It was not intended to be immune to corrosion, just to give a bonus to attack certain creature types. Not all oozes corrode weapons, only gray oozes and the puddings (black, brown, white, dun). Ochre jellies, mustard jellies, green slimes, olive slimes, crystal oozes and gelatinous cubes do not corrode weapons.
Likewise, rust monsters, an aberration, use the same scripts for corrosion and they are not a creature type the weapon has a bonus against.