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Letter 18070
Dull Electroless on Alum Castings
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Can someone explain why on aluminum castings, Electroless Nickel
Plating is sometimes shiny and sometimes dull. Do either appearances
indicate a compromise in the properties of the plating
James Polidori
- Rochester, NY
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Whether sand cast or die cast, the color of the final finish is a
function of the silicon and other alloying ingredients in/on the
surface. Silicon on the surface is a function of the cooling rate,
hence the thin areas will have more silicon imbedded in the surface.
For each alloy and each shape some experimentation is needed to
find the best mixture of acids (nitric-fluoride-sulfates) and the
optimum time of immersion to produce the most "aluminum-rich"
surface.
Without regard to thickness, sand castings need 100% Nitric Acid
with 2 pounds per gallon ammonium bifluoride, for about 20 seconds
with slightly increasing times as the solution ages.
Without regard to thickness, die castings need 50% Nitric and ONE
pound per gallon ammonium bifluoride. Come back with more information
and let's talk again.
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