Volvo 850 strange "ash" cumulation around seat mount

[JimBee]

This car was stored outside for the winter with no battery. When I started driving it recenly, I noticed a pile of ash almost an inch high around the left front seat mount bolt. There's a hole right next to that seat mount that goes through the floor panel. It almost looks like there was a different seat mount bolted there. The ash looks like some sort of electrical product. Any ideas?

[Ozark Lee]

The ash looking stuff is corrosion. I had a bum sunroof seal on one of the cars that kept the carpets soaked and I had the same stuff on the seat bolts.

...Lee

[JimBee]

Yeah, it looks like the corrosion that builds up around a battery terminal, only it almost looks like cigarette ash—a huge mound of it. There's a hole through the floor panel right beside the mount point that's corroding. Moisture might be coming up through there and keeping the carpet in that area damp. Why it's attacking the mount/bolt like that is puzzling. I'll clean it up, spray everything that's rusty with POR 15 and plug that danged hole. Hopefully, that will fix it.

[bmdubya1198]

Make sure there isn't any water getting into the car. I would agree that it's probably the cause of the corrosion.

It seems strange that there would just be a pile of it there, but anything's possible. Seat mounts tend to have a bit of buildup on them, especially if there is known water entry in the car.

[mrbrian200]

It tends to occur in the presence of salt or some other catalyzing agent.

When I pulled the front left carpet a couple weeks ago to clean it more thoroughly out of the car, I found 12 years of 'saltcake' buildup underneath from where salt solution water tracked in on shoes was getting through at the lower left corner where the carpet's rubber backing has cracks. I'm glad I pulled that carpet out , saw that, and cleaned it up good. Rust at this point was limited to 'minor/surface' just at the edges where larger pieces of sheet metal meet, but the paint on larger flat areas was not yet damaged. Left to be, that would have eventually cascaded into a total rot through of the floor pan. I'm brainstorming some sort of game plan to isolate any water that finds its way through the carpet from getting to the sheet metal floor pan. I'm leaning toward a poly based roof sealant/underbody/spray on bed liner sort of treatment sort of like a weathertech mat, but underneath the carpet and bonded. For the time being I applied a layer of poly roof sealant to the bottom of the carpet around that corner covered over with cellophane. But water will still seep around that through the foam. The seat bolt on that side had some white corrosion on the head as also did the DDM/mirror motor ground terminal which I occasionally see a DTC in vida for 'loss of communication', though I've never experience any functional problems with the DDM or power mirror. I addressed that ground terminal and am thinking 50/50 or better that will put a stop to that occasional DTC.

[RickHaleParker]

If the bolt and the mount are electrically dissimilar metals and there has been saltwater spraying up through the hole, you got the three ingredients for a corrosion cell. The principle non-rechargeable batteries work on.

The two dissimilar metals would form the anode and cathode, the saltwater forms the electrolyte. Galvanic corrosion (also called bimetallic corrosion).

[JimBee]

What's strange is there don't appear to be any water leaks, just that hole right beside the seat mount that could admit moisture. As a Minnesota car—new to me in 2011, but records from local Volvo dealer up to 2006 and local owner after 2006 known—the only salt exposure would be from road salt and things underbody don't seem badly rusted. So that remains a mystery. Comments, tho, prompt me to lift the carpet and try to stabilize any corrosion there.

[bmdubya1198]

It never hurts to take a look, you never know what you'll find. I wouldn't be too worried.

[abscate]

Corrosion protection on these cars is very, very good.

[JimBee]

True about the corrosion protection. There are a couple of rust spots on both of my 96's, plus exhaust and the top of muffler heat shield that was lying on top of the muffler. All 4 studs were rusted off. Not sure what to do with that.