Rehabbing an old hot water tank

[jeff47]

1. No idea if this applies to this kind of piping...

...but on copper potable water pipes, you don't want to remove the "rust". It's a thin layer that's permanently stuck to the metal, and keeps the rest of the pipe from rusting. (As opposed to iron rust, which flakes away and exposes new metal).

So in copper potable water pipes, the inside of the pipe quickly rusts, but it stays there. Removing that rust makes the pipe thinner, and the newly-exposed metal rusts again, resulting in a thinner pipe.

However, this isn't a potable water situation, so I don't know if the same applies.

If it's just build up from minerals in the water, soaking in a relatively weak acid should dissolve those minerals quickly without harming the pipes. Think vinegar-level acidity, but you'll probably want to mix it up from a cheaper acid, such as muriatic. Too strong of an acid solution will indeed eat the pipes.