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Dissimilar metal corrosion with chemical filmed (Alodine, Iridite) aluminum

  • 1.  Dissimilar metal corrosion with chemical filmed (Alodine, Iridite) aluminum

    Posted 4 days ago
    mil-std-889 dissimilar metal chart Attached is the dissimilar metal chart from Mil-STD-889. We have all seen this "dissimilar metal" chart in one form or another. 

    Aluminum (Group D/E) is listed as being incompatible with all other metals in every atmosphere-industrial, marine and sea water. 

    Aluminum is never used bare-the common coating for electrical conductivity is a chem film chromate conversion coating per Mil-DTL-5541 (Alodine, Iridite common trade names). 

    With a Mil-DTL-5541 coating under what group (A→T)  would chem film aluminum fall under? Since it is a chromate conversion coating would it be chromium Group J? 

    For example if the aluminum was nickel plated it would be Group P. 

    Thanks for any responses! 
      

      

      


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    William Pinzon
    BAE Systems
    Greenlawn NY
    (516) 735-5413
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  • 2.  RE: Dissimilar metal corrosion with chemical filmed (Alodine, Iridite) aluminum

    Posted 2 days ago
    Mr. Pinzon,
    Your statement of "aluminum is never used bare" is perhaps a little too broad.  Of course that got me some projects in failure analysis over my career!  I have seen remarkable examples, including a textbook exfoliation of an alloy aluminum I-beam bare end, used structurally beneath a power station generator to avoid magnetic interaction.  Unfortunately it was in a seacoast environment.  For what it was worth, it did take decades to deteriorate to the point that it needed to be replaced, but only because it was in a sheltered location.

    Also there was a series of failures in cast aluminum high-voltage conductor support shoes, where the supporting pin was galvanized steel, leading to corrosion product build-up in an enclosed volume and failure due to expansive forces.  Also on a seacoast.

    In my opinion, in a situation where aluminum is used in contact with another metal, you can only move it out of the classification of "subject to dissimilar metal corrosion" when you can 100% guarantee there will not be galvanic contact, ever.  Because of the fragility of any coating compared to the base metal, when the surface asperities come into contact the coating will likely fail locally.  The amount and rate of corrosion that results will totally depend on the service conditions of a conductive liquid film.

    I don't think that the intent of the ratings table was that it should be extended to coatings.  It is intended to guide the connection of two dissimilar metals, and only that.  A chromate conversion coating shouldn't be treated as a metallic surface for purposes of connecting it to a metal part.

    If a coating is 100% inert, full coverage, and impervious to all damage including assembly movement and removal by fastener contact, then the contained part's corrosion resistance would never matter.  As coatings rarely meet that standard, then the question of what the results of coating failure would be (failure mode effects analysis) must be considered.

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    Paul Tibbals, P.E.
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