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Red dye issues

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  • Red dye issues

    I had been having my share of problems with red parts, and needed to get to the bottom of it. I had seen where one of the Caswell?s said the red PH has to be between 5.5 and 6. I had tried to use sulfuric acid without success sometime back, but apparently there were other issues. Lance Caswell confirmed that muratic or sulfuric acid was suitable to adjust the PH down, and ammonia is used to go up.

    I didn?t want to risk spoiling a whole tank of red, so I used 5 paper cups so I could try several PH levels with a small volume of dye. I don?t have any buffer solution to calibrate my electronic PH meter. The base reading of the unadjusted red dye was about 7.5 with the un-calibrated meter. So, I simply adjusted each cup in PH from 5.3 to 6.3 (according to my meter) to test. I anodized a single small sheet of 5052 for 90 minutes at 4.5 amps per foot. I then cut enough test strips off the main piece, approximately ?? x 2?, and placed one in each dye cup. I also anodized the remaining sheet a little longer to compare the effects of anodize time also. Anodizing past PAR does make the color darker, and easier and faster to obtain, according to my tests.

    The dye PH that did best, by a significant difference, was between 5.5 and 6., so I proved to myself that it is indeed important, and that I could adjust the PH successfully after all, and adjusted the main dye tank. The PH did climb back up after some time, and was adjusted again. I had suspected this would all work, because I had noticed if I didn?t rinse the sulfuric acid solution from anodized part very well, it would take dye well, but would bleed out if left in the dye very long.

    Water PH will fluctuate, even when comparing distilled water from various sources, so it would be hard to sell a concentrate that guaranteed a proper Ph upon final mixing I would think. Plus, the PH of water will fluctuate when exposed to air, and I imagine other uncontrollable factors change it also.

    Depending on the dye tank size, straight battery acid may be too concentrated because just a drop will change a couple of gallons noticeably and it may be hard to zero in on the desired PH with out going too far. So I diluted some battery acid in a 1:3 mix ratio with distilled water in a small eyedropper bottle, to more easily adjust the PH.

    If you are having problems with red, adjusting the PH and possibly increasing anodizing time may be the answer.

    The part on the left, was anodized @ 4.5-amp density for 90 minutes and dyed when the PH was high, in the mid to high 7 range. You can see some little dark specks, I have found that other times when dying doesn?t go well.

    The part in the middle was anodized the same, but the PH was adjusted to the recommended range. Depending on the angle of light, the color may look lighter or darker than the part on the right, but is not as ?red?. The coating is thinner and it has more of a metallic look.

    The part on the right was anodized @ 6-amp density for 120 minutes and dyed with the PH corrected dye. It is a true red, very solid and even with no imperfections, and the color is not as subject to vary with light reflection.

    The picture isn't great, but hopefully good enough to help illustrate the differences.

  • #2
    Very nice work, MD. What was the cost of the pH meter?

    I have a feeling that we are very quickly chipping away at the last few remaining issues affecting DIY anodizer's consistency problems. CC anodizing was a big step in the right direction. I see these other issues to be important to quantify to ensure quality work:
    • Electrolyte Chemistry (Acid and aluminum concentration)
    • Electrolyte temperature
    • Dye chemisty (In the process of being solved!)
    • Surface preparation/cleaning
    • Possibly others

    Basically, I think that the DIY anodizer will eventually be monitoring and controlling almost everything that the 'big guys' are doing now, we'll just be doing it with smaller batch sizes.[/list][/list]


    • #3
      The PH tester was $39 from Caswell?s. It?s probably a little hard to use litmus (sp?) paper on dye, since you judge the Ph by the color the strip turns, but that?s just a guess on my part. It seems to me that to get consistent results all influencing factors need to be understood and controlled.

      I have some questions along some of the very things you mentioned, so I am going to start another post on acid ratio versus current density.

      One thing I will mention here since you listed it is part cleanliness. I spent probably 40 hours tracking a problem lately. The problem was spots that would not take color (none at all, like the spots were masked), or patterns that you get when a liquid dries and leaves a residue, and the flaws where mostly on the lower section or bottom section of the part (the clue that led me to the problem eventually). I had changed to a larger tank, larger cathodes, added the circulating pump (which introduced grease to the tank), as well as started using the heated cleaner and desmut, so there were plenty of variables all at once.

      Through a series of observations and not one part without some blemish or another, I finally deduced that it must be the desmut solution, and did a test where 1 part was soaked in the desmut solution and rinsed after cleaning until water sheeted. The other part was just cleaned. The part not soaked in the desmut turned out great, and the other didn?t. It turns out a simple dip and spray rinse was not adequate for my process after desmutting the parts. Since then I have been very careful to ensure water will sheet immediately before going in the anodizing tank, and I am very pleased at how consistent and predictable the parts are anodizing.


      • #4
        A very well organized and executed experiment M_D. Nice work!

        Who would have thought? We are now applying Scientific Method and the Rules of Experiment to anodizing, If we keep this up, we will advance small scale anodizing further in the next year than has been done in the previous decade.

        Regarding NeoMoses' to do list:

        Wernick's book, judging by the subject index has a lot on aluminum content. Unfortunately its in Volume 1, which I'm still trying to get. Montgomery's Light Metals Finishing Process Manual which I have, says this about aluminum content (verbatim).

        "It has ben estimated that in sulfuric acid anodizing, under normal anodizing conditions, 1 gram of aluminum are dissolved per square foot of work processed in producing an 0.8 mil (20 micron film). Aluminum that dissolves into solution decreases the conductivity of the anodizing electrolyte and as the concentration increases, the voltage applied to the work must also be increased to maintain a constant current density. This extra power generates heat which must be removed with chilling equipment. When the aluminum level reaches 15-20 g/l further voltage adjustments are not usually practical and the solution must be decanted to lower the aluminum content."

        In another place Montgomery discusses "mottling" in the anodizing if the aluminum content is too high. I can't find it now.

        Anyway, my EE's paraphrase of Montgomery's statement is this:

        Aluminum in solution increases the electrolyte resistance, because conductance is the mathematical reciprocal of resistance by definition. A Constant Current Source (CC) will compensate for this by increasing the applied voltage (Ohm's Law) up to it's maximum voltage compliance (until it runs out of voltage). This means to me that aluminum content is a nonissue if you have the necessary voltage available; up to the point where surface defects (mottling) starts to appear, or the dissipated power raises electrolyte temperature to a too high level. By Ohm's Law, the aluminum content is beneficial to provide some current limiting if you have none or not enough. The same goes for anode / cathode ratio BTW.

        Sorry I've gotten OT.


        • #5
          I've got 2 tanks, one large with 15 gals of electrolyte and one small with about 3 gals of electrolyte. So, assuming all of my anodizes are 0.8 mil, my 3 gallon tank should reach the 15 g/L aluminun after about 170 square feet of anodizing:
          3 Gal ~ 11.35 L. (15 g/L) *(11.34 L) * (1 ft^2/g) = 170.1 ft^2.

          That's not too bad IMHO. I doubt if I anodize that much in a given year, but I'll try to keep track from now on. Thanks for the info, Fiber. Do you happen to know how the pros measure this?


          • #6
            Montgomery describes two titration methods to determine aluminum content, both are pretty involved. He also mentions determining Al content by electrical conductance measurement and SG measurement, the latter may be the most practical on our scale. The details of these are not provided.

            As I have said previously; considering the relatively small electrolyte quantity we use in small scale anodizing, and the low cost of battery acid, it's more practical to toss the electrolyte before the Al content gets objectionable. You'll probably want to toss it before this point due to the sulfates and dirt that will accumulate in it anyway.