Anodized surface is powdery

A. Hi Nehemiah. People will try to help you but will need feedback and data. Please review the suggestions already offered and give us some comments; for example, what is the temperature of your anodizing bath and how carefully is it controlled please?
Also, a photo to mooney@finishing.com for posting here could probably help.

Luck & Regards,

A. There are about a dozen possibilities, one of which is that your pore is too large because of too high free acid, too high temp, or too low current, then you do not have a nitric acid post dip to clear the pore, so when you hit the hot seal the white aluminum sulphate pops out of the large pores.

A. The most common "greenish-yellow powder" is Nickel Hydroxide when the nickel acetate seal pH gets above 6.0. Go back and do the x-ray again and look for nickel.

A. We have had powdery anodize finish problems in the past. They have usually been the result of the anodize solution being too warm. I assume this results in more chemical attack on the anodize surface from the sulfuric acid. The powder would sometimes wipe off and dyed parts would fail a nitric acid seal test. The green color is most likely a dichromate seal.

A. It sounds to me that you have got a bloom on the surface. Check your operating parameters, it sounds as if something is out of kilter.

The first thing that is needed to knowbefore anyone can make a real stab at what the problem is will be what type of anodizing was done to the part? Next is what application are you using it for. From there some reasonable answers will come.

A. It seems your anodic film produced is softer than required.

You may check :

1- the temperature of anodising bath which should less than 21 °C. Higher temperature softer anodic film and dusty appearance.

2- do not hold the load in anodising tank after the power is switched off. Otherwise the anodising solution make anodic film soften. Anodised load must be taken from the anodising tank as soon as possible.

3- rinsing between anodizing and sealing. At least 2 rinses necessary to eliminate low pH solution from anodic film pores.

4- use a good quality seal additive to prevent dust that is formed as a result of chemical reaction of sealing. Also, take notice to the amount added to the tank, more additive softer film.

A. Hi May. I hope a knowledgable anodizer offers some insight here, because I really don't think a 1% difference in electrolyte concentration is the cause of your problem.

Regards,

A. One thing to monitor in your dichromate seal bath is that the pH does not stray out of your set range. We find that the best results are attained at a pH of 6.6. Our shop's allowable range is 6.5-6.9 and we have never failed a salt spray test on 2024 for this process. Others prefer to run at lower pH values. I'm speaking from my own experience. We run sodium dichromate (some folks use potassium dichromate so clarifying for informational purposes).

Some notes on pH maintenance in an aging dichromate seal bath is that you may start to see pH falling faster as the bath ages and builds up metals. When attempts are made to adjust an older bath back up to your desired range using sodium hydroxide, a precipitate will form and maintaining pH will be increasingly difficult. You can take a sample in a large beaker [beakers on eBay or Amazon [affil links] and check this in the lab.

Without knowing what you are running for a target pH level, or how much use the bath has seen over its life, I can't speak as to whether that is an issue in your case, and if you've got good plus undesirable finish coming out in loads run within a short time frame …that makes it seem even less likely as a culprit. But every little bit of tank optimization helps :)

Another place to look would be at whether there is variation in immersion times in the seal bath from load to load. Longer is not necessarily better, and an extended immersion can produce a poor quality finish.

Last, the water used for rinsing after seal must be kept quite clean. It should be deionized to begin with, and try to limit buildup of contaminants from dragout to below 20 ppm as total dissolved solids, and change the resin if you don't see that level drop back down within a reasonable amount of time after the rack of parts has physically been removed from the tank.

 Hi Rachel,

Thanks a lot for the guidance! Will try those method and let you know whether it works.

Regards,
May

A. Hi cousin Val. What if anything are you doing about pretreatment and about sealing?

Nothing against hobbyist and small-scale anodizing, it's just that when a professional is doing anodizing in an anodizing shop and reports a problem, we usually start with the assumption that they are at least doing all the routine steps involved in the anodizing process from cleaning to etching to de-smutting to anodizing and sealing. When a hobbyist or small-scale anodizer like yourself says they are having a problem, we don't know what they're doing and not doing, and what they are aware of and not aware of.

It's possible that your parts are overheating, that your acid concentration is a bit high, etc., but if you're not doing the other necessary anodizing steps we should start there :-)

Regards,