Ultrasonic monitoring of structural evolutions of corrosion product layers

Abstract

The corrosion product layer that forms on a ferrous material can protect the material against further corrosion. The protectiveness of a corrosion product layer is very much governed by its structure. While it would be highly ideal, in a research context, to be able to continuously monitor structural changes of corrosion product layers, so as to gain deeper insights into their growth mechanisms, none of the existing research tools possesses such capability. Herein, we present an in situ ultrasound-based research technique that can monitor, in real time and non-destructively, structural changes of corrosion product layers during corrosion experiments. By leveraging the sensitivity of ultrasonic waves to changes in material properties, the technique is able to characterize the key stages of the growth process of a corrosion product layer, including the evaporation of the electrolyte, and the nucleation, thickness increase, densification and collapse of the corrosion product layer. The technique is employed to reconstruct the growths of corrosion product layers on different steels during wet-dry cycling in different electrolytes. By considering the ultrasonic reconstruction results in collaboration with in situ EIS results and ex situ material characterization results, in-depth scrutinization of the growth mechanisms of the different corrosion product layers formed is carried out, shedding light on the underlying structure-property relationships between oxidation process, compactness, thickness and corrosion resistance. All in all, the technique can potentially make significant contributions to development of corrosion prevention and control methods.