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Use of electrochemical noise to monitor multiphase flow and corrosion
Y.P. Deva, , W.P. Jepson
Published in National Assoc. of Corrosion Engineers International
Volume: 1996-March
Electrochemical noise measurements were taken in a 7.5 cm diameter, 10 m long multiphase oil/water/gas flow system. Full pipe flow studies were conducted for liquid velocities of 0.8, 1.0 and 1.5 m/s and slug flow for Froude numbers 4, 6, 9 and 12, using two phase (ASTM standard saltwater and carbon dioxide gas) and three phase (oil of 2cP viscosity, saltwater and gas) flow. The pressure was maintained at 134 KPa and temperature at 40 C. Noise data was measured with a Zero Resistance Ammeter and analyzed using software provided with the instrument. The noise fluctuations show trends which can be related to the type of How. Full pipe flow studies show random fluctuations around the mean for both potential and current noise.This implies the occurrence of uniform corrosion. The Fast Fourier Transform (FFT) shows roll-off greater than -40 dB/decade in magnitude, characteristic of uniform or general corrosion. In slug flow the potential noise fluctuations take the form of characteristic transients. The frequency of these transients correspond to the observed frequency of slugs. The FFT of this data shows roll-off slopes less than -20 dB/decade shown to be characteristic of pitting corrosion. A second slope of magnitude greater than -30 dB/decade is observed at higher frequencies and this might signify the presence of a second time constant controlling the corrosion reaction. This is due to the unique mechanisms associated with the mixing zone of slugs. Peaks in the Fast Fourier Transform (FFT) are also seen at particular frequencies. One of these corresponds to the slugging frequency. An increase in the percent change in standard deviation of current with Froude number implies enhanced corrosion activity on the surface of the electrode. The electrochemical noise technique is extremely useful for studying corrosion mechanisms and flow parameters in multiphase flow pipelines. Its ability to isolate dynamic incidents and study them both in the time and frequency domain makes it a powerful tool for gaining an insight into mechanistic information on corrosion in multiphase pipelines. © 1996 by NACE International.
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Published in National Assoc. of Corrosion Engineers International
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