I ferment in my keezer when empty of beer. I'm using a johnson controls thermostat brass probe and I leave my CO2 tank in the keezer during fermentation. I have done this without incident over the past year and several batches of brew.
This time around, I tried two batches of all-grain for my first time. I always use a starter and had krausen blow out through the airlock a few days later from an active fermentation. There was a LOT of krausen spewing out of two - 5 gallon carboys. I threw a towel around the top lids to soak up the mess until I had time, 4 days later to transfer to secondary.
Four days later, I opened the lid to my keezer and noticed everything that was brass had turned a dark rust color. Even the thermostat connecting the the johnson controller. I keep a regular thermometer in there just to be sure my temps are right and noticed they were off by about ten degrees. When I adjusted, the thermostat thought it was warmer and kept the condenser running until I checked on it in time (30 minutes later) to see it had cooled to 20F. Yikes! So I used a peice of sandpaper to sand down the corrosion of the temp probe hoping that would help. Not sure yet if this will.
What would cause this oxidation or corrosion of my brass fittings? Really, the only thing wafting inside the keezer would be CO2 and ethers right? Here's a pic of the damage... these were shiny brass fittings days earlier....