More on Anodizing Die-Cast Aluminum Alloys

Q: We are anodizing (Type II) components that are made of alloy ADC12, which is equivalent to EN46100. Briefly, the process is shot blast, rinse, rinse, DI rinse, anodize 10–11 Vdc for 20 min at 200 g/L sulfuric acid and temperature of 14–18°C (57–64°F), rinse, rinse, DI rinse, DI rinse, room -temperature nickel fluoride seal for 15 min, rinse, air dry.

The anodized parts are a dark gray color. In addition, we perform a salt spray test and the parts show corrosion pitting over more than 10% of the test panel after only 72 hr. Do you know if this is normal? F.R.

A: The alloy you are using is high in silicon— 10.5–13.5% Si. Most die-cast alloys are high in silicon with the exception of the Aluminum Association designated 200 series alloys. Silicon is not soluble in the aluminum matrix and does not anodize. So with high-silicon alloys (usually above 7 or 8% silicon) the anodizing can result in a brownish gray or black, sometimes “sooty” appearance. This depends on both the chemistry of the alloy and the quality of the die casting. In some cases the anodic coating may not be dense or continuous and thus may cause failed salt spray tests.

Added silicon enhances the casting characteristics of the alloy. Unfortunately, it is a detriment to anodizing. One solution is to use an alloy that is as low as possible in silicon—certainly less than 7%, and lower if possible. Another method that might be used is called “modification” of the alloy. I’m not a metallurgist, so I’ll quote from a reference on casting alloys.

“Aluminum-silicon alloys have a special problem. Since a negligible amount of silicon is soluble in aluminum, the silicon forms a mixture with aluminum called the aluminum-silicon eutectic. In this mixture the silicon crystals are coarse, hard, plate-like particles and can grow relatively large. Also, large particles promote uneven distribution of the eutectic phase. The result of all of this can be unsound metal and a blotchy, sooty appearance after anodizing. The crystal size and distribution of the silicon can be controlled by the addition of such elements as sodium and strontium. The result of such additions is to convert the eutectic mixture containing coarse silicon flakes into a finely dispersed mixture. This treatment is called ‘modification.’ The finely divided and dispersed silicon promotes better ‘feeding’ of the alloy and a sounder metal structure is obtained. The appearance and quality of subsequent surface finishes are greatly enhanced by ‘modifying’ aluminum-silicon alloys. This treatment is accomplished during the melting operation.”

So, the quality of the anodizing will be determined primarily by the amount of silicon in the alloy, what (if any) “modification” has been done and the quality of the casting. I don’t find anything wrong with your anodizing process. Low voltage is good, but you may get slightly better results if you use a higher anodizing bath temperature. Try using 22–24°C (72–75°F). You are probably rinsing excessively. That certainly doesn’t hurt, but some of the rinsing steps could perhaps be eliminated. Also, you stated your DI water rinses vary in quality from 300–500 ppm total dissolved solids (TDS). There is nothing wrong with this, but I would not call water with this level of TDS DI water. The minimum-quality DI water that should be used in the anodizing process has 200,000 ? resistance (5 µ-Siemens specific conductivity). This is equivalent to approximately 2 ppm TDS. Water with 500 ppm TDS is equivalent to 1,000 ? resistivity.