An experimental technique for the study of the persistency of films

Hickey, Gregory Scott, Department of Engineering, University of Virginia

An experimental technique was developed for the study of the influence of hydrodynamic shear on the persistency of films in fluid mediums. This work was conducted in two parts: evaluation of the shear stress from an impinging fluid jet and a study of the effect of shear on two films, a corrosion inhibitor and a passive film on iron. The radial component of shear stress on an axisymmetric disk electrode is a function of a hydrodynamic constant to the 3/2 power and the physical properties of the fluid. The hydrodynamic constant was evaluated using a series of rmg electrodes at nozzle heights of H/d = 1 and 5 for jet velocities up to 4.35 m/s. The hydrodynamic constant was independent of radial position m the area of uniform mass transfer that extended to r/d = 0.45 and was linear with jet velocity for both laminar and turbulent nozzle exit conditions.

Knowledge of the critical shear for the removal of films would allow hydrodynamic strategies to be developed to minimize the effects of erosion-corrosion. The persistency and effectiveness of an industrial inhibitor being considered for use in brine geothermal wells was studied. The conditions were simulated by a 4.5 M NaCl, pH = 2.0 solution with and without an inhibitor concentration of 100 ppm. The effectiveness of the inhibitor decreased with increased fluid jet velocity. This decrease in effectiveness was associated with the removal of the inhibitor film by hydrodynamic shear. The critical shear was evaluated by determining critical radii for enhanced corrosion.

The shear from an impinging jet of 4.35 m/s was not sufficient to remove a passive film on iron m 1 M sulfuric acid far from the Flade potential. Within 30 mV of the Flade potential the passive film could be removed by the fluid shear from jet velocities less than 4.35 m/s. At potentials approaching the Flade potential a lower jet velocity was sufficient to remove the passive film. The current increased with jet velocity even when no removal of the passive film was apparent. At 30 mV from the Flade potential a ring structure was formed where the outer ring showed increased corrosion. This was due to local destabilization of the passive film by shear and repassivation.

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MS (Master of Science)
Hydrodynamics, Fluid mechanics, Shear flow
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