The outdoor artificial boxwood foliages have a 5-8 year outdoor and 15 indoor warranty before color fading and any other issues can occur or be apparent. We can only offer a lifetime guarantee on galvanized frames and that is just for the frames.
Unpainted raw steel should last the duration of the outdoor artificial boxwood foliages for both outdoor and indoor environments. Paint/primer just adds additional protection from rust. This just makes the frames last even longer. No matter if painted, galvanized, or powder coated, the frames would likely need some kind of touchup after 5-8 years. If regular paint, then the metal would need a recoat. The best route is to galvanize the metal to ensure a lifetime use of the topiary.
Galvanized metal frames work great for all almost every application except live plants and fertilizers with direct contact against the metal. A shrub is fine since the frame is for shaping the leaves/trunk and not in direct contact with soil. Artificial moss is fine too with no planting. For a metal frame that will be in direct contact with the soil/moss for planting and fertilizers, Powder coating is recommended over Galvanizing the metal.
Below is an example of a galvanized framed. Note the silver color.