Originally Posted by PorFin
Looks like both your flange and your elbow are aluminum.
You might want to try applying a little (maybe a lot) of heat to the pipe coupler (your "joint") to try and get the two apart.
As for cleaning out the inside of the mixing elbow: If it is aluminum, you'll need to be careful if you want to try boiling it out. Radiator shops usually have set ups to do this, but make sure you you talk to the owner/foreman and discuss the risk up front.
For DIY, I'd think about letting it soak in Spray Nine for a while first, to see if that helps loosen things up a bit. Carb and/or brake cleaner might also be somewhat effective.
The elbows do look aluminum in the picture, but by weight, they're definitely steal. The joint is stainless, I think.
The parts shop offered to use an acetylene torch when we bring it in next week, but if they tell me I can reuse it, then I don't need to remove it. As for the mixing elbow, I'll pass your de-crudding suggestions on to my buddy whose been working on it. Thanks.