Hi all, thanks for the responses!
Alaskastar said the blue substance was formed from ammonia contact with brass fittings and copper pipe (anamonious hydroxide I believe).
Alaskastar said the nickel is only a catalist and is not consumed, so
laying a roll of nickels in a section of one inch pipe should be fine. The heated liquids, when flowing over the nickels, rise to allow cooler liquids to replace those.
The suggestions with nickel plating, etc. of course should work. Any kind of pipe will work. But do you really see any advantage to making this process more expensive or complex? I don't.
I used a piece of 6"x1" pipe and as you can see in the photos utilized only a very small portion of the exhaust heat. You can utilize more of the available heat by using more pipe.
Alaskastar and I both agree on this- KEEP IT SIMPLE!