Acoustic emission for real-time monitoring of interfacial erosion-corrosion of austenitic stainless steel in polluted phosphoric acid environment

Erosion-corrosion is a complex problem causing severe damage of austenitic stainless-steel equipment used in phosphoric acid production. Herein, time-resolved analyses of the acoustic emission (AE) signal together with the corrosion potential and weight loss were performed to online monitor and evaluate the dynamic degradation and depassivation of the 904L stainless steel under the effect of abrasive SiC particles. Important insights into the erosion-corrosion process are gained from the analysis of the AE waveform and its parameters. The characteristic root mean square (RMS) voltage and acoustic energy of AE signals revealed the self-healing difficulty of the passive film, of which the waveform of AE signals showed the appearance of imperfect continuous high intensity waveform, ranging from 0.1 to 0.4 mV. Evaluation of the average frequency spectrum showed that the amount of SiC particles alters the damage mechanism from AE events dominated by ploughing to AE events driven by micro-cutting. This suggests that the imperfect continuous waveform is possibly a precursor to the emergence of new acoustic sources (burst type): sudden jumps of chips from the cutting lips. This response resulted in a quasi-linearity in the mass loss of the alloy with the respective acoustic response at high particle loading (24 g L⁻¹). The non-linear erosion-corrosion behavior suggests that the depassivation-repassivation events are dependent. These results render the combined AE and electrochemical analysis an effective sustainable approach for the online investigation of the synchronously changing erosion and corrosion rates.