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Help! Black Spots on Hard Coat Anodizing have me baffled!

We are experiencing a problem with black spots on Hard Coat Anodized aluminum. The black spots appear only on one certain area of all of the rejected parts, which confuses me. About 50% of the parts we processed exhibited the black spot condition. I inspected parts yesterday prior to shipment, and upon receipt by my customer the black spots had appeared!? I am certain the black spots are not rack marks. I have reviewed our chemistry parameters and everything checks OK.

Chris McCain
Job Shop Plating & Metal Finishing - Gadsden, Alabama
2003

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2003

Gadsden has high calcium water therefore your anodizing and dye should be made up with DI water. Normally hard coat is not sealed but if you are absorbing black dye then you must seal. The seal must be DI water. If you add Gadsden tap water for evaporation loss for a few days you have calcium left behind when the seal evaporates off, and if you black dyed, then the calcium compound has absorbed the black dye too.

Is the alloy wrought or cast. If cast then you are looking at bleed out from the pores. Try ,hot-cold-hot-cold etc. If wrought; then what is the alloy and what voltage are you using? Come back with more specific data.

Dissolved aluminum in the anodizing tank should be under 12 gm/L for consistent color of dyed or undyed hard coat.

Go to the paper entitled "White Spots" (sometimes white spots are dyed black) shown elsewhere on finishing.com.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina