UV Clean Coat Over Metal

Q: Our company is coating a high polished metal with a UV clear coat. Our precleaning process includes hot alkaline cleaner at 150°F, 80-psi impingement spray, city water rinse and DI water final rinse at 100°F followed by a 120° air blow off.

The parts are then sprayed with an electrostatically charged material in a bell. Our question is whether rust sometimes occurs under the UV coating. Can you explain why some may rust quicker than others in a non-humid environment when routine maintenance is performed on all tanks and not all parts rust? D.E.

A: Unfortunately, I don’t have an obvious solution to your dilemma, although there are plenty of opportunities to investigate. There are several potential causes, either alone, or combined that could initiate this problem. You do not mention the base metal, but I will assume since you are seeing rust, that it is steel. If you were seeing rust development on, say aluminum, it could be a little easier to diagnose. If that were the case, you would have an obvious contamination problem at some point in your process that could be identified and rectified.

Within your process, there are several potential places to investigate. Starting at the beginning of the process with the polishing, you should consider this and all steps related to it. What is the media and preventative maintenance at each step? Is there an opportunity for cross contamination or embedding a polishing media too deep in the metal? The alkaline cleaning process will likely not remove all embedded media since some will be shielded from the direct spray.

Even if the cleaning line is effectively removing the residue and media from the polishing process, it could be redepositing back onto the metal surface. This could occur out of the cleaning tank, or either rinse due to carryover from the cleaning stage.
Another source to consider is both water stages. The final rinse is always the most important rinse in any chemical process line, so it should receive the most attention. It is encouraging to see that you are using deionized (DI) water in the final stage. That will tend to minimize residue that can create a flash rust situation. Are you using a mixed bed system that will remove both anion and cation? If not, you will need to upgrade in order to remove both positively charge species (cations like Ca2+ or Mg2+). The negatively charged species will be even more important to remove from the water (anions like Cl- and SO42-).
You may want to take a look at your preventive maintenance program and schedule on this portion of the line. How is the DI water quality controlled? There should be regular monitoring of the system for conductivity to determine when the resin bed is full. Ideally, there are two cylinders, one to “pre-clean” and the other as a final polishing stage. A defined water quality should be determined and documented. If you receive rental cartridges, some have lights that will stay green until the conductivity exceeds a required limit and will then change to red. This conductivity can often be set by the user with a jumper on a very small board. Typical settings are 1, 2, 5 and 20 µsiemen. I would recommend the use of the 5 setting, because you are working through the potential sources of the problem.

The final stage of the process should not just be considered the drying, but all the time and events that may occur between the final rinse and coating. In that time you dry with hot air (120°F), likely travel on a conveyor, may go to a storage area for an undetermined amount of time, and then proceed to the coating operation. I would consider this in several steps. Is the dryoff consistently getting the parts sufficiently dry? Obviously 120°F alone will not be enough to boil water, so the blow off will have to be fairly forceful and the surface area of the part will have to be fully accessible in order to effectively strip and dry. Possibly a hotter or more consistent drying operation is necessary. What is the source of heat for the dry off? Electric is the cleanest. Gas combustion products could deposit residues on the surface.

Also, it is important to maintain a consistent storage environment, if applicable, for the parts prior to coating. Even with a low humidity, if the parts are subjected to relatively significant temperature fluctuations, that will be enough to precipitate a film of water on the surface and initiate the corrosion.

Finally, (and maybe I should have started with this), even with all that you may investigate, it is possible that the quality of your incoming material is variable enough that it could be causing you problems. For instance, do you have any specifications for the material you receive, and do you routinely check to ensure you receive what is ordered? If not, you may receive a hot-rolled material one time and a cold-rolled another time. You may get a low-carbon steel and later a higher-carbon steel. If ordering without a specification, the supplier or service center may be sending you any steel that is most available and still meeting the intent of your purchase order.

This isn’t a quick list to go through, but rarely do you “solve” corrosion problems by accident. I would suggest you try to involve your chemical supplier to see how they can assist you.