Pilling–Bedworth ratio

On the basis of the P–B ratio, it can be judged whether the metal is likely to passivate in dry air by creation of a protective oxide layer.

The oxide layer would be unprotective if the ratio is less than unity because the film that forms on the metal surface is porous and/or cracked.

Many of the exceptions can be attributed to the mechanism of the oxide growth: the underlying assumption in the P–B ratio is that oxygen needs to diffuse through the oxide layer to the metal surface; in reality, it is often the metal ion that diffuses to the air-oxide interface.

[citation needed] The P–B ratio is important when modelling the oxidation of nuclear fuel cladding tubes, which are typically made of Zirconium alloys, as it defines how much of the cladding that is consumed and weakened due to oxidation.

The P–B ratio of Zirconium alloys can vary between 1.48 and 1.56,[4] meaning that the oxide is more voluminous than the consumed metal.