White Spots on Anodized Parts Dyed Black
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:
Featured Content
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.
RELATED CONTENT
-
Cleaning, Pretreatment to Meet Medical Specs ISO 13485 or FDA 21 CFR820
Maximilian Kessler from SurTec explains new practices for industrial parts cleaning, metal pretreatment and decorative electroplating in the medical device industry.
-
Anodizing for Bonding Applications in Aerospace
Anodizing for pre-prep bonding bridges the gap between metallic and composite worlds, as it provides a superior surface in many applications on aluminum components for bonding to these composites.
-
What is the Correct Anodizing Specification?
My company fabricates aluminum and steel pedestrian bridge railing among other bridge parts. We recently got an aluminum railing job that called for “Type I” anodizing per MIL-A-8625. There was no anodic coating thickness called out. We are not anodizers and we are at a loss as to how to write up a meaningful anodizing specification for this railing.