Electrochemical Noise Measurement: The Definitive In-Situ Technique for Corrosion Applications?

A review is presented of electrochemical noise (EN), the generic term given to fluctuations of current and potential seen in high-temperature corrosion, molten salt corrosion, and aqueous corrosion. EN levels in corrosion and particularly localized corrosion are significantly greater than EN observed in redox systems. EN associated with corrosion is the result of stochastic pulses of current generated by, for example, sudden film rupture, crack propagation, discrete events involving metal dissolution at etch pits, grain boundaries and kink sites, and hydrogen discharge with gas bubble formation and detachment. EN in corrosion includes low-frequency, nonstationary, and weakly stationary processes; transients; and cyclic or oscillatory phenomena. The use of EN, obtained either by potentiostatic/galvanostatic measurements or at freely corroding potentials, has been shown to offer advantages over conventional DC and AC techniques in research studies, testing, and corrosion monitoring. In many cases, reaction mechanisms can be elucidated and corrosion rate information can be obtained. Under freely corroding conditions, current noise impulses give rise to changes in potential through the interfacial impedance, 〈In〉 → 〈En〉/|Z|. Correlation of current and potential noise from uniform corrosion may be used to obtain the electrochemical impedance of the corroding interface; these noise signals are produced by weakly stationary or deterministic processes. Corrosion rates may therefore be evaluated from this low-frequency impedance or from the associated but simplified noise resistance, obtained from the standard deviations of the current and potential, σE/σi = Rn. Localized corrosion events produced by Poisson processes can be easily identified from time records and spectral density plots. Localized corrosion involves nonstationary systems. The anodic and cathodic processes can be separated in both time (the anodic event occurring before the cathodic event or vice versa) and space (at different parts of the electrode surface). The electrons produced or consumed during these corrosion events are stored temporarily in the electrochemical double layer and corrosion product film capacitors. Assessment of individual transients, use of signal analysis techniques, modeling of ensembles of transients as developed for electrocrystallization studies, and use of the chaos theory have all been used in EN evaluations.