plating, anodizing, & finishing Q&As since 1989
Want even blackening on aluminum castings
DOES ANYBODY KNOW OF A WAY OF MAKING ALUMINIUM CASTINGS GO BLACKER WHEN ANODISING WE GET AN EVEN DULL BLACK COLOUR. THANKS STUARTstuart banks
I have had some experience in black anodizing cast Al. The main problem is, of course, that cast aluminum usually has a lot of silicon in the alloy which turns the parts grey when they are anodized, and which shows through the black dye. Also, cast Al becomes rough when anodized, and the light-scattering effect causes the "blackness" to look more grey. Did you ever notice how nice and black the parts look coming out of the dye rinse or out of the seal bath- until they dry, and the "greyness" appears? That is the secret- to make the parts look wet. Many anodizers know this, but few are willing to discuss it and their own little way of making the parts look wet. (These are the same ones who make rack marks disappear from black anodized parts with a quick flick of a black Magic Marker.) I no longer work for an anodizer, so I can discuss some of my secrets. One way, which can be less than ethical depending on the application, is to wipe the parts with a rag soaked with penetrating oil or other light oil. The trick is to leave as little oil behind as possible, but not enough to let the parts feel oily.
A better way is to dip the parts in a very thin water-soluble lacquer, which when it dries, leaves a very thin, slightly glossy film which dramatically improves the color. This is more permanent and quicker for high volumes. I suggest you repeat your stainless question in the Passivating section in this sight, where readers of the site more familiar with stainless are more likely to be looking.
- Madison Heights, Michigan
Thanks! That information is most forthright and interesting. Of course, as you said, the use of these techniques is mostly verboten, but useful to know when troubleshooting.
When I worked at Bendix, we used to black anodize pure aluminum castings for gyroscope housings, so we had to get a perfect coating without other dips. Which alloy are you anodizing?
Falls Township, Pennsylvania
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