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inconsistent results (mainly poor)

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  • inconsistent results (mainly poor)

    I have been trying to anodize 6061 aluminum parts, with highly mixed success.

    My first attempts were done crudely, using a battery charger and some high-wattage resistors to get rough current limiting. Amazingly, I got good results. The left-hand piece in the attached image has about 15 square inches of surface area. I ran it at about 0.8 A for 45 minutes, having roughly mixed up acid and Rit dye, with no measurement of the concentration of the former or the pH of the latter. It looks better in reality than it does in the picture.

    Then I had some failures, so I read through the information on this site, acquired a CC power supply and instrumentation, and tried some controlled experiments. The overall program (wash, rinse, degrease, rinse, 3 minutes of NaOH, rinse, anodize, rinse, dye for 15 minutes, rinse, boil for 30 minutes) has been constant. The positive connection to the workpiece has in each case been a length of threaded 6061 rod. The negative electrode has been a 6061 plate.

    Things are not going well. The middle and right-hand pieces in the picture look worse in reality than they do in the picture. These were treated with the same solutions, and for each I allowed 0.016 square feet for the support rod. I have no means of titration to get the acid concentration right. For what it's worth, its SG is about 1.085. The dye pH was about 5.8. Both pieces bubbled during anodizing.

    The middle piece (about 0.025 square feet) was run at 6 A/SF, which worked out (with allowance for the support rod) to 0.25A. The power supply started out at 6.15V and then steadily dropped. At the end of 2 hours (6A x 120 minutes for 720), the voltage was 4.24.

    The right-hand piece (also about 0.025 SF) was run at about 8 A/SF, or 0.34A. The voltage started at 4.35, rose to 5.00 after 5 minutes or so, and then very slowly fell. After 51 minutes, the voltage was 4.95, and I decided that the peak had definitely passed. The results are subtly better than those of the middle piece (very bad instead of very very bad).

    I want to experiment further, but I need a suggestion as to what experiment to do next.
    left to right: success (#0), trial #1, trial #2

  • #2
    I am still experimenting, and I think my problem may be related to the strength of my anodizing solution. I started with a cleaning product ( that turns out to be > 90% sulfuric acid. Its as-purchased pH is < 1.0 (I didn't wait for my pH meter to settle, fearing that it would be damaged), and its specific gravity is 1.85. Caswell's LCD Anodizing System document suggests (Paqe 15) that the pH of the anodizing bath should be 4, requiring a dilution of at least 1:1000, with the resulting SG of about 1.001. This is almost 2 orders of magnitude more dilute (by SG) than any mixture mentioned in the sticky "Quick question on old ano method." post.

    I tried anodizing yet another piece of 6061 at 6 ASF, using acid diluted to pH ~3.5, run for a little over 2 hours. The voltage to maintain constant current peaked after about 5 minutes. Dye (blue, pH ~6) at 135-140° left only pale, blotchy color. This trial was, in other words, no more successful than my earlier experiments with stronger (probably pH between 1 and 2) acid. What am I missing?
    Last edited by Fenichel; 03-29-2018, 08:57 PM. Reason: typo: 35 written where 135 intended.


    • #3
      Do you have our plating manual ? You would benefit greatly from it.
      Jason Vanderbroek
      315 946 1213 x116


      • #4
        Yes, I have tried to use your manual as a guide. I described my experience in detail because I hoped that someone on this forum would spot (as I have not) a way in which I have failed to follow the manual's advice.


        • #5
          Since I don't run the LCD method I can't comment on your electrolyte strength or amperage requirements but it looks like you are close enough at 1.085 SG. The only true way to tell is to titrate the bath.

          If you can provide larger pictures of your issue we might be able to provide some guidance. Get a Flickr account, upload the pics, copy the BBcode (with the size set to medium) and paste it into your post here. The we can see what's going on.


          • #6
            A full-size copy of the picture I posted earlier is on my Web site at


            • #7
              First part looks like it wasn't degreased properly.
              The second photo that even aluminum? Maybe its cast alloy?


              • #8
                It was sold to me as 6061, from a reputable source (probably Online Metals). Still, that my faiures have been from trying to anodize the wrong substrate is a possibility I hadn't thought of, and it's worth considering. Thank you. I will try once again, with a sample from the same stock that gave me (fairly) good results before. It will be a few days before I have the time to do it.


                • #9
                  The first one looks like a bad rinsing job IMO you can tell by the white ring around the threaded hole were you fixtured it, the acid leeches out when it hits the hot dye. The same happens (but at a lower rate) from the crevice of the dovetail corners only at a lower rate causing the dye to leave "sputtering" sort of marks.

                  As for the second and third I can't even begin to describe what's going on there. I have my doubts it 6061 though and if it is then possibly they succumb to dissolution.
                  Last edited by gardinhackle; 04-07-2018, 07:37 AM. Reason: spelling