RUMBLETiGER
Adventurer
Alright, if you want to play that way, consider this:I would argue that the Chain Golem is not one piece of metal, and at any given point only one piece is going to be striking or being struck by the Rust Monster, thus only one chain would be affected per strike.

A Rust monster is 5' long (Longer if they're one of my bred ones, but we'll go with standard), and they have antenna that are, what, atleast 3' long based on proportions in this picture?
The Rust Monster isn't going to tap the tip of one antenna to the metal and then retract, it's going to lay the length of both of them along the metal as the ravenous creature it is. You've got 2 strips of 3' long instant rusting antenna laying along the surface, *poofing* metal into rust dust, and then continuing to press downwards to the next layer under. In 6 seconds, these antenna are cutting swaths through metal, gouging massive chunks, even if you're ruling that it's only the specific inch of metal the Rust Monster touches, Which the entry clearly says it doesn't otherwise a 10' cube wouldn't instantly desintigrate.
A Fighter's entire armor gets distroyed in 1 round of attack. Armor is not a single, fuzed piece of metal, but multiple sections connected by chain links, leather straps, cloth, whatever. The fact that the breastplate is not the same as the neck protection, waist section or shinguards makes no different on how the Rust Monster makes it all go bye-bye.
Rust Monster is a powerful beast. It's just how it rolls.