What Caused this rusting?

This space is an old shower that has been converted to a storage room/pantry with a 6 ft high metal restaurant style shelf.

Something fully rusted the safety bar, and I'm wondering what. The space has no light so I just didn't see it until a while back. I couldn't say if this happened slowly or in a matter of weeks or months.

Various pool chemicals and cleaning supplies have been stored in here over the years, but I don't store those together because I read that's dangerous. Bleach, baking soda, muriatic acid, and a product called Zappit 73, (calcium hypochlorite), and packets of shock have been in there over the years. Free unstabilized chlorine is stored outside of this area but near it in jugs from the pool store. My understanding is that those jugs are vented somehow.

Looking at the picture I now see that the old floor drain cover, which was surely made of some sort of no-rust metal, is covered in rust.

Can anyone tell me specifically which chemical caused this aggressive rusting? If anyone has a brilliant quick fix idea on reversing this mess I'm all ears.

The space is really tight, and to install the shelf it was brought in piece by piece and put together inside the shower.

IF IT was ME, I would move all pool chemicals OUTSIDE the house....???

Chlorine is corrosive. I would guess if that is vented to the room then that is what caused the rust.

Among other reasons, one of the reasons pool rooms are kept at a negative pressure is because the vapour of the chemicals used are corrosive. It’s also why pool components are a high grade stainless steel (not the magnetic stuff).

I also recommend moving the chemicals to either a room that is designed for it or to the outside in a safe place that can be locked away from little hands

I finally got over to Trouble Free Pool website. They have complete info on pool chemical storage. I'm glad I'm finally reading about this.

Chlorine is corrosive, but it's the muriatic acid that is much more aggressively corrosive. That shower bar would have been designed to resist rusting, that's why I was surprised when I noticed that mess.

Apparently heat doesn't affect potency free chlorine so a garage is ok for that. Muriatic acid is not recommended for the garage as it could corrode something on your car or other things.

I now have the MA in 5 gallon buckets with tops on them outside in a place that is 100% in shade.

I'm usually a cautious person, but you don't know what you don't know.