State-of-the-art review of electrochemical noise sensors

There are a number of different techniques capable of being used to measure corrosion within equipment. The most simple, the use of metal coupons, usually causes the process to be shut down, is manpower intensive, and has a time delay in getting the required corrosion information. Electrical Resistance (ER) techniques are often used but their response is very sensitive to temperature and they cannot differentiate between general and localized corrosion. Electrochemical techniques, such as linear polarization resistance (LPR), electrochemical noise (EN), electrochemical impedance spectroscopy (EIS), harmonic distortion analysis (HDA), and electrochemical frequency modulation (EFM), have the capability of solving most of those drawbacks. Electrochemical probes can be mounted permanently in most equipment, give regular measurements of the intensity of corrosion, and some can detect localized corrosion. Of all of the electrochemical techniques, EN has the most potential for being used successfully to measure general and localized corrosion rates of equipment. The EN technique was studied in the late 1970s and early 80s as a means of detecting localized (stochastic) corrosion phenomena, such as occurs with pitting, crevice and cavitation attack. EN measurements are based on fluctuations in electrochemical potential and corrosion current that occur during corrosion. Electrochemical potential is related to the driving force (thermodynamics) of the reaction, while corrosion current is related to the rate of reaction (kinetics) of the reaction. The idea is that random electrochemical events on the surface of a corroding metal will generate noise in the overall potential and current signals. Each type of corrosion (for example general corrosion, pitting corrosion, crevice corrosion, and stress corrosion cracking) will have a characteristic “fingerprint” or “signature” in the signal noise. This “fingerprint” can be used to predict the type and severity of corrosion that is occurring. By comparison, conventional electrochemical techniques such as LPR, EIS, HDA and EFM rely on a steady-state analogy for the determination of general corrosion rates. Early studies were carried out using potential EN measurements, using time domain, statistical and frequency domain analyses to characterise the electrochemical response of systems undergoing localised corrosion. Current EN measurements followed quickly using zero resistance ammetry to study the current noise between two identical electrodes. For general corrosion processes, EN has been demonstrated independently by several workers to provide information similar to LPR. Noise technology has been used to study systems undergoing very low to very high rates of corrosion, for example, coatings performance, passive systems undergoing pit initiation/propagation, condensing systems, systems undergoing stress corrosion cracking, and general corrosion through to the very high corrosion rates experienced during chemical cleaning processes. This review will describe: state of the art methods and probes used to measure EN, data acquisition requirements, theory to analyze the signal and to relate the signal to corrosion rates and types, the results of EN field trials, and laboratory results in environments similar to gaspipelines