Search

Abstract: Erosion–corrosion is a complex materials degradation mechanism involving the combined effects of mechanical erosion and electrochemical corrosion. It is known to be induced by high shear stresses in strong flows. One way to mitigate this phenomenon is to use additives reducing the turbulent Reynolds stresses. Oleyltrimethyl ammonium cationic surfactant and sodium salicylate as counter-ion known to form thread-like micelles behaving as drag reducers were tested in this work and successfully imaged by using AFM technique. A shear stress mapping in the rotating cage using an electrochemical method was implemented in the absence and in the presence of surfactant, showing drag reduction amounts up to 65%. In a second step, erosion corrosion of Cu in Na2SO4 0.1M in the presence of small amounts of NaCl (1mM) was investigated on Cu microelectrodes located at different positions on plastic coupons in the cage. The surfactant effect was found to be beneficial by preventing the removal of the pre-existing oxide films, otherwise observed in the absence of surfactant. Tests performed with benzotriazole known as a good corrosion inhibitor for Cu, could not prevent either the breaking of the oxide film in strong flow conditions. [Copyright &y& Elsevier]

Abstract: The electrochemical method has been used to study the wall turbulence in a rectangular channel flow and to assess the role of high polymers in the drag reduction phenomenon. Experiments were performed with aqueous solutions of two high molecular weight polymers: polyethylene oxide (PEO) and polyacrylamide (PAA). High levels of the drag reduction (up to 75%) were observed for small concentrations of these polymers under turbulent flow conditions (Re=5000–35,000). The photolithography technique of the probe fabrication was improved to turn out very accurate and complicate patterned electrodes. Double semi-circular gold electrodes of very small diameters (d=100μm) located close to each other on the channel wall (the minimum center-to-center distance was 125μm) were simultaneously working in the regime of the limiting diffusion current. Eight electrodes provided information on two components of the fluctuating wall shear rate at the wall. The frequency and the amplitude of flow fluctuations were greatly reduced by addition of polymer and the corresponding power spectra exhibited a shift towards low frequencies. Spatial correlations were calculated from the probe signals and an increase of the size of the coherent flow structures due to the polymer addition was observed. [Copyright &y& Elsevier]

Electrochemical method was used to study in elongational laminar and turbulent flows the mass transfer toward a rotating disk in different drag reducing solutions of two cationic surfactants (Arquad 16-29, Arquad S50) together with counter ions (Para-chloro benzoate, Nasal) by varying the concentrations of their mixture. The mass transfer in laminar flow conditions displays a first order transition characterized by a significant drop accompanied by strong instabilities and a hysteresis cycle at a critical angular velocity which depends on the counter ion to surfactant concentrations ratio ζ . The local wall shear stress in turbulent flow was measured using a microelectrode inserted flush with the cross-section of the rotating disk. The ζ ratio influences the drag reduction onset: Arquad 16-29 was shown to be more efficient at lowest concentration values than the Arquad S50 for both the critical velocity transition in laminar flow and the onset of drag reduction in turbulent flow. [ABSTRACT FROM AUTHOR]

The mass transfer given by an electrochemical reaction towards a rotating disk in a solution of surfactant having drag reducing abilities exhibits a first order transition. Beyond this transition marked by a significant slump of the mass transfer rate, the flow becomes unsteady and is characterised by an hysteresis cycle. This phenomenon was earlier observed for viscoelastic dilute solutions of linear polymers of high molecular weight and is induced by the elongational strain prevailing in the flow around a rotating disk. [Copyright &y& Elsevier]

Abstract: The local wall shear stress, in turbulent flow of drag reducing and corrosion inhibiting surfactant solutions, was measured from polarographic mass transfer experiments using microelectrodes. A good agreement was observed between these values and those obtained by pressure drop measurements in a rectangular channel of high aspect ratio (10:1). Irreversible degradation was not observed in these systems at variance with polymer solutions. [Copyright &y& Elsevier]