You might check out OSHA's site, or call your local University and talk to a Chemistry Professor. Describe an E-Tank, the D/C charger, stainless steel and ask about hexavalentchromium. Be prepared though, my professor laughed.
Now in saying that if you do not feel comfortable using stainless steel...don't. I'm notoriously tight and for many years used flattened #10 food cans. They don't last long, but will work.