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Corrosion under e-coat on sand cast copper based alloys

We manufacture small sand cast parts that we grind to a satin finish and e-coat. We are experiencing brown spots under clear e-coat over a copper based alloy. Occasionally, the spots develop immediately after e-coat but usually they don't appear until after our product is shipped. Pre-baking for three hours at 500 degrees F. brings spots out on some parts but does not appear to prevent spotting after e-coat. We tumble our parts after casting in alkaline cutting and burnishing solutions (Oakite FM 183 & 103 - Chemetall). Machining is done without cutting oils except a dab of Acculube for riveting. We are working to improve our casting quality but we have functional castings for all of our purposes except for finishing. We use DI water for all of our e-coat solutions. Ultrasonic cleaning has been recommended. Does anyone have suggestions on where else to look to solve this problem?

Debbie Flood
E-coating of non-ferrous castings - Grapevine, Texas
2004

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2004

Debbie,

Likely porosity entrapped liquids saying hello at the wrong time.

1) Use deionised water rinses, 5 micro siemens conductivity, 3 to 5 minutes, with Ultrasonic cavitation as the last step before e coating. It may help..

2) Alternatively Plate a dull Nickel strike of 2-4 microns (See Woods Nickel ) and follow with 12 to 20 Microns of acid copper plate. The plated layers will blank off, put a lid over the porosity and hold for a long time. These are alternatives to squeeze casting. Or Forging. E coat as usual.

Regards,

Asif Nurie [deceased]
- New Delhi, India
With deep regret we sadly advise that Asif passed away on Jan 24, 2016