Latest News: Forums Technical Stainless Chain, Aluminium Anchor

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  • #4410
    Fundoctor
    Member

    Guys

    Is there a problem with attaching a stainless chain (stainless only due to love of shiny things) to aluminium anchor? Is a swivel connector worth the dosh?

    Trevor
    W9002

    #10715
    admin
    Member

    There is a corrosion issue, see http://www.bssa.org.uk/topics.php?article=89

    I copied the relevant text below:

    Corrosion risks with aluminium and stainless steel in contact

    Aluminium and stainless steel together also appears to be a bi-metallic corrosion risk, from the ‘nobility’ table.
    With this combination the affect of relative surface area on corrosion is important.

    A large area of ‘cathode’ relative to ‘anode’ will accelerate the anodic corrosion. Although aluminium is anodic to stainless steel, large relative surface areas of aluminium to stainless steel can be acceptable, dependant on local conditions.
    Stainless steel fasteners in aluminium plates or sheets are normally considered safe, whereas aluminium rivets or bolts holding stainless steel parts together is an unwise combination, as there is a practical risk of corrosion.

    An example of the safe use of stainless steel and aluminium together is where stainless steel fasteners and hold down bolts are used to secure aluminium roadway or bridge parapet guards.
    Even with no insulation between the metals, there should be little risk of corrosion.

    In contrast, in a marine environment, severe localised pitting corrosion to the aluminium treads has been observed where un-insulated stainless steel bolts were used to secure the treads in place.

    On the same ladder however, bolts with sound insulating washers did not show any pitting on the surrounding aluminium.

    This illustrates the beneficial effect of breaking the corrosion cell by isolating the two ‘dissimilar’ metals in marginal cases.

    On the Nobility table on the BSSA website you will see that aluminium and zinc are next to each other implying no significant problem. So that will be why we mostly use galvanized chain then!

    However, since you have money to burn how about this? http://www.force4.co.uk/7074/Force-4-Stainless-Steel-5kg-Claw-Anchor.html

    #10716
    Swiebertje
    Participant

    Mike what you say is true but the galvanic voltage between aluminium and galvanised steel is even bigger.

    I have been sailing for over ten years with an Aluminium Fortress anchor shackled to a galvanised chain by a galvanised D-shackle. When I got it, it was already used for years by another Wayfarer sailor (Boat jumble). Back then the idea was to ditch it when it got rusty. I can buy five galvanised chains for the price of one SS one. Since the galvanised chain is still OK it will take me at least 50 years until a SS one is cheaper.

    Since I only use 5 meters of chain and because it is connected to 25 meters of rope I don’t need a swivel. If anything twists it will be the rope. I carry the rope and chain on a drum (on an extended mast pin). This ensures the rope and chain are never twisted when I present the anchor.

    #10717
    admin
    Member

    Well I am no expert although I am an enthusiastic internet surfer 😛 . The following was also found on the web:

    Problems can also occur where the metallic combination is galvanised steel and aluminium. The zinc coating of the galvanised steel will, at first, prevent the aluminium being attacked. However, this protection disappears when the steel surface is exposed after the consumption of the zinc.

    As it has a thicker zinc coating than electroplated material, hot dip galvanised material gives longer protection. Thus, in combination with aluminium in aggressive environments, hot dip galvanised material should be used.

    The suggestion is, I guess that old worn galvanized links of a chain may expose the raw steel underneath, at which time corrosion occurs. I take the view that like sh*t, corrosion happens, we can only do our best to limit its effect but with everything damp and possibly salty, eventually it’s your bank balance that corrodes as you buy replacement gear. Wash everything off with fresh water and don’t keep stuff in waterproof bags and there is a chance it might still be there tomorrow.

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