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Anodize Aluminum passes conductivity test but will not take black dye

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  • Anodize Aluminum passes conductivity test but will not take black dye

    Hello, I am new to anodizing and have a question. I anodized a small 6061 aluminum part. The part passes the conductivity test but when I dyed it (Caswell Black) it only turned grey. I left the part in the dye for about an hour. I am thinking that I didn't have hot enough. I tried re dye the part making sure that the dye temp is 150 degrees. Nothing changed. My question is, did I "hardcoat" anodize the part and that is why it wouldn't take the dye. Or did I seal the pores with the "cold" dye bath?

  • #2
    Can you walk me through the process you used, including amperages and surface areas ?
    --
    Jason Vanderbroek
    315 946 1213 x116
    www.caswellplating.com

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    • #3
      I used the 720 calculation to get my amperage and time. My part is about 10 sq inches. I used a CC power supply set at .4 amps for 70 minutes. Before I started I dipped my part in Easyoff for about 15 minutes. I then washed off the Easyoff with distilled water and let it dry. After the anodizing process, I am washed the part in warm (70 degree) water. After drying, I placed it in the dye. I did not have a temp gauge but the solution was steaming slightly. I left the part in the dye for 45 minutes. The part came out this grey color. I thought that I didn't anodize it correctly so I tested the part with my multimeter. There was zero conductivity. I thought maybe my dye was not hot enough so I heated it up to 150 degrees and tried it again. The part did not get any darker.

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      • #4
        Don’t let your parts dry out between steps. Go straight from the anodizing tank into the rinse, then directly into the dye.
        Last edited by KevinB; 02-09-2021, 02:11 PM.
        Process control doesn't give you good quality, it gives you consistent quality.
        Good quality comes from consistently doing the right things.

        Process control systems for Anodizers
        If a post helps you out LIKE the post
        I'm not an Amateur Metal Finisher. I've just been around the industry for a dozen years or so helping and consulting when and where I can.

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        • #5
          KevinB, do you think I need to strip off the anodizing and start over?

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          • #6
            Yes. As a general rule you should always etch the parts before running them especially if you are running parts over.
            Last edited by KevinB; 02-09-2021, 02:11 PM.
            Process control doesn't give you good quality, it gives you consistent quality.
            Good quality comes from consistently doing the right things.

            Process control systems for Anodizers
            If a post helps you out LIKE the post
            I'm not an Amateur Metal Finisher. I've just been around the industry for a dozen years or so helping and consulting when and where I can.

            Comment


            • #7
              lots of thing to consider here.

              dye concentration and pH at optimal concentration? usually 10 g/L for black, 5.5 - 6.0 pH. you'll need a calibrated digital pH probe to measure this. use a temp probe to ensure that you are getting the dye to 140 - 150F. dye time with optimal concentration, pH and temp should be 10 minutes for any reasonable quality black dye.

              oxide thickness at necessary range for black? - should be at least 0.7 mils, more preferably 0.8 to 1.0 mils thickness unless the dye is designed for thinner coatings less than 0.7 mils. Most black dyes need a relatively thick oxide to reach full saturation and depth of color otherwise you will end up with a grey color.

              Looking at your amperage and run time, we can estimate oxide thickness. It looks like you were running about 6 ASF. using 720 rule: 720/6=120 minutes to achieve 1 full mil oxide thickness. You ran 70 minutes which puts you at a theoretical 0.58 mils. It's possible you achieved that but also possible that you came up a bit shy due to the dissolution effect. If your black dye is not designed for thinner coatings, you will end up getting the grey color.

              If it was me, I'd bump the amperage up a bit to 8 or 10 ASF. Make sure your ano tank is 65 - 70F. Anodize for time to achieve at least 0.8 mils. Get dye tank dialed in and dye for 10 - 15 minutes. Seal in boiling water so that you do not have the dye leach out during sealing.

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