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General Trends in Anodizing

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  • General Trends in Anodizing

    I'd like to make sure I understand the general trends in sulfuric acid anodizing so I can do some testing of different operating conditions. Please correct me if any of these are wrong, and feel free to add any that may be missing.

    1. Smaller pore size = harder surface.
    2. Colder bath temperature = smaller pore size.
    3. Lower bath acid concentration = ? (I'm thinking smaller pore size).
    4. Higher current density = lower pore size.

    What's the general trend for anodizing layer thickness, assuming you anodize to PAR?

  • #2
    1. Smaller pore size = denser coating, not harder. Density is not interchangeable with hardness, they are different things. I am trying to get a "hardness" figure, comparing the "hardness" of Type III "hardcoat" to ordinary Type II, in the Brinell or some other hardness scale. No one seems to know what the difference in "hardness" is, or even if there is a difference.

    2. Electrolyte temperature has little to do with pore size, temperature affects dissolution rate. Colder = slower dissolution.

    3. Lower acid concentration = smaller pore size.

    4. Higher current density = smaller pore size.

    Some recent coating thickness measurements shows LCD producing a layer of 0.5 to 0.7 mils thick. The value of PAR (2.5 Ohms/sq.ft.) holds up well at 6 A/sq.ft. it may need some adjusting for the lower current densities.

    The pore size is so favorable for dyeing in LCD, that coatings as thin as 0.3 mils provide excellent dyeing. LCD also has the advantage of producing a colorless transparent anodic layer, where standard anodizing produces a yellowish anodic layer.


    • #3
      Fiber, I have noticed that the LCD plating is virtually impossible to distinguish from bare aluminum. The first time I took a part out, I thought it had failed completely until I saw it soaking up dye.

      As far as hardness goes, here's my thoughts, although they may be completely wrong. Isnt' the anodized layer Aluminum oxide, or Al2O3? My assumption is that the Al2O3's hardness is roughly constant, and you are actually only changing the thickness and density of the Al2O3 layer when you change your anodizing parameters. How far off does this sound?

      Edit: spelling and grammar.


      • #4
        That's my understanding of anodic layer "hardness" as well.

        There is a lower limit in current density however; at some current density below 2 A/sq.ft., the pores become so large that a reasonable pore structure won't form, and the anodize falls off or fails to form at all. At 2 A/sq.ft. the pores are too large for dyeing and/or sealing, and the part won't retain the dye. This is why the lower limit for LCD is specified as 3 A/sq.ft.

        The colorless characteristic of LCD will permit dyeing in shades previously thought impossible. The commercial dyes will need to be adjusted, because they have been blended to account for the yellowish standard anodizing tint.


        • #5
          real dumb question

          I'm just getting started and have not got set up yet. When I ordered my kit it was simply labled "Aluminum Anodizing Kit" What is LCD that you refer to? and is it different from what I ordered. I ordered the kit from CAswell Canada . . . thanks for your time


          • #6
            LCD is a process of anodizing, using Low Current Density (LCD) rather than high current density which was the norm. As far as I know, Caswell only sells a kit for LCD and it is the only process he supports, so the Aluminum Anodizing Kit and the LCD Kit are the same, just different terms.