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David C. Silverman |
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Overview of the Experimental Technique Fluid motion can influence the corrosion mechanism and from that, the corrosion rate. The hydraulically smooth rotating cylinder electrode can be used to assess such sensitivity under appropriate conditions. This apparatus is best suited to examining those situations in which the rate at which a critical component moves between the bulk fluid and the surface equals the corrosion rate. That component is said to be the rate controlling species and the corrosion process is said to be under mass transfer control. Under these conditions, the corrosion rate can, in theory, be calculated from the product of the mass transfer coefficient for that geometry and the difference in concentration of the rate limiting species between that at the surface and that in the environment. Often, the mass transfer coefficient can be estimated from the fluid velocity through well-documented relationships among velocity, characteristic length, and fluid properties.The rotating cylinder electrode
The two characteristics, turbulent flow achieved at reasonable rotation rates and the existence of empirical correlations that relate transport properties have led to developing methods to use the results found in the rotating cylinder electrode to predict the effect of fluid motion on corrosion in other geometrical configurations. The fact that the fluid boundary layer is uniform means that the smooth rotating cylinder electrode is most applicable to those situations in which the fluid boundary layer is fully formed, fluid velocity is uniform on the surface, and no boundary layer detachment occurs. If any of these requirements are not fulfilled in the field geometry, the rotating cylinder electrode might not be applicable to that flow situation. Many designs exist for the rotating cylinder electrode. One scheme is shown in this figure CYLEXPERT assumes that the rotator apparatus is capable of under 100 rpm to as high as 10000 rpm. Many times, the highest rotation rate allowable before splashing is about 5000 rpm. The electrode can be attached to all types of electrochemical measuring equipment and can be used for obtaining cyclic potentiodynamic polarization scans, polarization resistances, electrochemical impedance spectra, and even mass loss, all as a function of rotation rate. Before initiating the first corrosion evaluation, the apparatus must be confirmed to have hydrodynamics that conforms to that expected for a smooth rotating cylinder electrode. This confirmation can be made by using a system whose reaction is under complete mass transfer control. One example is the reduction of oxygen on alloys such as nickel or Monel 400 when the electrode is polarized at about -0.8 to -1.0 volts (SCE) under near neutral conditions with a sodium chloride or sodium sulfate electrolyte. Oxygen is usually sparingly soluble in aqueous media. Under appropriate conditions, the reduction of oxygen is limited by (equal to) the rate of mass transfer of dissolved oxygen to the surface. It can usually be assumed to react immediately when it reaches the surface. Under these circumstances, the oxygen concentration at the wall can be assumed to be zero. The rate of reduction per unit area can be estimated by the product of the oxygen mass transfer coefficient to the surface and the bulk concentration of oxygen. The mass transfer coefficient can be estimated at each rotation rate by the methods discussed in Assessing Mass Transfer Control using the correlations discussed in the section Mass Transfer Correlations. This calculated rate of oxygen reduction can be compared to the actual measured current density under controlled potential at the same rotation rates. The apparatus and approach is discussed in more detail in D. C. Silverman, "Rotating Cylinder Electrode for Velocity Sensitivity", Corrosion, Vol. 40, No. 5, p. 220 (1984)1  (466k)
and D. C. Silverman, "Rotating Cylinder Electrode An Approach for
Predicting Velocity Sensitive Corrosion", in Flow-Induced Corrosion: Fundamental Studies and
Industrial Experience (K. J. Kennelley, R. H. Hausler, and D. C. Silverman, ed.), p. 20-1,
NACE, Houston, Texas, 1991. Once this agreement has been verified, the rotating cylinder
electrode is ready to be used in conjunction with CYLEXPERT for practical assessment of
velocity sensitivity of corrosion processes.
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Previous Page: Introduction-What is CYLEXPERT? Next Page: Using CYLEXPERT-a step-by-step procedure 1 © NACE International publication and year shown in citation above. All rights reserved. Displayed with permission from NACE International, Houston, TX (http://www.nace.org). Published in Corrosion, in the month and year shown in the citation above. |
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David C. Silverman, Ph.D. - Primary Consultant |
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