Chlorine & Cast Iron Don't Mix

Last Spring, we had a severe wind storm that blew our pool deck umbrella into the swimming pool. The base is cast iron, and weighs about 50 lbs, so it was impressive that the wind could move a closed umbrella about 3 meters and plunge it into the pool.  The next morning, I discovered the submerged umbrella and stand, and fished it out.  Many months later, with my water-proof Olympus Stylus Tough, I took a close up photo underwater of the permanent iron oxide stain that resulted from the hours of chlorine exposure to the painted cast iron base.

I tried scrubbing it off (albeit underwater) with no effect. I'll just have to chalk it up to permanently etched oxidation.  And from now on, I keep a deck reclining chair between the pool and the umbrella and it's heavy stand, as an added buffer of safety margin.