An electrochemical cell consisting of a double horizontal Impinging Jet Cell (IJC) has been conceived and characterized. The purpose of this system is the simultaneous electrodeposition of a composite metal/particle coating on both surfaces of a metal sheet. The silica particles imprint in the nickel matrix has allowed to distinguish four different flow areas onto the electrode namely the stagnation area, the radial flow area characterized by a higher flow speed, the return flow area that involves gravity effect, and the drainage area with a constant draining speed. Based on the limiting current evolution as a function of the Reynolds number, three flow modes were extracted: the Laminar Low Flow (LLF), the Laminar High Flow (LHF) and the Disturbance. The IJC investigated ensures a laminar flow for a large range of flow rate from a nozzle-to-sample distance of 19 mm and creates an laminar flow ovoid plan merged with the sample for the high flows.
Particles embedment into metal matrix by electrocodeposition is increasingly used to improve mechanical properties, corrosion resistance [
Previous works have focused on the characterization of a vertical impinging jet cell [
The cell designed for this study consists of two parts. The body of the cell is a glass cylinder of 70 mm diameter and a height of 45 mm (
two different heights, and each pair was placed at 90˚ from each other. The two upper holes, in which tubes with an internal diameter of 4 mm are introduced, guide the electrolyte stream towards the working electrode (electrolyte inlet) on the two faces of the substrate. The excess of electrolyte is evacuated through the lower holes pair. The flow rate, ensuring a proper agitation of the electrolyte, is controlled by a peristaltic pump. The internal volume of the cell is 150 cm3. The cap (
The working electrode stands in the middle and two counter electrodes are placed on each side equidistantly with the working one. Such a positioning allows a simultaneous treatment of both surfaces of the working electrode and insures a homogeneous distribution of the electrical field lines, at the same distance to the cathode with the flow jet. It also enables the investigation of the effect of the distance d between the nozzle and the working electrode on the flow mode.
The complete system of the jet cell and the peristaltic pump (Isamatec) is shown in
The working electrode and the two counter-electrodes were made of a pure nickel foil (Alfa Aeser) of 1 mm thick cut in a T-shape according to the dimensions indicated in
The mass transfer coefficient K associated to the working electrode limiting current
where n is the number of electrons exchanged in the electrochemical process, F is the Faraday constant and C the concentration in electro-active species.
The Sherwood number Sh, which represents the ratio between the total mass transfer and diffusion transfer to the surface of an electrode is expressed by:
where R is the radius of the surface of the electrode and D is the diffusion coefficient.
The Reynolds number Re, which represents the significance inertial forces associated with the flow rate in contrast to the kinematic viscosity strength and allows distinguishing the limits of flow regimes is established by:
With V being the flow velocity in cm/s, ν the kinematic viscosity in cm2/secand d0 the nozzle diameter in cm. In the case of this work where the liquid is not subjected to heat transfer, the Sherwood number is expressed as:
With a being the hydrodynamic coefficient characterizing the flow, and Sc the Schmidt number which is the ratio of the kinematic viscosity ν and the diffusion coefficient D.
From Equation (2) and Equation (4), one establishes the relationship between the limiting current and the Reynolds number as follow:
The terms constituting the current variation coefficient versus the square root of Re are all constants except the hydrodynamic coefficient a. Thus, any change of slope will reflect a flow regime change. In the current study, an equimolar solution of potassium ferri- and ferrocyanide of 5 mM ions in an aqueous solution of potassium hydroxide 100 mM has served as electrolyte for characterizing the flow conditions in the IJC.
For each flow rate in cm3/s, the flow velocity V was determined based on the following dimensions and parameter values: R = 0.2 cm, n = 1, F = 96500 C, D = 7.9 × 10−6 cm2/sec, and Sc = 1215. The kinematic viscosity of the electrolyte ν was determined to be 0.99 mm2/s using a viscometer (Viscometer sv-1a).
For different flow rates, the corresponding Reynolds numbers was calculated by Equation (3), and the limiting currents for reduction and oxidation processes were measured with the technique of linear sweep voltammetry, where voltammograms were recorded using a galvanostat-potentiostat (Autolab PGSTAT302N, Metrohm). The reference electrode was an Ag/AgCl in saturated KCl solution.
To describe the flow on the working electrode surface, its imprint was collected during the codeposition of a nickel/silica composite coating performed at room temperature. The composition of the Watt type suspension used for this process is provided in
The size of silica particles (Sikron-B600) is in a range of 1 to10 µm.
The different flow areas and the flow profile have been visualized through the silica particles distribution in the nickel matrix examined by scanning electron microscopy SEM (Philips XLF-30 FEG).
In
In view of the particles distribution, the overall shape of the imprint may be set together with the history of their movement. Four areas, here called A, B, C and D of the jet flow at the electrode surface are characterized by different flow rates:
Area A: This area is the impact zone. It presents a high amount of particle encrusted. The particles in the
Chemicals | Concentration (g∙L−1) |
---|---|
NiSO4; 7H2O | 250 |
NiCl2; 6H2O | 90 |
H3BO3 | 30 |
Sodium dodecylsulfate (SDS) | 0.14 |
SiO2 particles | 30 |
electrolyte support the same force and, the residence time over this area is being longer, the probability of embedment is necessarily more important. Area A corresponds to the stagnation zone.
