20% DowFrost (PG) freezes at 19.2F.
20% DowTherm (EG) freezes at 17.9F
if you slow your glycol flow down below the recommended minimum flow rate, the glycol next to the tubes can freeze and the glycol in the middle of the tube will not freeze. so then your chiller tries harder and gets less cooling out of it.
i don't know about glycol breaking down, but pudding......
Once you go black...you never go back to galvanized.