That's one hell of an assumption. It's not gonna break down equally across the entire spring. Whatever the weakest point is will eventually wear away first and cause it to break because of all the tension in it.
Even if it could dissolve equally across the entire spring, the outer parts would go first and it eventually will dissolve away from the things holding it in place and release that tension. If it doesn't just break due to the dissolving metal weakening the structure while still under tension.
I feel like to get the meat and potatoes of the question a better way of asking would be what would happen to the potential energy if the spring was instantly vaporized, like by a Star Trek phaser.