CO2 Tank Fittings

I picked up a 20 lb CO2 tank and need to hook up my regulator to it. They both have 3/4" male ends. Can I use a regular black 3/4" union to hook them together that I picked up at Tractor Supply? Or, do I need to get the brass fittings? Didn't know if the non brass union would hold up to the pressure or temperature.

Thanks,
William Crawley