Metal finishing Q&As since 1989

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Aluminum casting that "won't take hard anodize"

I have an aluminum casting of 355 T7 - 1 piece out of a multi piece order - that I am being told by the anodizer won't take the hard anodize. It was attempted a couple of times. Is someone pulling my leg or are there conditions where aluminum won't oxidize?

Alan Orban
Engineer - St. Louis, Missouri, USA
March 31, 2009

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Somebody that probably does not know how to prepare castings is gently tugging on your leg. With proper surface prep which is very different from wrought metal prep, die and sand castings can be hard coat anodized. The special surface prep is supposed to remove "most" of the silicon from the surface so as to expose more aluminum surface - we anodize the exposed aluminum, we do not anodize the exposed silicon. Many shops will not set up the very noxious procedure to limit the silicon exposure. Others just do not know how.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

April 3, 2009

Is this 1 part salvageable? My anodizer is saying that the treating solution has sealed the surface and won't allow the part to coat properly. This is about a $400 machined part. The coaters final cause for the failure was the treating solution became polluted.

Alan Orban
- St. Louis, Mo., USA
April 9, 2009

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There is no question that at this point things are more difficult but as Bob said proper pretreatment is what is needed. At this point if the dimensions are still in tolerance stripping and re-anodizing can be accomplished. Using a caustic strip will only exacerbate the problem a Phosphoric strip should be used. Bob is really an expert on this subject and contacting him is the first step.

Drew Nosti, CEF
Anodize USA

Ladson, South Carolina

April 25, 2009