Aluminium Brass Copper Corrosion experiment

Discussion in 'All Things Boats & Boating' started by zimbodave, Feb 17, 2016.

  1. zimbodave
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    zimbodave Junior Member

    Greets All,

    For my own education and curiosity, I decided to do a little experiment on galvanic corrosion of aluminium in salt water.

    As I live about 600km/400miles from the sea, I had to make salt water. Not able to use sea water. So for the experiment I put 35 grams of fine Atlantic table salt in to one litre of fresh water and dissolved it. (Well, that's what it said on the packet anyway :) )

    I then put a few pieces of 1050 Aluminium in to the jar.
    One of them has a piece of copper and a piece of brass on it.
    When I inspected it this morning (Been in there for two weeks) I got a bit of a surprise. The brass has caused corrosion and the copper has not.
    So, two weeks on, I'm not seeing the kind of corrosion I expected to see from what I have read on various forums.

    I have attached some pictures.

    Interestingly, there is significant corrosion under where the brass is sitting, but not even a mark under where the copper is sitting.
    Bottom surface of the copper is pretty rough so the contact surface area will be very small but I was expecting the copper to really eat in to the aluminium in short order. So far, after two weeks, nothing at all.

    The bottom surface of the brass is fairly smooth, machined in a lathe. I wonder if this is the reason for the significant corrosion under the brass.

    I will machine and polish a new piece of copper today so that I'm comparing apples with apples.

    Any comments from you guys? Any ideas for the complete lack of corrosion under the copper?

    I don't believe I'm seeing the kind of corrosion I would see if the same experiment was done with sea water. I know from personal experience on the Mozambique coast, that aluminium and brass in contact in sea water gets eaten away very quickly down there.
    What am I missing in my seawater recipe which would make the experiment more accurate?

    Any thoughts or advice welcome :)

    Cheers

    Dave
     

    Attached Files:

  2. zimbodave
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    zimbodave Junior Member

    And can anyone please tell me what all the white "snow" is that has formed and has precipitated to the bottom of the jar?
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Is it refined table salt ? that would be pure (ish) Na Cl with an anti-caking additive, so not strictly the same as unrefined salt that contains a large range of constituents beside Na Cl. And none of that Aluminate flowing agent. Which may have no bearing at all on your experiment, but it isn't entirely the same.
     
  4. zimbodave
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    zimbodave Junior Member

    It was actually course salt. The stuff you put in a grinder.
    In original post I meant fine quality, not fine grain size. Sorry for confusion.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    I stopped using refined salt a decade ago, after it was suggested to me unrefined salt could assist a certain medical condition (gout). Seemed to have worked pretty well for that, so maybe it affects other experiments as well.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    There are many mechanisms at play here.

    1) The oxide layer. Is the oxide layer still inert? This is related to the solution it is immersed in. This provides the pH value. Since in a certain range of pH values the solution does not attack the passive oxide layer. This can be seen better on a Pourbaix diagram. Typical example shown below:

    Pourbaix diag of ally.jpg

    2) This leads to the solution, the electrolyte that carries the charge. Artificial solutions have been shown to provide very wide varying results.

    For example, Copper in artificial solutions has yielded an an electrical potential range of +140mEV in phthalate +10mEV in artificial seawater. Whereas the correct and accepted potential is that of -360mEV.

    3) The electrical potential. The potential of copper and brass is both -360mEV. Whereas 1050 ally is -750mEV. Thus given the same potentials, the surface roughness, size and weight of the samples, hence the available surface area for 'contact' plays a part too. The rate of dissolution is a function of:

    cathodic surface/anodic surface

    ..and can also play a role. However the potential difference is circa 390mEV which is sufficient for any form of corrosion to take place.

    Thus it is not surprising that you do not get exactly the results you expected. Next time you are down the coast, collect several large buckets of water.

    The white stuff is most likely a hydroxide by-product of the corrosion taking place.
     
  7. zimbodave
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    zimbodave Junior Member

    Thank you AdHoc

    So I need to get one of my mates to bring a drum of seawater back from the coast.

    I guess the corrosiveness of sea water changes according to temperature.
    Is it true that warm tropical waters have a higher salt (and other compounds) content than cooler temperate waters? Therefore tropical waters are more corrosive? Or make a better electrolyte anyway?

    Thank you

    Dave
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    It is not the temperature per se, but the salt content. The salt content of the world’s oceans is almost the same the world over, 32~37gl-1, which of course includes 30gl-1 of NaCl, or ‘salt’. In coastal areas this can vary according to tide and time of year.

    The Baltic Sea, for example, has a total salinity of 8gl-1, the Black Sea 22gl-1, the Mediterranean 41gl-1 and the Persian Gulf 57gl-1. This is because of the lack of refreshing the sea water has, it is 'trapped' in essence. The salt content determines the specific electrical conductivity of the water and thus how corrosive it is to the metals.
     
  9. gonzo
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    gonzo Senior Member

    Temperature is also a factor. The chemical reaction is accelerated at higher temperatures.
     

  10. CDK
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    CDK retired engineer

    A more scientific approach is required here to avoid random results.
    Both copper and brass samples must be identical in shape and size, clamped or screwed to the clean aluminum plate to ensure electrical contact.

    There is a vast difference in electrical potential between Al and Cu, so corrosion will show around the samples within a few days. The amount of NaCl determines the conductivity of the electrolyte and consequently the place where most corrosion takes place: a weak solution causes corrosion nearby, a strong solution corrodes the whole surface.
     
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