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Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2007

Zinc Phosphate

Q: I am zinc phospahting 1010 steel, the problem I am having is the CARC paint system is failing adhesion testing. We have had the adhesion of the zinc tested and found that it holds up to 5000 to 5300 psi but that is on store bought panels and not the actual production parts, which brings up my question if I am zinc phosphating steel that has not been time saved or DA sanded to rough up the surface would this cause the zinc and paint to come off? When talking about paint coming off it fails the cross hatch test as well as if we were to bend the part 90 deg the paint would sheet off. I have started testing the theory of a rough surface curing the issue but I am looking for more insight.

A: You should not need to sand the steel to get good adhesion. When FMC  was still in California we used to shot blast the armor plate because we did not want to immerse the high strength steel into an acidic solution, thus avoiding hydrogen embrittlement.   However, when not using a high strength steel you should be able to simply pass it though your zinc phosphate system and get both good adhesion and corrosion resistance.  Therefore, it sounds as if you might have some oil or contaminant on the steel that is not coming off in the zinc/phos process.  

Clearly, when you test the zinc/phos system on purchased panels you appear to be getting acceptable results. I assume, therefore, that either you are purchasing panels that have already been degreased by your vendor, or the contaminants on the surface can be removed by your degreaser. In conclusion, I suggest that you focus your attention on the steel itself. If you still run into problems, please get back to me.   

(PS. From 1977-1984 I was the paint technology manager at FMC ... long before it became BAE Systems.)  


Ron Joseph

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