Red Rust Under E-Coat

Question:

We have noticed red rust under the electrocoat on some parts. There is no visible rust on the raw parts, but “bubbles” on the parts after coating and curing. What could be the source of this rust? P.H.

Answer:

One of the advantages of e-coat is that a dry-off oven is not normally required between the pretreatment system and the coating tank. Because of this, it is usually difficult to observe the parts between these two parts of the process.

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My first suspicion for your problem is line stoppages during the coating process. Assuming that you are using an alkaline cleaner and an acidic phosphate pretreatment system, line stoppages can result in rusting of parts in or between washer stages. Once parts are clean, but not phosphated and sealed, they can rust very quickly. It has been my experience that keeping all pumps on and all doors closed on the washer during any line stoppage will minimize rusting.

Of course, eliminating or minimizing line stoppages is most important. When they do happen, have someone watch the parts as they move from the washer to the e-coat tank to look for rust. If rust is observed, attempt to calculate the location of the parts in the washer during the stoppage. Your chemical and equipment suppliers may be able to help the problem, but identifying and eliminating the cause of stoppages is paramount. If you do not have a line stoppage alarm and/or recorder, I strongly recommend one. If line stoppages are planned for breaks or lunch periods and you can identify the problem location, try gapping the line to remove parts in the problem area(s).

Another possible source of your rust might be rust on raw parts that may be “hidden” with lubricants or soils. You might try a “bounty award” system for the employees that are loading the parts to look for rust or other defects that you think might be on parts that are not fully visible as received at the coating system.