The electrochemical noise (EN) measurement has been received great attention as a promising electrochemical method for corrosion monitoring. This article gives a grounding in electrochemical noise (EN) for corrosion applications. The definitions, electrochemical models, and the typical waveforms of EN are described. The differences between the measuring system for EN and that for conventional electrochemical measurements are also described. The author hopes this article will serve for better understanding on the papers in this special issue.
The corrosion monitor by using electric chemical noise method (ENA) was developed. And it was applied to corrosion monitoring at a chemical plant. The application of the corrosion monitor by the electric chemical noise method is thought to spread out more.
This paper describes the monitoring test result that we conducted in order to review the possibility of practical application of electrochemical noise analysis to a plant. That is, in salt manufacturing plant, we monitored corrosion condition of type-316L stainless steel pipe with potential noise measurement. As a result, we could obtain the characteristic potential noise and rest potential change corresponding to crevice corrosion.
Application of ENA Technique for actual plant in abroad was investigated. High-level nuclear wastes at the Hanford Site are stored underground in carbon steel tanks. The installation of a three-channel prototype electrochemical noise monitoring system for pitting, SCC into double-shell tank was completed. And a monitoring system of a biofilm growth under the coolant water was developed with the disk electrode of the pair. But, it can't be applied well with the ocean environment. Application of ENA technique to the plant doesn't proceed even in abroad compared with Japan.
This paper shows some applications of corrosion monitoring techniques (metal ion analysis, linear polarization resistance method, corrosion potential measurement) in chemical plants. However, the targets of these existing techniques are almost limited to general corrosion. On the other hand, ENA is expected as promising corrosion monitoring technique for localized corrosion because it is the technique to analyze active-passive corrosion behavior continuously.
Electrochemical Noise Analysis (ENA) has been used as laboratory technique for the fundamental study of form of corrosion. In recent years, the development of instrumentation technology as in measurements of potential and current makes it possible to apply ENA to commercially available corrosion monitoring technique in practical use for the field monitoring. In addition, since recent maintenance strategy for process plant requires a new role for corrosion monitoring technology, ENA-based online corrosion monitoring technique is being expected to be applicable to proactive corrosion control. In this paper, the importance of ENA for the latest maintenance strategy as an on-line measurement for real-time data collection was discussed. Issues to be solved for wide use of ENA in the practical field was also discussed.
Corrosion potential of sensitized type 304 stainless steel has been monitored in a 0.35% chloride solution at 313-353K in order to analyze the embryonic stage of stress corrosion cracking (SCC) using a U-bend specimen. The drops in corrosion potential indicated ruptures of passive films and they recovered sooner or later. The cathodic process was analyzed based on a simple RC parallel equivalent circuit model, where R is resistance for the electron transfer (Faradic process) and C an apparent capacitance for the cathodic electrode (non-Faradic process). The values of R and C were determined experimentally as a function of electrode potential so that the local anodic currents could be estimated for every event of potential drop. The histograms of the amount of electricity for each event showed that the distribution shifted to larger values with increase of temperature. Among the apparent parameters of potential drop, such as drop duration (Δt), drop depth (ΔE) and bottom potential (ER), Δt showed the closest correlation with the amount of electricity.
Possibility of the application of the electrochemial noise analysis to aluminum crevice corrosion was studied. Anodic polarization curves with various scanning rates of potential were measured for the aluminum electrode in the artificial crevice. Current noises were measured for the aluminum artificial crevice electrode at various anodic potentials. On the other hand, current noises were measured for the platinum artificial crevice electrode at various cathodic potentials. The short-circuited current noise and the potential noise of the aluminum artificial crevice electrode coupled with an external aluminum electrode were measured. Any current noises were not obserbed in the region of the potential less noble than crevice potential (Ec), above which the noises were generated due to the hydrogen evolution. This was verifyed by the fact for the hydrogen evolution on the platinum crevice electrode. Notable noises of synchronized short-circuited current and potential appeared after 50 minutes of immersion being due to the hydrogen evolution. Thus, the natural crevice corrosion was simulated with this short-circuited coupling cell. The potential noise will be considerd as an easy monitoring tool for these crevice corrosion detection, because the potential noise was changed with the current noise in the same period.
