White Spots on Anodized Parts Dyed Black

Ask an Expert From: Products Finishing, from Anodizing Technologies

Posted on: 5/1/2005

Question: We are having a problem with very small “white” spots that appear on our parts that we anodize and then dye black.

Question:

We are having a problem with very small “white” spots that appear on our parts that we anodize and then dye black. We call them “sparkles.” The sparkles are sporadic and non-uniform in their distribution over the parts. We have tried a number of things to get rid of the problem, but we have not been successful in eliminating them. Do you think we have a problem with contamination? Our black dye bath is about 10 years old. J.B.

Answer:

This is a fairly common problem for anodizers. Here are some things to look for or try:

1. If the racks are titanium and there is a large area of titanium relative to the area of aluminum parts, this can set up a “gal- vanic” effect that can cause electro- lytic corrosion on the parts. Electrolytic corrosion is most likely the condition you are seeing that is exhibited in the white spots or “sparkles.”
2. Make sure the dye is at the correct pH—probably around 4.5–5.0, but check your supplier’s technical data informa- tion to make sure.
3. Make sure the rack or load bar is isolated from the tank itself if the tank is metal. If it is plastic, then it doesn’t matter.
4. If the dye is contaminated, there is a greater likelihood that this condition will occur. Contaminants can be just plain “dirt” from the plant settling in the dye tank over a period of months, or it could be excessive levels (> 200 ppm) of sul- fates and/or chlorides (> 50 ppm). It could be dragin of city water or well water rinse water over time.

Hanging magnesium anodes in the bath sometimes works for a while, but it’s only a temporary fix until the real prob- lems are found and corrected.

5. If the dye tank is more than a year old, chances are that it is contaminated. Make sure when making up a new tank to use only high quality DI water (five micro siemens specific conductance or 200,000 ohms resistance). It helps to have a DI rinse before the dye tanks to lessen the possibility of dragin from a less than clean rinse.

 


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