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Pinholes in color

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  • Pinholes in color

    I've encountered a problem in my first anodizing trials. The dyed parts are covered with pinholes where the bare aluminum shows through. Has anyome encountered this as well?
    I am preping by degreasing with Simple Green and etching in a sodium hydroxide solution. The electrolyte is 16.5% sulfuric acid, powered by a 6 amp / 12 volt battery charger with a 6061 aluminum cathode. The dyes are Rit clothing dye. All water used for rinsing and solutions is deionized filtered water (maybe not as pure as distilled, but cleaner than tap water).

  • #2

    What alloy are you using? I don't use an etch. Maybe I'm wrong here but what you are using as an etch is the same chemical as a stripper, not sure if that is what you want. I do use a De-ox/de-smut before anodizing. I say lose the Simple Green and clean the part with plain old dish soap and a tooth brush.



    • #3
      If the bare spots are actually pinhole size, and are all over the work, I'd bet that you used no agitation at all when you anodized. Each pinhole was a hydrogen bubble that stuck to the work and blocked anodization. The effects of no or poor agitation can also be pale areas on the work.

      Your anodizing parameters are meaningless without specifiying the most important one; current density.

      A 12V 6A battery charger will supply about 10.8V at 3A to a grounded load. The 12V 6A spec is valid only when its driving (charging) a partially discharged battery.

      RIT dye was designed for cloth, and has a long and ugly history of poor results as an anodization dye. It's resistance to fading is particularly bad.


      • #4
        The alloy on this part is 7075, but I've encountered the same effect with 6061 on my only previous attempt. All my info so far has come from various "home anodizing" articles found on the Web. I understood an etch to be necessary to remove the oxidiization layer which naturally forms on aluminum over time.
        The 130 sq in part was drawing 8 amps according the the charger meter. This drove the tank temp to 84 degrees. Reanodizing the part today, in a tank surrounded by ice and water, the current drawn is 4.75 amps at 60 degrees. I don't have a multimeter to determine the voltage. I'm not seeing any hydrogen bubbles being generated this time but I'm stirring the bath as you sugested. The Rit dye was just to give this anodixing a try cheaply. I just ordered a catalog from Caswell to get the correct products.


        • #5

          You have to control the temp better. First you are way to high at 84 then way to low at 60. I do all my work at 70.



          • #6
            A 130 sq. in. part at 8 Amps calculates to a current density of 8.88 Amps per sq. ft. In order to grow a layer to maximum thickness would require:

            2.5 Ohms x 8.88 Amps = 22.2 Volts. Your SOL with the battery charger.

            If you actually got 8 Amps from your 12V 6A charger (real numbers, closer to 10.8 V at 3 A) the voltage has been pulled real low, maybe as low as 3 or 4 V. All battery chargers are unregulated power supplies.

            I'll bet the dyeing sucked.

            You better get a multimeter, you're working blind. A cheap $15.00 one will do.

            If we assume the current readings are reasonably correct from the battery charger meter (not likely) your second attempt was at a current density of 5.28 Amps per sq. ft. This would have required 13.2 Volts for maximum layer thickness. Your still SOL.
            This is only academic however, if you didn't change anything else, that large of a change in current indicates that your electrical connection to the work was bad, no bubbles means it was so bad little if any anodize was grown. The temperature has nothing to do with it. You probably have no anodization at all, right?

            Where did I get these numbers? Download and read Caswell's new LCD directions, it's all explained there. Are these numbers correct? I can prove that they are. The math still works properly even if you aren't using the LCD process.


            • #7
              First, let me start off by saying I'm no expert. I've had my fair share of troubles with anodizing, but lately I've produced very consistent finishes. I'll give you some advice that will hopefully help.

              1. Lose the RIT. I used RIT dye for about 3 months, and it worked only in the loosest sense of the word. Do yourself a favor and buy one of the cheap bottles of dye from Caswell. They're well worth the $9 plus S/H you pay for them, and after using them once, I guarantee you'll stop using RIT. Here's a link to the anodize dye page:

              2. Control your temperature. I do not have any way to actually control the temperature, so I always start with my bath at 68-70 degrees. My bath is large enough that during the course of anodizing, it does not warm up above 75 degrees. If you do not have enough acid to accomplish this, you can surround your acid container by a much larger container with water at about 68-70 degrees. It'll stabilize your bath temps.

              3. My preparation steps consist of:
              a) get desired surface finish
              b) if surface finish is fine, but part is old, a light rubbing with steel wool or scotch brite will help to clean off many inconsistencies.
              c) wash VERY WELL with dish detergent and hot water. I use a toothbrush to accomplish this step.
              d) While washing, attach electrode, then rinse in very hot water to remove any last bits of grease/oil/etc...
              e) You may want to then rinse with cool water so you're not submersing hot parts into your acid bath, as this can only raise your bath temperature.

              All I can say is that this method works for me and produces consistent results. I wish you the best of luck.

              If you're having troubles, try keeping a journal. Log room temps, bath temps, preparation steps, etc... Eventually you'll find a method that works for you. Those people who say that anodizing is an art simply have not identified and controlled all of the variables necessary to get consistent results.

              Good luck!


              • #8
                Those who say "anodizing is an art" do so because they do not understand the process at all. If you reduce process to the physics, math, and chemistry that govern it completely, you will find it simple, easy, and predictable.

                Avoid using abrasives that contain iron, copper (the worst two) or any other metal other than aluminum. If you bead blast, the blasting media must not have been used on steel or brass previously. Bead blasting with iron impregnated media can imbed steel particles in the aluminum, resulting in surface defects.