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Color depth vs. anodizing time

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  • Color depth vs. anodizing time

    Color depth vs. anodizing time:

    I've been playing around more and more with constant current anodizing at 12 A/ft^2 in a 1:1 (battery acid:water) electrolyte tank. My test pieces are 0.25" 6061-T6 rod, 6" long. Surface area is 0.0334 ft^2. All test pieces are bead blasted, then cleaned in dish soap, then Simple Green before anodizing.

    I am testing color depth vs. anodizing time. Having no equipment to accurately measure the anodized layer thickness, I theorized that relative layer thickness could be judged by color depth. That is, the part with the thicker anodized layer, (assuming all else is equal) will have a richer, deeper color.

    Correct me if I'm wrong, but as I understand the LCD anodizing paper, layer thickness can be judged by the anodizing voltage or resistance, thus the reasoning behind monitoring and detecting PAR. Once the peak voltage has been reached, you have reached the peak layer thickness.

    So... in theory there should be little to no difference in color between parts if the same voltage is reached each time you anodize. (Again, assuming all else is equal). To test this theory, I first anodized a part to PAR (slightly beyond, actually, just to ensure PAR was actually reached). This first part was anodized for a total of 45 minutes, yielding the below Voltage:Time plot. Using the first V:T plot, a suitable time would be selected for the second test piece. 20 minutes was selected, as it would give 97% of the peak voltage reached in the first test.

    Now to the results. Here's what I've seen so far:

    As you see, these graphs look much like the LCD graphs at 4.5 A/ft^2. Anodizing to PAR with the first piece took approximately 28 minutes, yielding a peak voltage of 15.2 volts. PAR then would be 15.2/0.4 = 38 Ohms. Normalized per unit area gives ~ 1.27 Ohms/ft^2. Why is this number not 2.5? Keep in mind the acid bath is much more concentrated than LCD.

    Here's a picture of the finished parts:

    The part on top was anodized for 20 minutes, the part on bottom was anodized for 45 minutes. As you can see, the part that was anodized for 45 minutes achieved a deeper blue color. Both parts were submerged in the same blue dye at 120F for 15 minutes, but had reached their full color within 5 minutes.

    - It is clear that the part that was anodized for 45 minutes achieved a darker color, possibly indicating that its anodized layer is slightly thicker.
    - The part anodized for 20 minutes, although not as dark as the 45 minute part, did achieve a good solid anodized layer that took color well.

    Fibergeek, I'm especially looking for your input on this. Does the above sound correct? What methods are available to accurately measure anodized layer thickness?

  • #2
    Wow! Pro experimental technique NeoMoses. By all means, continue!

    Coating thickness measurements:
    MIL-A-8625F describes a method to ascertain anodize coating thickness. Its based on determining the weight of the coating. This requires the use of a scale capable of accurate measurement to a small fraction of a gram.

    Another way (what I'm using) is measuring thickness directly using an electronic eddy current gauge. This is cheaper than a precision scale, not to mention easier to do. "Easier" is a relative term; measuring a coating 0.0001" - 0.001" thick is tricky at best. I'm using an Elcometer 345, purchased used on Ebay for $425.00, plus factory calibration "shims" (0.50 and 1.00 mils thick) purchased from Elcometer. New instruments like this go for over $1K.

    I have seen that a value for PAR of 2.5 Ohms/sq.ft. holds up well at 6A/sq.ft. current density. It's off at lower and higher current densities, I can't say yet how much off. To me this means that coating resistance isn't the only parameter, the coating density also plays an important part. This means that PAR should have different values based on the actual current density. It will take some time to work this out. Coating density means here, the percentage of pore area vs. solid area over the entire surface of the work. Smaller pores yield higher densities; which show higher values of PAR? I wouldn't swear that it goes in this direction yet. I expect to find some research addressing this if I dig deep enough in the technical literature. I don't want to rely on experiment alone if I don't have to.

    As we all know; acid concentration and current density affect pore size, these two parameters seem to dominate, temperature is a second order parameter (weak effect) from what I've seen. There may be others.

    We need to have a handle on coating density; in NeoMoses' experiment, we may be seeing the effect of too large of a pore size for deep color dyeing.

    NeoMoses, you need a means to measure coating thickness to resolve this, I'll see what I can do.


    • #3
      below is a picture comparing color obtained with the LCD method and the above 12 A/ft^2 method. (SCD)

      As you can see, the color obtained from LCD is very comparable to that obtained by 20 minutes of SCD anodizing. 45 minutes of SCD is darker than both. This is something I've noticed in all of these tests: 12A/ft^2 tends to produce a deeper, richer color than the LCD does. How does this relate to pore size/coating density? I don't really know. Again, having no means to measure this, I can only theorize at this point.

      Fiber, I agree that I need a method to measure coating thickness. If I sent you some of these test pieces, would you be able to measure the anodized layer thickness?


      • #4
        I'd be glad to help.
        The gauge is out on loan for the moment, but I can get it back.

        The LCD sample in the latest picture, was this one done with 1:1 or 1:3 battery acid/water?

        When convenient, could you post your connection method? The more detailed the better. Please include what the wire is and where you got it.

        I'll be in touch.


        • #5
          the LCD sample pictured was anodized in the 1:3 bath as described in the LCD instructions. The 2 others were anodized in the 1:1 bath, as described above.

          The wire was bought at Home Depot, it's standard aluminum electrical wire, I forget what gage. (10-3, perhaps?)

          The electrical connection on the test pieces is to place the wire through a small 1/8" hole in the part and tighten it thoroughly with a pair of pliers, nearly to to the point of breaking the wire (which happens sometimes. ) The connections are quite tight, even though they are not threaded connections.


