According to the Pilling-Bedworth rule, if the volume of the oxide layer is greater than the volume of the metal then the oxide layer is ______ in nature.

The volume of the oxide layer is greater means the layer is non-porous and protective in nature. 

The Pilling-Bedworth ratio (P-B ratio), in terms of metal corrosion, is the ratio of the volume of metal oxide to the volume of the equivalent metal where the oxide has been created.

By using the P-B ratio, it can be determined if it is possible for the metal to undergo passivation in the presence of dry air by building a protective oxide film.