HDG or Duragal for Fence frame?

Duragal would be fine but beware that cement (particularly leaching tile adhesive) can corrode it at the baseline. Having said that I have had duragal posts in concrete for more than 20+ years. Also I have come across these posts that are not gal inside and will rust out if water pools in them.

depends where you live and how long you want it to last and if can easily repaint it. If you want it to last use black and get it sandblasted and industrial epoxy 3 coat system or HDG. Gal is tricky with hollow stuff, needs drainage holes etc. The epoxy coating will outlast gal in really corrosive environments.

Otherwise just duragal and spot prime the welds after a wire brush. Then paint the whole thing. Should last 10 years if you are away from the surf