Al casting alloys can be anodized.  There are some limitations, such as total alloying elements.  According to the ASM Handbook Volume 5, Surface Engineering:

The chromic acid process should not be used to anodize aluminum casting alloys containing more than 5% Cu or more than 7.5% total alloying elements, because excessive pitting, commonly referred to as burning, may result. The sulfuric acid process can be used for any of the commercially available alloys, whereas the hard anodizing process is usually limited to alloys containing less than 5% Cu and 7% Si.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

If your doing it for cosmetic reasons, aluminum castings generally don't look very good when anodized (compared to wrought aluminum).

My previous employer did anodizing of permanent mold 319 and 356 alloy and sand casting alloys 355 and 201. All done with the sulfuric acid anodizing process.

From the Aluminum Anodizers Council:
"Casting Alloys. Several aluminum casting alloys also can be anodized. Unfortunately, the characteristics that make good castings are not necessarily the best for anodizing. Alloys with the best casting characteristics are those containing up to 12-percent silicon. High-silicon alloys do not anodize well because silicon is not readily soluble in aluminum. Only the aluminum on the surface of the part anodizes, leaving areas with higher silicon unanodized and the entire part with a black or gray silicon powder.

Aluminum casting alloys containing relatively low amounts of silicon and iron and higher amounts of magnesium, chrome, and zinc tend to anodize well. Aluminum-magnesium alloys such as 514 and 535 anodize well. Aluminum-zinc alloys like 712, 713, and 771 also respond well to anodizing. Some higher silicon casting alloys, such as 356, can be anodized using special techniques and processes. It is best to check with an anodizing expert before specifying casting alloys for projects that call for anodized finishes."
http://www.anodizing.org/html/bulletin_4.html

In addition to the alloying effects noted above, a smooth surface & small grain size give a more uniform anodized appearance, and a non-porous microstructure is essential. The etching pretreatment and the anodizing process itself can exposed hidden porosity, which entraps the sulfuric acid anodizing electrolyte, creating appearance and corrosion problems.

Aluminum handbooks, e.g., Casting Kaiser Aluminum, note the suitability of each alloy for anodizing (appearance, corrosion protection). Also, 319, 356 & tooling plate (MIC-6) are more suitable for hard anodizing than the high-silicon alloys, e.g., the 16-18%Si 390 .