Black pipe airlines and the crud that develops in them.
Seems aluminum piping the air compressor companies are offering is the way to go these days if you are building a new shop or thinking about redoing your airlines. The picture shows a line that was only a few years old. Granted the picture probably depicts a worse case scenario but the science is real. At least a 1/2" air hose strung along a shops' wall on an angle with a filter and water separator on the end won't develop rust and hold moisture. Copper corrodes, stainless has carbon in it that could react to acids over time.

aluminum airline