Blue Anodize for Die-Castings

Question:

We have a customer who wants to anodize a 383 alloy die-cast part and color it blue. The shade of blue is not important as long as it is uniform in appearance. Is there such a process? How will the uniformity of color be? How would the part best be processed? J.M.

Answer:

None of my references of chemical composition specifically calls out 383. We did establish that 383 alloy nominal chemical composition limits fall somewhere between 380 and 384, both of these alloys having 3.5-3.8% copper and 9-12% silicon as the major constituents. I have no hands-on experience with anodizing 383 alloy. Alloy 380 can be anodized. However, the quality of the anodic finish depends somewhat on the quality of the casting. Homogeneity of the metallurgy is important; this is difficult, as silicon is not very soluble in aluminum when in solution. The silicon wants to leach out of the casting and “float” to the surface during anodizing. This results in an anodic finish with concentrations of silicon, which can “stain” the coating grayish or brownish in those areas. This indicates segregation in the casting in roughly the same pattern as the concentrations of silicon in the coating.

This is a situation where nothing speaks like actually anodizing some pieces to see what results can be obtained. Try anodizing in a 15% by weight, sulfuric acid bath at 75F, 13-14 volts for 10-20 minutes and evaluate the results. Dye the anodized parts and observe the overall appearance to see if it is acceptable. Note that you could end up with a “sooty” black part with no measurable coating, or it might be dark gray at best and incapable of taking an acceptable blue color. Perhaps some of our readers who have had experience with anodizing this die-cast alloy would be willing to share their results with us. In the meantime, feel free to call or email me with questions, or simply to tell me how you (your anodizer) made out.