantime, or short of sending the frame to the framebuilder for his diagnostic (he is super busy in the next months with all the orders for the next season), is there anything I can do to determine if the rust inside the tube is so bad that it compromises the frame?
One thing that most people don't know about rust is that when steel oxidizes, the rust created occupies about ten times the space that the steel originally did. This is why rusting structural steel will cause masonry to bulge, and eventually fail -- it gets pushed off the building by rust! So the amount of stuff you're seeing looks like a lot more than it actually is, volume-wise.
Well, here's another chestnut: a few things that most people don't know about stainless steel. It is in fact protected by a very thin, invisible layer of chromium oxide that develops "passively," that is within a few hours of exposure to (uncontaminated) air. Problem is, if that layer is disturbed, by abrasion, say, and salt spray or some other contaminant inhibits the re-formation of the oxide layer, it will rust just like normal steel, and the rust will just keep on truckin' through the material.
People who "finish" stainless using tools that are not dedicated to stainless fabrication -- such as have been used to work carbon steel -- end up embedding particles of rust into the stainless, and then the rust just keeps getting worse! And the "higher" (more mirror-like) the finish, the greater the resistance to corrosion.
Stainless steel is generally lower in yield stress than good carbon steels; as much as a third. They have good ductility and tensile strength, though. Also, not all "stainless" alloys are non-magnetic, only those that are "austenitic" (about 70% of tonnage produced), meaning that they have a higher chromium/nickel content (which makes them the least corrosion-prone).
62 Falcon, 58 Raleigh Lenton Gran Prix, 74 Raleigh Pro, 75 Raleigh Int, 75 Raleigh Comp, 76 Colnago Super, 75 Crescent, 80 Peugeot PX10, plus others too numerous to mention!
Get a gallon of Oxalic Acid (deck wash at Lowes or Home Depot), find a suitable container that can hold most of the bike in fluid (cheap kids wading pool?) mix Oxalic Acid with water (maybe 3-1 water, or 2-1 if you want more rapid results), place bike frame in solution so it is completely submerged, wait 3 days, take frame out of solution, clean out inside of frame by rinsing in some "basic" (anti-acid) solution (baking soda), clean out, treat entire interior of frame with "Frame Saver", and you should be good to go!