Acoustic emission behavior of 316LN stainless steel during oxide film formation in high temperature aqueous environment

Acoustic emission (AE) technique was used to study the oxide film formation of 316LN stainless steel (SS) in high temperature aqueous environment. The morphology and chemical compositions of oxide films of 316LN SS base metal and weld were also investigated by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The results indicated that 316LN SS weld had a higher oxidation resistance than that of base metal due to higher amount of chromium and nickel in the weld material. AE activity and intensity were high during the oxide film formation and decreased gradually with increasing immersion time. The evolution of AE hits was consistent with the oxidation kinetics of 316LN SS base metal and weld. The formation of Cr-enrichment layer and Ni(OH)₂ contributed to the enhancement of oxidation resistance, leading to a continuous decrease of AE activity and intensity. Two types of AE waveforms were detected and the frequency spectra of both types mainly ranged from 0.3 to 0.6 MHz. According to the analyses of AE based b-values and parameters, it could be deduced that AE sources during oxide film formation were the plastic deformation of base metal/weld and oxide film.