I'm cleaning out two big bin storage containers that have been outside, near the pool, shaded by a big tree for several years, basically undisturbed. It gets sizzling hot here and this winter we had several hard freezes. With the big freeze we had at least an inch of ice on the pool. We usually have at least one hard freeze every winter; down in teens. This is HD MA 3x %.
When I got down to the bottom of bins I found several bottles of muriatic acid left over from an acid wash in 2004.
Just out of curiosity I used about 1/2 of a bottle to pour on the steps of pool, let it sit for a few minutes and then swish off, to see if it would do anything to the scale. (pH was 7.8 and I wanted to lower it to 7.2 anyway.) I thought, it's got to be so deteriorated it can't make much difference. Well.............. next day my pH was way down below 7.
I think keeping it out of the sun and somewhat cool helps a lot. Mind you I'm not advocating storing it that long in extreme conditions.
I normally keep MA in a big metal shed (metal container like used on ships) that is totally dark but no heating or cooling other than being in constant shade. I've been using the MA that has been stored in there, left over from 2008 acid wash. According to the testing results, when I use it, it hasn't deteriorated much at all.
BTW..... I keep the MA stored totally away from other pool chems.
In the second bin I found a bucket of Soda Ash (for raising pH) that has to have been in there for at least 6 years. I used that to raise the pH after I dumped the the old MA in the pool yielding a pretty drastic drop in pH. Worked just as it should have. It was sealed very well so was still in the powder form without any clumps.
gg=alice