Industry Insights · Technology · Sep 29, 2026

Anodizing Die-Cast (Al-Si) Alloys — Development Trends and Limits of Alloys for Anodizing

ADC12-type die-cast alloys make it difficult to obtain uniform anodic coatings because of their silicon and copper content. This article summarizes the causes, practical limits and the direction of die-cast alloy development for anodizing shown in Korean patents.

Al-Si-Cu die-cast alloys such as ADC12, widely used in automotive parts, offer good castability and strength but are unfavorable for anodizing. Coatings are thin and dark, and stains form easily. As insulation and corrosion resistance requirements for die-cast housings grow in electrified components, responses on both the process side and the alloy side are under way.

Why It Is Difficult

Silicon: Eutectic Si does not oxidize during anodizing and remains in the coating or falls out. ADC12 contains 9.5–12 % Si and 1.5–3.5 % Cu, and unoxidized Si causes a brownish-gray to dull black appearance and soot-like defects (Finishing & Coating).
Copper and zinc: Cause heat generation from interfacial resistance and discoloration, and make the coating thinner.
Cast structure: Rapidly solidified structure, segregation and porosity make the coating nonuniform and cause dye to bleed (MIL-PRF-8625 paragraph 3.6 attributes dye bleed-out and color nonuniformity in castings to porosity).
Specification limits: MIL-PRF-8625 paragraph 3.4.3 excludes alloys containing more than 5 % Cu or more than 8 % Si from Type III (hard), and permits alloys with more than 8 % Si only with approval. Riken Alumite data likewise do not apply the standard performance table to alloys with 5 % or more Cu or 8 % or more Si.

Achievable Levels in Practice

According to industry sources, anodic coatings on ADC12 are typically 4–8 µm, or 7–10 µm under low-current conditions (Finishing & Coating), while field Q&A (finishing.com) cites practical levels of about 0.001 in (25 µm), and up to about 0.002 in with special additives. The AAC specification guide states that Si up to 12 % is the limit for castings and that 356-series alloys require special techniques. Pretreatment generally follows the sequence alkaline cleaning → fluoride desmutting (Si removal) → nitric acid desmutting (Cu removal) → rinsing, and the quality of smut removal determines appearance.

Development of Die-Cast Alloys for Anodizing

On the alloy side, the approach is to greatly reduce Si and compensate for castability with other elements. Korean published patent application KR20120084640A (published 2012) proposes a low-Si alloy containing 1.0–5.0 % Mn, 0.1–0.4 % Fe, 0.02–0.3 % Si and 0.01–0.06 % Ti, with optional Zr and Mg, citing the difficulty of anodizing conventional ADC12 due to its high Si content. Registered patent KR101709472B1 (registered 2017) claims a die-cast alloy capable of dyed anodizing with Si ≤0.2 %, Cu ≤0.1 %, Mn 1.5–2.5 %, Mg 0.1–0.6 %, Zn 0.5–1.5 %, Ti and Zr 0.05–0.15 % each, and a Zn/(Mg+Zn) ratio of 0.6–0.9. This patent also notes that applying wrought alloys such as 5052 and 6061 directly to die casting degrades anodizing properties due to rapidly solidified structure and segregation.

Limits and Selection Criteria

Low-Si alloys have poorer castability than ADC12, so thin and complex shapes require redesign of molds and casting conditions.
Parts where appearance grade is critical generally call for an alloy change, while parts intended for corrosion resistance or insulation are generally handled with ADC12 and process optimization (desmutting, low current).
Wrought alloy criteria (such as ISO 7599 AA classes) are difficult to apply directly to coating thickness and color assessment, so dedicated criteria for castings should be specified on the drawing.

EF Technology confirms the alloy composition and required performance of die-cast parts, then agrees on the pretreatment combination and target coating thickness with the customer in advance.

References

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