Hull Cell Test for Plating Solutions

 

A Hull Cell test is a diagnostic tool used in the electroplating industry to evaluate the performance of a plating bath. It involves plating a small panel (typically made of copper or zinc) in a miniature plating tank called a Hull Cell. The cell is designed to simulate the conditions of a full-size plating tank, allowing operators to observe the effects of different current densities on the plating process.

The test panel is placed at an angle in the cell, with one end closer to the anode (positive electrode) and the other end closer to the cathode (negative electrode). This creates a gradient of current densities across the panel. The operator then applies a specific current to the cell for a set amount of time, typically around 10 minutes. After the plating process, the panel is removed and examined for various characteristics, including: 1. Deposit thickness: The thickness of the plated metal is measured at different points on the panel to determine the uniformity of the plating process. 2. Surface appearance: The operator looks for signs of defects, such as roughness, nodules, or pits, which can indicate problems with the plating bath or process. 3. Color: The color of the plated metal can provide clues about the composition and condition of the plating bath. By analyzing the results of the Hull Cell test, operators can make adjustments to the plating bath or process to optimize the quality and efficiency of the plating process.

Typical Acid-Sulfate Bath Hull Cell Panel Results

The schematic shows the cell has two sides that are parallel, and two that are not parallel. The side that slants off has a blank plate installed to receive the plate, while the other side supports the anode. The plate’s closest side receives more current than the side that is further away. In effect, it receives the plate at the end experiences a higher current density. It is easy for the deposit to be “burned” (note the darker color in the drawing).

On the other side, the current density is generally lower than what is recommended for bath operation (note the light coat in the drawing). Quality plate is laid between the two extremes. How wide the usable coat in between is tells the technician if he needs to adjust the bath. It also shows what the problem might be, whether with the low current or the high current end. The commercial bright acid-copper bath we used had two brighteners, one for low-current and the other for high-current density performance.

Other Acid-Sulfate Bath Hull Cell Panel Results

On rare occasion, other problems might arise. One of these was tiny spots where plating did not take on the cell panel. This proved to be caused by tiny particles from the anodes that effectively insulated their point of attachment, effectively preventing plate from depositing.

Another problem was a swirling pattern on the plate. The panel was shiny and there were no pits, so this difficulty did not suggest the bath was completely dysfunctional, but it suggested the bath needed carbon filtration, followed by introducing fresh brighteners. After doing that, a repeat Hull Cell performance indicated normal bath behavior.

Finally, there was the rare occurrence of plating at the upper and middle ranges of the cell panel, but little to no plate at the lower range. This problem can be more serious if initial low current-density plating is indicated. It is good to check for anode to cathode current density ratio. Yes, the cathode (the piece receiving plating) and the anode (the negatively charged electrode) function best when the ratio of their current densities falls within a given range as well. For bright copper, a ratio of between 1:1 and 2:1, anode: cathode provides the best performance.

In Conclusion

The information provided here must surely suggest to the reader the practical wisdom of buying a Hull Cell test kit to assure a “healthy” properly maintained electroplating bath as a first line of defense.