How-To: Anodize Aluminum Without Strong Acid

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How-To:  Anodize Aluminum Without Strong Acid

If you’ve ever wanted to experiment with aluminum anodizing but were put off by the conventional requirement for concentrated sulfuric acid, you will be very interested to read of Ken’s successes with an alternate process using the acidic sodium salt of sulfuric acid. This salt, sodium bisulfate (NaHSO4) is much safer to transport, handle, and dispose of than the strong acid, and appears to give anodizing results that are just as good or better. It is widely available as a pool chemical. [via Hack a Day]

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16 thoughts on “How-To: Anodize Aluminum Without Strong Acid

  1. Anonymous says:

    Interesting, wonder if the mechanical properties are different.  The aviation community discovered the hard way that anodizing aluminum parts that are used structurally is a very bad idea, due to reduced strength or increased brittleness (can’t remember which, but in either case part failure was a problem).  For most applications, like bike parts etc, no big deal since they’re not as weight conscious.  So I’m now wondering if this new process is better/worse? 

    1. Anonymous says:

      It really depends on how thickly you Anodize the product. Anodization is just the controlled formation of protective oxides on the outer layer of the aluminum. Unless you have a very very thick layer of oxides, they shouldn’t effect the strength at all.

      I will say though that it makes it very very difficult to weld on, you need to clean the oxides off before you can effectively weld the metal.

      1. Anonymous says:

        Well, I would have thought the same, but the evidence speaks otherwise (but it’s more complicated, so we’re actually both right, sort of). 

        Keep in mind that the applications I’m talking about are very highly stressed and carefully designed.  The parts in question shave off as much material as is considered safe for the application, to save weight, which means that a small change in a critical property can have catastrophic consequences.  High performance parts do sometimes have fussy care and feeding rules. 

        Specifically, a helicopter fell out of the sky due to mechanical failure of a control rod, and it was discovered that the critical part had been anodized, so tested further it turned out that the anodiziation had caused the part failure due to changes in the properties of the part. 

        Here’s what I found on a quick google:

        Short version: Never anodize critical structural members. 

        Fatigue resistance is compromised by the anodization process, which isn’t exactly the same as tensile strength directly (but has a direct effect upon it over time). 

        This property is just as relevant as tensile strength in any application where vibration occurs.  Cracking, once started, will usually lead to rapid failure in a structural part under load. 

        So I thought of all of this when a new anodization process came up, but even if it were proven to be less of a problem, I’d never do it for something structural.  Well, in a couch or something, sure, but not in an airplane, where failure is often fatal. 

        1. Anonymous says:

          After looking into that a bit, I would want to see a bit more research into the area to actually declare that the Anodization itself is actually responsible.

          Let me clarify a bit. If you took a rod of the same thickness as the un-anodized portion of the control rod, would it last longer? I ask this because the people who say that the Anodization caused the failure point to the consumption of metal by the anodization process. Which makes sense. Aluminum does not have a cyclic limit, no matter what it will eventually fail. By removing the amount of material in the actual rod, you’re placing that much more stress on the remaining Aluminum. I believe it was a fatigue failure, but only because of a change in cross sectional area, not because of chemically changed physical properties.

          I personally think piss poor engineering was to blame for that wreck. The part should have had a greater safety factor built into it. And the Builder should have done more research into it before putting it in the air.

          1. Anonymous says:

            That’s not the conclusion reached by people far better versed in the matter than I.  It has been specifically said that it is in fact a material properties change that is not related to cross section but chemical surface changes on the piece. 

            Once you have a small surface area change that leads to a crack, that crack will propagate through the entire piece like a crack in glass, just a lot slower. 

            When making aircraft parts, they are often polished not for show, but to avoid any stress risers or crack potentials, resulting in a much longer part life. 

            But believe whatever you like. 

          2. Anonymous says:

            It may be entirely possible that it was a small surface crack propagating that caused the failure. Like I said, I would like to see test results directly comparing a few samples before reaching a definite conclusion.

          3. Anonymous says:

            Those tests were done, as part of the assessment of anodization in that context, unfortunately I don’t know where the results are stored. 

  2. Daniel Nelms says:

    Wonder if this would work on a MBP

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I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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