Stress corrosion cracks initiation can be detected by an electrochemical noise analysis (ENA) method. The electrode potential of the specimen is continuously measured by using the reference electrode in the sodium chloride solution. When a stress corrosion crack occurs, it can be detected by potential decrease. A stress corrosion crack initiation to propagation process, such as initiation of the micro-crack, the individual crack to progress, coalessence, and steady state propagation is separated based on the change of potential. Also, we can measure the potential inside the bittern spot for the case of high humidity sodium atmospheric condition. Stress corrosion cracking can be detected by monitoring of the potential even in the atmosphere.
A unique approach would allow to estimate the threshold state of stress corrosion cracking (SCC) was developed by using potential fluctuation measurement. The potential fluctuation (noise) resulting from the initiation of corrosion cracks was measured in a chloride solution at various temperatures. A direct tension specimen to which a constant stress was applied and a U-bend specimen were employed for the working electrode, and senstzed type-304 stainless steels were used for their material. The local anodic currents were estimated from the form of the potential fluctuations by a calculation using a double-layer capacity and the constants of the Tafel's equation obtained from separate experiments. The feature of the electricity quantity's distribution of the estimated local anodic currents was examined. The peak height of the obtained distributions changed significantly between the temperatures above and below the threshold state of SCC. This suggest this method would be a potential one to monitor the occurance of SCC in chemical plants.
An electrochemical noise analysis to evaluate corrosion rate of mild steel was studied theoretically and experimentally. It was revealed that a signal source involved in an external measurement system and that in corroding electrode were equivalent to calculate impedance of the electrode. Sudden and large fluctuations in potential and/or current were not suitable to estimate the corrosion rate by noise impedance, however, these should be applicable to monitor occurrence of crack propagation of SCC or initiation of pits. Based on theoretical analysis, both the correlation coefficient of potential and current data in time domain and the coherence of those of power spectrum densities in frequency domain are considered as a fundamental index to estimate the corrosion rate from the impedance spectrum. High correlation coefficient in time domain was required, but not enough to judge the reliability of impedance from noise data. The impedance calculated from noise data, that was measured for a mild steel in sodium chloride solutions, coincided well with that measured by AC method, when both the correlation coefficient and the coherence showed high values. Limits of estimation of corrosion rate from the noise impedance and the noise resistance defined by the ratio of standard deviations of potential and current fluctuations are discussed.
The potential noise (fluctuation) generated by the local film breakdown of carbon steel electrodes was measured in deaerated chloride solutions. Sodium nitrite of up to 50mmol·dm-3 was added to the solutions as a passivator. Carbon steels have a high resistance to general attacks in this solutions. However, they also develop susceptibility to localized attacks because of being passivated. The potential noise was measured under the conditions where the electrodes were naturally immersed in the solutions. The noise when the electrode's surface was scratched with a glass tube was also measured to clarify the relation between the potential fluctuation and the film breakdown. The type of the noise measured in the experiments was changed with the corrosion potentials; the RD-type appeared at the potentials higher than -0.5V (vs. SSE), and the RR-type occurred at those around or lower than -0.5V. The current transient that generated the RR-type noise was estimated by using the reverse potential setting (RPS) method. Obtained results showed the RR-type was produced by the cathodic current transient. This suggest the local current generated by he film-breakdown was a cathodic current not a anodic one.
A modified noise resistance method was developed, and it was applied to corrosion monitoring at a chemical plant. An electrode unit and a data-acquisition system capable of measuring the electrochemical noise under the site's conditions were also developed. The noise resistance, Rn, has been regarded as an equivalent factor to the polarization resistance, Rp. However, it was considered that this estimation might not be appropriate because the real dimension of the anodic and cathodic area possibly vary with immersion time as well as with the combination of materials and environments. Therefore, a new factor, F, was introduced instead of the Stern-Geary constant, B, for the conventional polarization-resistance technique. The F was named corrosion factor. In the modified method, the corrosion rate, CR, is defined as follows; CR=F·I/Rn. The F was experimentally determined from the relation between the mass-loss rate and the Rn of the electrode made with an equivalent material to the objective under monitoring. A test solution which simulated the environment of the site was also used for the experiment. Applying this modified method, the occurrence of unexpected and intense general corrosion at a plant was successfully detected. The corrosion loss estimated by the method fairly agreed with that at the site in thickness of the damaged tubes.