is rust a safety issue?
 

i'm trying to make a really crappy beater bike, if possible without spending any money. i've got a frame that i got for free. it's got a lot of rust... lets say the top tube has more than 50% surface rust. i know that's a lot of rust, but what's gonna happen? is the frame really going to fall apart because of it?

Well put it this way. Maybe I'm over-cautious but I retired a bike because it has a rusty crack on the weld where the seat stay and seat tube join.

Well put it this way. Maybe I'm over-cautious but I retired a bike because it has a rusty crack on the weld where the seat stay and seat tube join. The rust went deep. Surface rust shouldn't be that bad.

Surface rust is OK, if the rust is dep enough that it is leaving gouges where it is removed or is actually flaking...that's bad.

Cracks, flakes, and rust on fragile parts is bad. Please make sure any rust around brake housing braze-ons are good...last thing you need is to have one of those pop when you really need it.

Taking the material that your frame is made of, then converting it to another type of material that flakes off, noooo, that can't possibly be bad for it.

I would have been more worried about the rack in the weld, that the rust in that crack...

Couldn't you have just re-welded it?

Welll, uh yes I guess I could have. I still could. I have the bike in the back...






psssst! (That was my excuse to buy the Kona!! Don't blow my cover!)

Occasionally, I have to account for rust in stress calculations. It's necessary to reduce the as-designed thickness of the part and see how much material will be left over after a period of time. Here's what General Dynamics Electric Boat has to say:

Corrosion allowances may be necessary for some calculations. Recommended corrosion allowances for items wetted by seawater are:

Nonferrous Materials 0.050 inch
Ferrous Materials 0.065 inch
Copper Nickel 0.050 inch

Double these values if in contact with seawater on both sides. This corrosion allowance shall not be considered as contributing to strength

Of course, we are talking about ferrous materials; materials containing significant amounts of iron. As Catatonic wrote, surface rust is not a problem. Sand to bare metal and prime and paint.

If you are talking about a 1" plate, the corrosion allowances given above are neglegible. For the thickness of tubing used in a bike frame, however, they are very significant!

I'll spare you the gory metallurgical details, but the method of welding can contribute to corrosion. The heat of welding affects the metal as it cools, and can make the weld or adjacent material more prone to corrosion. This is especially true with "stainless" steel. I'm guessing a cheap bike was simply welded and allowed to cool. Not the best senario.

Brazing temperatures are not as hot and will not contribute to corrosion the way welding can.

If there is excessive pitting, you may consider ditching the frame.

If you have a photo, we may be able to give you better advice.