I'm playing quite the odd game here, not killing anyone, but I have run into something strange..
For the first time ever I have used disintegrate weapon and armor to steal peoples stuff but I'm at loss of how they work. When I fought a dark brotherhood assassin I threw a whole bunch of disintegrate armor spells on him and they all hit him but after a while I just couldn't wait any longer so I killed him just to get rid of him and when I looked at his stuff, the only thing damaged was his cuirass and it had only been lowered some 150 points. I must have thrown more than a thousand points worth of disintegrate on him..

So. How does it work? Clearly not as I thought...

How high was the magnitude of your spell? Disintegrate needs to be pretty high to be effective. I went back to test ingame and my Blade Chewer with a magnitude of 150 was doing 100 points of damage to my enemy's weapon. But I think the conversion gets worse if the magnitude is too low.

I have to warn you though, the vanilla spell cost for the disintegrate effects are ridiculously high and will require a gigantic amount of magicka to do anything useful.

I started using the standard armor eater spell, it has something like 10-20 points of damage but after a while I bought a bigger spell that (should have) made 45 damage. I'm still pretty low level so I can't throw much bigger spells without them failing to often..

But if what you say is correct, then it's pretty pointless to even try. I think I just have to test it the next time I catch an assasin in the mages guild. Save, make spell, throw it once and then kill him and check the armor. Rinse and repeat. Maybe it's just a pointless spell.

Just so you know all of the armor together has one thousand, seven hundred and fifty points of health. That's at least forty castings of your custom spell.

Be sure to kill him with pure magicka, as any weapon-inflicted armor damage will poison your data.

Yep. Or I will rather kill him with pure console damage since I have no damage potential whatsoever.
My assasins mostly have netch leather armor. But anyway.. 40 castings is nothing. That probably doesn't even break a netch cuirass..

So I caught an assassin in the mages guild again and did a series of tests on him. With straight 1 second spells i got these numbers:
20 damage did nothing at all.
25 damage did 10.
30 did 13
50 did 33
60 did 43
100 did 79
With two seconds spells I got:
30x2 did 31
50x2 did 69
With three seconds spells I got:
20x3 did 0
33x3 did 62
With four seconds spell I got:
25x4 did 13

So that's how it works. I can see some patterns, 20/second or less results in no damage at all. The straight 1 second spells are fairly linear with real damage close to X-20.
After that it doesn't make any sense at all.. It seems like several seconds of damage gives a small bonus compared to the same damage/sec over 1 second but not in the case of the four second spell..
Taken the spell cost into account it's the 33x3 second spell that's most efficient at a cost of 39 magicka so I will go with that one for now. Maybe I will try some more in that area tomorrow but for now it's time for bed.

And here comes a related oddity. It seems like skeletons (or rather their weapons/armor) aren't affected by the disintegrate spells. I really have no idea why. They get immobile by the damage strength spell so they are really carrying their stuff but they are just immune to disintegrate.. Not a big deal since a skeleton with 0 strength really doesn't do much damage even with a weapon but it's still odd.