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topic 5543

Excessive Growth of Phosphate Crystals in E-Coated Galvanneal Steel




(2000)

Recently we had samples of our e-coated galvannealed parts analysed for paint defects. The report from lab states that defects were caused by excessive phosphate crystal growth. Enery Dispersive Spectroscopy (EDS) analysis of the crystals showed significant peaks of zinc, phosphorous and oxygen leading them to believe that the crystals are phosphated base.

During the pre-treatment of e-coating process, the parts are phosphated for a certain period of time. Would anybody know of any causes of "excessive phosphate crystal growth"? Any help is greatly appreciated.

Henry Nazeri
- Concord, Ontarion, Canada


(2000)

I assume that the large Xstals are causing a surface roughness in the EP coat. Is the phos process dip or spray? The use of a titanium compound based refiner prior to the phosphate process will enable the formation of finely crystalline coatings. If the system has worked OK before then the problem must lie with a change in chemical balance of the phos process, problems with the cleaner or ageing of the refiner prerinse (if used). Call in your pretreat co. support and get them to check the process over. R

Roger Bridger
- Croydon UK


(2000)

I hope that the phosphate you are using is a microcrystaline type and not a macrocrystaline one. The addition of a grain refiner is a good recommendation but not instead of using the micro, on top of it. The phosphate that can be used as a base for e-coat is iron ( you are not using iron as they cannot give growth crystals) or zinc phosphate. Manganese cannot be used.

sara michaeli
sara michaeli signature 
Sara Michaeli
chemical process supplier
Tel-Aviv, Israel



(2000)

If your process worked before and not now I'd look at cleaning first. Lack of cleaning, poor rinsing, or the rinse ahead of the phosphate being either too alkaline or acidic can all cause this problem. If your process never worked then you should check your chemistry and make sure it has a chance.

Jeff Watson
Jeff Watson
- Pearland, Texas



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