I remember reading about corrosion elsewhere, I think the problem can be solved by making note of weather, Weezing gets flamethrower but would naturally use corrosion since it's stronger, but if corrosion doesn't get the sun boost and is say 80 bp there are plausible explanations for why it'd be weaker. Fire types as well would love a fire type attack that isn't dampened from the rain as well as many other types. Corrosion could also work against flash fire mons, though if the purpose is to hit steels I view this as a hypocritical use taking the best of both worlds.
Flavor wise it's a perfectly logical move and others like scald could mimic its properties. There are acids that melt steal shouldn't there be at least one poison move that could do the same,
A poison move that has the ability to burn or paralyze makes sense. Making it a fire move does not make sense.
Most people agree that poison needs a buff. I think that poison does just fine poisoning people to death, but that's me. Giving poison essentially three more things it is strong against and one less thing that is immune does not fix balancing. It weakens the balance that the type chart tries to create.
It reduces the usefulness of both bug, which has enough things to worry about, ice, which does too, grass still loses, and steel (which was included as a way to counter poisoning without needing a poison type) would be crippled when fighting against one of the things it is supposed to counter. Steel is supposed to be a counter to poison type, and making it weak to poison essentially detracts from why it is on the type chart in the first place.
Edit: Poison is underpowered. The solution is not to make it overpowered by giving it a loophole around important game mechanics such as typing, weather, abilities, and other moves. That would break the game.
To the pay shock argument. The effectiveness of that move varies from battle to battle. It does go around stats, but this can be just as much of a bane as a boon.
Giving three types a new weakness, two of which have four weaknesses already, with the last one supposed to be a counter to poison is game breaking. Not to mention a fire move that isn't affected by rain dance on a pokemon not weak to water and is already strong against grass, and could care less about electricity. Please explain how this move is not game breaking? --- "Hello, um, you..." That awkward moment where, after talking to someone every day, you still don't remember their name.