The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Removing calcium and zinc from rinse water on aluminum anodizing line

Q. Hello,
I am the lab manager at an aluminum anodizing facility, however this is my first time working a position in the anodizing field. I come to you today with a specific question in regards to the general issue of white spots on type II anodize.

My process:
slightly alkaline cleaner (city) > alkaline etch (city) > three-acid de-smut > anodize > dye > seal (rinses w/cflow)
All tanks after the etch rinse are DI water.

Where I think the problem is:
I believe my problem is either from 7xxx series aluminum leaving behind zinc or from calcium in the city water. The specific parts that saw white spots were 6xxx series aluminum, but Mr. Probert's wonderfully helpful "WHERE DO 'WHITE SPOTS' ON ANODIZED ALUMINUM COME FROM?" led me to consider this as my most likely source.
I ran a very small pick over the spots to ensure they did receive an even anodize and seal; No pitting large enough to catch with my pick occurred. Since the white spots appeared to be underneath the seal, I wasn't able to do any sort of chemical testing to determine a culprit salt specifically. We do use a glycol in our anodize tanks, which can form a salt with calcium. We also do have some nitrates present in our de-smut, which can form a salt with the zinc.

Other possible salts that could've created this:
Calcium acetate
Zinc acetate
Zinc sulphate

My proposed partial solution:
I would like to have the deox (de-smut) rinse tanks plumbed for chilling to drop the zinc out before reaching the anodize to eliminate one source of the problem. I do understand that this will hold onto the calcium, but I am unsure at what point it would be best to have a high temperature rinse, so that this can be mitigated.
I'm also unsure if the zinc or the calcium is the larger issue and if I lose the 50/50, I am afraid it will make the problem worse.
We seldom see white spots like the ones on these parts, so I'm unsure if changing our degrease and etch over to DI water is necessary/called for. We are very close to throughput usage on our DI, so I would prefer to try other methods first.

Cayden Centers Inquirer often helps others, so especially deserves our help
Lab Rat - Fort Wayne, Indiana
December 19, 2024