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Hard anodizing of extrusions gives necessary chem resistance, but will cast aluminum?  

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We have a few products that have DI water and/or wet hydrogen running through them that are either 6061 or 6063 aluminum. We hard anodize to MIL-A-8625, Type III, Class 1 to prevent both corrosion of the aluminum parts but more importantly to prevent the aluminum and its alloys from leaching into the DI water or wet hydrogen as contaminants.

We are planning to move to a cast aluminum manifold but I haven't been able to find much on anodizing cast aluminums. I have heard that Aluminum A356 is anodize-able but don't know how well and if there are any other grades that are.

Any assistance would be appreciated.

Avi Feinberg
Manufacturing Engineer - Richmond, BC Canada

Anodized appearance of A356 is gray due to many Si inclusions and is rated 4 on a scale of 1 (best) to 5 (worst). Corrosion resistance is rated 2, very good.

Aluminum casting alloys rated excellent for both anodized appearance and corrosion resistance include the Al-Mg alloys 514.2, A514.2, 518.2, 520.2, 535.2 ['Almag 35'] and the Al-Zn alloy C712.1. From Casting Kaiser Aluminum, 3rd Edn. (1973); some alloy designations have since changed a bit.

>From the Annual Book of ASTM Standards, vol. 02.02 (2001):
ASTM B26/B26M, 'Standard Specification for Aluminum-Alloy Sand Castings,' lists 514.0, 520.0 & 535.0 as excellent for anodizing and corrosion resistance.
ASTM B85, 'Standard Specification for Aluminum-Alloy Die Castings,' lists 518.0 as excellent for anodizing and corrosion resistance.
ASTM B108, 'Standard Specification for Aluminum-Alloy Permanent Mold Castings,' lists 513.0 and 535.0 as excellent for anodizing and corrosion resistance. Alloys 705.0, 707.0, 711.0 and 713.0 are excellent for anodizing and very good for corrosion resistance.

Note: the alloys best for anodizing and corrosion resistance are rather poor in castability.

Ken Vlach - Goleta, California

Thanks for the response. In your 1 (best) to 5(worst) scale what is the criteria? I don't care about appearance at all. The purpose of anodizing is purely to protect the DI water that will be flowing through the manifold from pulling contaminating ions from the cast metal.

Avi Feinberg
- Vancouver, BC, Canada

The Surface Finishing and Treatment of Aluminum and Its Alloys, 6th Edn., page 432, gives a chart with additional anodizing characteristics for BS 1490 cast alloys. Ratings are E (excellent), VG (very good), G (good), M (moderate), U (unsuitable). No cast alloy is rated E in any category.

Alloy LM25 (~356A) is rated G for protective anodizing, M* for dyed appearance, U for bright anodizing. *dark colors only.

Alloy LM5 (~514.0), is rated VG, VG and G.

The only other casting alloys in this table having VG ratings for protective anodizing are 99.5% Al and LM10 (~520.0), both probably less popular with the foundries than LM5 (514.0). Also, LM10 must be fairly homogeneous for good anodizing; a bit difficult with 10% Mg!

Anodize appearance correlates fairly well with corrosion protective. The anodizing on A356 is dark due to many Si inclusions which disrupt the continuity and hence protective value of the anodizing. Use another alloy such as 514.0.

Ken Vlach
- Goleta, California

Ken,
Thank you for your responses. Much appreciated.

Avi Feinberg
- Vancouver, BC, Canada