If ALL weapons contributing to an attack are Corrosive and the Attack equals or exceeds the target’s Damage Rating, the target model gains a Corroded Marker.
A Repair Attempt MUST be made to remove each Corroded Marker that a model carries during the End Phase of each Turn, on a 4, 5 or 6 the attempt succeeds and the Marker is removed, on a 1, 2 or 3 the attempt fails, the Marker remains and the model loses one Hull Point.