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Cleaning of high silicon aluminum die cast parts for stannate

I want to do stannate high silicon parts. These parts have two type of surface one is diecast heat treated, and other is machined. I am using 50% NaOH solution to remove heat treated oxides. After treating with NaOH the machined surface becomes dull, and some gray mass has appeared on the surface. After this I have treated the parts with 30% HNO3 for dissolving gray mass produced during caustic dip, but I don't see any good change in color. Please do tell me what is wrong with my procedure. I have to do stannate plating after preparing the surface.

Salman Aziz
piston manufacturing - Lahore, Punjab, Pakistan
2006

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The problem is uneven attack by the NaOH. It removes aluminum oxide and aluminum but not silica or silicon. Remove the heat treat scale before machining, either by mass finishing (blasting, tumbling, etc.) or with a fluoride-containing acid descaler.
A suitable acid descaler is concentrated nitric acid + ammonium bifluoride (1 pound/gallon). Add a little concentrated hydrogen peroxide as needed to prevent toxic reddish NOx fumes. The solution is quite hazardous, and ventilation to remove nitric acid fumes is necessary; a fume scrubber may be required by the government.
ASTM B253, 'Standard Guide for Preparation of Aluminum Alloys for Electroplating,' contains additional information.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2006