Galvanic corrosion

Discussion in 'Materials' started by urisvan, Jun 11, 2020.

  1. urisvan
    Joined: Nov 2005
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    Location: istanbul

    urisvan Senior Member

    i have an aluminium mast of my dinghy and i need to attach stainless fittings on it by rivets. Before i was using "tefgel" to prevent galvanic corrosion, i was also putting tefgel on rivets too. Actually it was working fine...

    What else can i use instead of tefgel??

    Regards
    ulas
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    fallguy likes this.
  3. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    I've cut spacers out milk jugs.
     
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  4. urisvan
    Joined: Nov 2005
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    urisvan Senior Member

    Hi Blueknarr,
    But what about rivets?
     
  5. Blueknarr
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    Blueknarr Senior Member

    Didn't claim it was a perfect system
     
  6. David Jones
    Joined: May 2020
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    David Jones Junior Member

    Your stainless rivets are more noble than your aluminum mast and the area ratio is in your favor. It may not be "perfect" - but it works.

    dj
     
  7. fallguy
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    fallguy Senior Member

    okay, hold on a second

    would it be preferred for the hole or the rivet to fail?
     
  8. Will Gilmore
    Joined: Aug 2017
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    Location: Littleton, nh

    Will Gilmore Senior Member

    The nature of galvanic corrosion was first discovered when ships were plated with copper sheeting. They were using iron bolts to hold the copper sheeting on. When ships were brought out of the water to have their plating repaired, they noticed that some suffered more corrosion than others, which often suffered from almost no corrosion around the iron bolts. The difference, it was discovered, was that suppliers of the copper sheeting would cover the copper with heavy paper to protect it. Some shipyards would peel the paper off before bolting the copper to the bottoms, while other yards left the paper on and bolted right through the paper before cleaning the paper off. The bottoms that were bolted through the paper trapped just enough paper between the bolt heads and the copper sheeting to isolate the two metals from each other. It really doesn't sound like it takes much to isolate from galvanic corrosion. It is also important to reduce contact with the electrolyte. Two dissimilar metals don't generally corrode without an electrolyte. Mastheads see little salt spray and dry quickly in the sun and wind. A masthead should have less problems then bowrails, for example.

    -Will (Dragonfly)
     

  9. Will Gilmore
    Joined: Aug 2017
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    Location: Littleton, nh

    Will Gilmore Senior Member

    What David is saying is that the more noble metal pulls electrons from the less noble metal, but it can only do that at a given rate determined by a number of factors including path to ground, how disparit the two metals are from each other and how much material each is made up of. A small amount of the more noble metal surrounded by a much larger amount of the less noble metal will result in a very slow corrosion rate. All the surrounding aluminum material contributes to feeding the relatively tiny steel bolt. Also, your mast isn't pure aluminum, it's an alloy that has much more resistance to corrosion than pure aluminum.

    -Will (Dragonfly)
     
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