          • #6
            Just a note on your color variance, understand I am not familiar with dies, only 2 step tin coloring.
            Voltage as you said will play a vital role. Anodized coatings at 10 volts will give you a smaller pore structure but will be more porous, but an anodized coating at 20 volts will be less porous but will have a larger pore diameter.
            We have found that anodizing no greater than 12 amps/sq.ft. will give us the best color uniformity depending on load size. If amp rating on our rectifier is maxed we may run at 18 volts for best color uniformity and adjust our anodize time to the running amps.
            Acid concentration, temperature, air agitation and aluminum content can all play a role in the voltage.
            Voltage will control the pore volume, while the current as a function of time will control film thickness.
            As for your mill thickness issue, we use mill meters that we buy from the Paul Gardner Company or Defalsco. Very easy to use but kind of pricey. (Up to $900.00)
            If your using the 720 rule and all of your anodize parameters are inline, ie: Acid Concentration, aluminum content, temperature, cathode ratio and proper amps/sq.ft. you would have to be very close to the mill spec you are trying to acheive. This will very depending on alloys but should be very close.
            I am also very impressed by your reseach you shared.


            • #7
              I hope this isn't too off topic for the post, but the mention of different shades and current densities promted me to comment.

              I am getting some pictures sized and will upload them to the album page soon. I have been getting a nice dark blue and black with 4.5 amps per square foot. The red from my setup is not bad at 4.5 amps, but could be better, although it may be the ph which I am also working on. I am about to start testing more at higher amps too, as I need to cut the cycle time down.


              • #8
                This is the first I've heard of the 720 rule, so I did a quick google search. Is the following correct?

                Originally posted by
                You can get a good estimate of the processing time using the '720 Rule' which states that it takes 720 amp minutes per square foot to produce one mil of oxide. The application of this rule is simple. Take 720 and divide it by your current density in amps per square foot. That will be the time required to produce a coating that is one mil thick. It works fairly well for Type II and Type III coatings and for most wrought alloys. Example - how long to produce one mil at 24 amps per square foot? 720 divided by 24 is 30 minutes. So if you want 12 microns it will require 14.2 minutes. Our experience show that this will get you pretty close, say within 10%. You must use current density control, not voltage control, though.

                Sjon Westre
                METALAST International - Minden, NV, USA


                • #9
                  Your intrpretation of the 720 rule is correct. The formula is used for most commercial anodizers and I cannot see a reason it would not work the same for you.


                  • #10
                    I have been wondering about how long the part could go after PAR. Although I have been getting a fairly decent medium red, I was looking for a darker red so did a little test yesterday. I followed the 720 rule, and anodized @6 amps per square foot current density for 120 minutes. It hit a 13.8 V peak at about 30 some minutes, and held until I stopped the anodizing. I didn't have time to record the exact curve, so I don't have that to post like Neomoses' nice chart.

                    I was very pleased at how much better the red dye colored, after a few seconds the part had a nice pink hue, a sure sign it was taking the dye well. After 5 minutes or so, it had reached the color my reds had been getting after a 30-60 minute dye period. I left it in the dye for a couple of hours, just to see how dark it would get and it is a nice, deep red.

                    I am going to experiment some more at other current densities too.

                    Just a related a note about color depth for those having troubles with red: I talked to Lance Caswell to confirm what to use when a red dye PH adjustment is required. He recomended muratic or sulfuric acid to lower, and ammonia to raise the PH. I had done this previously and it helps. The longer anodize time and getting the red PH right makes a world of difference.


                    • #11
                      M_D- I'd like to see somebody mix up a new batch of red dye and measure the Ph to see what it is. Something tells me it will be higher then they recommend before you even use it.

                      Also, i'd be interested in seeing how all your reds turned out, esp. the ones you did previously before letting the part anodize longer.


                      • #12
                        One more comment about connections and then I'll shut up (I promise).

                        A peak of 13.8V @ 6A/sq.ft. is a PAR value of 2.3 Ohms/sq.ft. That's within 92% of 2.5A/sq.ft. No connection problems here. People having problems with connections have seen 3.5 Ohms/sq.ft. and up; and lousy results, probably regardless of anodization time.

                        Regarding pH of newly mixed dye:

                        I'm no dye expert but it seems clear that anodizing dyes are sensitive to pH. It also seems to me that if the dye manufacturer could have made the dye more tolerant of pH, he would have. It's to his business advantage to make his product as easy to use as possible. I've not seen any anodizing dyes available premixed; maybe because even if they were the pH would still be off after some time. The end result is higher costs due to the increased labor and the volume of liquid, and no improvement in pH stability. Pushing the premixing and pH adjustment tasks on anyone else doesn't solve the pH problem, it just raises the dye prices.


                        • #13
                          Fibergeek- thats not really what I meant when talking about the dye. I simply have a feeling that even newly mixed red dye wouldnt fall into the range that the manufacturer recommends.


                          • #14
                            That wouldn't suprise me either.


                            • #15
                              Fibergeek, one of the things I have learned is you are right and that the connection is important. I haven't got a lot of experience, but for the last 6 weeks have been anodizing parts every day almost. When I first got a "real" power supply, I was seeing peak voltages of 13-15 at 4.5 amps per square foot. The area calculations of individual parts are from a CAD program where the parts are modeled in 3D, so that part of the equation is likely pretty good. I thought the connections were good, and questioned your figures and power supply meter accuracy. I was using wires with aligator clips to make the connection to the hanging wire (away from the sollution). When I changed that, the voltage dropped. Then I realized the aligator clips were not making as good of a connection as I had believed.

                              Even though it is somewhat related to color depth, I'll start a new post about the dye, I believe I can share some expeience and insights I have learned that may help those having problems.