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Acid concentration versus current density

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  • Acid concentration versus current density

    The post by Neomoses regarding color depth, and the issue brought up about current density and acid ratio got me to wondering something. That is, how is it determined what the ratio should be? It seems that there must be an optimum acid concentration for a given current density. Obviously, it is somewhat forgiving.

    But for example, if one were to use 8 amps per square foot as their standard, shouldn?t the ratio be somewhere in between the LCD standard of 5% acid and 15% which is commonly used at commercial current densities? I guess I should just give it a try since I seem to have many variables under control, but I was wondering what the results would be if I used 8 or even 12 amps per square foot at the LCD acid mix. In other words, I am wondering at what point does the ratio need to be higher, and what will the effects be if it isn't withing range or is borderline?

  • #2
    Fibergeek did some testing at 12 ASF in his original paper. According to comment number 2 on page 24, "Dyeing is poor, but the surface is very hard..."

    I believe you'll see dye absorption start to suffer at the higher current densities with the 1:3 LCD electrolyte. I don't know the exact point where it becomes unsatisfactory, but I have successfully anodized and dyed parts at 6 ASF in the LCD tank.

    You might give it a try at 8 and 10 ASF and let us know how it turns out.


    • #3
      We are seeing the effects of the highly analog nature of anodizing. This means that almost everything; good and bad, is a matter of degree.

      The upper limit for the 1:3 ratio LCD electrolyte is 6 A/sq.ft. as surmised by NeoMoses. If you anodize at a higher current density than this, you will find it necessary to increase the electrolyte concentration. The exception would be if your goal was a denser/ smaller pore structure coating.

      In the over 6 A/sq.ft. to 12 A/sq.ft. range, I would have tried a 1:2 electrolyte ratio first. Those experimenting at 12 A/sq.ft. with a 1:1 ratio should be wary of excessive pore size. The signs of this are dye leaching out when sealing, and a "softer" anodic coating. The latter can cause mechanical strength problems with the coating when it is grown "too thick". I don't know how much durability you need for your application, but you should be aware of this.

      Another consideration in higher acid concentrations is corrosion issues. Besides being noticeably more noxious, higher concentrations will cause a coating of rust on iron or steel that can be reached by the acid vapor. Again, this is a matter of degree. Since none of us are operating in an acid proof industrial environment, rusting equipment and tools in the same room will become a problem. If you have machine tools in your basement like I do, this is intolerable. It will take months for this to show up. You may not have a lathe and milling machine, but what about your hot water heater and HVAC? Large tanks; warmer ambient temperature, and higher humidity all make this worse, as you would expect. A tank cover will improve, but won't eliminate this problem. I have had rusting occur in close proximity to straight battery acid stored in polypropylene chemical jugs. These containers are designed to store strong mineral acids like sulfuric. It takes about a year in this case but it still happens.

      It is not my purpose to discourage the use of stronger electrolyte. I don't think you can get the results you want without it, but you do need to be aware of the downside. I don't claim that the LCD ratio (1:3) eliminates corrosion issues. But 6 months later my milling table has no rust, it's 20 ft. from the tank (3 gals. plastic tank cover).


      • #4
        I tried a 12 AMP test at 1:3 electrolyte ratio, but made a mistake that makes the results invalid. It didn't work in any case, which probably isn't surprising. I'll play around and post back on it either way.

        Fibergeek, one of the reasons I was interested in the LCD method was the lowered acid ratio, weaker fumes and related hazards, and etc. I know it (LCD) isn't probably going to used by many commercial anodizers, but I think for my application it may be the way to go. I'll just need larger tanks and more racks to make up for the slower cycle times. I have a lot of machines to consider keeping in good shape.


        • #5
          It probably would be a good idea to look into what it would take to get an acid rated fume hood or booth. LCD or not. You'll certainly experience less corrosion problems with LCD, but I can't see corrosion going to zero with LCD, particularly if you are going to scale up your operation. The value of your machine tools alone might justify this.

          You probably expect this, but operating at higher current density will generate more fumes than lower current density (more electrical power in the electrolyte) regardless of electrolyte ratio. The 1:3 ratio will produce fumes with a lower acid content however.


          • #6
            I am planning to use a closed system with exhaust, although I don't have any particular design or product chosen yet. Years ago when I was doing this with a battery charger and nearly pure battery acid I discovered how noxious the fumes could be. The fume suppressant really helps that. I wanted to get a feel for it would take to build a production system to fit the need first, and to evaluate the fumes and etc., as you know there are lots of options.

            Just the lower acid burn risk is attractive with milder concentrations, although a higher current density may be the final choice. I can get more serious about testing the higher current densities now that things in general are mostly consistent and predictable. I'm sure I'll discover a few more ways to make parts fail QC before I'm done though.


            • #7
              What about the use of mist balls and/or fume suppressants. Do either of these things help to lower the corrosion factor?


              • #8
                Mist balls and fume suppressant can only help reduce the problem, they can't cure it. If your using just a few gallons of electrolyte; and its only in use occasionally, it would be good idea to store it in air tight plastic containers.