Summary for color anodizing diecast aluminum Discussion
Color anodizing of diecast aluminum is difficult because high-silicon die castings often anodize to an uneven dirty gray, and dyeing can make the problem more obvious rather than hiding it. In the thread, Ted Mooney’s warning about hydrofluoric acid is echoed by Paul, who says HF etching can help black dye look better on high-silicon 380 die castings, though it will not make them match lower-silicon 356 castings. Chris Jurey explains that sulfuric-acid anodizes on high-silicon castings tend to turn gray quickly, but says fair cosmetics may be possible with proper pretreatment and darker dyes. He also lists alternatives: use low-silicon alloys such as 518, apply an IVD aluminum coat before anodizing, use plasma/microarc anodizing with integral black color, or chromate pretreat and paint. Jess Wannasin adds that slurry die casting may enable low-silicon alloys to be die cast successfully.
this text gets replaced with bannerText
Q. Hello Ted and Company,
We have had issues with anodizing aluminum castings. From what I understand, a poor anodized finish is something to expect with cast aluminum.
The vendor has suggested that it could be silicon dioxide formation during anodizing. Aluminum’s high silicon content (>6%) and the solution heat treatment these parts undergo make it susceptible to incomplete coloring.
Does anyone know what can be done to eliminate this issue?
Hydrofluoric acid burns
Never use HF anywhere unless trained!
If you do, you MUST have the antidote gel on hand for instant use!
Calcium Gluconate
for HF acid burns
on Amazon or
(affil link)
Paul Buckley
Quality – Stoughton, Massachusetts
March 5, 2025
A. Hi Paul. As far as I know, discoloration of silicon alloys can be somewhat reduced by etching with acids which include hydrofluoric acid.
We have more than a dozen threads about this subject, so you can view many second opinions by searching the site for “anodize casting”, and if you tell us the cast material you are using, a reader may be able to offer more exacting experiences.
Jess Wannasin says on this page that “slurry die casting” allows the use of lower silicon for better appearance but I’m not familiar with it.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
A. Paul
Ted’s response is spot on. Hydroflouric acid helps tremendously when dyeing castings especially in black. 380 die cast material has a high silicone content. The black color will probably never dye as true as 356 casting material but the h/f etch will lessen the visibility of the grey overtone and allow for the black to be more prevalent.
Philip J. Verzal
Retired – Newport, Tennessee
Thanks for that Phil. I appreciate it! Paul
Paul Buckley [returning]
– Stoughton
⇩ Similar, related, Q&As — oldest first ⇩
Q. Does anyone have any experience, comments on decorative color anodizing or die-cast aluminum? Is an intermediate plating of zinc or chromate required (because of the high silicon content of the aluminum)?
Any suggestions, tips, references, samples would be most appreciated.
michael rose
– Boxborough, Massachusetts
1998
A. Michael:
We are metal finishers specializing in aluminum anodizing and you picked a very difficult alloy to anodize, especially in colors. A few suggestions: Tumble the parts in a stainless steel media prior to anodizing. The colors you pick should be dark (Black, Blue, Red). You cannot zinc plate or chromate finish prior to anodizing aluminum.
adv.
If you need further assistance please feel free to E-Mail me.
Regards,
Bob Bramson
B&M Finishers / Prismatic Stainless Steel
Kenilworth, New Jersey
A. See the responses to Letter #
2053. Whether it’s conventional or hard coat anodize, the coating still comes out grey, and it is difficult to have an aesthetically pleasing appearance with any dye with die cast. I wonder if anyone has ever IVD-coated a die cast part, then anodized the IVD aluminum. You can’t remove the silicon, so maybe it can be covered up.
Phil Johnson
– Madison Heights, Michigan
“The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys”
by Wernick, Pinner
& Sheasby
on Amazon
or
or
(affil link)
A. The high level of silicon in most aluminum diecastings makes sulfuric acid based anodizes turn an uneven dirty gray almost immediately. Color dyeing does not cover this up, in fact, it often highlights the problem. Several approaches are possible:
1)With proper pretreatment, fair cosmetics are possible with anodizing and the darker colored dyes.
2)Alloys without silicon, such as 518, produce much better results. However, they are difficult and expensive to diecast.
3)IVD coat(vacuum deposited aluminum)the part prior to anodize. We have done that here and it works very well. However, this is also expensive.
4)There is a new kind of coating becoming available, known as plasma or mircoarc anodizing. They are naturally off-white but some versions are available with an integral black color. Our high silicon diecastings have a very uniform satin black coating.
5)Chromate pretreat and paint the piece.
Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.
Wadsworth, Ohio
A. Another solution is to die cast low-silicon alloys using a slurry die casting process. These low-silicon alloys can then be anodized to give nice colors.
The issue is how to die cast these low-silicon alloys with high quality and low production costs. These alloys tend to have hot tearing and die soldering problems. Slurry die casting may be used to solve these problems.
Jess Wannasin
– Bangkok, Thailand
August 12, 2015