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One of the PCs in my group is trying to go through every locked door in a dungeon with his acid breath weapon. They're supposed to find the key or lock pick it open, but the player wanted to just melt every door. Does the acid count as corrosive to metals? Or just flesh?

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    \$\begingroup\$ Just out of curiosity, is your party really taking a short rest between every locked door? \$\endgroup\$ – Miniman Aug 31 '16 at 5:26
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    \$\begingroup\$ The other answers below seem more correct to me (coming down to you as the DM deciding if that makes sense), but if you want to be realistic, acids really do eat through metals to produce (usually) water and some kind of salt. It depends on how thick the door is, the pH of the acid breath, how long it is exposed, and other factors, but it is realistic to think that this even has a chance of working, whether or not the specifics may or may not prevent it. bbc.co.uk/schools/gcsebitesize/science/add_ocr_21c/… \$\endgroup\$ – Cody Aug 31 '16 at 16:23
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    \$\begingroup\$ @Cody (1) why not make an answer out of that? (2) how much time does one have to use the proposed acid available in the question to achieve the objective? \$\endgroup\$ – KorvinStarmast Aug 31 '16 at 18:39
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    \$\begingroup\$ @KorvinStarmast I didn't really see it as an answer since this site is Role Playing, not Chemistry. It just seemed like an interesting point that none of the answers suggested, and something worth considering. I still maintain that they all have more correct answers, all variations of "You're the DM, you decide if its fair". For your second question, I'm not a chemist, just a nerd who happens to know a bit of chemistry. That question should be taken here: chemistry.stackexchange.com \$\endgroup\$ – Cody Aug 31 '16 at 18:43
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    \$\begingroup\$ @KorvinStarmast Rhetorical or not, now I want to know the answer. Maybe it should be posted there. I wonder how they'd react to "How hard would it be for a dragon to vomit through a metal door?". I bet that question would trend pretty quickly. \$\endgroup\$ – Cody Aug 31 '16 at 18:47
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TL:DR - Even if acid does "melt" metal, it may not open a locked door

Although not a direct answer to the question, it strikes me that in this situation whether the acid is capable of melting metal or not is irrelevant.

Consider the term "melt". Acid does not really melt things: it corrodes/dissolves them from the surface downward, with the acid losing potency as it goes, since it's reacting with the substrate to form an inert product. Powerful acid is an effective weapon because skin is thin and is essential to the body. Dissolving the surface of it down to the flesh is agonizing and debilitating.

Now think about applying the same logic to metal armour. It's thicker than skin, and so will take longer to dissolve. It also has a greater chance of rendering the whole volume of acid inert before it eats through to the other side.

Now apply the same logic again to a lock. The visible portion of a medieval level lock on a door is a chunky bit of metal. To actually damage the internal mechanism you've got to eat through that outer casing. That's likely to require a lot of acid.

Furthermore, trying to dissolve a lock from the outside of a door isn't an easy task even if you do have enough acid. If you apply it to the visible lock and it does corrode through the outer plate, all you'll likely achieve is to damage the internal mechanism so much that you'll never open the door, even with a key. To open the door you need to corrode the bolt itself which, if you think about it, has almost no surface area accessible in a locked door.

In short, you would seem to have plenty of options to discourage this sort of behaviour should you wish to do so, regardless of whether you and your players agree that the breath weapon can corrode metal. The first time your players find themselves stuck because they can't now open a locked door they tried to corrode even once they do have the key will probably also be the last.

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The 5e SRD says about Objects:

...given enough time and the right tools, characters can destroy any destructible object.

So, is acid breath "the right tool" for destroying a metal lock, or a wooden/metal/stone door?

Objects and Damage Types: Objects are immune to poison and psychic damage. You might decide that some damage types are more effective against a particular object or substance than others. For example, bludgeoning damage works well for smashing things but not for cutting through rope or leather. Paper or cloth objects might be vulnerable to fire and lightning damage. A pick can chip away stone but can’t effectively cut down a tree. As always, use your best judgment.

It doesn't say they're immune, and it doesn't say they're vulnerable. In the end, it falls to DM judgement.

If you decide it is able to be destroyed, the Objects page gives the stats for objects' Armor Class:

Substance           AC
Cloth, paper, rope  11
Crystal, glass, ice 13
Wood, bone          15
Stone               17
Iron, steel         19
Mithral             21
Adamantine          23

and Hit Points:

Size                                    Fragile     Resilient
Tiny (bottle, lock)                     2 (1d4)     5 (2d4)
Small (chest, lute)                     3 (1d6)     10 (3d6)
Medium (barrel, chandelier)             4 (1d8)     18 (4d8)
Large (cart, 10-­ft.-­by-­10-­ft. window)   5 (1d10)    27 (5d10)

eg, an iron lock: AC 19; HP 2-5 (1d4-2d4)

But even then, he can't use it on every door, unless your party takes a lot of rests. From the 5e SRD:

After you use your breath weapon, you can’t use it again until you complete a short or long rest.

