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.
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David Sapiro
Engineer
King George VA
dosapiro@gmail.com------------------------------
Original Message:
Sent: 05-08-2021 07:37
From: Waleed Khalifa
Subject: tunneling corrosion
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Waleed Khalifa
Principal and CEO
Arabic Consultancy Center for Engineering Materials, Inspection and Welding
Maadi, Cairo, Egypt
01098163293
accmiw@ymail.com
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