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  • Peak voltage ?

    When should peak voltage occur?



    Rusty

  • #2
    R-wells,
    Hows it going?
    Peak voltage occurrs when coating growth and dissolution reach equilibrium. However if you stop anodizing at peak voltage, you have stopped too soon the majority of the time. This is because although the coating continues to grow, the pore size has started to get large enough to reduce resistance and drop voltage. If all conditions are optimum you will be able to slow dissolution to a point that will allow you to hit peak voltage at the end of your anodizing time or past that point. Decreasing tank temp will slow dissolution but will increase the value of peak voltage. Better circulation also slows dissolution by decreasing temps in the pores.

    At 72* tank temp. I usually hit peak voltage 15-20 min. before time is up.

    This was learned from Fibergeek and has been verified by my own anodizing.

    Comment


    • #3
      Hi sswee,
      Thanks.
      I am trying to dial my system in as close to optimum as possible,
      I have been trying to figure out the relationship between temperature and voltage.
      Seems if I run above 72° I peak to low to soon, if I run 68° I peak to high.
      My system seem to like 71.257286°
      When I asked when, I really meant growth wise .1 or .5 or 1mill

      Thanks
      Rusty

      Comment


      • #4
        The colder the temp, the more resistance and more voltage to push the amps. To shoot for optimum coating, don't worry about voltage level on the peak but the time of the peak. You want to postpone the peak as long as possible within reason to allow maximum growth. The only time to take concern on the voltage is on a spike or an extremely high peak that would indicate high resistance. ie. burnt connection At 72*, I don't usually go for more than a .5 to.7 mil coating. I've been able to get 1 mil + only by dropping tank temp down to 60* to 64*. This causes voltage to peak a little higher than calculated but not usually more than 2 or 3 volts. In general voltage only varies +-3V max with temp variances on different runs from 60* to 74*. The only problems I've had since I starting using the 720 rule have been due to screw ups in prep. I've dyed 7 different colors so far and no problems with taking good color in about 15 min. Let me know how things go. SS

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        • #5
          Hi Swwee,
          I am going to mess around and ask the right question eventually.
          If my tank temp was Ideal and my conections were ideal and my circulation was ideal and my shoes were on the right feet, what thickness of growth would I should I reach peak voltage,
          Should it peak at .1mill? and then decrease or should it peak at 1mill?

          Thanks
          Rusty

          Comment


          • #6
            OK. This is not exactly easy when not in person. If everything was optimum like it is in the perfect world. Aluminum hydroxide(Anodize) can be grown to 1 mil thickness in 720 amp minutes with 1 amp CD. This is where the 720 rule comes from. I understand what you are asking but am not sure that can be answered with a cut and dried answer. I believe theoretically, peak voltage should hit at 1 mil growth. Having learned early in life, in theory and in practice are not always the same. I think voltage peak in relation to coating thickness is variable depending on the combination of the other variables. Fibergeek might need to field this subject in that he has done extensive test runs in more varied CD's under a more controlled evironment than I have. Nomally I hit peak at different times in relation to coating thickness depending on temp. Voltage usually stays at peak an average of 25 min. before starting a .1V per 5 min drop.

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            • #7
              Hi sswee
              I understand about the variables.
              I am not really having a problem, just just curios.
              Yesterday I did 4 parts, 4.5asf for 90 minutes, none of which ever reached peak voltage..
              Thanks
              Rusty

              Comment


              • #8
                How much below peak voltage was it? And what temp was the tank?

                Comment


                • #9
                  I cant really say, since I dont really know what peak voltage would have been for sure.
                  At 90 minutes I was at 11.3 volts.
                  The parts I did Monday I ran for 120 minutes peaked at 11.5v. and decreased to 11.4

                  Thanks
                  Rusty

                  Comment


                  • #10
                    Using voltage to gauge conditions can be misleading. If you know what the voltage should be for a given setup, and monitor all conditions, it can help to troubleshoot. If the voltage is suddenly way high (possibly caused by a bad connection or over estimation of amps needed), or even low (under estimation of amps needed), it may point you in the right direction.

                    The problem with comparing voltages from two individual setups (at the same current density of course) is everything changes the voltage some. A well-deoxidized part will run a different voltage curve compared to an oxidized one. Various alloys change the peak and curve also. Low acid concentration raises voltage, and higher acid concentration lowers it. The effect of general tank temperature was mentioned by sswee.

                    One thing else that is not common knowledge is the agitation level changes the voltage peak and curve, by as much as 2-3 volts and commonly 1.5 volts, at least in our operation. If you want to know why, the sentence before this paragraph contains a clue. (As a note, we run higher current densities for type II which amplify things such as voltage and temperature issues which make them easier too see) I will comment more latter, but thought it would be interesting to see if someone else either knows or guesses what the connection is between agitation and voltage is. It is one reason why the voltage sometimes goes up as time progresses, and other occasions it never does but just drops from the beginning. I’m fairly certain Fibergeek knows, as he has all but said it in other threads.

                    When you add up all the potential variables you can see why a seemingly large variance in voltage shows at times when comparing notes across 2 individual setups. Sometimes it points to a problem and sometimes it doesn’t.

                    Comment


                    • #11
                      Hi M_D,
                      My lillte brain says that temp has to be higher on the surface of the parts,
                      At the moment I am playing with everything, trying to learn.
                      I am using air to circulate, I started of with aqaurium pumps, now I am using shop air .
                      I am wondering if there is not a way to eliminate all or most of the variables you mentioned.
                      For instance, could we measure the resitance across the electrolye at a given temp.

                      Rusty

                      Comment


                      • #12
                        Hi ,
                        I didnt finish the last post, I had to go take my pants off, a spyder crawled up my leg

                        I did notice a volatge increase when ading more air to my circulation,
                        So I have to believe it is from moving the hot electrolye away from the part.

                        Rusty

                        Comment


                        • #13
                          You're right as usual, M_D.

                          The big, fat hint I dropped several times (the temperature in the pores, not in the electrolyte is what actually counts) wasn't lost on M_D, and Sswee I suspect.

                          You can see this if you study my anodization curves (1 through 3). Pore temp is obviously difficult to measure (but possible, its been done) all we can do is measure electrolyte temp. (better than nothing).

                          The best case is to have the anodization curve slope unwards as the anodization progresses, I was able to achieve that with 6xxx and 1xxx alloys. 2xxx and 7xxx alloys showed a downward slope anodized under the same conditions. The closer you can get to an upward sloping curve the better, difficult with 2xxx and 7xxx alloys.

                          This Spring (its Spring here now) I will start another series of controlled anodization experiments; looking deeper into cathode area and into the use of electronic feedback (a "smart rectifier") I will design to automatically correct down sloping anodization curves. I will look at Ti racking effects in more detail too.

                          (edited for spelling)

                          Comment


                          • #14
                            Well that wasn't hard for you to figure out. There is a boundary layer that forms since the heat comes off the part, and even with mild agitation it is still significantly warmer on the part surface. Assuming your electrical and mechanical connections are robust and aren't hanging on by a hair, you can demonstrate how it changes by wiggling or swishing the part around and watching the voltage. Unless your tank is exceptionally well agitated, the voltage should go up immediately as the boundary cools down, and it will start to drop as soon as it begins to warm up. With LCD the temperature difference will be less between the boundary layer and general or average tank temperature, which is one of the many reasons it is well suited for hobbyists.

                            This may have been slightly off topic, but I guess the point I was trying to make is that comparing voltages is tough unless all factors are controlled to the same value.

                            Comment


                            • #15
                              No M_D, it wasnt of topic at all.
                              Thanks guys

                              Rusty

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