Will this rust? Pictures inside.

[F82ny]

Hey all,

About 3.5 years ago I was doing work in the trunk of my m4 coupe where I pulled back the padding for some rewiring. In the process, the inside of the wheel arches were scratched. Not deep at all, but they could be felt. I didn’t think much of it back then, until last week when I accessed the area again. To my surprise, they looked exactly the same, which is obviously a good thing. After 3.5 years I’m VERY glad these didn’t rust (exposed metal), but it got me thinking….COULD this rust in the future and WHY hasn’t it rusted yet?

Surely this has been a long enough period where, if something were to happen, it probably would have, right? It’s been through 4 winters, 3 summers, rain, humidity, etc. I went ahead and applied touch up paint on all the scratches. Do you guys think this could still rust or is it safe to say if it hasn’t already, it won’t?

Thanks in advance!

[b_w.]

Fenders might be aluminum. Put a magnet on them.

[F82ny]

Originally Posted by b_w.

Fenders might be aluminum. Put a magnet on them.

The magnet sticks to the inside of the wheel arch, so it’s not aluminum

[jdmpimp]

IMO, the only way it would start to rust is if it was exposed to the elements. But being that the scratches are inside and covered up you will be fine. I wouldn't worry about it unless it was getting wet from time to time, but then you would have a real issue if that were the case.

[Yuille36]

Aluminum... No Steel on the other hand... Yes

[M3SQRD]

Originally Posted by Yuille36

Aluminum... No Steel on the other hand... Yes

Technically, aluminum does rust and it forms aluminum oxide on its surface. Aluminum is an oxygenated environment will develop aluminum oxide. Steel forms iron oxide which is rust. So aluminum oxide is aluminum rust but how aluminum responds to its rust is very different from how steel acts to its rust.

A major concern for many aluminum alloys is stress corrosion cracking (SCC) in a salty environment. Fewer steels are susceptible to SCC and they are more likely to be the more exotic forms of the material. I believe none of the aluminum alloys used by BMW are table III materials. MSFC-STD-3029 REVISION A I’d the primary source for SCC materials.

[F82ny]

Just trying to understand if I’ve avoided any chance of rust occurring, since these scratches are almost 4 years old, and nothings happened so far. I know it’s nearly impossible for everyone to say with certainty though. I do know that a magnet sticks to that area so it is steel, and even though that area is shielded from the elements, it’s not protected from humidity. Still, maybe I just need to tell myself that if rust hasn’t formed yet, then it likely won’t, especially now that I’ve applied touch up paint on top of those scratches.

If this was a body panel like a door or hood I wouldn’t be as concerned, but this is part of the unibody which is a little more worrisome to me at least

[F82ny]

6 weeks since I covered those scratches up with touch up paint. Looked today and couldn’t see any type of rust form… am I in the clear?

Sorry for being paranoid. I can live with dents and damage where panels can be replaced, but a potential rust issue on the unibody is a huge cause for concern for me.

[NSX2M4]

BMW recommends having the under carriage cleaned every 2 years in the manual (I think) when in a snow/salt filled climate. I would just have that done if you're trying to avoid any type of long term exposure.