I would worry about it. All new springs come painted. It wasn't corrosion that broke the rear spring, its a common problem on our cars. It happened to me at 37k miles and I replaced it with a new OEM one. It was on the car until I put new Eibach springs on at 96k miles. Just replace both rear springs and you should be fine for quite a while.
2003 BMW 325xi
2010 BMW M3 Sedan