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New to anodizing aluminum...please critique my process

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  • New to anodizing aluminum...please critique my process

    I do cnc work and I have done extensive anodizing with Ti. It is, of course, a lot easier.

    I tried some aluminum the other night and got some mixed results. I'm using sulfuric acid mixed at a 1:1 ratio with distilled water (for now.) I used two large surface area pieces of aluminum with aluminum wire tied to them for cathodes. Aluminum wire to hold the aluminum parts as well, which would be the anode.

    Originally tried to use a battery charger bought specifically for this purpose, but could not get the clamps to put out any current because if the lack of load.

    So moved to my variac and ran it on about 12 volts DC. I anodized for about an hour in a very active bath, and the temp went from about 85 degrees to 130 within this time frame.

    When the parts came out they appeared bead blasted, so I knew something worked. Rinsed with distilled water and stuck the first of the six parts in distilled water mixed with red RIT dye. Results were great. About five minutes of dying and then into a pot of boiling distilled water.

    At this point about 20 minutes has passed and I move to the remaining five parts. They have air dried at this point. Stick them in dye doing the same process as mentioned above, and nothing. The dye would not even touch them.

    So, thinking that maybe there was some strange window I had to be in when getting them out of anodize, I dunked them back in the anodize tank and ran for another 15 minutes. Pulled them straight out and rinsed with distilled water and directly into dye..same thing. No dye will touch them.

    What did I do wrong? Is there in fact a window that I have to get the parts into dye after removing from the anodize bath? It is puzzling because all parts appeared the same after coming out of the bath the first go around, and the first part looks great.


  • #2
    There are so many things wrong with what you were doing I'm surprised you even got results the first time. Your electrolyte strength needs to be around 180 g/L for 12ASF if that's the method you choose to use (I don't know what it is for the LCD format, somewhere around half of that I would imagine. You bath temp should be between 68 - 72 deg MAX! Tank time should be between 36 mins and 60 mins depending on how thick you want your coating. With a bath strength of 180g/L at 12 ASF for one hour you should achieve 1mil thickness. It also sounds like you lost connection on those 5 parts as well. Once in the tank you need to be careful not to disturb them so the connection is not lost. Good agitation is important as well to help avoid dissolution and burning.

    As for your question about window of opportunity regarding dying parts. There is no standard rule but most operators prefer to never let the parts dry between steps.


    • #3

      I was concerned about a lost connection on those other five parts too, but they all turned the same color. I figured they all anodized the same.

      Can you elaborate on your description of how the ratio in the bath should be? I don't understand your explanation.


      • #4
        If you are going to be taking this a bit more serious then you'll need to learn how to titrate your bath so you know how strong it is. You can use a Hydrometer style battery tester (Just pop it out of the cylinder and set it in the electrolyte) to give you a very basic idea which if I remember correctly should read about 1.1 at 72 degrees for 12ASF anodizing. Someone else might need to confirm that though. And I have no idea what it should read for the LCD method.