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SS Muffler Flange Bolts Corroded in Half


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This is my daughter's 2008 Legacy 2.5i.  This exhaust is stock taken off a 2008/9 LGT.  When I put this together ~6 years ago I figured I would use stainless steel bolts, washers, and nuts for the muffler flange connections so as to avoid corrosion.  Car has been loud for a bit now.  Got under there today and the flanges are REALLY rusty and I am not surprised about that but I am surprised to find one 3/8" SS bolt on each side has corroded completely in half right in the middle.  Any idea what forces caused stainless steel bolts to corrode like this?  The only thing I can think of is the exhaust gases and leaky gasket result in acidic corrosion on the bolts.




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Not enough nickel in the stainless alloy.  Of course, the heat will accelerate the corrosion, but without enough nickel the SS is mostly iron, which oxidizes easily.  

I have found most hardware store SS hardware doesn't hold up that well.  Go to mcmaster-carr and get true SS hardware if you want it to last.  

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Yep and that is where they came from - the local hardware store.  Tomorrow I'll clean them up and see how much steel flange is left - if any.  I did order 4x of the split flange repair things with 1.75" ID in case there is not enough material left when I get it apart.

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I had my flanges rust out in the same spot a year or two ago.  I cut the flanges off and used some exhaust flex pipe to clamp it together.  Held pretty well through the winter, though I did have to replace the flex pipe section before getting all the way to exhaust replacement.

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I posted the before pics so here are some after pics.  The exhaust leaking at the flanges must have some corrosive qualities - or the moisture that comes out of there too.  The rusty mess cleaned up far better than I anticipated.  I used new gaskets and permatex ultra copper which says good for exhaust and up to 700*.  I've never used silicone anything on exhaust before.  I guess we will see how it holds up.  I used M10 x 1.25 bolts/nuts to hold it together.  Car is nice and quiet again and daughter is happy.




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  • 3 weeks later...

Fwiw - all high grade SS is created equal no matter where you get it. (And if it is being labelled accurately, etc….)

Just saying, if it is 300 series stainless, with lots of nickel and chromium + vanadium, you’ll be good. Ie: 304 SS

If the grade isn’t listed, good chance it is 200 series and won’t hold up to the corrosive effects of an exhaust leak, no.

I like McMaster Carr as much as the next guy, but their stuff does not come cheap. If you can find a local place that carries good grade SS, maybe you can save some $$$.

I buy a lot from Fastenal locally, tho they require you to have a commercial/business account. Amazing how much car related stuff the place I work for buys… on my CC. Weird.

Edited by KZJonny
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