Box springs can be repaired if you tie up the springs with wire and/or change or add new boards for strength.
It will rust bad
Experience here sharing. We love doing this and try to search out old springs that are discarded! When primed and painted we find they will last years. Our choice of colors run from dark brown, black, and have two shades of green. These colors blend better in the landscape of our gardens and are not at all obtrusive re curb appeal as long as not specifically roadside or frontage. They are strong and great for putting right smack in the bed (ideally before planting, else take care with the existing plants OR wait until the existing plants have 'died back'). They especially serve well with being useful in supporting those types of flowers that typically 'fall over', such as peony bushes! Haven't used them with bulbs yet, thinking that is next garden project since have 2 boxsprings yet awaiting usage. Happy planting!
Love this idea thanks!