Salt, Calcium Chloride, used as a de-icer up here in Canada, will absorb so much moisture it will literally dissolve itself into a puddle of goo if left to the atmosphere and it's own devices. Have a pan at the bottom of the barrel that can accommodate the volume of slurry (if there is that much moisture in the air inside of the barrel to get it to that point) and that can also take some heat. If the salt ever gets enough moisture to get all soupy, pull out the tray and heat it (slowly or the trapped moisture might 'spawl' ie explode) then once it's dry salt again just put it back in the barrel and you're good. One small cavoite to consider though is that heat is released as it dissolves, to what degree that would be when it's just exposed to 55 gallons of air i'm unsure, and would require a temp test vs time in comparison to a barrel without the salt added. Also safety note, wear proper PPE as the dried salt will want water from anywhere really really badly and it doesn't care if that's water from your skin, eyes, other mucosae membranes etc so avoid contact with the salt and don't make it into a powder as the dust will also dry out anything it touches causing damage, the wiki also mentions the powder causing internal burns if it gets inside you.
There's also lab grade stuff "Calcium sulfate, Gypsum" under the brand name Drierite that doesn't dissolve in the water it collects, but it's pricey compared to the stuff we just throw on the road to melt ice.
https://www.sigmaaldrich.com/CA/en/substance/drierite136147778189