Chromate Conversion of Aluminum Casting Alloys
I am trying to establish if there is any limitation associated with chromate conversion of aluminum casting alloys such as 355 or 356. Thanks, GaryGary Wonacott
composites - San Diego, California
355 - 1.3 copper, 5 Si, 0.5 Mg. 356 - 7 Si,0.3 Mg. These alloying constituents create difficulty in chromating, and may reduce the protection, and affect the appearance of the chromate. Proper cleaning and pretreatment can improve the effect of a final chromate conversion coating. See www.finishing.com/chemicals/ for chemicals, and the Testing/ directory if you have some parts you want to test.
Degease using non-etch alkali cleaner. Warm phosphoric acid based etch cleaners may be used with care, possibly fluoride accelerated. Hexavalent chromate processes are the only ones that will give satisfactory performance bare or painted.
Use high pointage chrome content and maximise coating weight without getting loose coatings. Aim for a rich golden yellow colour. Do not use hot water rinsing and keep dry-off below 100 deg C RogerRoger Bridger
- Croydon UK
The only limitation I know of surface wise is the limitation on corrosion resistance associated with castings, its around 1/4 to 1/6 the corrosion resistance associated with wrought material. Other than that, there's no downfalls per se with chromating casting material, it does provide increased resistance, but to what extent is questionable.Matthew Stiltner
plating company - Toledo, Ohio
Casting aluminium is a problematic becouse of the hig silicone content in the alloy . Special treatment for removing the silicone obstcle with properietry formulation can remove the obstcle , in my career I developed and sold product for that problem.
YB Plating Engineering and Quality - Haifa Israel
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