Color anodizing of diecast aluminum

A. Hi Paul. We have more than a dozen threads about this subject. As far as I know, discoloration of silicon alloys can be somewhat reduced by etching with acids which include hydrofluoric acid.

We appended your inquiry to a thread where Jess Wannasin claims that "slurry die casting" allows the use of lower silicon for better appearance (I'm not familiar with it).

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.

Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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,

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

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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.

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.