18G is pretty small. I'd definitely go up a size or 2.
I've been using mostly .062" (almost 14G) MIG wire with no problems at all.
I picked up a 6 lb spool at a surplus yard for $12. There's probably 1500' or so on a spool that size.
The alloy is unknown, but it's fairly hard and springy, has a very smooth shiny surface and anodizes nicely. If you bend it too tight it'll crack.
It also welds nicely with my new capacitive discharge welder.
That little bit of wire just above the surface of the acid bath has the hardest time. It gets the full corrosive action of fumes and acid being sloshed on it without the protective benefits of being submerged in the acid where it can "see" the cathode and get a protective anodized coating built up to protect it.
You want to make sure the wire is sized large enough that it can afford to get a small amount eaten away and still survive, plus be large enough that the current flowing through it DOES NOT CAUSE ANY RESISTIVE HEATING.
The warmer the wire is, the faster the acid will react and eat it up. Even a few degrees greatly increases the speed of destruction
That bit just above the surface of the acid gets eaten fastest. As it gets eaten and the diameter gets smaller it's resistance in that spot increases, which causes localized heating. That heat increases the speed that the acid eats it, which causes it to get thinner faster and the resistance to increase faster which causes it to heat more...
It's a vicious circle that ends when your part hits the bottom of the tank.
Once the wire has entered the surface of the tank, it is both cooled by the acid bath, and quickly protected by an anodized coating. It's that bit just above the surface that's a bitch. Keep it fat and keep it cool...
steve