SS heat exchangers

[srb7f]

Were there such a thing as Stainless Steel heat exchangers where the exhaust pipes were made of normal steel?
I am curious because I bought two exchangers separately. The shape of the metal is identical on each, but one appears to have much more corrosion/dark discoloration on the actual pipes only than the other one does. The ducting surrounding the pipes is definitely stainless on both as it is still bright and shiny on both of them.
However, the surprising thing is that both of them have significantly more corrosion on the plate which bolts to the muffler (which I know is not stainless on both). So my question is, did I buy one with normal steel pipes with stainless ducting and one where the pipes are stainless as well, or do the stainless steel pipes discolor and develop some bizarre surface corrosion after years of use? The one with this on it does appear to have been through more service. I'm just curious. Steve

[tshih]

What do you mean by dark discoloration? Is it dried baked on oil+dirt or rust (brown and flaky)?.

[Dave at Pelican Parts]

Some early SSIs had mild-steel muffler flanges. At one point, SSI would replace the flanges free of cost (except you had to pay shipping, I think).

I don't know of any that had mild-steel tubing. That's the part that can cause serious problems if it rusts ("serious problems"==carbon monoxide poisoning), so I would doubt that anyone would use SS for the wrapping and mild steel for the tubing.

Check the parts with a magnet. The better grades of stainless are non-magnetic.

--DD

[srb7f]

Before I mounted the SSI exchangers, I tested them with a magnet. It stuck strongly on the flanges of both, loosely on the exchanger pipes with the surface rust, and did not stick at all to the pipes which were obviously stainless. It did not stick at all to the heater duct work on either exchanger.

So I think that this tells me that the company must have used a lower grade of stainless steel on the pipes than on the ductwork, and at some point they must have upgraded the pipes to the quality of the duct work. I assume that this would explain the discoloration/slight surface rust on the one set of pipes but not the other. At least that's my assessment. Steve