Aluminum oxidation on frame, still worth it to use?
 

Dear all,

Recently I got a very intensively used electric bike and when I removed the chain guard I noticed the frame was subject to oxidation. Especially on the lower chain stay. I was planning to use this frame to build a normal bike since I expected it was a very nice frame to start with. However, now I'm doubting whether the strength of the lower chain stay has been effected by this oxidation. As you can see on the images that I attached, I scratched some of it off the frame, and I am planning to remove the oxidation and paint it again, but I am wondering if this still is useful with this frame, or try searching for another better frame?

i.imgur.com/k8h880L.jpg
i.imgur.com/CUYNJOU.jpg

Aluminum will oxidize very quickly unless painted or protected some other way. It will not decrease the strength of the frame.

That is pretty extensive corrosion.

I don't know why one might think an AL ebike frame might be "very nice". The vast majority of those I see are simple thick walled, built for a price point constructs made with various designs to deal with the non human power and weight that aren't needed for you or I. The others are either worse or way too expensive for the first owner to just let go without some thought.

As to the corrosion: who knows from a couple of photos. But since you have the frame anyway why not go ahead with your plans and report back on your results. At worst the frame will crack/break, at best you'll get many miles of function (note I didn't say "fun"). You will want to remove the corrosion completely enough to reach clean and smooth metal, or else the cancer will return even sooner. Monitor the areas over time. I have less concerns about safety as the chain stays are still held in place by the unbroken other ends. Complete fractures in these locations generally result in some funny bike feelings and you are able to slow and stop with little drama. (Forks are a very different concern!). Andy

Thanks for the comments. I will plan to strip the frame and see if the oxidation has occurred in other places as well and if there are any cracks or other damages. With very nice I was more reflecting the geometry and the size of the frame, which is spot on for me and has been something I was looking for for some time to build a nice city bike. I don't think the quality or the construction of the frame is superb and I hoped the condition of the frame would have been better, but that's ok for me since I'm not planning to use it to make an MTB or road bike.

I'd be more concerned about the corrosion in the bottom bracket, might not be able to get it apart.

Normal aluminum 'corrosion' is oxide a few thousands of an inch thick. What you have looks like prolonged exposure
to deicing salts on winter roads without cleaning or rinsing. Best bet is mild media sand blasting to clean the frame and
repaint, powder coat if you can justify it (they will do their own prep). Doing this by hand will entail many hours of
work.

I can't see the pictures. In general, though, aluminum corrosion is only a problem if there is some chemical or mechanical process removing the oxide layer. If it is left alone, the oxide layer is extremely thin but is actually a hard ceramic 'oxide' material that will prevent further corrosion. Each time the oxide layer rubs off it quickly forms another, taking away another thin layer of aluminum. This can be from something rubbing on the aluminum (or maybe a chain hitting it) or from some chemical process - another person mentioned chemical ice-melter - around here they put rock salt on the roads and it corrodes everything.

My advice would be to feel with your finger if there is noticeable pitting or other removal of material. If there is then I would scrap the frame - cut it up and put it in the recycling with aluminum cans. Replacement aluminum frames can be found very cheap on the internet.

Aluminum can corrode severely in the presence of other metals. The best example I have seen was my Mercury Zephyr(Ford Faimont). This car had very thick(1/4") aluminum bumpers. I was once hit from behind at a stop light pretty hard with no damage to my car. Yet, after 5 years of ownership those very thick bumpers were corroded right through where the steel mounts were bolted to the fenders

Thanks for the replies! I'm planning to use the bottom bracket and the headset as well, as long as it will work. Maybe I will put another crank on it, but so far the moving parts still work ok. In the meantime I acquired another frame, with less oxidation, and I plan to use this one for a build first, but I will strip the paint of the more damaged one later and inspect the frame. Since I don't have any electrical tools at my current place for grinding and sanding it will have to wait some time, but sanding by hand I noticed it's best to do the whole frame and not only the parts that look affected in the first place.

Pic assist:

http://i.imgur.com/k8h880L.jpg

http://i.imgur.com/CUYNJOU.jpg

Beginning to buckle? Lots of decent frames/bikes available.

This is correct. Aluminium (aluminum) oxidation is self-limiting. At the same time, aluminium is easily corroded by a multitude of chemicals, both acidic and alkaline, as well as a number of salts.

Thanks noglider for the pictures! The bike has indeed been exposed to a lot of rain and likely snow mixed with salt during the winters. However, some of my bikes are used during these conditions as well, with low to zero maintenance during the winter months and some are much older than this recently acquired frame. I was quite shocked when I saw the paint chipping off and the oxidation on it, never seen it to this extent. It was my plan to build a bike with leftover parts, no intention to spend any money on it haha. Since I still have some decent wheels and parts, my plan was to build a bike with a 3 or 7 speed hub to use for traveling around the city. But I will certainly take a look at this frame later and remove the paint completely to see to what depth the frame has been affected. As mentioned by others acquiring used but decent frames is possible for a limited amount of money, so if this frame seems to be too far gone I can use the one I obtained recently.
Btw, this forum is really great, so much information on it and nice users, plan to stick here around for a long time!