What Does Primary Current Distribution Mean?
Primary current distribution is a type of current distribution that is completely controlled by the resistivity of the electrolyte solution between the working and counter electrodes. The primary current distribution establishes itself when the influence of overpotential is negligible.
Primary current distribution:
- Accounts only for losses due to solution resistance
- Neglects electrode kinetic
- Concentration-dependent effects
Primary current distribution can be applied to electrochemical systems, including:
- Heat transfer by conduction
- Diffusion in solids
- Potential (ideal) flow
- Conformal mapping
Corrosionpedia Explains Primary Current Distribution
The primary current distribution is the current distribution obtained across an electrode in the absence of polarization. Current always follows the least resistive path; consequently, a non-uniform current distribution results if the geometry of the electrodes is such that the resistivity of the current path is not the same to every point on the working electrode. Every other effect that may influence the current distribution is ignored in this case, or assumed to be negligible. Therefore, the primary current distribution only depends on the geometry of the anode and cathode.
Characteristics of primary current distribution include:
- Large gaps
- Low conductivity
- High current density
- Fast kinetics
Primary current distribution can be used for modeling cells where there is a relatively high electrolyte concentration (in relation to current density) or vigorous mixing in the electrolyte, allowing the assumption of a uniform electrolyte concentration. It can be a valuable approximation for electrochemical processes involving relatively fast reactions like the oxidation of chloride ions in the chlor-alkali process.
The primary current distribution shows that the more inaccessible parts of an electrode receive a lower current density. Since the highest rate of corrosion is achieved at a maximum current density, high primary current distribution causes higher levels of corrosion.
To minimize the primary current distribution, it is recommended that anode-to-cathode surface area ratios should not exceed 2:1 and anode length should be three to six inches shorter than the cathode. For example, rack design and cathode spacing on the rack also influence the primary current distribution in electroplating of copper.
Primary current distribution is not unique to electrochemical systems, as other physical systems exhibit the very same distribution.