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How to remove silica smut?

2007

I have an ongoing problem with cast tooling plate and the silica smut generated after the slightest amount of caustic etch.
this smut is very hard to remove with conventional nitric de-smut.
also I appear to get blackening of the substrate pores when anodising this material.
Operating parameters in the anodising are: 185g/l sulfuric. Aluminium 12g/l. Voltage 20V

Bill Taylor
Plating shop employee - Ireland

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First of two simultaneous responses --

By going into the alkali etch you are decreasing the exposed area of aluminum and increasing the exposed area of silicon - the opposite of what you want - therefore you get a darker more silicon laiden oxide.

Stay out of alkaline etches. Clean in non etch. Dip in 100% Niric Acd ,with 2 pounds per gallon ammonium bifluoride until the part turns "frothy white" all over, about 20 to 30 seconds, rinse two times, keep moving, do not stop to look, anodize.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2007

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Second of two simultaneous responses --

Better do not etch it. if you have to do the etching,you could use concentrated Nitric acid to de-smut.As for anodizing condition, my personal experience is that the voltage should be lower than 17V and temperature no higher that 16 degree C.

John Hu
- Singapore
2007