Giant Robots Vs. Giant Monsters is fine- that's the premise of the movie.
But when it came to little details like what the robots are built of- they could have said anything. ANYTHING.
And they say "Solid iron- no alloys".
Alloys are created to maximize the desired properties of a material. That's why we fucking invented them.
And the robots spend a huge amount of time in salt water. And we all know that that has proven to have no negative effects on iron whatsoever. No siree.
There's unrealistic, and then there's just plain fucking stupid.