Low pH in E-Coat Bath
We are coating parts in a cathodic epoxy electrocoat with a very low pH. How can we increase the pH in our tank and improve the quality of our coating?
Q. We are coating parts in a cathodic epoxy electrocoat with a very low pH. How can we increase the pH in our tank and improve the quality of our coating? M.R.
A. You do not mention the exact pH you have in your system and what is the acceptable range that your electrocoat supplier has determined for your specific electrocoat material and operation. Also you do not mention the process difficulties or quality issues you have as a result of your low pH.
Low pH in your bath indicates that excess solubilizing acid is present in the system. Excessive solubilizing acid can lead to aggressive post rinses and low film builds due to paint re-dissolution. The amount of acid released during coating deposition in the electrocoat bath is directly proportional to the amount of material being coated in square feet or square meters. In other words, the more you coat the more acid the system generates, and thus the more acid that must be removed from the system to maintain the pH within the required limits. Maintaining your electrocoat bath within the recommended ranges will enable you to operate a very stable electrocoat system and obtain the best possible quality and corrosion performance online, as pH is a measurement of paint solubility and overall bath stability.
pH values are expressed as logarithmic values in a logarithmic scale, so what may appear to be a small change in pH actually represents a large change in the overall chemistry of the paint bath. Long term charting and SPC is used to detect unusual pH swings or trends and prevent future problems.
To increase your pH, you must do one or more of the following:
- Dump anolyte
- Dump permeate
- Add new material to the bath.
The best way to increase the pH will depend on many variables, such as the actual pH and the target where it needs to be, the volume of ecoat tank and permeate post rinses, the type of electrocoat technology and the color of electrocoat, the capabilities of your UF system to generate permeate, the capabilities of your anolyte system to generate and store anolyte, the type of parts you coat, and the minimum and maximum mils your quality system allows.
There is no single answer fits all for all low-pH problems. The time and risk your system is allowed to take to correct the pH will drive the direction to take. A system is not designed to drain large amounts of anolyte or permeate, especially over short periods of time, so proper steps must be taken prior to taking a final decision on how to proceed. Excessive permeate draining can lead to an electrocoat system that does not want to coat at low voltages.
Also, you have to be careful to correct the root cause of the problems that got you where you are now with the low pH. If you don’t eliminate the process or systemic root cause(s) that created the low pH, then your system will return to this condition again sometime in the future, and the correction will be temporary and wasteful.
Typical low-pH conditions encountered in an electrocoat system are related to the anolyte system. Leaky anolyte cells with holes and tears or plugged anolyte membranes are typically the root cause of most pH problems in electrocoat systems. In this case, you must fix or replace the damaged anolyte boxes or membranes and allow the system to reach a new equilibrium in pH. Also, you must check the conductivity set-point and control system to make sure it is functioning properly and that you have the proper set-point for your operation that is consistent with your electrocoat supplier recommendations. Your supplier must be closely involved in any decisions you take and must keep a close eye on the stability and recovery of your bath.
PPG launched the first use of waterborne compact paint technology in a U.S. automotive manufacturing plant at the BMW assembly plant in Spartanburg, S.C. This painting process has turned out to be a 2012 award winner and has opened up a new way for auto manufacturers to go leaner and become more efficient in their operations.
This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 13, 2012.
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