Austenitic stainless steel is widely used in various fields such as petrochemical, aerospace, transportation, pharmaceuticals, and construction due to its excellent corrosion resistance. To maintain long-term corrosion resistance under diverse and challenging working conditions, the stainless steel requires passivation treatment. During the manufacturing of stainless steel equipment, verifying the quality of passivation is a critical link of quality control. Among the methods available, electrochemical detection offers high precision compared to visual or chemical methods. However, the electrochemical potential detection method tends to have a long testing time and is likely to be significantly affected by environmental factors, leading to considerable fluctuations in test results. To achieve rapid and accurate detection of surface passivation quality, the work aims to propose a new method of using potential step impedance in the time domain. The effects of potential step time, data smoothing techniques, and step potential on the time domain impedance spectrum were compared, with those observed in conventional frequency domain impedance analysis. 316L stainless steel samples were treated by various passivation processes in nitric acid solutions at different temperatures (25 ℃ and 45 ℃) for different time (10 and 30 minutes). The samples were prepared by cutting 2 mm thick stainless steel plates into 20 mm×20 mm specimens and then polished, cleaned, and passivated according to ASTM A380 standards. The electrochemical measurements were conducted through a three-electrode system with Ag/AgCl as the reference electrode and Pt as the counter electrode. The potentiostatic step-time domain impedance measurements were performed with step potentials ranging from 0 to 300 mV, and the current responses were recorded at a sampling frequency of 10 kHz for up to 2 seconds. The data were processed with various smoothing techniques, including adjacent averaging, FFT filtering, and the newly proposed filtering variable-window smoothing denoising method, which combined Savitzky-Golay polynomial fitting with FFT filtering and variable window-width smoothing. The results demonstrated that the filtering variable-window smoothing denoising method outperformed other techniques in preserving the transient peak and subsequent current characteristics, yielding time-domain impedance spectra that matched the frequency-domain impedance spectra. The optimized measurement parameters resulted in an error of less than 2% in the interface charge transfer impedance values compared to frequency-domain impedance results. Further analysis of the polarization curves and time-domain impedance spectra revealed that the pitting potential increased with the increasing time-domain impedance values, indicating improved corrosion resistance. It was also found that increasing the passivation temperature and duration enhanced the surface impedance, with the highest impedance observed for samples passivated at 45 ℃ for 30 minutes. The SEM images confirmed that step potentials above 200 mV caused electrochemical corrosion on the sample surfaces, with the severity of damage correlating with the passivation quality. The reliability of the proposed method was validated by comparing the time-domain impedance spectra with frequency-domain impedance spectra under different passivation states. The findings indicated that the filtering variable-window smoothing denoising method, combined with optimized measurement parameters, provided a robust and accurate approach for assessing the passivation quality of stainless steel surfaces. This method offers a significant advancement over traditional techniques by reducing measurement time and improving precision, making it suitable for industrial applications where rapid and reliable surface quality assessment is critical.
Key words
stainless steel /
potential step method /
time-domain impedance /
smooth denoising method /
quality of passivation
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Funding
Science and Technology Key Project of Henan Province (232102230043)
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