Increasing Low Current Density (LCD) Coverage in Trivalent Chromium

Q: Our company plates decorative nickel/chrome parts and recently we have noticed that we aren’t getting chrome coverage in some of the recesses. How can I increase the low current density (LCD) coverage in my trivalent chromium deposit?

A: There are many factors that can influence the low current density (LCD) coverage in a trivalent chromium system. Since you didn’t mention which type of system (chloride or sulfate) you are using, we will discuss some ideas for both systems.

• Metallic Contamination: The main offenders involved are copper, zinc and nickel. It is important to keep the concentration of these interfering metals as low as possible by use of ion exchange or low current electrolysis. For consistent deposit quality, ion exchange is recommended.
• Bath pH: Maintaining the pH at the low end of the recommended range will increase the rate of deposit, but it will decrease the LCD coverage. Conversely, maintaining a pH at the upper threshold will decrease the rate of deposit, but increase the LCD coverage.
• Temperature: Reducing the temperature in sulfate-based systems will decrease the deposition rate but will increase the LCD coverage. Care must be taken not to reduce the temperature below a certain threshold or the conductivity salts will begin to lose solubility, resulting in deposit roughness.  
• Chromium Concentration: Increasing the chromium metal concentration can increase the LCD coverage, both by maintaining a higher content in the cathode film and by increasing the limiting current density of the high current density area, allowing more current to be applied without burning.
• Conductivity Salts: Maintaining the conductivity salts at the top end of the spectrum increases the LCD coverage by increasing the current-carrying capacity, and thus efficiency, of the solution.
• Anodes: If the anode surface area is not properly sized and the anodic current density is excessive, hexavalent chromium can be generated, which will decrease coverage in the LCD area at first. Then, if not corrected, it will stop the deposit completely. For sulfate-based systems, damage to the anode coating can also induce hexavalent chromium generation.
• Solution Movement: Excessive air agitation will reduce efficiency and decrease LCD coverage (mainly in chloride-based systems). Conversely, a properly designed mechanically agitated system will increase the efficiency of the deposit.
• Proprietary Additives: There are other factors specific to each trivalent chromium system that can affect LCD coverage, such as proprietary complexors, organic additives and so on but these are generally the most applied solutions.

Overall, keeping your bath parameters within recommended ranges along with minimizing your contaminant metals and optimizing solution movement will give you success with plating most part geometries. If you are still having problems with low current density coverage or you have very difficult geometries to plate, it then becomes a balancing act of adjusting bath parameters to fit what works best for you.