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  • 1.  Corrosion of zinc electroplated 4140 steel fastener

    Posted 09-21-2022 09:06
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    I have a 4140 steel set screw with a 5 micron zinc electroplated surface. It is exposed to gulf coast atmosphere. We often see some rust on the non-threaded top of the screw where the zinc may have been scraped away during installation. Would the rust continue to attack the steel down through the fastener, or would the initial iron oxide layer prevent further corrosion?

    John Merrill
    Principal Engineer
    Matthews NC
    FAS - education

  • 2.  RE: Corrosion of zinc electroplated 4140 steel fastener

    Posted 09-22-2022 15:42

    It will keep right on rusting.  For what it's worth a zinc electroplate usually has a trivalent chrome topcoat.  The expected salt spray test minimum is 96 hours to white corrosion.  There are other coatings out there (zinc/aluminum flake) that offer over 720 hours of salt spray results and eliminate hydrogen embrittlement failures.

    Michael Connelly
    Connelly Consulting
    Chicago IL
    (312) 972-8556

    FAS - education

  • 3.  RE: Corrosion of zinc electroplated 4140 steel fastener

    Posted 09-23-2022 15:09
    I am not confident that even using a zinc-aluminum flake will eliminate hydrogen embrittlement, because it depends on the yield strength of SAE4140 bolt, and whether it is in a saline environment from either deicing salts or ocean exposure.  If the galvanic current from the Zn-Al coating is sufficient to create hydrogen evolution at the bolt cathodic sites and the media is acidified due to chlorides, you may get bolt failure, especially if the bolt is tightened to near yield using turn-of-the nut for torque.  This occurred with hundreds of bolts on the Tappan Zee Bridge when A490 bolts were galvanized.

    Christopher Hahin
    Engineer of Structural Materials & Bridge Investigations
    Illinois Department of Transportation
    Springfield IL
    (217) 522-4023

    FAS - education