Since armor can't be the target of the special attack sunder, when would it ever take damage?
Weapons and shields have listed hardness and hit points that may be adjusted by its material composition, and they can be targeted by a sunder attack. Armor, however, has a hardness and hit points solely from its material and thickness...but when and where would this mechanic ever come into play?
Do attacks that are absorbed by the armor, or penetrate it to damage the player, not mar it at all?
Weapons don't get damaged when they strike, armors and shields don't get damaged when they intercept a hit... or at least the game rules don't care about these sort of nicks which yes, your character might need to fix between adventuring days (for weapons, this is made clear by the presence in the game of the whetstone, which, unlike other mundane equipment, has no rules attached; it's easy to assume the same level of detail is to be used for armor).
There are, anyway, situations where the HP and hardness of a suit of armor are used.
You could try to sunder or otherwise attack an armor that is not being worn, sometimes to break a curse, sometimes to get rid of an evil magic item, sometimes to break the item upon which an unconvenient spell has been cast, sometimes to expose a demon that is possessing it, sometimes to prevent some kind of undead to come back the next day (I'm sure this kind of undead exists in Pathfinder 1e but I think 3.5e also has it).
There are enemies, usually some kind of ooze, that deal damage to your armor when they hit you.
Some spells or class features might be able to specifically damage armor.
If you roll a natural 1 on a saving throw against a spell, you need to identify the four items that are more exposed (according to a table) and randomly roll which one item might also be damaged if it fails a separate saving throw (the table starts with shield, then armor, so if a character is wearing armor there's at least a 25% chance that it risks being damaged).
Therefore... no, armor is not invulnerable.
Well, since armor provides an "Armor Bonus" to your AC, we would consider that your armor deflects any and all kinds of physical attacks when your enemies are hitting on your armor rather than on you at the right angle. Same as the shields which provide a "Shield Bonus" and require enemies to get around it in order to hurt you.
And there's a spell that can hurt all your equipment, including armor: Acid Fog.
Each round on your turn, starting when you cast the spell, the fog deals 2d6 points of acid damage to each creature and object within it.
It doesn't mention whether the object can be worn or held by a creature, so both used and unused objects would be affected normally. And it does not have a save throw. So you may expect an NPC to walk out of the fog naked if they stay there for too long (since all their equipment that don't have immunity or resistance against acid, including armor, are destroyed). Almost sounding like a cheaper version of Mordenkainen’s Disjunction.
