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304ss exhaust to manifold corrosion / help needed


3046moparcoupe

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This past year I stumbled across a fella who had been mandrel bending large diameter stainless steel tubing here in the metroplex for years making custom exhaust, frames, roll cages, etc. Anyway - the fella said he was done but agreed to use the material he had left over on hand and bend up one more final system for my Plymouth project.  I took him the old stock rusted steel exhaust system (before and aft  of the muffler), and he bent up a complete new front to rear for me out of 304 stainless steel.

 

Next step was to find a 2" I.D. 304 stainless 2 bolt flange to weld on to the exhaust to connect up to my exhaust manifold. Found one at Summit Racing, made by Vibrant Performance PP#VIB1470S. 

 

I'm not there yet, but when the time does come to bolt this new exhaust system to the car, I'll have to make a one shot decision on where to cut it, and how best to bolt up the ss flange of this exhaust system to the oem cast iron exhaust manifold

 

My concern here, is in regards to the dis-similar metals making contact and GALVANIC CORROSION. I WOULD GREATLY APPRECIATE ANY AND ALL INPUT FROM THE FORUM IN RESPECT TO THIS, SO PLEASE ALL INPUT IS WELCOME AND APPRECIATED.

 

As you can see in the attached pictures, the front section was bent to allow me to duplicate the way the oem pipe slid up inside the collector end of the exhaust manifold, by locating the flange down some on the vertical straight section of pipe going into the manifold. ( I've physically inserted the ss pipe up inside the exhaust manifold and you couldn't ask for a sweeter, more snug, fit. ) At present the exhaust manifold is painted with rattle can VHT exhaust manifold/header 1300-2000 degree high temp paint, and I do have a thin coat of paint inside the collector opening where the pipe would insert. In theory I suppose the paint should act as a barrier to prevent the stainless and the cast iron from coming into contact, but in the real world - I doub't that I'd bet the farm  on that holding true. The manifold was cleaned up nicely when it was painted, and the paint will have had a few years to cure out, so that's a plus I suppose, but just the fact that on the description itself, which reflects a 700 degree variance window on how much heat the paint will handle ? seems odd to me , and kinda hard to believe...I'm just guessing here, but I'd guess that the manifold on this flat head six might run 600-700 degrees hot max ?? Maybe leave the input tube - but also sand off a few thousands off it, and also paint the exhaust input tube with this extreme high temp exhaust paint ??

 

I definitely like the idea of having the guide tube slip up inside the manifold for both pipe alignment and also for gasket alignment and longevity. Chrysler thought it was necessary and Walter was wat smarter than me :)But if it's gonna eat away at the female opening of my exhaust manifold, I need to try and find another way to mate this up.

 

Thoughts I've had in regards to trying to keep these metals separated are as follows: 

 

1: Eliminate the section of tube that inserts into the manifold and cut the pipe off flush with the top of the flange (maybe only leave a 32nd or so of pipe above the flange, just enough for the exhaust manifold gasket to center on),...and depend on the 2ea exhaust manifold bolts to hold the exhaust centered and in place.  This would eliminate the ss of the exhaust from touching the cast iron manifold, due to the manifold gasket seperating the two....my exhaust bolts would be the only steel touching both surfaces,....and easy enough to replace when necessary. But - you'll see that the 2ea bolt holes in the new ss flange are both slotted rather than spot on round holes. In my mind spot on round holes might do a better job in keeping the exhaust in place, and I suppose these slots could be welded up and re-drilled in effort of achieving that. Maybe un-necessary, don't know - I do not have the experience, maybe someone out there can share back with me on this ?

 

2: I had thought about not using the oem style flange gasket and trying to use a donut gasket, ( if I could find one the correct size) it would keep the metals separated and the donut gasket would keep the setup centered in place, but - I can also see where I would be restricting the size of the exhaust pipe down as the donut gasket would need to have a smaller ID to stay in position and seal...

 

There's lots of talent and experience here on the forum,i truly appreciate those of you whom are willing to, and whom have, shared it with me.

 

Your thoughts ?

 

Thanks again, 

 

Steve

 

 

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I wouldn't even be concerned.

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I've not heard of stainless steel and cast iron reacting to each other, certainly using a copper head gasket on an aluminium head is asking for trouble but stainless and cast iron is not something I'd be concerned about...........I'd certainly be using stainless bolts with brass nuts and a wipe of neverseize on the bolt threads but with that nice what looks to be 3/8"thick flange I'd think it would be fine.............check with a local exhaust supply/muffler fitting company or maybe an engineering supplier but as I have said I've never heard of stainless and cast iron reacting and would make the new stainless pipe flange exactly the same as the mild steel one...................regards....andyd 

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stainless pipes and cast iron manifolds have been the go-to since early 90's for most cars on the highway today.....there is no need to be concerned about anything except wrecking the vehicle  in an unfortunate accident.   One thing you did not address...welding the flange to the pipe as these are not ball socket connection to the manifold....get that flange welded...

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 … This reference pretty well covers the topic

https://www.fastenal.com/content/feds/pdf/Article%20-%20Corrosion.pdf

 

 

Article - Corrosion.pdf

Edited by T120
added link to article
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The link from T120 sums it up pretty well and while there is some potential for galvanic corrosion of the cast iron it should be low due the minimal moisture present in the exhaust manifold.  You could further decrease/eliminate the risk by getting your exhaust manifolds ceramic coated.  That would give you an insulator between the two metals.

Personally I like the idea of the donut gasket and you could take the exhaust manifolds to a machine shop to bevel the outlet.  Then you could slip your donut over the extended pipe (or better yet a flared end on the exhaust pipe) and it would increase you alignment tolerance.

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Personally,  I wouldn't worry about that at all. By definition, galvanic corrosion requires dissimilar metals in contact in the presence of an electrolyte.   That last item is almost nonexistent in an exhaust application.  Couple that info with the fact that stainless exhaust and cast manifolds are the most commonly used materials in today's auto production and my conclusion is:  No problem.

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