Well that 20 AC was fun while it lasted. 

Previous versions were said to go into a feeding frenzy around steel, which is the most refined type of iron. For those who were wondering why they would be so aggressive towards steel, it's just that they crave ferrous metals, and steel is the best!

The effect is magical in nature, and they smell the ferrous metal under the silver, so touching the object is what triggers the effect- not "touching the ferrous metal", but touching the object. One assumes that a monster that can eat rust has no problems with silver or gold. Probably a bit like gum. Also, note that if you hit with a metal weapon of non magic origin, it corrodes. It appears to mean ANY metal weapon of non magical origin.Tin and other such ferrous metals are also fair game.

Our Warforged Paladin, Lv9, is scared to death of this thing.
Haven't told her that she's magical and so immune.

The most refined type of iron is just iron.  steel is iron with impurities (a very small amount of carbon)

Just introduced one of these (as an infant version of it) to my level one party as an encounter; they behaved passively towards it, and our half-orc barbarian sacrificed both of his hand-axes to the creature. It is following him as a companion. Fortunately for the party, they just entered a mineshaft with hundreds of metal spikes driven into the floor of the cavern as anchors for the minetrack. The party is already brainstorming creative ways of training the creature they lovingly named "Rusty."

Got a mini of this little guy & didnt know who he was. Now i see him. Cool

Quote from Trewill91 >>

The most refined type of iron is just iron.  steel is iron with impurities (a very small amount of carbon)

Not quite. Pure iron is just iron, but steel is relatively pure iron. Basically as pure as they could get it back in the day. Carbon was (and is typically) used to extract the impurities. The carbon present in mild steels is often just remnants of the purification process. Pure iron is softer than pretty much any kind of steel, but most/all steel is softer than what most people would consider iron. You can add more carbon to make it harder for various tool steels, and you can add other metals to create alloy steels (I consider stainless steels to be in this category).

Steel: 0.002%-2% carbon by weight
Cast iron: 2-4% carbon by weight
Pig iron: 4-5% carbon by weight

Quote from NedyBonz >>

“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?

I don't know of an explanation in the lore, but... They locate their desired metal by detecting magnetic fields, which are produced by ferrous metals. Steel is an alloy, and only part of it is iron, which probably what the rust monster really wants, otherwise they wouldn't be adapted to specifically finding ferrous metals. Also, steel is a better metal overall, hence the historical value of steel weapons over iron. The rust monster definitely doesn't understand metallurgy, but its senses are probably enough to tell that this thing it's detecting is what it might think of as 'bad food'. It can eat it, and it will, but it's also more dangerous to get. If it were a little smarter, it might avoid steel altogether, but as it is, it's just focused on its hunger and angry that it's not sensing the 'good food' (regular iron). It would also explain why it wouldn't particularly care about weapons coated in silver, for example. Silver isn't ferrous, but it also isn't a weapon metal. And rust monsters aren't vulnerable to silver. In physical terms, it's just decoration to the rust monster, like parsley.

Probably just want to eat it

Are there any rules for gold cost/materials and time to repair a degraded weapon?

The 1 monster the wizard is less scared of than the rest of the party.
......although ....1d8 +1 sound like it hurts

Probably because they have the metal, which they convert into rust which is there food

It might just be a oversight but with the rust metal trait, unlike antenna, it doesn't specify Ferrous metals?

Wrought iron is an iron alloy with a very low carbon content (less than 0.08%)

Kaine, carbon isn't the only impurity. Wrought iron is horrible quality. It's filled with slag. Steel is much closer to pure metallic iron than wrought iron.

When it says “destroys a 1-foot cube of it” does it turn the object to a pile of rust?

would these damage a warforged's integrated protection armor?

Quote from Shinjithegale >>

What about silvered weapons?

 A bit late, but here is an answer:
Silver is not a ferrous metal and doesn't rust, so the rust monster couldn't remove the "silvered" property from the weapon. It would still take the -1 penalty though, because the weapon is probably still mostly iron/steel (silver is really soft so a fully silver weapon would suck).

nice