You don't do this test on a finished product, you do it on a test piece. It is a destructive test. Clamp the welded pieces in a big vise, and grab a big hammer. You just bang on stuff until it comes apart, then you look at what broke: did the weld simply peel off the metal again (failure)? Or did the weld hold long enough for the metal to break somewhere else (pass)? If you simply mangle the heck out of it and can't get it to break, that's also a pass.
Yes.
Not quite. According to Lincoln, the xx15, xx16, and xx18 rods are low hydrogen. There are plenty of other rods in the 7000 series are not. There are also low hydrogen rods that are not in the 7000 series.
7014 and 7024 are "high deposition rate" rods with extra iron powder in the coating. 7014 is very easy to run, especially on AC, and is typically available even in the smaller 1/16 sizes. It is sometimes dubbed "farmer's rod" because even inexperienced welders can usually manage to make acceptable welds with it without a lot of practice.There are formulations of 7018 which are designed specifially for use on AC machines, and are typically labelled either AC7018 or 7018AC. Often available in 3/32, but I've not seen it much in 1/16. Near me it is more common at hardware stores, it can be hit-or-miss trying to find it at the big home centers.
Once 7018 has gotten damp and lost it's special low-hydrogen properties, it will still weld much like any other mild steel rod, it just won't protect you against hydrogen embrittlement anymore. If you need some for a project, and don't have a way to keep it dry, don't worry about throwing away the unused part when you're done, just save it for non-critical stuff.
With any rod, if it picks up enough moisture to actually get rusty, it's junk. Also, if the coating pops off easily or has degenerated into fuzz and fluff, toss it.
If you stick a rod trying to get it started, and the whole length glows red and the coating turns brown, you've pretty much killed it too. When you stick a rod, it can be easier to quickly yank the rod holder off instead of fighting it. Let it cool, break the rod off the workpiece, cut it back to a clean end if you have to, and try again.
--Glenn Lyford