Can I Use A Welding Helmet To View An Eclipse
The short answer is Yes you can look at a solar eclipse with a welding helmet. According to NASA, you will be safe using a welding helmet with at least Shade 12, "Experts suggest that one widely available filter for safe solar viewing is welders glass of sufficiently high number. The only ones that are safe for direct viewing of the Sun with your eyes are those of Shade 12 or higher." Although most people (including myself) find that Shade 13 gives the most optimal view. Just don't go up to Shade 14 or else the solar eclipse will end up being too dark.
Welding Helmet For Eclipse
Even though you are able to look at a solar eclipse with a welding helmet, NASA still recommends using actual eclipse glasses. They state that your eclipse glasses just need to be up to date with the ISO 12312-2 safety standard. If your eclipse glasses don't have this safety standard printed on them, then do not use them under any circumstances. Otherwise, if you do, you could end up with temporary or even permanent eye damage.
Now, most of us handymen don't just have eclipse glasses laying around. Nor do we want to spend a ridiculous amount of money on something we might only use just once. Luckily you'll be safe relying on one of your welding helmets laying around.