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New Hard Anodizing setup

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  • New Hard Anodizing setup

    Hi, I have a few questions about hard anodizing.

    1. How many volts do I need to use?
    2. What is the current density?
    3. How well will the dyes come out, compared to room temperature anodizing?
    4. What is the sulphuric acid concentration?
    5. For how long do you leave the part in?
    6. How can you test the thickness of the coating?


  • #2
    Hard anodizing isn't dyed, so I'm not sure what you're really trying to do. Let us know what your application is and we'll be able to help.
    Mike Caswell
    Caswell Inc
    Need Support? Visit our online support section at


    • #3
      I was reading about anodizing standards and they have a standard for type III class 2, which is hard anodizing that is dyed (I'm sure that you knew that already). I understand that there is a big difference between a standard on paper and what happens in the real world. I'm wanting to know if there is a way of dying hard anodizing. I've seen some job shops put it on their web sites that they can do it or maybe they have some strange in-house thing.

      We are trying to anodize bike parts. Room temperature anodizing is not hard enough. We are trying to anodize a brake calliper gold and a number of other parts. We have a desmut for castings and would really like to add some color to them.


      • #4
        Hi Mike

        I really don't want to disagree with you about the dying of hard anodizing. So lets forget about that. Can you help me with the other questions I asked in the first post please?


        • #5
          Hard anodizing is acheived by weakening the acid bath.

          4 parts distilled water to 2 parts battery acid (SG 1.26 - approx 33% acid).

          12-24 volts

          0.1 - 0.15 amps/sq in area

          Anodize for approx 60 mins for a good thickness and maximum wear resistance. The film forms at the rate of 0.10 mil per 2-3 minutes.

          As we said before, hard anodizing isn't generally dyed, so we don't have any experience in how it will look. Try some sample pieces and experiment a little. You might also want to download our instructions on the anodizing page at
          Mike Caswell
          Caswell Inc

          Need Support? Visit our online support section at


          • #6
            For the record; MIL-A-8625F does not list a "Type III Class 2" (dyed hardcoat) at least not in revision F which is the current one. Milspecs read like they were written by a "Philadelphia lawyer", so they're very easy to misunderstand. The same spec specifically does not recommend dyeing or sealing Type III layers, as this reduces it's mechanical abrasion resistance. In practice: because in Type III layers the anodize is dark yellow to bronze colored, very dark dyes, like black or dark green can only be used. The dyeing process is quite difficult.

            The standard recipe for Type III (hardcoat) is 24 A/sq.ft. current density, at 32 deg. F, for an extended anodization time. I don't know what PAR would be for hardcoat yet, certainly higher than 2.5 Ohms per sq.ft.. The acid/water mix is the same for Type III and Type II. Reducing the acid concentration for Type III would serve to make the pores, already small from the high current density even smaller. Dyeing would become nearly impossible. Without this level of cooling (32 degrees), the dissolution will overpower the anodization and you will fail miserably. This requires very powerful agitation and a refrigeration type chiller. You would have little chance using ice.

            Edited to correct 3 typos.


            • #7
              When I started anodising at 24v I was getting 25 amps per sq ft and dying after 30 minutes of anodising left a really nice deep colour but when you looked closely the surface had become pitted and also the anodising had added .015mm to the surface. That was at 62deg. F. That might suggest the pore size created by this current density is still ok to accept dye but the effect of changing temperature obviously remains to be seen.

              Not sure if this really helps but I thought I would throw it into the ring anyway.


              • #8
                All information helps.

                In an earlier post, you indicated that you were using a 24V/12V 30 A battery charger. When driving a grounded load like an anodizing setup, the actual voltage and current would be much less. If I assume you used no current limiting, the actual voltage might have been 12 V or less on the 24 setting. Did you actually measure 25 A per sq. ft.? You need to measure the voltage and current when anodizing, you are working blind if you aren't doing this.

                Just curious, how did you measure a coating thickness of 0.015 mm (0.0006")?


                • #9
                  Yeh I realised I did not have a voltage measurement after I posted the message. The ammeter on the battery supply read 25 amps - this would be accurate I assume? Unfortunately as I have just got the light dimmer in place I probably won't be going back to test the voltage!

                  The anodising thickness I simply measured over the diameter of a 25mm tube - before and after the process. I could also compare the results to previous layer thicknesses carried out at lower voltages/current densities (which were negligible)


                  • #10
                    I recently acquired a used Elcometer 345 digital coating thickness gauge, and calibration "shims" for 0.5 mils and 1.0 mils. These things aren't cheap. $425.00 on ebay.

                    The problem with measuring an increase in diameter after anodizing, and than dividing the increase by 2 for the thickness, is that for Type II, 1/3 of the anodize is below the surface, and the other 2/3 rds. is above the surface. For Type III, it's about 1/2 above and 1/2 below.