[pterion]

I saw in a thread a while ago someone posting about having strange burn like marks on the outer rim of their wheels:

http://forums.mbnz.org/forums/forums/thread-view.asp?tid=1198047&posts=20&hl=clearcoat+burn

I just noticed a similar (but much worse) problem on one of my winter wheels (right front).

pics are below. I checked my tire pressure and it was low (like 22) and bumped it up to 34, but all 4 wheels were low (forgot to check them when i took of the summer ones [xx(]) but this problem only occured on the front right. I also noticed the car seems to vibrate at higher speeds (like one of the rims might be bent?). Any ideas?

PT

 

[Bruce R.]

That is filliform (sp?) corrosion, not a rim "burn" ...

Corrosion has worked it's way under the clearcoat and will continue to do so with little worm like trails.
The only repair is to clean off the protective coating, clean off the corrosion and recoat the wheel. It's a job best left to the professionals.

 

[pterion]

Thanks for the info Bruce. It looks pretty bad, so maybe I will just replace that wheel. They are just winter wheels after all...

PT

 

[YellowSLK230]

Ditto what Bruce said. Its called filiform corrosion, and can occur under a clear coat or even under ALclad aluminum plated alloys if the surface coating gets nicked, which are much more vulnerable to corrosion than pure aluminum. You will probably also see it at the corners of the bolt holes on the stock MB wheels, where the edge is fairly sharp, leaving the clearcoat very vulnerable to damage from removing the wheel bolts. Its one reason to like chrome* plated wheels with no sharp edges.[]


*Where "chrome plated" means a copper base layer, followed by a nickle overplate, followed finally by polished chrome.

 

[Bruce R.]

Not to argue with you, but unfortunately chrome plating has some strong shortfalls as well. If not properly handled after the plating, plating will weaken the aluminum alloy in the casting, from what I understand, as much as 30%. TUV doesn't allow it in most European wheels made for Mercedes. The ones you see here in the States are done in the States, and there are no guarantees that they were done "right".
I understand there is one exception, an older Benz in the SL Class, can't remember which one tho'.

 

[YellowSLK230]

Bruce R. - 2/6/2005 7:51 AM

Not to argue with you, but unfortunately chrome plating has some strong shortfalls as well. If not properly handled after the plating, plating will weaken the aluminum alloy in the casting, from what I understand, as much as 30%.

I am not personally familiar with any way that copper-nickel-chrome plating can damage aluminum. It is routinely done on stressed motorcycle parts, including frames, engine castings, brake components, wheels and rims. I suppose if high temperatures were used somewhere along the way they could do damage to the heat treatment of certain aluminum alloys, but I am not aware of any such processes used in decorative plating and there are quite a few chrome wheels out there in the aftermarket.

If you (or any other posters with knowledge on the topic) are familiar with any technical references on the failure mechanism at work with respect to aluminum wheels, I would be interested in reading them; I would suspect the effects of salt induced corrosion on wheels to pose the greater threat.

 

[Bruce R.]

It's called hydrogen embrittlement in steel, and I believe it's a similar problem in aluminum.
High strength steel has to be baked at 400 deg. F for a period of time to release the hydrogen post plating, or the steel is weakened substantially. I'm not as familiar with aluminum, but I'm told the failure mode is similar.