ASM Online Member Community

  • 1.  tunneling corrosion

    Posted 05-08-2021 07:37
    I would like to share some images for MIC of St.St. 304 pipe.
    The SEM image shows some bacteria inside the corrosion pit, while the optical micrograph shows the tunneling corrosion paths. I am wondering if someone knows if there is any preferred path for this tunneling corrosion?

    tunneling parallel to the longitudinal axis of pipe

    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection and Welding
    Maadi, Cairo, Egypt
    Metkon USA - Technology behind Specimen

  • 2.  RE: tunneling corrosion

    Posted 05-09-2021 10:49
    I have not dealt with tunneling corrosion, but I do have experience with localized corrosion in 304.  Generally, corrosion will follow defects, which in 304 are generally MnS inclusions and sensitized regions.  If sensitization occurs (removal of Cr from the matrix, generally as M23C6 carbides), it tends to be along grain boundaries.

    Look at the direction the tunnels propagate relative to the rolling direction and grain boundaries.  I have seen stress corrosion cracking transition to crevice corrosion along MnS inclusions parallel to the rolling direction.  To check for sensitization, polish the surface and electrolytically etch in oxalic acid; if it looks like flat grains with the GB trenched, it's sensitized, if individual grains etch rather than the GB, it's not sensitized.

    David Sapiro
    King George VA

    Metkon USA - Technology behind Specimen

  • 3.  RE: tunneling corrosion

    Posted 05-10-2021 08:16

    From my past experiences in MIC, mainly in cooling towers, the preferred paths were more driven by the geometry of the system, not the microstructure of the base material. Anaerobic bacteria tend to form a covering (not remembering what this is called) where they live under. The ability to form this colony seemed to me to be more a function of the geometry of where in the system they could form these more easily. 


    David Jones
    Senior Principal Materials Engineer
    Stryker Orthopeadics
    Mahwah NJ
    (914) 469-2958

    Metkon USA - Technology behind Specimen