I am not familiar with HDG tanks, but have quite a number of Fabers.
As long as the coating is protected from scratching and other stuff with a net, and the interior has no oil/water etc, their lifespan is quite long. One of my faber was manufactured in 1988, and passes hydro and visuals w/o problems. Sometimes the rust inside looks a bit ugly, so I put them in a tumbler for a couple hours and that´s it.
Thank God, nowadays most of the tank inspection companies in my area do not engrave by punching the inspection date in the tank shoulder, but use stickers or tags. Punching was terrible for the coating in that spot.