Poor Salt Spray Resistance of Epoxy Coating

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Q: We are processing compressors made of mild steel as follows:
Compressor --> Zinc Phosphating (C.Wt- 3~4 gm/sq.mtr) --> black solvent based epoxy liquid paint by dipping --> Stoving @140 deg.Cemt 20min. --> wait for 24 hrs --> expose to salt spray as per ASTM B 117 --> Check adhesion after 2hrs.
Specification is no failure in adhesion after exposing for 300 hrs.

The components are failing in adhesion test even after exposure for 60 hrs.  We request you to inform us the reason and countermeasure to be taken to this problem.

A:  Although your process comprises zinc phosphating followed by a solvent based epoxy coating, this does not guarantee that you are removing all the contaminants from the immersion process.  What type of quality to control do you have on rinsing. In fact, how many rinse stages do you have? Does your zinc phosphate process comply with TT-C-490?

You mention that you stove the epoxy for twenty minutes at 140°C, but is this adequate to fully cure the coating?  Are you performing any quality control tests to confirm this?

If you are conducting the process correctly and if surface preparation does in fact meet TT-C-TT-C-490 Type I you should quite possibly pass the 300 hours of salt spray exposure.

One other possible problem is the method by which you mix the epoxy.  Is this a single component or two component coating?  And if it is the latter, what quality control do you have to ensure that the coating is mixed in the prescribed proportions?

Based solely on the description that you sent me the process should work; therefore, I must assume that there is some flaw in your system that results in the poor salt spray performance.

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