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E-Coating of Magnesium Castings

I currently have a customer that has requested E-Coating of the Magnesium Castings we supply. We have attempted to have the components plated using our customers Plater and have not had good success. The problem we are experiencing is that the E-Coat bubbles during their testing (90% humidity testing - no salt spray). They currently clean the castings with a sulfuric acid rinse and then zinc phosphate prior to e-coat. Is this method of plating incorrect? Can you suggest an alternative method to have the castings E- coated? Any help would be appreciated.

Perry Longhurst
FisherCast Global - Zinc & Mag castings - Peterborough, Ontario, Canada
2004

The e-coat is likely bubbling due to the porosity that is present in the casting. The sulfuric acid etching process will remove ~ 50 micrometers from the surface, which may result in exposed porosity. However, you should investigate the exact process used by the e-coat supplier. Is the sulfuric acid rinse in-line with the rest of the coating process, or is it completely separate? The Mg surface needs to be etched prior to zinc phosphate, which can be done using either an alkaline etch (pyrophosphate alkaline cleaner) or acid pickling. I am not sure if all of the phosphate conditioners used for processing steel are appropriate for Mg. The pre-treatment supplier should be consulted on this.

Toby Padfield
Automotive module supplier - Michigan
2004

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