Amazing

“They ignore creatures not carrying such metals, but can become aggressive toward those bearing steel weapons and armor.”

Is there an explanation in the lore for the aggression toward steel in particular?

Probably because they eat the rust and see those wearing iron/steel weapons and armor as getting in between them and their food (the metal items).

or its because the fact that iron is needed for there blood and steel would hurt them like if you take all the iron out of your blood and then replace it with steel so maybe its just that many years ago the rust monster was happy eating humans and other stuff and so the gods were like nah stop eating our worshipers and so they cursed them to have no iron in there blood and so now they eat every thing that is iron or iron based and that is the reason they look weird and eat iron they look weird cause they don't have iron in there blood so that is why the get mad at iron carrying people cause they hate us cause our gods cursed them and cause they think we wont let them have there iron

Quote from bortle1456 >>

or its because the fact that iron is needed for there blood and steel would hurt them like if you take all the iron out of your blood and then replace it with steel so maybe its just that many years ago the rust monster was happy eating humans and other stuff and so the gods were like nah stop eating our worshipers and so they cursed them to have no iron in there blood and so now they eat every thing that is iron or iron based and that is the reason they look weird and eat iron they look weird cause they don't have iron in there blood so that is why the get mad at iron carrying people cause they hate us cause our gods cursed them and cause they think we wont let them have there iron

 

 That might be the case.

One of my players is a half-orc cyborg. It's a long story. But I plan to throw a rust monster at them, sense their cybernetics are the only thing keeping them alive, they fear rust monsters. So I plan to throw a thing a call a "Grand Rust Monster" at the players. It's basically the same thing but size class huge and much more powerful.

Hungry

Seeing as it's unaligned, could one have it as a pet?

Like a wyvern, but it eats metal and could tell real metal from false

what does it do with magical weapons/items?

Quote from JoJo108 >>

Seeing as it's unaligned, could one have it as a pet?

Like a wyvern, but it eats metal and could tell real metal from false

 The monster manual mentions they can become pets

makes since

If a weapon was affected by a rust monster, can I have it be more susceptible to breaking? lso, could I make it so that those hit by it make a Constitution saving throw (DC 11) or take 1d6 poison damage and have tetanus (Basically within two days of getting the disease, they cannot eat because of lockjaw, and begin taking levels of exhaustion.)? Weapons with the gleaming  property as mentioned in chapter 7 in the DMG will not be affected.

These are perfect for a Mad Max style campaign where the party are using guns and driving cars or a good thing to throw at them if supplies are scarce

Boil em mash em stick em in a stew. It's like tolen when you spoil a good meal

Quote from Yeomans33 >>

Boil em mash em stick em in a stew. It's like tolen when you spoil a good meal

The shortage of skilled chef PCs really is criminal.

The Rust Metal trait and Antenna attack is a decent framework for damaging weapons (same with the black pudding).

Mapping the the debuffs to object HP might be more trouble than it’s worth, but it seems like (-1) corresponds to the lose of a fifth of their HP.

cool beans

What about silvered weapons?

Love these things. Had an NPC who hired the party to exterminate a nest of them from a cave system after he tried to do it himself, disregarding what they were and losing his legs.

In a campaign I ran a while back one of the players found on of these in a wasteland and adopted the fellow it got him out of so many scrapes but fighter in heavy chainmail hated it