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topic 957

Impurities in cast Aluminium



I have a few cast aluminium boxes that has gone through a humidity test (no salt spray).

After this test, the aluminium shows some signs of corrosion. I have found from EDS analysis in an SEM that the corrosion products contains sulphur (S) and potassium (K). I will do some powder x-ray later.

The alloy is a casting alloy with 8% Si, 3% Cu and <1% Fe. I also had some machined dummies in the test made from pure Al. These does not show any signs of corrosion. I suspect that the boxes have been phosphatized (iron phosphating?) but I'm not sure.

Does anyone have any idea regarding the origin of Potassium and Sulphur impurities in the phosphating bath? Some type of residues from a blackening treatment or some release agent from the casting operation?

Any help or suggestion is highly appreciated.

Hannes M [last name deleted for privacy by Editor]
- Sweden

January 3, 2009

Hi, Hannes. I don't quite understand your situation. Some boxes you are looking at have corroded after testing, but you don't know what, if any, anti-corrosion treatment they received, and you are instead studying their sulphur and potassium content?

Aluminum requires a corrosion resisting pretreatment -- you cannot expect freedom from corrosion without treatment. A chromate conversion coating would be best. Do the pretreatment, then test the parts, then investigate if they still fail. Good luck, and please clarify if you get a chance.


Ted Mooney, Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

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