The Rotating Cylinder Hull Cell: Design and Application
In our first paper this month, featuring the latest work from AESF/NASF Research, the rotating cylinder Hull cell (RCH) was used to study the behavior of a Ni-Mo-W plating bath. Originally published in 1993, this second paper outlines the basic concepts and work behind the development of the RCH cell.
#basics #plating #nasf
C. Madore and D. Landolt
Laboratoire de Metallurgie Chimique
Originally published as:
Plating & Surface Finishing, 80 (11), 73-78 (1993).
Editor’s Note: A printable PDF version is available by clicking HERE.
A new cell geometry, the RCH cell, can replace the classical Hull cell in applications where mass transport is of importance. The cell includes a rotating cylinder electrode and an asymmetrically placed counter-electrode with a suitably placed concentric separating wall. The primary current distribution resembles that of the classical Hull cell. The usefulness of this cell design is demonstrated by studying the reaction distribution on the cathode during deposition of a copper nickel alloy from a citrate-based model electrolyte.
The Hull cell1 is a trapezoidal structure in which the cathode is placed at an oblique angle with respect to the anode. The cell allows exploration of the variation in the appearance of an electrodeposit over a wide range of current densities along the cathode surface in order to determine optimal electroplating conditions. It is commonly used for maintenance of plating solutions to insure continuous production of satisfactory deposits and for the development of new electrolytes. A shortcoming of the traditional Hull cell is the absence of controlled mass-transport conditions. Indeed, for many plating processes, mass transport is a crucial step; examples include metal plating in the presence of leveling agents or alloy plating with one component being present in low concentration. In such cases, interpretation of a Hull cell plate is difficult and the cell cannot be used effectively.2
To overcome this problem, several authors have proposed Hull-type cells that include controlled hydrodynamic conditions. Graham and Pinkerton built a rotating cylinder placed coaxial with a stationary conical anode.3 The current distribution along the cathode examined in their studies, however, was significantly more uniform than that obtained in the traditional Hull cell. Kadija, et al. developed a hydrodynamically controlled Hull cell using a rotating cylinder with the anode placed below the cathode on the cylinder.4 Using a partially submerged cathode and insulator baffles, they obtained a current distribution similar to that of the Hull cell. In this cell, however, a well-defined current distribution is obtained only if effects resulting from a vortex can be avoided. Lu proposed several designs using conical and cylindrical electrodes.5 The described cell configurations, however, were either empirical or of complex geometry and therefore had limited applicability.
In the absence of mass-transport effects, uniformity of the current distribution in electrochemical cells depends on the ratio of the polarization resistance at the electrode surface and the ohmic resistance of the electrolyte. This ratio is expressed by the Wagner number. For Tafel kinetics, the Wagner number is given by Equation 1:
where η is the overpotential, ρe is the electrolyte resistance, κ is the electrolyte conductivity (κ = 1/ρe), iavg is the average current density, and L is a dimensionless length, here the cathode length h. The most non-uniform current distribution, the so-called primary current distribution, corresponds to Wa 0. The empirical Eq. (2) used in practice for the determination of the current distribution i on the classical Hull cell corresponds to this case.6
where z is the dimensionless cathode length measured from the low current end and iavg the average current density in the cell. The insensitivity of practical Hull cell experiments to kinetic parameters is readily understood with Eq. (1) because the evaluated average current density is usually quite high, leading to a value of Wa close to zero.
The goal of the present study is to design an alternative Hull cell having a primary current distribution close to that of the classical Hull cell, but with well-characterized mass-transport conditions. The mass-transfer rate should be uniform over the entire cathode surface and the magnitude of the limiting current should be easily varied. The new cell should be relatively simple to build and to use, without the need for elaborate pumping systems or of a large electrolyte volume. Its primary and secondary current distribution should be well known from numerical simulation.
Theoretical and practical design considerations of a cylindrical Hull cell with controlled convection have been published in a recent paper.7 In the present investigation, a simple cell design is described and its performance tested with two model systems - electrodeposition of copper at almost primary current distribution, and electrodeposition of a copper-nickel alloy involving mass-transfer-controlled deposition of copper. The cell consists of a rotating cylinder electrode with a counter-electrode placed at one end. An outer stationary insulating separator is placed coaxially with the rotating cylinder electrode and its diameter determines the current distribution along the cylinder electrode. The primary current distribution was simulated for the above geometry with a software based on a boundary element method (BEM).8,9 The cell geometry was optimized in order to yield a current distribution similar to that of the Hull cell.7 The optimal geometry was found to correspond to a ratio of the electrode length over the distance between the electrode and the insulating separator equal to three. Moreover, the calculations showed that position of the anode, placed outside the insulating separator, has no influence on the current distribution.
Figure 1 compares the primary current distribution for the optimal geometry of the RCH cell described above and the Hull cell. The two distributions are quite similar. For small values of z, the current density in the RCH cell varies slightly less than in the Hull cell because the current density at the far end does not go to zero. The simulated primary current distribution for the RCH cell is described by the following expression:7
where z is the dimensionless distance along the cathode from the lowest current density side (z = l - x/h in Eq.  of Ref. 7), i(z) the current density along the cylinder cathode and iavg the applied current density.