Is this a dissimilar metal issue?

[Horn]

Rear bumper on the Volvo is up for repair.  Last weekend it popped off on the driver’s side.  Under the plastic bumper cover is an aluminum bumper bolted to 2 pneumatic shocks.  The aluminum is brittle where the bolts went through.  I just pulled the passenger side right off.  I have a plan to fix with some scrap but curious if the problem is a result of the bolts, bumper, and bracket being 30 year old dissimilar metal.  It looks like the area where the bracket touched the bumper is compromised. The rest of the bumper doesn’t appear to be brittle, though I’m not going to smash it with a hammer to see.

 

[Horn]

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[JohnyPro]

That’s annoying, I’m no metal expert, but I think you’re probably correct. At least from what I’ve seen and heard from others about dissimilar metal corrosion.

 

[YJScrounge]

Yep

 

[redjeep]

Or enough salt got up in there and the aluminum oxidized. 

 

[A new level]

I’m going with both

 

[massxj]

Road smoo

 

[beaker]

Aluminum and salt is a bad mix.  Add in steel.  It's worse.  Isolate the bumper electrically from mounting bolts and flange.  Urethane paint while not reliable worked most of the time in a job long past.

 

[Horn]

Well, I didn’t isolate, it aside from a layer of Rustoleum, but aside from that it’s done.  Some JB Weld, 2 L shaped brackets supporting it from underneath 1 other section supported and some new bolts and it’s back on. One of the existing studs is still in place on the driver’s side.  The one that didn’t have a nut on it when the bumper came off.  That one still seemed solid.

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[YJScrounge]

AND.... NO ONE KNOWS!!!!!!!