And, destroying the lock or door may have unfortunate consequences. Maybe a trap will trigger. Maybe you'll want to re-lock the door later - easy if you've used the key, impossible if you've melted it.

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    \$\begingroup\$ The dragonborn acid breath attack is an area of effect, not a targeted attack so there is no attack roll, making the object's AC irrelevant to the question with regards to the breath attack. \$\endgroup\$ – LegendaryDude Sep 1 '16 at 12:30
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    \$\begingroup\$ The AC information is useful to the OP in general for adjudicating other attempts to destroy objects. \$\endgroup\$ – Bloodcinder Jul 24 '17 at 11:27
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Not all Acid Damage corrodes metal

If you take a look at the most infamous creatures that deal Acid damage, Oozes, you'll see that the Monster Manual is specific on what kind of oozes can corrode metal. Two kinds of ooze, namely the Gelatinous Cube and the Ochre Jelly, though they deal Acid damage, do not corrode metal.

In the case of a Dragonborn's Breath Weapon, they are modeled after a Dragon's Breath Weapon, and both Copper and Black Dragons lack the ability to melt metal objects.

Conclusion

No, a Dragonborn's breath weapon cannot be used to melt anything other than flesh simply because the game doesn't say so.

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    \$\begingroup\$ I'm not an acid expert but I know at least some acids are ineffective on metal. Since the game doesn't say that Dragon acid melts steel beams, I assume it's one of those kinds of acids. \$\endgroup\$ – daze413 Aug 31 '16 at 10:14
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    \$\begingroup\$ Visit youtube and search acid with metal. Your stomach acid can melt metals. A quick google should bring you to BBC which says "more (kind of) metals are able to react with acid than with water (rust)". No need to be an expert. \$\endgroup\$ – Sheepy Aug 31 '16 at 10:53
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    \$\begingroup\$ I'm guessing these reactions are from the metal being submerged into the acid, and not the acid being sprayed unto the metal? I've sunk myself too deep into the acid pit over this answer and have reached the limit of my knowledge (so this is my last argument) but wouldn't an acid that is applied over a vertical surface have less time to react with the metal and would be ineffective at corroding it? Of course, this is fantasy acid, and you can say that it is much much stronger but the rules indicate that: yes, it is strong, but not strong enough to melt metal, apparently. \$\endgroup\$ – daze413 Aug 31 '16 at 12:29
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    \$\begingroup\$ Regarding the chemistry: The acid is consumed in the reaction with the metal. The more metal, the more acid you need to corrode through it. Even a weak acid (say, vinegar) will dissolve the Eiffel Tower, given enough time and quantity. But a single spray of even the strongest acid will only mar the surface of a large item like a door simply because the acid is almost immediately consumed. \$\endgroup\$ – pokep Aug 31 '16 at 15:48
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    \$\begingroup\$ @KorvinStarmast Thanks. But I'd rather not go into complicated real life acid discussion because my main point is that having non-metal-eating acid doesn't mean those are normal/standard/average acid or that make these metal-safe acid the default in D&D world. \$\endgroup\$ – Sheepy Sep 1 '16 at 2:07
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No

The dragonborn's Breath Weapon racial trait explains how it works:

When you use your breath weapon, each creature in the area of the exhalation must make a saving throw, the type of which is determined by your draconic ancestry. The DC for this saving throw equals 8 + your Constitution modifier + your proficiency bonus. A creature takes 2d6 damage on a failed save, and half as much damage on a successful one. The damage increases to 3d6 at 6th level, 4d6 at 11th level, and 5d6 at 16th level.

Target choices for spells and other game features are deliberate. If a dragonborn's breath weapon was intended to affect objects, it would say so. Note that this is also consistent with the breath weapons on the stat blocks for dragons.

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Each acid reacts differently to each metal. Copper reacts with practically everything. Chrome and aluminum resist most acids unless there is electricity. Platinum and palladium react very slowly with HN and HCl. Gold can only be dissolved by agua regia...

Some metals will dissolve instantly, others sit and bubble for a long time.

Piranha will dissolve steal, but it is probably going to explode when it hits the floor with sulphuric fumes...

Not sure about brass.

Make the locks out of other metals...

(source: work with acids and metals.)

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