This is yet another example of stress corrosion cracking. The bending stresses induced by the imperfect flatness of the stacked sheets caused tensile stresses in surfaces to which some stray ammonia had access.
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This specimen was taken from a pile of slightly cold worked
sheet brass stored for a long time near a nitriding furnace, which used
ammonia to produce atomic nitrogen to surface treat heat treated steels
(about which, more, later). The magnification of the
photomicrograph is 500X. This is yet another example of stress corrosion cracking. The bending stresses induced by the imperfect flatness of the stacked sheets caused tensile stresses in surfaces to which some stray ammonia had access. |
| SUMMARY:
Corrosion failures such as those you have seen here often involve
synergistic (mutually reinforcing) interactions between the stresses,
chemical environment, temperature, and microstructure (i.e., processing
history) of susceptible metals. Go back over each specimen in
this set to be sure you understand the synergism in each case. |