We never like to see rust or any signs of corrosion, but I don't see anything extremely odd in your numbers that would indicate a corrosive atmosphere. Of course your CYA is zero from opening, so the chlorine is much more harsh, but I'm sure you're working that soon. Some parts in our systems are simply prone to corrosion. I have a metal e-ring under the lid of my Hayward filter that seems to corrode every 1-2 years and I can't see why. But keep an eye on the remaining parts of your system and pool. Hopefully this is just an isolated component (clamp, ring bracket, etc) that would have otherwise began to rust no matter what you did.