A New Method to Measure the Oxide Layer Thickness on Steels Using Electromagnetic Acoustic Resonance

Abstract

Electromagnetic acoustic resonance of 2.25 Cr–1Mo steel plates with (Fe, Cr)₃O₄ oxide layers of thickness from 0.03 mm to 0.18 mm were investigated. Due to the phase shift at the interface between the metal and the oxide layer, the resonance frequency, f _n, was not proportional to resonance order, n, but n/f _n varied depending both on the frequency and the oxide layer thickness. A model to derive the amount of the phase shift at the interface was developed, and a good agreement with experiments was found. It was shown that the oxide layer thickness, h, could be obtained as h = 0.33 × c _oxide/f _max, where c _oxide is the shear wave velocity in the oxide layer and f _max is the frequency at which n/f _n had a maximum. The shear wave velocity in the oxide layer was determined as 3400 m/sec and the reflection coefficient at the interface as 0.09.