What Causes Increasing DC Rectifier Voltage for Ecoating?

Q. In my cathodic ecoat system, why does the DC rectifier voltage requirement keep increasing?

Electrocoating requires significant DC current at a modest voltage to cause a film to form on the cathode (i.e. ware). For a brand-new ecoat system, the required voltage of 175 to 225 V is typical to achieve the required ecoat film thickness and, over time, the ecoat operator will observe that the voltage requires increases. While some increase can be expected, when there has been an unexplained 80 to 100 V increase over baseline, many ecoaters report problems achieving desired film thickness. 

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As part of the troubleshooting process, ecoat operators should look at these possible causes of low film thickness:

• Low rectifier voltage
• Low ecoat bath temperature
• Increased conveyor speed
• Increased ware surface
• Low % NV
• Poor ground connection
• Low DC current
• Low ecoat bath pH
• Low ecoat bath conductivity
• High phosphate coating thickness

One item missing from the above list is the consideration of the anode cells, which act as the conductor between the DC rectifier and the ecoat paint bath. When the anode cell is new, the ion-exchange membrane is practically invisible to the DC current as it passes through from the anode to the ware. With time and operation, the resistance of the ion-exchange membrane increases to a level that is significant. The flow of DC current to the ware is then impeded.  The response must then be to raise the voltage of the DC rectifier.

Measuring ecoat film thickness as a quality check will confirm that the actual thickness is within specification. In addition, a best practice recommendation is to begin budgetary planning for replacement anode cells once the voltage of the DC rectifier is 50 V over the baseline (assuming ware area, line speed and paint parameters have remained unchanged). The new equipment should then be installed when the required voltage is 80 V above the baseline.

Recording baseline parameters and regularly comparing system data is key to operating an optimal ecoat system. By staying on top of trends, the ecoat operator can avoid unexpected and troublesome setbacks.