Electrochemical Noise (EN)

The non-intrusive use of EN for corrosion monitoring is very attractive, e.g. aircraft corrosion and gas scrubbing tower monitoring examples. Fluctuations of potential or current of a corroding metallic specimen are a well known and easily observable phenomenon and the evaluation of EN as a corrosion tool has increased steadily since Iverson's paper in 1968.  A new revolutionary technique called the Self Linear Polarization Resistance (SLPR) method brings us back to the basic of electrochemical approximations and measurements.   

The extensive development in the sensitivity of the equipment for studying electrochemical systems has rendered the study of oscillations in electrochemical processes, that translate into measurable EN, increasingly accessible. The study of corrosion potential fluctuations was applied, for example, to monitor the onset of events characterizing localized corrosion such as pitting or stress corrosion cracking (SCC), exfoliation, erosion-corrosion in either laboratory or diverse and complex industrial environments. No other technique, electrochemical or otherwise is even remotely as sensitive as EN to system changes and upsets.

During localized corrosion EN is believed to be generated by a combination of stochastic processes, such as passivation breakdown and repassivation events, and deterministic processes which can be caused by film formation or pit propagation processes. 

The most traditional way to analyze electrochemical noise data has been to transform time records in the frequency domain in order to obtain power spectra. Spectral or power density plots would thus be computed utilizing fast Fourier transforms (FFT) or other algorithms such as the maximum entropy method (MEM). Another very useful mathematical model has been proposed to specifically reveal the fractal characteristics of signals. The following battery of techniques  were initially adopted by the Corrosion Doctors for the analysis of their EN measurements:   

• Maximum entropy method
• Stochastic process detector
• Rescaled range analysis
• Statistical analysis: standard deviation, kurtosis and degree of skewness

Numerous pages of the Corrosion Doctors Web site discuss passivation related topics: Beer, Biomaterials, Blocking, Calcareous deposits, Electrochemical noise, Electrode passivation, Galvanized, Inhibitors, Iron, Nickel aluminum bronze, Oxidizers, Passivation layer, Passive curve, Passivity, pH, Pickling, Pitting, Potentiodynamic polarization, Rouging, Stainless steels, Steel, Stress corrosion cracking, Surface contaminants