Corrosion Under Powder Coating

Q. We’re facing problems in powder coating on sheets of cold-rolled closed annealed parts. We found that, after some days, the color is peeling off, and we have found corrosion beneath that. We do a three-in-one cleaning process and then powder coat at our job shop. Our product is an acoustic enclosure of a diesel generator set, so we are using polyester powder for sun resistance. Is there a solution, or should we switch to another process? T.S.

A. Given your description, the problem is not with the paint type or application, but seems to be related to your pretreatment process. You need to consider if the three-in-one sort of pretreatment is adequate for your product performance. That type of pretreatment will clean and coat in the first step, followed by rinsing and then a seal or passivating rinse. That first stage is formulated to only remove light-duty lubricants and soils. If you have heavy lubricants or a significant load of soils to be removed, this type of pretreatment isn’t suited for your requirements. It is imperative to remove all the contaminants, since that is necessary to expose the substrate for subsequent conversion coating in the same step.

If you have anything except light-duty lubricants, I would suggest you consider working with your chemical supplier to evaluate the possible improvements you could realize through the use of a full five-stage cleaning and conversion coating system. With this scenario, you will have much more flexibility to use a more heavy-duty cleaner that will adequately clean your surfaces prior to the third stage of conversion coating. If even greater pretreatment corrosion protection is necessary, you would then have to consider a zinc phosphate conversion coating process that may include more than the traditional five steps of an iron phosphate process.