[bet if it is its not near as bad as calcium]
Its not any worse than beet juice either and alot cheaper,[probably don't want to wash with either],
Calcium,don't have no experience with it,and we all know it rust steel,but,if it covers rim it shouldn't hurt rim,if you could get a stainless steel valve stem[maybe they are,never seen them rust?] you shouldn't have any trouble with it,people has been using it for years,probably takes many many tears for it to harm rims.
It won't freeze period I don't think,if it mixed strong enough,its probably got a much lower freeze point than beet juice if water/methinal mixture is strong enough. I got[I think] 5 gal to about 35-40 gal water,thats supposed to be good down below zero I think,[forget the ratio to freeze point now,but its probably on this site somewheres.
Now yes its nasty stuff compared to beet juice and it don't weigh as much as beet juice[I hear],it weighs same as water more or less,but for your average small tractor,thats what 5 lb difference? a tire,? between juice and methinal,not much more than that anyways.
Calcium,rotted your tubes,didn't know it would do that,[believe you,just didn't know that],heard it rusted out stems?But 20 years later,thats not to bad a deal than,20 years,most people don't have anything now a days for 20 years[stuff don't last that long].
So your saying pure methinal will freeze at minus 20? [When I got mine filled had done a search on here I believe,and kinda maybe figured 5 gal to each tire with water would keep it from freezing down to well below zero,and thats about 1/7 or so.] .Your going to make me do another search.[Not saying anyone would put pure methinal in their tires!]
Speaking of methinal,I would be more worried about the effects it could have on the tire itself,supposed to be none,but[and mines filled with methinal],don't believe it will ever hurt the rim,but wouldn't bet you the same with tire in say 20 years.Will let you know in 20 years.