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  • Anodise tank Titration question

    Hi to everyone,
    I am doing Titration PH tests to measure my Anodise tank for acid and aluminium content over time with use, and have been doing some tests and it seems that when I run the anodise tank over time it appears that with use the acid G/L rises and also the Aluminium content rises.

    This seems strange I would have thought Aluminium content would rise but I would have expected the Acid content to fall with use ?

    Can anyone explain why it does this as it seems I am making Sulphuric acid? so based on this when it gets to a high acid level do I dump a portion of the tank and refill with DI water, but if this is the case will I ever need to replace the acid as it seems I am making it ??

    If anyone could throw some light on it for me I would be grateful as I would like to know whats happening.

    Many thanks in advance
    Regards
    Richard..

  • #2
    When you pass current through a sulfuric and water solution(especially if you are using lead cathodes) it does convert some of the water to acid. Not sure how old you are, but in the old days we used to top off the battery in the car with distilled water before you charged it.. This coupled with the faster evaporation of the water would leave your bath acid heavy for sure. Just top it off with DI water..

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    • #3
      Hi Thanks for that I didnt know that it created acid by passing current thats interesting, yes I have lead cathode.
      My tank is pretty much full of liquid so I suppose I could decant some acid and refill with DI water, being as battery acid is hard to find round here would it be wise to keep the decanted acid for future re use ?
      Thanks for your reply.
      Regards
      Richard..

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      • #4
        Here is the chemical stoichiometric equation of the anodizing process...

        2Al + 3H2O --> Al2O3 + 6H+ + 6e-

        I basically says that 2 aluminum molecules plus 3 water molecules will react to provide 1 molecule of oxide with 6 Hydrogen ions and 6 electrons left over. Since the reaction yields free Hydrogen you could see a measurable change in the bath pH while the process is running (and shortly after ward I suppose). After the process is complete and the bath returns to equilibrium, there is no net gain of Hydrogen in the bath. There will however be a net loss of water which, if not replaced, will increase the concentration of sulfuric acid.

        Edit: pH is a measure of the relative amount of free Hydrogen(H+) and Hydroxyl(OH-) ions in a solution. If the Hydrogen concentration goes up, the pH goes down and the bath becomes more acidic. Hence, removing water from the sulfuric bath will increase it's acidity.

        Introducing lead cathodes could (likely will) provide a reaction that could consume the sulfuric ions to create lead sulfate. I can't speak to this amount or degree this occurs because none of my customers use lead cathodes. Lead is toxic and if you are using it please make sure your waste is treated properly. It's easy to remove the lead reaction from the bath by simply replacing lead cathodes with 6061 Aluminum. I encourage all of my customers to remove lead from their facilities. It's toxic and shouldn't be used unless it is absolutely unavoidable.
        Last edited by KevinB; 03-03-2018, 04:37 PM.
        Process control doesn't give you good quality, it gives you consistent quality.
        Good quality comes from consistently doing the right things.

        Process control systems for anodizers
        If a post helps you out spread the love and LIKE the post
        _____________________________________________
        Last edited by kevinB: Now. Reason: superfluous typo's

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        • #5
          Thanks for that Kevin I think I will go over to using Aluminium 6061 like you say it does sound better to get away from lead cathodes.
          I dont pretend to understand the formulae of the chemical process but your explanation does make it a bit easier to see whats going on.
          I wonder if using aluminium cathodes would the sulphuric acid have an almost limitless life span in the anodising tank? maybe it would I dont know ?
          Thanks for your reply

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          • #6
            No. your sulfuric will still deplete and you still need to check the concentration regularly. The Aluminum cathodes will also require cleaning and replacement now and then.
            Good luck Rick!
            Process control doesn't give you good quality, it gives you consistent quality.
            Good quality comes from consistently doing the right things.

            Process control systems for anodizers
            If a post helps you out spread the love and LIKE the post
            _____________________________________________
            Last edited by kevinB: Now. Reason: superfluous typo's

            Comment

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