Passivation study of AISI 316L under PWR PW conditions via Electrochemical Noise (EN)

Abstract

Long-term current vs. time experiments (100–200 hrs) were performed in boric acid/lithium hydroxide buffer at 320 °C & 130 bars for five anodic overpotentials, ranging from 0.1 to 0.4 V, to achieve the steady-state anodic current at each overpotential. The remarkable observation was that all currents were noisy, i.e., consisting of a stationary baseline and more or less intense anodic transients. This electrochemical noise was characterized using the Power Spectral Density spectra over a frequency ranging from 3 Hz down to 0.1 mHz. These spectra exhibited a low-frequency plateau and a high-frequency roll-off described by a power-law function with a single slope, except for $\eta$ = 0.1 V. The infinite integral of the PSD spectrum quantifies the current noise level. This initial approach enables a comparison of the DC anodic current and the average anodic current noise intensity on the same scale (A). This comparison revealed that their variations with overvoltage differed significantly, suggesting that the total current consisted likely of two distinct anodic components. Based on the Point Defect Model, we suggest that the oxide growth phase at the expense of the metal could be a time-discontinuous process, consisting of a series of elementary anodic events characterized by exponential time decay and distributed over time. In contrast, the DC current would be associated with metal-atom oxidation, which is considered a time-continuous process.