correct. If at any time you have less than 0 armor corrodes will be removed until you have exactly 0 armor.
If you got Rhino hide and Dragonscale Hauberk + 4 corrodes on your mage, then you will have a total of 0 armor.
If you disarm yourself (dragonscale) then 2 corrode will be removed from your mage.
In the next upkeep the dragonscale is no longer 'disarmed' and your armor will now increase to 2.
If you had Rhino + Dragon Scale and only 1 or 2 corrodes on you, then disarm alone would not remove anything.