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David C. Silverman |
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That a measurement in one configuration can be used to predict if velocity sensitive corrosion exists in another configuration is an assumption. Part of its foundation comes from the observation that for the large Schmidt Numbers normally encountered in liquids the fully developed mass transfer boundary layer for hydraulically smooth surfaces is much thinner than the fully developed hydrodynamic boundary layer. The Schmidt number (raised to a fractional power) provides an order of magnitude ratio of the thicknesses of the hydrodynamic boundary to the mass transfer boundary layer. This relationship for liquids is depicted in this figure Both boundary layer thicknesses are usually much smaller than the radius of curvature of the surface under study. Under these circumstances, the coordinate system within and immediately adjacent to the boundary layer may be assumed at least to a first approximation to be independent of the geometrical shape. For example, in the pipe and rotating cylinder electrode, these thicknesses are much less than the radii of curvature. That assumption is one of the main justifications for considering use of one geometrical configuration to predict the effect of fluid flow in another configuration. Such an approach is predicated on the additional provisos that the boundary layers in both geometries are fully developed and no detachment has occurred. The objective, then, is to define experimental conditions in one geometry that can enable that geometry to model and predict the effect of fluid motion on corrosion in another geometry. Note that the word "predict" refers to corrosion mechanism, not corrosion rate. Two possible approaches have been proposed to establish flow conditions (rpm) within the rotating cylinder electrode that could enable predictions of velocity sensitive corrosion mechanisms in other geometries. These are similarity in wall shear stress (Silverman, D. C., Corrosion, 40(5), 220 (1984)1  (467k))
between geometries and similarity in mass transfer coefficients between that
in the rotating cylinder electrode and that in the configuration of interest
(Silverman, D. C., Corrosion, 44(1), 42 (1988)1 (522k))
and Nesic, S., Solvi, G. T., Skjerve, S, Br. Corros. J., 32(4), 269 (1997)).
These references suggest that using the equivalence of mass transfer coefficients
seems to provide a way to use one geometry to get a handle on corrosion in another
if such corrosion is controlled by mass transfer and surface roughness is minimal.
The linkage between geometries may be established, therefore, by determining fluid
velocities in each that create equal mass transfer coefficients. The objective is
to determine flow conditions under which to operate the rotating cylinder electrode so
that the resulting corrosion mechanism and rate information will provide good insight about
the mass transfer influence on corrosion mechanism in the modeled geometry. Good insight
refers to understanding corrosion mechanism, not accurately determining corrosion rate.
After reviewing the equations that have been proposed (Silverman, D.C., Corrosion, 55(12), 1115 (1999)1 (271k))
those in the following table have been found to offer
reasonable predictions for the geometries considered to relate fluid velocity in one
geometrical configuration to that in the rotating cylinder electrode at equal mass transfer
coefficients for hydraulically smooth surfaces. CYLEXPERT has extended this analysis
to include several additional flow configurations important in industrial settings.
Additional information can be found in
D. C. Silverman, "The Rotating Cylinder Electrode for Examining Velocity-Sensitive
Corrosion - A Review", Corrosion, Vol.60, No.11, p. 1003, 2004.
1 (1560k)
This figure As discussed in the section entitled Effect of Surface Roughness surface roughness changes the Sherwood number vs. Reynolds number relationship because the mass transfer rate is usually increased by roughness. If increased roughness is suspected or observed, one might use the information provided by CYLEXPERT as a starting point. But instead of examining corrosion at only one or two rotation rates, the measurements should be made across a wider range of rotation rates and a plot generated in terms of Sherwood number versus Reynolds number. The line generated by curve-fitting the points may provide additional insight into the corrosion mechanism especially if the exponent deviates significantly from those values corresponding to a hydraulically smooth cylinder. | ||||||||||||||||||
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David C. Silverman, Ph.D. - Primary Consultant |
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