This system proceeds as a spray coating. A major advantage of such system is the possibility to coat a given surface with a predicted particle amount into the metal matrix.
Area B: In accordance with particles imprint, this area characterizes the radial flow of the liquid after impacting the working electrode. The upper part formed above area A looks like a crescent moon with a weak rate of embedded particles. In fact, particles move away from the working electrode carried by the liquid back motion or by elastic shocks. The spacing between the flow lines increases betraying an increasing radial velocity
Area C: It concerns the return of the suspension after the impact and the radial flow. In this area, the distribution of particles is intense and homogeneous due to an extended living time of the particles in the layers which have their speed annihilated at the surface of the electrode. Indeed, under the effect of weightlessness, the radial velocity of the liquid suddenly annihilates. This forms a thick layer of solution at the surface of the substrate that flows down around the radial flow area B.
Area D: After impact, the liquid layers in the lower plan normally flow down under the effect of gravity. To these waves, those flowing back from area C are added. Then, these homogeneous layers formed, drains particles with a constant drainage speed
In the light of this description, the flow profile at the working electrode surface can be sketched as presented in
The limiting currents of oxidation and reduction plotted as a function of the square root of corresponding Reynolds number for d equal to 5, 10 and 15 mm are presented in
All graphs present the same shape irrespectively of the electrochemical process considered and the nozzle-to-sample distances investigated. Three flow regions denoted I, II and III stand out. The limiting current evolution is characterized by a linear increase in regions I and III interrupted by a plateau in region II.
The slopes of regions I and III exhibit the same value as summarized in
This indicates that the flow regime is the same in both regions I and III. Low flow velocities conditions in region I are related to a laminar flow. Since the hydrodynamic coefficients remain identical for the higher flow velocities in region III, it is conceivable that the flow in these conditions also remains laminar. Moreover, the liquid renewal in the system occurs by the peristaltic aspiration and not by overflowing. This particularity results from the cell configuration. Indeed, the electrolyte that reaches both faces of the sample is sucked by the peristaltic pump from the bottom, ensuring a wave regulation: The waves are absorbed, stabilizing the flow and maintaining it laminar. The flow lines are not disturbed and do not create turbulences as commonly observed [
d | ||||||||
---|---|---|---|---|---|---|---|---|
Oxidation | Reduction | |||||||
5 mm | 10 mm | 15 mm | 5 mm | 10 mm | 15 mm | |||
Regions | I | 0.88 | 0.88 | 0.84 | 0.79 | 0.69 | 0.69 | |
II | 0 | 0 | 0 | 0 | 0 | 0 | ||
III | 0.88 | 0.88 | 0.84 | 0.79 | 0.69 | 0.69 | ||
higher Reynolds numbers are necessary for the return flows to disturb the incident jet. At theses Reynolds numbers, the incident jet with a horizontal velocity
This result indicates that the IJC allows keeping a laminar flow on a working electrode whatever the Reynolds number as from 19 mm. The designed cell is so suitable to improve the uniformity of composite coatings and for electroplating spray applications.
The voltammograms presented in
After the impact, the fluid spreads increasingly onto the sample surface with increasing velocity according to Glauert [
To assess δ evolution, one considers a conical trunk between the section of the nozzle S0 of radius r and the sample surface facing S1 of radius R1. If the volume of fluid coming out of the nozzle VS which is equivalent to the volume of the conical trunk is found at the surface S1, δ and Vs are given by:
By solving the quadratic equation in R1 from Equation (7), R1 and S1 for each flow are:
For one face of the cathode, the graph of the change of δ as a function of the half of the limiting current of oxidation
The thickness of the diffusion layer decreases as
This study has consisted in the design and hydrodynamic characterization of a horizontal electrochemical impinging jet cell. The flows on the working electrode have been successfully mapped using the silica particles imprint in the nickel coating. Thus, the stagnation area, the radial flow area, the return flow area and the homo- geneous drainage flow area have been characterized. The amount of embedded particles is the highest in stagnation area.
The change of the limiting current as a function of the square root of Reynolds number has allowed to identify three flow regimes namely the laminar low flow, the laminar high flow separated by a disturbance region. The impinging jet cell developed is capable to ensure a laminar regime regardless of the flow from a nozzle-to- cathode distance estimated at 19 mm. Based on the evolution of the diffusion layer thickness, the existence of a hydrodynamic ovoid plan merged with the working electrode has been considered.
The authors are grateful to the Swiss Government for their support through the “Swiss Government Excellence Scholarship” (No. 2012.067/Côte d’Ivoire/OP).
Désiré M. K. Abro,Pierre Dable,Fernando Cortez-Salazar,Véronique Amstutz,Edith Kouassi Kwa-Koffi,Hubert Girault, (2016) Design and Characterization of a Horizontal Double Impinging Jet Cell: Determination of Flow Modes at the Surface of a Flat Electrode. Journal of Materials Science and Chemical Engineering,04,18-28. doi: 10.4236/msce.2016.48003