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Glycol is not corrosive unless it's left in the system too long (typically over 6 years) without a corrosion inhibitor being added periodically. But unlike water which evaporates from minuscule leaks before it's noticed, glycol leaves a residue. That's likely what you're seeing and it's nothing to be concerned about. I've had glycol in my heating system for 30 years with no ill effects and yes I do have some of the same deposits you describe. One thing I would mention, unless to live in an area where is gets extremely cold a 60/40 (60% glycol) mix is perfectly adequate. I'm in NH and that what I have in my system.