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Letter 17026
380 Anodize "Smutty" Finish (Florida)
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We anodize a die cast composition 380 part. On occasion the parts
have a "smutty" film on them that can be wiped off. We've been told
that this condition happens when the silicon content of the 380 is
high. Is this accurate? Is there any way to prevent this occurrence
outside of changing the material?
Frank Coates
- Naples, FL
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Apparently every 380 recipe is different from caster to caster,
also the wall thickness causes a different amount of silicon to be on
the surface, thinner areas tend to have more silicon (and other JUNK)
than the thick areas.
Never alkali etch a casting, alkali removes aluminum and not
silicon, you want to do the opposite. On most heavy wall 380's a
simple 50% Nitric Acid with 1 pound per gallon ammonium bifluoride
will remove the smut.
Some anodizers down in Florida swear by the Tri-Acid which adds
sulfuric acid to the above, however going back to September 1950 I
have never seen a casting that was improved by tri-acid versus simple
Nitric/F.
In all cases the electrolytic action of the anodizing should pull
off the smut and render as good finish as can be formed on the given
"380" casting. Try wiping off the smut on one and leaving the smut on
another to see if the smut is interfering with the anodic film
formation, if not then the problem is the casting and not the smut.
We only worry about smut when we are chromating.
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IF U ETCH THE 380 PLEASE FORGET IT THE BEST REMEDY IS MASS
FINISHING WHICH REMOVE SCALE AND APPROACH TO A BRIGHT SURFACE.
TO AVOID DESMUT IN ANODIZING U HAVE TO FIND BEST CURENT DENSITY
(ALWAYS LOWER THAN ORDINARY CD) AND LOW VOLTAGE.
WORKING WITH FLORIDE IS DANGEROUS.
HADI KHOSRAVI
- Tehran, Iran
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