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Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2007

Zinc Phosphate & Oil Coating

Background: zinc phosphate & oil coating is specified for a small (automotive) steel strip spring, for 25 hrs corrosion resistance.

Q: a) With the coating weight being specified as 1000 - 2000 mg/ft2, should an actual coating weight of 2069 mg/ft2 be considered a manufacturing defect?

b) Can this heighter coating weight cause "phosphate holidays" allowing corrosion in situation were 1000-2000 mg/ft2 would resist corrosion?
Thank you in advance!

A: I am not familiar with coating weights of zinc phosphate as high as 1,000 - 2,000 mg/ft2.  My concerns would not be holidays, but rather that if one were to apply a coating over the phosphate layer, it would split due to inadequate cohesion. However, since you are not applying a coating but applying oil instead, I don't why you would expect a defect, especially at only 69 mg/ft2 over the maximum specified limit. From the question I assume that your customer is rejecting your work?  If so, I would be pleased to examine the zinc phos for holidays using a stereo microscope, or at worst, a scanning electron microscope. Have you subjected the coating to salt spray? After all, 25 hours corrosion resistance is very, short. 

Please get back to me if you need further assistance.


Ron Joseph

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