Metal finishing Q&As since 1989

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Test to determine corrosion rate of powder coated steel

I need some assistance on what tests may be valid in determining a corrosion rate of powder coated mild steel.

The application lies in a refrigeration system. The accumulator on a compressor has been changed from copper to powder coated steel. The accumulator will see cold temperatures from the refrigerant and therefore condensation. The concern is possible warranty impacts if corrosion to failure (rust through) occurs.

Salt spray test seems most common, but would only tell me if there is a problem area or if we need a greater mil thickness of the powdercoat before signs of corrosion. It won't tell me, once corrosion begins, how long the ~12 gauge steel might last (is there a standard reference out there?). Life expectancy is 10 years.

Any direction is appreciated.

Coleman Retzlaff
Manufacturing - Madison, Wisconsin, USA
November 4, 2009

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November 6, 2009

I have not heard of such a test¹ however, if you follow correct procedures in powder coating the item of your concern, then I would write into the warranty that applies to the refrigerator... that the specific component is inspected for any defects during the annual service contract. To ensure the best possible resistance to any coating problems I would ensure that the coating process is adequate to meet your requirements.
1) The metal substrate is clean and rust free prior to applying a good zinc phosphate pre-treatment.
2) Ensure that the following water rinse and drying of the item is satisfactory.
3) As the aesthetic value of the powder film is only second to its resistance to corrosion¹ I would choose to apply a minimum of 4 mils of an Epoxy powder...(has excellent resistance to corrosion).
4) Even following these recommendations the coating will fail if the powder film is not fully cured.
You could write into the annual service contract that a thin coat of bees wax [on eBay or Amazon] is applied to this item if in the opinion of the service engineer, it is/maybe required.

Terry Hickling
Birmingham, United Kingdom