How to remove the surface oxide of aluminum diecastings ?
August 16, 2010
I have a problem in removing the surface oxide skin from the aluminum die castings. The castings are intended to be used as the LED light seat so that the heat dissipating coating that we have developed can be applied on effectively. We have tried the normal alkaline dipping,however, the surface will be left with gray smut and thus shall influence the adhesion of the coating.
No No. do not alkaline etch die castings. Alkali etching removes aluminum and increases the area of silicon - exactly opposite of what you want to do - stay out of the alkali etch.
Clean in a non-etch or silicated cleaner, rinse, dip in 50% nitric acid plus one pound per gallon ammonium bifluoride until the surface turns frothy white all over, about 20 to 30 seconds, remove, do not stop to look, rinse two times, anodize.
It seems that you have a smut residue on the surface, probably from the non-dissolvable alloying elements in your Aluminium alloy. If you have e.g. a Si-rich alloy, you should try to remove this smut in a (1) HNO3/HF acid dip (e.g. 3 vol. parts of 1,42 D HNO3 and 1 vol. part 48% HF) or in (2) 50 g/l. Chromic acid/100 ml/l. D=1.82 sulfuric acid dip. Some people even recommend dipping in (2) first, followed by (1) and again followed by dipping in (2). Another possibility is: seek assistance from the big supply houses and ask if they have a reliable process. The solutions I mention are unpleasant to maintain and to handle both from safety and environmental point of view.
Harry van der Zanden - Budapest, Hungary
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