Corrosion Copper and Aluminum

Discussion in 'Boat Design' started by Katoh, May 3, 2011.

  1. Katoh
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    Katoh Senior Member

    Friends
    We all know for some reason or another we can not mix copper and aluminum on boats, especially those with aluminum hulls, due to the adverse effects that the copper creates to cause corrosion to the aluminum.
    After posting a couple of the threads asking for opinions on different matters the subject has reared its head again, and the question has crossed my mind.
    OK we know any copper directly in contact with the hull is going to cause trouble. is this only below or above water line? What about copper insulated from the hull but in contact say with a motor, a raw water cooling line, or even a fuel line, will this have the same effect of corrosion to the hull?
    If so my argument is that shouldn't we use then only stainless or aluminum wiring on an aluminum hull, why do we use copper wiring even if tinned its still copper?
    So why cant we use copper for fuel lines, cooling lines, raw water pipes?
    To those who Know excuse my ignorance, I am a civil Engineer, not Naval engineer as stated before, I just like playing with boats.
     
  2. Jeremy Harris
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    Jeremy Harris Senior Member

    The basic problem with having dissimilar metals in contact with one another, particularly in the presence of an electrolyte (and dirty water /seawater makes a good electrolyte) is that the two different metals create an electric cell, just like in a battery.

    The result is that one metal will act as a cathode, and the other will act as an anode, depending on their relationship to each other in the electrochemical series. The anode will dissolve over time if the two metals are electrically connected together by some means; the cathode will remain virtually untouched.

    All metals have a place in this electrochemical series and the aim when building any metal structure is to try and ensure that metals that are in intimate contact with each other are similar in terms of their electrical potential. Here are some examples of the electrochemical potential for some commonly used metals:

    Zinc = -1.11V
    Aluminium = -0.86V
    Steel = -0.68V
    Stainless steel = -0.61V
    Copper = -0.43V

    In crude terms, if you place aluminium and copper together in the presence of an electrolyte then you'll set up a cell with a voltage of about 0.43V. This will rapidly cause the more negative metal (aluminium) to corrode.

    This corrosion can only take place if the two metals are electrically connected in some way (so that a current can flow) and if they are in the presence of an electrolyte. Copper wiring is OK because it's always insulated from the hull material; copper fittings aren't OK because they may well become connected electrically to the hull.

    You can use dissimilar metals together provided you either insulate them very well, or you seal them in such a way that an electrolyte (typically water) can't get to them.

    FWIW, even stainless will cause aluminium to corrode, particularly the "wrong" sort of stainless. Stainless steel comes in different types, some of which have a potential voltage much closer to aluminium (so better for an alloy boat) than others.

    If you do an internet search on the electrochemical series and metals you'll probably unearth mountains of further information.

    Jeremy
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Jeremy has given you a very good summary.

    For a more detailed info on ally v copper of the eletrical potentials, see here:
    corrosion-ally-copper.JPG

    We paint the pipes of the copper piping and also expoy paint the bilges of our ally boats.
     
  4. viking north
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    viking north VINLAND

    While the field of electricity is my trained profession in life I can't add anything more than that given above. This is a very complicated and in dept topic that would require a professional study in electricity to fully understand. The best thread i've read on this actually has to do with "zincs" and the thread starter CDK in my opinion is the foremost expert on this subject. My advise use the chart and avoid widely seperated galvanic metals like the plague and remember the dissimilar metals don't have to be emerged in water, acid rain, fog, moist salt air is and can be just as good an electrolite. --Geo
     
  5. Frosty

    Frosty Previous Member

    You say alluminium and copper would generate 0.43 V

    Is that becaue copper is 0.43?

    or is it by coincidence that alluminium is 0,86 -0,43 which is still 0.43

    So steel and copper would generate 0.68-0.43= 0.25V
     
  6. Jeremy Harris
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    Jeremy Harris Senior Member

    First off, those voltages are nominal and relate to a reference that's a copper sulphate cell - other tables may use a different reference and quote different voltages, but these are typical of the figures used when looking at galvanic corrosion.

    In the case of aluminium and copper, the figure is the difference between the two voltages, so for aluminium & copper respectively = (-0.86) - (-0.43) = -0.43V

    For zinc and copper it would be (-1.11) - (-0.43) = -0.68V

    For stainless steel and copper it would be (-0.61) - (-0.43) = -0.18V

    As I mentioned above, stainless isn't clear cut, as different grades of stainless, and particularly whether or not the stainless is in active or passive state, determine the actual potentials and position in the series. In general, using metals together that are close to each other in the series is OK, using metals that are far apart isn't.

    Jeremy
     
  7. Katoh
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    Katoh Senior Member

    Gentlemen that's quite an in-depth answer, nearly straight to the point.
    In layman's terms you can use dissimilar metals like copper and aluminum together along as they are totally isolated from each other., But not recommended.
    I take it if you had water passing through a copper pipe that was insulated from and to an aluminum pipe the water would act like an electrolyte even though the water is moving. So basically no copper is best at all on an aluminum hull, and should be avoided at all costs.
    Just to throw another spanner in the works,
    why is it recommended that a bare copper wire be placed from from the motor to the hull to make sure that there is a solid bonding earth between both.
    Even if not you are still meant to bond all your negatives to the hull from the batteries side to bus to hull to again achieve the same earth bonding. To make no confusion here all electrical fittings still need to have a positive and negative feed, and be isolated from the hull. But it is recommended to earth the boat through the negative. Not picking at straws but doesn't that mean that the copper wire connected to the hull.
     
  8. Frosty

    Frosty Previous Member

    Good question Katoh
     
  9. Jeremy Harris
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    Jeremy Harris Senior Member

    You can get around the problem of connecting a copper wire to an alloy hull by using plated or terminals of a different metal. For example, if your copper ground wire is crimped into a stainless lug, then the stainless lug is bolted to the hull, you've effectively reduced the corrosion potential at the hull to that between stainless and aluminium alloy.

    Alternatively, you can bolt a copper lug directly to the aluminium alloy hull as long as you completely seal the joint so that no electrolyte (moisture mainly) can get in, although personally it's not something I'd be that comfortable with.

    Jeremy
     
  10. Katoh
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    Katoh Senior Member

    To sum up, moisture of any kind is our enemy. simply Fuel lines, raw water lines, cooling pipes in copper are a big NO NO. If moisture connects the two metal's its all over. Copper wiring is fine if the lug or connector is not copper, and the cable should be sealed.
    Many thanks for the education.
    cheers
    Katoh
     

  11. viking north
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    viking north VINLAND

    On my alum. hull build I operated with a 12v. system only and as such did not bond anything to the alum. hull. I simply bonded everything to a common ground buss bar.You must run a two wire system to everything and not use the hull as a common neg. return. If you install a 115 system , I.E. shore power connection it only then becomes necessary to bond to the hull for safety reasons and you can do this as Jeremy suggest.
    Katoh-do yourself a big favour as electrical will be a never ending project- pick yourself up "Boatowner's Illustrated Handbook of wiring by Charlie Wing, you'll never regret doing so, it's a must for every boats library.--Geo.
     
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