Temperature Tolerance of Chrome Pretreatment

Q. We powder coat aluminum extrusion using a five-stage spray washer with a chrome phosphate pretreatment conversion coating. At what temperature will the chrome conversion coating deteriorate when exposed to heat prior to powder coating? We are concerned that heat may make the chrome pretreatment ineffective in protecting the aluminum from corrosion and bonding of the powder coating. —F.P.

A. The temperature is an important part of the discussion, but exposure time is as critical as the temperature. During the dry-off oven cycle, you may see temperatures of 350º to 400°F. If you sustained a temperature of 400ºF, the conversion coating would deteriorate after a prolonged exposure. The dry cycle is usually less than 10 minutes, and it takes some time for the surface temperature to rise. Since the conversion coating typically does not see extreme temperature for a prolonged period of time, it is typically not damaging to the conversion coating. Running the dry-off at the lowest effective temperature is a good idea. If you can avoid excess heating during the dry-off, you protect the conversion coating, save energy and have a cooler part at the exit.

Dry-off temperature should be around 250°F if the cycle time and air movement are adequate. However, many dry-off ovens do not have enough time or air movement to dry, and they often run higher. In addition, some parts trap water and may require a higher temperature to boil out that water. Still, even at a higher temperatures, the conversion coating should hold up if the time is not extremely long. In the cure oven, the conversion coating is under the powder film and not as sensitive to temperature. You should be good with normal cure temperatures up to 400ºF (200°C).