Ask yourself: "why does not every person I know (in real life) carry lots of acid? It seems so handy!"

Your answers to that, which may include:

• acid doesn't actually melt everything^*;
• acid that melts things can be very hard to control in application;
• acid that melts things is hard to store, transport, and handle;
• acid can take much longer to deal with locks than, say, a key;
• acid can create noxious fumes;
• acid generally irrevocably damages things, which isn't always desirable;
• acid can be hard to procure in large quantities;
• &c. &c. &c.

Your artificer doesn't have to worry about procurement or storage, presumably. But the rest (and whatever else you think of!) could all be reasons that a PC might eschew the use of acid, too--use them to inform your world, your designs, your rulings.

^{* - The stuff described at the bottom of this WhatIf basically does, though. A quick scan of the linked articles should provide you all the narrative fodder you need for D&D's "step 3": The DM narrates the results of the adventurers' actions.}

This Isn't a New Problem...

The Acid does a number of damage to the objects following what the rules say. This damage is no different from a weapon damage, game wise. This question is the same as asking "My barbarian smashes everything". In that regard this might be a duplicate, except this is play test material instead of a core class.

Acid in DnD isn't special. It does damage just like any other weapon. You'd solve it the same way as raging barbarian tearing through stuff.

Object Have AC and HP per RAW...

Objects in 5e, have HP. Look up in the PHB or SRD the HP of chests and doors, and from the listed HP numbers for various things, figure out how much HP the other stuff he's splashing with acid. I doubt the bars would melt with reasonable HP.

A critical hip might burn some mundane objects in the chests, and I'd calculate that out, too, to make sure.

Object AC

$$ \begin{array}{l|c} \text{Substance} & \text{AC} \\ \hline Cloth, paper, rope & 11\\ Crystal, glass, ice & 13 \\ Wood, bone & 15 \\ Stone & 17\\ Iron, steel & 19\\ Mithral & 21\\ Adamantite&23\\ \end{array} $$

Object HP

$$ \begin{array}{l|l|l} \text{Size} & \text{Fragile} & \text{Resilient}\\ \hline Tiny (bottle, lock)& 2 \; (1d4) & 5 \;(2d4)\\ Small (chest, lute)& 3 \; (1d6) & 10 \; (3d6)\\ Medium (barrel, chandelier)& 4 \; (1d8)& 18 \; (4d8)\\ Large (cart, 10'\text{x}10' window)& 5 \; (1d10)& 27 \; (5d10)\\ \end{array} $$

Unearthed Arcana is play-test Material -- Tweak As Needed.

Lastly, remember the Artificer is playtest material. I personally think this problem isn't a problem of the class, but one of player-DM relation -- but any problem in play test material can easily be remidied by a rule change.

Inform Players Ahead of Time

If you change the way it works (either Object HP as RAW or a rules change), don't spring it as a surprise in game. "Hey, this thing is broken in our game. We're going to try a change tonight. I'm doing X differently, and see how it works."

There's a few things you can do.

1. Remember that glass is (generally) unaffected by acid (though there are acids that will eat away at glass). But also have the glass be sturdy enough that it can't simply be shattered, either (thus causing the characters an alternative way of doing things).

2. Put obstacles in the way that acid won't help with (a pit for instance) or have something that actually eats acid. (For instance if the character is trapped in the stomach of something… the acid would start breaking HIM down (or later down in the digestive track aid in his own digestion!)

3. Have the acid do something that's unexpected and cause more problems. For instance in the door scenario, you might simply have the lock and its opening mechanism fuse together, thus the door becomes inoperable through normal means.

4. Do exactly as you said. If there's important contents inside the chest, they're simply destroyed.

5. Let the acid use come back to bite them. Let's say that he uses acid in battle… but forgets where he threw it. Suddenly he steps where he threw it and now he has a disadvantage because there's a hole in the floor! Or they cross a rickety rope bridge, and to prevent others from following he uses acid on the ropes. Now the party's stuck with a very hungry dragon… and the PCs have no way of escape!

Create Different Obstacles

You seem to be presenting your characters with lots of "Lock-and-Key" problems. These types of problems don't always require a literal key, but more often a particular item that solves an issue that impedes progress. This player has a sort of "Master Key" that works effectively on a wide array of "locks," literally anything that can be melted with acid. These forms of locks are not the only things that can impede progress, however.

Consider when planning your locks what may impede progress without being solved by the Artificer's acid. Thick stone walls, moveable only by a magical artifact might be a possible solution. Another might be a powerful sphinx asking a riddle, or another powerful beast who wants a particular item, in order to allow the group to proceed. Other locks can block progress less literally, as certain information may be needed in order to proceed, rather than an item. Perhaps a maze blocks progress without a map, and players need to find that first.

Remove the Key

The Artificer can only produce his acid so long as he has his satchel. If there's a puzzle or dungeon you have planned which may require literal keys, or some other solution which the acid seems to solve too easily, consider stealing the Artificer's satchel from him. This should be used with extreme caution, however, as it's his primary source of damage as well. I'd also suggest stealing gear from other players if you are to attempt this solution, as to prevent the Artificer from feeling singled out. Additionally, be sure to allow a way for any and all gear to be recovered after the completion of this dungeon/adventure.

I think most of the other answers cover most of the possibilities, but I would like to add that it can really suck as a player to take away or nerf their class abilities without extraordinarily good reason. As such, I think it's very much worth it to ask whether you really have a problem or is this a creative solution.

Your described problem is not really any different than a barbarian with an axe and the will to use it or a sorcerer using fire bolt endlessly. In our current Strahd game, my character has a crowbar and uses it whenever leverage seems helpful (which is a lot if you put some thought into it). These are all, effectively, unlimited use problem solving devices.

Your specific question asks

This artificer has taken to melting any obstacle with acid, locked doors, bars >they were meant to find a switch to open, chests.

I'm struggling to not be spiteful and say "Your acid ruins everything in the chest!"

This isn't necessarily a bad thing to do as it's a reasonable consequence of using acid. If your barbarian was using his unlimited battleaxe on chests, he'd probably break what was in the chest as well. Same with a sorcerer using an unlimited firebolt to bypass the problem.

If your concern stems from locks being bypassed, you should ask if your problem is still present if a Rogue used a crowbar or a thieves' tools to bypass the lock. Bear in mind, that destroying the lock doesn't necessarily remove traps that might be present, so you always have the opportunity to catch them with that, but you'd also have the same chance with a Rogue.

In summary, your artificer has a brute force method. Sometimes it's the perfect solution, other times it will cause problems, but overall, I'm not sure you have a problem so much as a 'working as intended'.

The means of XP gain is by overcoming obstacles. More often than not, there's more solutions to overcoming an obstacle then the one that you planned for as a DM.

As a final note, I would point out that some things of substantial size can have a damage threshold which essentially functions like hardness from 3.5 (see page 247 in the DMG). I wouldn't use it excessively, but it can provide a means to ensure that something can't simply be bypassed using the acid trick. Perhaps a massive portcullis on a castle's drawbridge has Damage Threshold 20 so only a particularly high level artificer can even hope to harm it.

Talk to your player is the short answer.

If you find your players are being one trick ponies and using the same thing to overcome all challenges then the best way to handle that in my experience is to talk to them. Tell them that they shouldn't use the same thing over and over to solve problems. D&D is in part about players playing ball with the DM.