'99 Legacy roof rack corrosion warning

[Scottbaru]

In case you haven't followed the other roof rack thread, there's some corrosion in my and another lister's roof racks. The threaded inserts in the aluminum roof rails look like hardened aluminum, and the screws holding the cross bars onto the rails are steel. The combination of steel, aluminum, and road salt has caused some serious corrosion in the threads on mine. I took mine apart, and had to use a lot of force to get some screws out. A lot of white corrosion rained out as I turned them. The last screw was stuck well enough to break loose the threaded insert inside the rail, rats. There's no way to get penetrating oil into the threads while the thing is assembled, I can't get them apart. Luckily the insert still has enough resistance, I think I can drill the screw out to finish dissasembly. I don't plan to use the cross bars again, but I'll track down stainless screws to replace the originals, and use lots of anti-sieze. If you have one of these roof racks and can still get it apart, you might want to replace those screws.

[Scottbaru]

Ok, I drilled the screw head off and got it apart. The crossbars are corroded on the bottom, the powdercoat is bubbled up in several hand-size areas. This Legacy has 78k on it, all in Northern Indiana. The only other rust or corrosion on it is the wheel balancing weights, it's in amazingly great shape for it's mileage.

[friendly_jacek]

Thanks for the tip. I don't have this problem in my 00 but live in milder climate. Sounds like galvanic corrosion. I think water sits in those bolt holes and does its thing. Your stainless steel will not solve the problem. Anti-sieze is not a bad idea, another thing I did in mine is to cover and seal the screws with anti-rust spray, you know, the type boaters use (not the cheap WD40 solvent).

Since I have my boat, I learned that you have to cover every metal surface (painted or not) with anty-rust to keep the salt water from eating metals. I extended this custom to cover every bolt and screw in engine bay and undercarrage in my cars.

[Scottbaru]

Your stainless steel will not solve the problem.

I guess I've never fully understood galvanic corrosion, despite decades of working on boats, racing aluminum bikes, flying aluminum aircraft, and now designing mostly aluminum industrial machinery. I assumed stainless would contribute less to whatever is going on, and maybe stick less to the crusty aluminum. Perhaps my good results in the past have just been from cleaning it up and using anti-sieze? I use Boeshield over dry lube on my bike chains, but I've never been convinced it lasts. It'd probably be perfect on the roofrack 'till I run through the carwash.

[friendly_jacek]

For example, when you put a stainless steel propeler in aluminium drive, aluminium will function as an anode and will be consumed (=corrosion in the outdrive). This is why I paint my stainless prop and make sure the sacrificial anodes (zink) are in good shape.

 

If you cover your bolts with paint or antirust film it should limit the process (less current). Zink plated steel is even a better option as zink will be consumed before aluminium.

[Scottbaru]

It looks like galvanized steel does better with aluminum. I've used zinc-based anti-sieze with good results in the past, copper-based for high-temp applications. I've always hoped the zinc would help with corrosion, maybe it did. My Thule rack came with what looks like black-oxide coated hardware, I've since determined it's black zinc coat. I haven't found black zinc coated screws in metric, I think that would be best for my roof rack.