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Galvonic Action Between Steel & Aluminum

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  • Galvonic Action Between Steel & Aluminum

    One of my friends said that steel and aluminum may induce a galvonic response that will accelerate corosion in the metals.

    I plan to press fit a soft steel shaft into aluminum (6061).

    Do I need to take special precautions if the parts are used indoors under room temperatures?

    Marv

  • #2
    Four entities must exist for corrosion to occur on metals. A cathode, an anode, an
    electrolyte, and a conductive path. Eliminate any one of these four and you stop corrosion on metal. In this case the cathode would be the more noble metal of steel and anode would be the more active metal of aluminum (galvanic series), the conductive path is obviously the press fit of the metals together. If this is going to be aboveground or out of doors than the electrolyte is any moisture in the presence of these two metals. Thus it is difficult to eliminate any one of the four in most real-life situations. One suggestion is to cadium plate the steel bushing or or just paint both surfaces minimizing their exposure with moisture. If you live in an arid or dry area you should be able to get away without doing this though. Especially if the aluminum surface area is much larger compared to the surface area of your steel bushing. What's going to happen in the presence of moisture is that the aluminum will tend to corrode protecting the steel. In essence the aluminum will sacrifice itself for the steel. But only if both surfaces are simultaneously exposed to moisture. If the aluminum area is very small compared to the steel bushing area then the aluminum will corrode itself very quickly or sacrifice itself over a short period of time. The short answer paint or coat both surfaces or plate the bushing or both.

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    • #3
      Anodize the aluminum part.
      I just need one more tool,just one!

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      • #4
        Excellent post Jim. All you say confirms my experience with metals in galvanic pairs in a salt water environment.

        The operative word here, however, is is "indoors." If indoors in a dry environment I'd say absolutely, positively debequem need not fear corrosion - except for trace rust on the steel if its grease free.

        Add moisture as in humidity from the dryer or rapidly changing temperatures and the steel will likely rust. The aluminum may show some white corrosion but only token amounts over many months.

        Take this bi-metal couple gadget out on the salt water and all bets are off. The steel will rust all over in a week but as Jim says the alum will protect the steel next to the bi-imetal interface by corroding preferentially.

        In six months exposure the steel will have lost significant volume as rust and the alum if unprotected will suffer some piting and a general attack of white corrosion.

        Aluminum loses about 1/20 the volume per unit of exposed area compared to steel in a salt spray environent but naturally different alloys and surface treatments will scotch this rule of thumb.

        [This message has been edited by Forrest Addy (edited 03-08-2005).]

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        • #5
          Best way to go about what you're doing is to Cadmium Plate the Steel Shaft and Anodize the Aluminum Plate.

          If you are trying to hold the cost down you can surface treat the Aluminum Plate with Alumaprep 33 followed up with Alodine 1201.

          Then press the Steel Shaft into the Aluminum in a bed of wet Zinc Chromate Primer. The problem is that the majority of the Zinc Chomate will be squeezed out but a fillet of Zinc Chromate Primer will seal out the moisture negating the possibility of Electrolytic Corrosion.

          A light coating of Liquid Cold Galvinize (99% Pure Zinc) will prevent the steel shaft from oxidizing.

          Basically speaking, you want to seal the metal from moisture


          [This message has been edited by jlrsn (edited 03-09-2005).]

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          • #6
            .

            [This message has been edited by Wirecutter (edited 01-28-2006).]

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            • #7
              Why not use some stainless for the steel? That should change the equation.

              Did a project for the Government on Kwajelein atoll where rust is a way of life, and everything corrodes immediately. They wanted Alclad aluminum siding and 316 SS for all fasteners. Tried to explain that Alclad would require a special run by Reynolds and any future repairs would also require a special run -- all VERY expensive. They wouldn't consider vinyl siding or anything else. They didn't consider that the tech screws used to install the siding couldn't be 316 because it is too soft to achieve the self-tapping function in light gage framing. I'd be interested to see how that job looks now, but I don't care to go there to find out.

              Anyway, think about the shaft material instead of expensive plating, etc.
              Lynn S.

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