Over-etched anodized parts and a ton of other problems

A. Since we don't know what your anodizer is doing , anything is really just a guess. But the combination of complaints sounds like what I've seen if the line tech leaves parts in the nitric acid pre-dye rinse too long; burnt appearance, dull/off-shade light colors, greying of dark colors.
As for new colors...
Look at the reliant 'outdoor colors' web page for chips because color, too, is so subjective. Pink tej is a great looking bright pink that I'm experienced with. Not sure if thats what you were thinking of. But you need to get the scorching/greying under control first or it's sadly irrelevant.

A. Hi Graeme- Just noticed the problem description includes burning on the CORNERS. Missed that when I was scrolling through on my tiny-screened phone. One more thing that you should look into is whether they are racking multiple different sized parts on the same load, and not being able to maintain consistent current across the whole load. It would be interesting to see where the scorching is occurring on the load, but if you aren't seeing them come out of the bath yourself, and not until they've been processed and unracked- it's even more of a challenge to troubleshoot!
Sorry about that! Both my previous suggestion and this problem are potential sources of dye uptake problems!

A. If the material is different in a single load, "current robbing" is a strong possibility. So your anodizer may be correct as to the cause. That being said, it does not absolve your anodizer from responsibility if he's aware of the differing alloys. For best results, only the same alloys should be ran in a load.

A. Do you know the thickness of the anodize? This also plays a role in the final color, you cannot get a black color with a low thickness anodize, and you can get different tones with different thickness for the rest of the colors.

A. Excellent points, Yohands! You are certainly correct that differing thicknesses will give differing orange tones, and that substantial anodizing thickness is required for good black anodizing (probably 0.0005" or more). But my own limited personal experience is that the color is a rather obvious purple rather than gray when the anodizing is too thin. Are there some dyes which yield gray in place of black if the anodizing is too thin?

Regards,

A. I'm with Mr. Green on the different alloys, for instance, 7xxx series, if anodized together with 6xxx will develop a film that is thicker than the 6xxx parts in the same batch, that is on top of the film on the 7xxx being more porous and absorbing more dye thus having a much richer tone than the 6xxx parts, and at the same time 6xxx will pass less current resulting in thinner film that expected, resulting in a fainter color shade.

Depending on the cleaning/etching process used for that batch, it might have been possible to catch the "wrong" alloy parts and pick them off to run separately if the anodizer was aware of the possibility, 7xxx and 2xxx will develop dark/black film if etched in hot caustic, and 6xxx will not, or just a slight hint of gray (depending on etching time), here the operator could spot the "wrong" alloys and pick the parts off to run them separately.

But it is my strong opinion, as anodizer, that it is 100% customer responsibility to supply this information with the parts, and if the customer doesn't know, then say so, because when you process hundreds or thousands of parts a day, there simply is no time to deal with these issues, once the parts are racked, they stay there till the lot is complete, deracking, reracking, etc., running on a separate rectifier, or even in a separate lot takes extra time, and the customer 99% of time will not want to pay the extra for that effort.

There are several different black dyes, the one I'm currently using doesn't produce the purplish hue, it gradually goes from light gray to jet black, and minimum thickness that a full black tone can be achieved with regular Type II anodizing with this dye is around 12 micron, below that the part looks sort of like "black chrome", it is black but somewhat translucent, more so when the surface is wet. Usually cheaper black dyes will require thicker film coating, heating, longer immersion etc.

A. Hi Graeme. Limited budget or not, if I were you I'd retain an anodizing consultant or put off the project until you have more experience under your belt. An experimental lab can "make do" in ways that a production facility, even a very low production facility can't.

1. I personally can't imagine a welding machine as the power supply for hard anodizing -- have you ever seen this?
2. Yes, lead pipe can be directly put in the tank for cooling, but depending on how low a temperature you can hold in your cooling loop, it might be an awful lot of lead pipe. Every Watt/BTU you put into the tank with your power supply must be removed by cooling.
3. Although far from ideal, yes, it is possible to do Type II and Type III in the same tank with the same solution.
4. Sorry, "geyser" doesn't mean anything to me. But yes, electric heaters can be used.
5. Limited space, on top of unrealistic budget and limited experience, may be the straw that breaks your back. Are you planning on a de-smut/de-ox tank?
6. It's not impossible to seal everything in one tank, but it sounds like you may be building a house of cards. Each shortcut is a problem; stacking up shortcuts may prevent success.
7. I'm not sure exactly what you have in mind for 'brightening'. A bright dip tank contains nitric acid as well as phosphoric acid, and they are one heck of a lot more than 'a little fuming' -- it's horrible stuff; don't use it :-)

Regards,

1. The voltage required to hold 35 ASF (you'd better start at 24 ASF) varies as the coating thickness resistance builds up, you must be able to keep increasing the voltage as the thickness builds. I have done a lot of anodizing with a welding generator, BUT NO HARD COAT!!
2. An external plate/frame exchanger is the "state of the art". The BTU requirement calculation is available in books.
3. I have many clients who do II and II in the same tank.
4. You will not need a heater if you do Type II on Monday and Tuesday, and Type III on Wed thu and Fri, By the next Monday it will be warmed back up, Do II at 72 °F, Do III at 32 °F
5 Etching before removing the organic spoil makes a messy pattern in the metal. NO, you must clean first; cleaning is more important than etching.
6. One seal will work on clear soft and hard, BUT, if you are sealing dye, you need a separate sealer for the clear.
7. From your above general questions I can tell that you are not technically ready to take on the complexity of bright dipping.

adv.

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