500-HR SALT-SPRAY REQUIREMENT
We have been experiencing some coating failures lately and we are trying to figure out how to find the root cause and what tests we can use to prevent them. We need to pass a 500-hr salt-spray requirement, and we cannot always meet the specification.
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Q. We have been experiencing some coating failures lately and we are trying to figure out how to find the root cause and what tests we can use to prevent them. We make plow blades used on large vehicles for snow removal. We need to pass a 500-hr salt-spray requirement, and we cannot always meet the specification. We clean and iron phosphate the parts in a five-stage process and apply around 3 mils of TGIC polyester. We seem to have more trouble with heavier gauge parts.
We’re in danger of losing our best customer. Help! K.L.
A. First, a brief word on salt-spray testing. Salt spray is meant to compare samples of different treatment methods and coating materials. It is not a measure of field performance. The substrate matters as much as the way it is treated.
A bad substrate can affect salt-spray results, and there are other variables, too. Failure in salt spray may be related to poor cleaning, poor phosphating, poor rinsing or inadequate cure. You need a total review of the process to find out why you cannot reliably meet the salt-spray requirement with a five-stage iron phosphate washer and a good powder. Go back to basics and make sure you have enough time in each phase of the process and that all operating parameters are in good control.
The first thing I would do is run sample panels though the washer and test them. Use a good-quality ground and polished panel. Test adhesion after cure and also test the cure with a solvent wipe test to make sure the oven is doing its job.
When you run the control panels you should also run panels of your own steel with them and run the same tests. If the test panels pass and your steel fails, it may be the substrate is the problem. That would force you to improve your process by adding blasting or better pretreatment to improve the substrate.
If your test panels fail the solvent wipe test, the problem is inadequate curing. Be sure to compare the sample with a known cured sample for accurate results. You may show some powder removal and still be cured. The powder may not have good chemical resistance.
Somewhere in your process control or design, you are not able to support the salt spray requirement. Find the weakness through testing!
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