Letter 18070

Dull Electroless on Alum Castings [New York] 

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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

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.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

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