Oblivious is better than Corrosion in Gen 7, while Corrosion is more popular in Gen 8.
First off, if a Corrosion Pokemon uses a Poison move on a Steel type Pokemon, it will still have no effect. Corrosion just allows the ability-bearer to poison a Steel / Poison type? Don't believe me?
Corrosion allows the user to inflict poison on any Pokémon regardless of type, allowing them to poison Poison and Steel-type Pokémon. However, the user cannot conventionally poison the target with a physical move or a special move, and can only be conventionally done with a status move that inflicts poison.
If a Poison- or Steel-type Pokémon with Corrosion holds a Toxic Orb, it will badly poison itself. A Pokémon with Corrosion can poison Poison-type and Steel-type Pokémon if it uses Fling while holding a Toxic Orb or a Poison Barb.
This Ability has no effect on Toxic Spikes. It does not allow the Pokémon with this Ability to poison a Pokémon with Immunity. Corrosion does not allow the Pokémon with this Ability to damage Steel-type Pokémon with Poison-type moves.
If the Pokémon with this Ability uses a move, and that move gets reflected by the target due to Magic Coat or Magic Bounce, the reflected move will not be able to poison Steel- or Poison-type Pokémon. If a Poison- or Steel-type Pokémon with this Ability poisons a target with Synchronize, Synchronize will not be able to affect the Pokémon with this Ability.
If the Pokémon with this Ability uses Magic Coat to reflect a status move that inflicts poison, the reflected move will be able to poison Steel- or Poison-type Pokémon.
Now that that's cleared up, this is why Oblivious is better in Gen 7.
First, Salazzle commonly runs Nasty Plot to deal decent damage. Taunt cripples this, so it's valuable to have Oblivious since it blocks Taunt. Corrosion isn't that good since it's very frail, and Poison resists Sludge Wave while Steel is immune to it. This means Corrosion is only good for Toxic, but, again, Salazzle is very frail and using a Focus Sash for Toxic will
Probably be shut down by entry hazards.
Not allow it to use Acid Downpour.
As for why Corrosion is more popular in Gen 8...
Salazzle's great Speed, Special Attack, and Corrosion ability turn it into a great wallbreaker, as it has a very strong STAB combination and is able to cripple most of its checks with Toxic.
Yeah that's not very specific lol.
Anyway, basically, a lot of things check it, so it needs Toxic to wear down the opponent. It also gets Heavy-Duty Boots, so it's no longer crippled by Stealth Rock, allowing it to pivot more reliably. This means that it won't usually set up Nasty Plot, and it will mostly be firing off fast Toxic's. Salazzle also runs Knock Off a lot now? So Corrosion can chip down opponents that have the Leftovers knocked off.
So it kind of depends on what format/game you're playing.
Hope this helps! :)