Summary for white powder on anodized finish Discussion
White powder on anodized finish is described as a random, brush-off residue appearing on top of otherwise perfect type II anodizing on straight knurled knobs. Chris says the parts were CNC machined and returned from anodizing with white powder in a few spots, but the coating underneath looked fine and the material cleaned off with a toothbrush. Scott Dordickre replies that knurling is formed by rolling dies on a spinning part, so the surface finish is difficult to control. He notes the shop has made the same part the same way for 10 years, and this is the first time the issue has appeared; the same dies were later used on another batch without trouble. The thread does not identify the exact cause, but it suggests the problem may be tied to the knurled surface or a one-off process variation rather than a coating failure.
August 12, 2021
Q. I am a machinist and we CNC machine parts. We received a batch of straight knurled knobs back from type 2 anodizing. In a few random places there is a white powder substance on top of the anodizing. The anodizing is perfect under the powder. The powder cleans off with a toothbrush.
We run these same knobs with the same process all of the time without issue.
My anodizer says it is our fault because a burr on the knurl entrapped the seal coat. I contend that if a burr was the problem then there would have been other imperfections in the anodizing due to entrapment of other chemicals in the processes. It is simply a matter of the seal coat not being rinsed off properly.
What do you think?
Scott Dordick
– Pomona, California
A. `I am no machinist, but I don’t think knurling is a very clean process. It results in folded edges and smeared material. These little void areas will wick in process solutions which are nearly impossible to rinse off. I would bet that you are seeing sulfuric acid electrolyte solution which is drying out and blooming sodium sulphate crystals
Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.
Wadsworth, Ohio
September 2, 2021
September 7, 2021
Hi Chris,
Yes, knurling is formed by rolling dies on a spinning part so it is difficult to control the surface finish. We have been making this same part with the same process for 10 years and this is the first time we have had a problem. We have even used the exact same dies on another batch since the above parts and did not have a problem.
Scott Dordickre
– Pomona, California