Effect of Velocity on Dissolved Oxygen Cathodic Polarization using a Rotating Cylinder Electrode

The aim of the present work to study the effect of changing velocity (Reynold's number) on oxygen cathodic polarization using brass rotating cylinder electrode in 0.1, 0.3 and 0.5N NaCl solutions (PH = 7) at temperatures 40, 50 and 60C. Cathodic polarization experiments were conducted as a function of electrode rotational speed and concentration.


INTRODUCTION
A brass alloy, 39%-Zn-Cu, has been employed to investigate the cathodic polarization of dissolved oxygen [3].
Polarization methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential.For anodic polarization, the potential is changed in the anodic (or more positive) direction, causing the working electrode to become the anode and causing electrons to be withdrawn from it.For cathodic polarization, the working electrode becomes more negative and electrons are added to the surface, in some cases causing electrodeposition.[6] The limiting current is defined as the maximum current that can be generated by a given electrochemical reaction, at a given reactant concentration, under well-established hydrodynamic conditions, in the steady state.This definition implies that the limiting rate is determined by the composition and transport properties of electrolytic solution and by the hydrodynamic conditions at the electrode surface [4].In general, for a rotating cylinder, when Reynolds number is greater than 200 the flow is turbulent [2].The aim of the present work is to study the effect of changing the velocity (Reynolds number) on oxygen cathodic polarization curve using on surface of brass rotating cylinder electrode at different concentrations of NaCl: 0.1, 0.3 and 0.5N at 40, 50 and 60 0 C.

EXPERIMENTAL DETAILS
The chemical composition (in wt %) of the brass alloy used analyzed in Ebn Siena labortary was (Cu = 60.24,Zn = 39.22,Sn = 0.52).The dimensions of the cylindrically-shaped metal specimen was 3 cm long and 3 cm in diameter.The specimen was connected to a rotating shaft driven by a motor.The brass specimen was ground sequentially with Sic papers to 600, 400, 300, 250, 200, 150 and 100 grit and immersed in alcohol 1 min.and acetone 1 min and dried by paper tissues and placed in desiccator over night before the electrochemical tests.The polarization runs were conducted in 0.1, 0.3 and 0.5N NaCl solutions (PH = 7).A saturated Calomel electrode (SCE) was used as the reference electrode and Figures 2 to 10 show experimental results conducted to demonstrate the effect of rotational speed on O 2 cathodic polarization curves.On increasing the velocity, the limiting current will be increased at constant temperature and concentration this appear in table (1 to 9).These results are in agreement with Stern and Uhlig [5,7].Velocity primarily affects electrochemical reaction rate through its influence on diffusion phenomena.It has no effect on activationcontrolled processes.The manner in which velocity affects the limiting diffusion current is a marked function of the physical geometry of the system.In addition the diffusion process is affected differently by velocity when the flow conditions are laminar as compared to a situation where turbulence exists.For most conditions the limiting diffusion current can be expressed by the equation: Where (K) is a constant, (U) is the velocity of the environment relative to the surface and (n) is a constant for a particular system.The value of n varies from 0.2 to 1 [5,7].
Figures 11 and 12 show experiments conducted to find the effect of concentration on O 2 cathodic polarization curves.When the concentration of NaCl is increased the limiting current will be increase at constant Reynolds number (rotational velocity) and temperature show in table (10) which is in agreement with Fontana and Greene [1].The effect of oxidizer additions or the presence of oxygen on electrochemical rate depends on both the medium and the metals involved.The rate of (limiting current) may be increased by the addition of oxidizers, oxidizers may have no effect on the corrosion rate, or a very complex behavior may be observed.By knowing the basic characteristics of a metal or alloy and the environment to which it is exposed, it is possible to predict in many instances the effect of oxidizer additions [1].
For diffusion-controlled process, an increase in concentration of the diffusing species in the bulk of the environment increases the concentration gradient at the metal interface.The concentration gradient provides the driving force for the diffusion process.Thus the maximum rate at which oxygen can diffuse to the surface (the limiting diffusion current) would be essentially directly proportional to the concentration in solution.Figs.11 and 12 are examples of the cathodic polarization diagrams which are operative for this system [5].

CONCLUSIONS
On brass alloy when increasing rotational speed limiting current will be increased.For diffusion controlled process, an increase in concentration of the diffusing species in the bulk of the environment increases the concentration gradient at the metal/solution interface.The concentration gradient provides the driving force for the diffusion process.Thus the maximum rate at which oxygen can diffuse to the interface surface (the limiting diffusion current) would be essentially directly proportional to the concentration in solution and temperature in presence of increasing NaCl concentration.